Irrigation, drainage and salinity

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Irrigation, Drainage and Salinity ' 11

A N INTERNATIONAL SOURCE BOOK, FAO/UNESCO

e

H UTCHINSON/FAO/UNESCO

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Throughout history,irrigation in arid zones has often led to soil salinization,thereby creating a problem which has faced m a n for centuries.With the increase of the world's population and the rapid development of irrigated areas,this question becomes more and more urgent: not only have unproductive lands to be put under cultivation but also the fertility of irrigated areas has to be m a i n ta i ned . The experience acquired throughout the centuries and the considerable amount of research which has been carried out on irrigation and soil salinization have produced a great variety of data. In their work, Unesco, within the framework of its Arid Zone Major Project,and F A O in its activities on Land and Water Development, have already m a d e constructive efforts in the collection,synthesis,and dissemination of such knowledge. However, m a n is often hesitant when attempting to select the best method for the improvement of existing irrigation schemes which are being spoiled by waterlogging and salinity and for the reclamation of virgin lands where these hazards may arise. Such a situation is obviously detrimental to newly developing and emerging countries where the shortage of experienced staff makes it imperative to provide simple and clear guidance on the procedures to be followed.During a joint meeting of FAO and Unesco Secretariats in 1960,it was proposed'to carry cut a world-widesurvey of the knowledge available and eventually to publish the information obtained. The aims of this publication would be: ( a )To provide a summary of modern scientific concepts in a formconvenient for use by administrators,engineers,agronomists,hydrotechnicians, soil scientists,and other specialists dealing with irrigation and drainage methods and practices in relation to salinity and alkalinity of arid lands. (b)To review procedures for forecasting water requirements of crops and the irrigation and drainage needs for irrigation projects under various conditions,giving particular attention to the prevention 01' salinity and alkalinity. (c) T o discuss irrigation and drainage systems and ma IIage m e ti t pro ble nis i ti relation to s a I in i ty and a I ka1 in i ty . (d)To assemble data required by the specialists concerned in the design and operation of irrigation and drainage works in the arid zone. In other words, it was proposed to prepare a sourcebook which would provide (1) a synthesis of modern scientificknowledge,(2)a guide to the practical experience obtained by leading countries i11 the field of irrigation and drainage methods in relation to salinity and alkalinity of arid lands, ;irid (3)generalizations useful in the solution of practical problems in this field. coiitiniieil o11 Duck pap

This proposal was adopted by the General Conferences of Unesco in November 1960 and of F A O in November 1961.To attain the proposed goal, a special procedure was needed in order to provide for general consultation and exchange of opinions on the subject.It was pointed out at the beginning that such an integration of knowledge on an international basis would require numerous contacts in order to make sure that all opinions were duly taken into consideration;moreover,this was the first time that such a work had been undertaken on a world-widescale.At the first meeting in R o m e in 1960,it was decided that a group of three editors should be appointed jointly by FAO and Unesco and that authors and coauthors for the din'erent chapters should be selected by mutual agreement from among the most experienced specialists working personally o n the problems under consideration.In addition, contributors selected from member countries of eiher organization would be invited to co-operate. The source-book would thus be a synthesis of all the information gathered and no effort would be spared in order to ensure that the book gave a complete review of the knowledge available in the world, making clear the points of agreement and indicating subjects where research is needed. Accordingly, FAO and Unesco asked Prof. V. A. Kovda, Prof.R.M .Hagan and Dr.C.Van den Berg to act as editors.They generously accepted this responsibility and nominated Prof. I Q 3. unstabilised negative,non-compensated;I< Q

The above three basic types of groundwater balance satisfactorily explain the processes of natural and secondary salinisation and swamping of irrigated soils and provide a basis for the planning of irrigation and land improvement projects. E

43

I R R I G A T I O N , D R A I N A G E A N D SALINITY (a) Stabilised compensated type (I= Q) This type of groundwater balance exists in cases when the total annual groundwater inflow is offset,more or less completely, by the total groundwater expenditure. In this case,I=Q. With this type of balance, the groundwater reserves remain roughly the same,subject to small variations,from year to year,i.e. it remains stable.Seasonal changes in groundwater level follow the samepattern every year,with slight deviations due to general meteorological conditions or irrigation. Because themain itemsofgroundwaterexpenditureare subterraneanouflow,transpirationand evaporation, occurring in varying proportions, a distinction must be drawn between the following sub-types1. compensation by subterranean outflow when I= Qgw. This sub-typeofgroundwater balance is established on well-drained territories with deep lying groundwaters (over 10 m). Soils in such territories are at some stage of natural de-salinisationand do not,in the majority of cases,undergo secondary salinisation due to irrigation. 2. Compensation by subterranean outflow and transpiration when I= Qgw+Qr.Territories with this subtype of groundwater balance usually have a groundwater table 2-5 m or less in depth, with the result that outflow is supplemented by transpiration.Typical ofthis sub-catetoryare fertile meadow soils little subjectto salinity except for the accumulation of lime,gypsum and,sometimes,soda or sodium sulphate.The groundwaters in this case are as a rule fresh or slightly mineralised. 3. Compensation by subterranean run-off,transpiration and evaporation when I= Qgw+Qr+ Qe.This sub-typeof groundwater balance is met frequently in irrigation farming. Thanks to the combination of outflow and partial evaporation,the salt balance is fairly stable and the soil fairly fertile. This type ofbalance is often established as a result of the installation of an extensive system of deep drains.Table 2.2contains data on this sub-typeof groundwater balance, for the eastern part of the Hungry Steppe. Table 2.2. Groundwater balance in the eastern part of the Hungry Steppe-(A fter RESHETKINA)

Items of balance

Mean annual (data in ma/sec)

Volume millions

40 1 a7 12.7

1260 53 400

54.4

1713

25.5

803

46.8

3.2

101

5.9

21 4.7

661 148

38.65 8.65

54.4

1713

%of annual total

(m”Yd

Influx

Subterranean inflow Atmospheric precipitations Irrigationlosses Total

73-2 3.45 23.35 100

Expenditure

Evaporation and transpiration Outflowvia collector and drains (into Syr-Daryariver) Outflow intQ the Syr-Darya (subterranean) Outflow into the Kazakh SSR Total

100

4. Compensation by transpiration and evaporation.In this case I=Qc+Qe.This sub-typeofwater balance is found in territories with inadequate drainage, formed on little permeable sedimentary rocks. The groundwaters lie close to the surface (1.5-3 metres) and have a wide seasonal fluctuation range (50-100 cm). They are highly mineralised and the soils are extremely alkaline or saline.Irrigationcauses intensive waterlogging and salinity.For purposes of successful development,lowering ofthe groundwater table and improvement of the circulation and outflow of groundwater,drainage has to be installed. 5. Compensation by evaporation when I=Qe.This sub-typeof balance is not as widespread as the preceding sub-type.It occurs in naturally undrained territoriesformed of little permeable heavy sedimentary rocks in deltas, depressions and lake and sea shores. The groundwaters occur at a depth of 0.5-2 metres and are subject to sharp seasonable fluctuations as much as 1.5 metres. They sometimes intersect the soil surface.

44

W A T E R A N D SALT B A L A N C E S

Groundwater evaporates directly and this causesintensive salinisation ofgroundwatersand soils.Themineralisation of the groundwaters is of the order of 70-150 g/l and more. An illustration of such a groundwater balance is given below (Table 2.3). This type ofterritory can only be developed satisfactorily by the installation of a dense network of deep drains and intensive leaching. Table 2.3. The balance of an irrigatedpart of the Yaz-Yavanlcssteepe of the Fergana valley-(After KRYLOV)

Month XI XII I

II III IV

V VI VI1 VI11

IX

x

Total

Precipitations

1"( 3.0 7.0 4.0 6.0 12.0 5.0 2.0

seepage of

irrigation water ( " )

0.0 0.0 0.0 0.0 0.0

8.2 7.2 3.0 2.0 12.0 25.0 35.0 50.0 80.0 73.2 34.0 5.0

39.0

334.6

Evaporation and transpiration

1"( 8.0 2-0 2.0 5.0 15-0 38.0 52.0 60.0 72.5 68.0 40.8 14-0 377.3

Balance ( " )

+ 3.2 + 12.2 + 5.0 + 3.0+ 9.0

- 8.0 -15.0

-10.0 + 7.5 + 5.2

- 6-8

- 9.0 - 3.7

(b) Unstubilisedpositive non-compensatedtype (I> Q) The main feature of this type of groundwater balance is the systematic increase of the groundwater reserve, due to the fact that the total annual groundwater influx exceeds the total annual disposal by which it is not, therefore,compensated. This type of groundwater balance leads to an average in groundwater level from year to year. Over-compensationmay be due to various factors: 1. increase in influx items 2. decrease in groundwater expenditure with influx remaining unchanged 'Decompensation' ofthe groundwater balance through increased influx (seepage from canals and on fields) may affect groundwaters lying at depths of as much as 30-40metres.As the groundwatersdraw nearer to the surface,they will become more mineralised (because the saltsin the subsoils are dissolved in the rising water), and evaporation and transpirationwill constitute an increasinglylarge proportion of their total expenditure. When the groundwaters pass the criticallevel,consumption through evaporation and transpirationwill be so intensethat compensationmay set in and the groundwater level may be stabilised at a constant mean depth. When this happens,however,the entire territory of the irrigation project will undergo intensive secondary salinity accompanied,in some cases, by waterlogging. The establishment of an over-compensation type of groundwater balance in an irrigationproject will in most cases have catastrophic effects on soilfertility,roads and buildings,and also on the health of the population. The main way of controlling groundwater fluctuations and preventing the establishment of this type of balance is to decrease influx of all kinds (construction of a water-tightirrigation network,improvement of quality and method ofapplication ofwaterings, and,where economically feasible,utilisation ofsprinkling). If these measures are inadequate,it will be necessary,in addition,to install an extensive network of drains and collectors or pump-wells,so as to provide an intensive artificial groundwater run-offand so stabilise the balance. (c) Unstabilised negative non-compensatedtype (I< Q) The main feature of this type of groundwater balance and regime is the systematic decrease of the groundwater reserve and lowering of the groundwater level, year by year. This is due to the fact that the total annual influx is less than the total annual expenditure of the groundwaters.The establishment of this type of

45

IRRIGATION, DRAINAGE A N D SALINITY

groundwater balance on irrigated territory brings about a fast improvementofany irrigation project suffering from waterlogging and secondary salinisation. A distinction must be drawn between at least two sub-typesof this type of groundwater balance:

1. non-compensated balance due to decrease in groundwater feeding 2. non-compensatcdbalance due to improvement of outflow Thus if measures are talten, within an irrigation system, to improve water management by planned, restricted watering norms,improve the technical standard of canals and introduce watering by sprinkling, the sourcesofgroundwaterfeeding will be reduced;and this will lead to the establishment ofthe desired undercompensated water balance,with a drop in the groundwater table.This sub-categoryof groundwater balance is extremely favourable for irrigated territories because it :

(i) (ii) (iii)

decreases the degree of waterlogging decreases the intensity of the salinisation process, which may sometimes even be replaced by desalinisation improves the general productivity of the land

This is one of the cheapest means of bringing about a general improvementin irrigated territory affected by secondary waterlogging. When groundwater feeding is decreased, a non-compensated groundwater balancé will gradually be transformed into a compensated type of balance,since a new ratio between groundwater influx and consumption is established. As the groundwater level drops,evaporation decreases,which in turn results in the stabilisation of the groundwaters at a definite depth. A non-compensatedgroundwaterbalance duetointensification ofsubterraneanoutflowisusually established as a result of installing a drainage canal system or else of pumping the groundwaters off through a system of tube-wells.Itmay also ariseinnatural conditions,however,in rising areas,where the system ofstream channels has been lowered as a result of erosion. Generally speaking,this type ofgroundwater balance is characteristic of irrigated farming areas where collector drains have been installed and are in full operation. After the installation of the collector drain system,the groundwater balance undergoes a radical change, owing to increasingoutflow. This type of Balance provides extremely favourable conditions for the success of land improvementmeasures. It has been observed to exist in newly drained irrigation projects in Central Asia, North Africa and North America.The loweringofthe groundwatersis accompanied by a slowing down ofthe rate of the capillary rise of salt solutions from the groundwater table. The predominance of downwardflowing streams causes desalinisation of the soils.After severalyears ofthis process,this type ofgroundwater balance changes and a compensated groundwater regime is established, since the amount of evaporation and transpiration drops and the mean groundwater inslow and outflow are becoming practically equal. General analysis of available data shows that the compensated type of groundwater regime and balance remains stable for long periods of time; whereas non-compensatedtypes of balance last for comparatively short periods (5-15 years) after which, as a result of the levelling out ofinflow and expenditure factors,they give place to secondary compensated types of balance. To conclude this section,Table 2.4gives a review of the main types of groundwater regimes and balances existing in the irrigated farming projects of the USSR. 2. Salt balances of soils and territories The salt balance of a territory is closely connected with the water balance ofthat particular area;though it is more than merely a reflection of the constitutent elements and trend of the groundwater balance. The salt balance is greatly inffuenced by the concentration of the solutions. Groundwater outflow may be small, representingonly a few per cent of the total expenditure;but, if the groundwaters are highly concentrated, the total outflow of salts will be large.Even a minimum amount ofmineralised groundwater outflow may be suficient to regulate the salt balance of the area.A vital factor in the salt balance is the original reserve of easily soluble salts in the groundwater and soil layers,Even extremely large reserves of soil and groundwater may be dried up by evaporation and transpiration.Reserves of easily soluble salts contained in the soil,on the other hand,remain constantly in the soil horizons or groundwater,though undergoing extensive seasonal re-distribution.The second very important factor in the typology of the salt balance is the concentration oí' salts in irrigationwater, soil solutions,and,in particular,in groundwater.The rising or sinking ofthe level of the groundwaters and of the capillary moisture within the soil layers may give rise to a very extensive re-

4G

W A T E R AND SALT B A L A N C E S distribution ofthe easily solublesalts,and to an increase or decreaseinthe salinity ofthe various soilhorizons. The following items in the salt balance must therefore be taken into account: total reserve of easily soluble salts at various periods influx of salts from various sources over a given period outflow of salts in various ways over the same period

Table 2.4 Main types of groundwater regime in irrigated zones of the USSR (KOVDA)

(a) Stabilised,compensated (primary,secondary)

Sub-category

Probable trend of salt-accumulation processes

(1) Compensated by

Desalinisation.Little danger of secondary saliisation

subterranean outflow (2) Compensated by subterranean outflow and transpiration (3) Compensated by outflow,transpiration and evaporation Compensated by transpiration and evaporation Compensated by evaporation

(b) Non-stabilised ‘decompensated’(+)

(c) Non-stabilised

‘decompensated’(-)

‘Decompensated’by increase of feeding ‘DecomDensated’by decrease of out- flow (1) ‘Decompensated‘ owing to decrease of feeding (2) ’Decompensated‘

owing to increase of oufflow

Desalinisation;CaCO,and Caco, may accumulate in subsoil; secondary salinity may occasionally occur Slight salinisationwith accumulation of Nazco,; Na2S04;if methods of farming are poor,there may be strong salinisation

Strong progressive salinisation with accumulation of large quantities of NaCI, MgClZ, MgS04, Na,SO, When the groundwaters reach the critical level,an intensive,rapid process of salinisationsetsin, sometimescomplicated by waterlogging With groundwaters around and above the criticallevel:increase of salinity;with groundwaters below critical level:desalinisation De-salinisation,increasingas the groundwater table drops

The main items of salt influx are: Capillary solutions rising from the groundwater.When there is intensivetranspiration or evaporation of the groundwater,this will be the largest item of salt influx. Inflow of saltswith irrigationwaters. This factor does not operate in non-irrigatedareas.In irrigated soils,it is of little importance when the water used is fresh (0.1-0-3g/l); but when brackish waters are used, the importance of this factor is very great, sometimes decisive for determining the whole character of the salt balance. The inflow ofsaltsfrom atmosphericprecipitationsisusually not large,exceptin the vicinity ofseas,oceans or salt lakes,where this item of inflow may assume local significance.The inflow of salts through wind and atmospheric precipitationsis greatest over a belt severalkilometres in width;but the effects are often feltto a distance of dozens of kilometres from the sea coast. 47

IRRIGATION, D R A I N A G E A N D SALINITY

There may be a certain limited inflow of salts through the use of fertiliser,but the significance of this item is mainly theoretical.

A certain part is played by the products of mineralisation of vegetable and animal organisms. The expenditure side of the salt balance is constituted by factors operating in the opposite direction. In non-irrigatedsoils,the main factor is the migration of salts out of the soil into the groundwaters,together with atmospheric water seeping in. In irrigated soils,this process is supplemented by the removal of easily soluble salts together with the irrigation waters;also during leaching or when rice is grown. With a drainage system in operation,these itemsmay assume vast proportions and bring about a radical change in the degree of soil salinity.The removal of easily soluble salts with the harvest ofagricultural crops does not constitute a factor of any great importance.The washing or blowing away of salt crystals from the soil surface may sometimes play a certain role. The practical significance of these forms of removal of salts from soils is, however, very small. In very simplified general form,the equation of the salt balance of soils may be expressed as follows:

s=s,+(Su,-suw)-I- s,,, where : S=the changed total salts reserve S,=the total salts reserve at the beginning of the salts balance period Su,=inflow of salts from groundwaters suw =removal of salts into groundwaters Si,=inflow of salts with irrigation waters

It is possible to divide the salt balances of soils,areas and landscapes into the three following main types, according to the ratio between inflow and disposal elements in the balance: Stable type of salt balance With this type of salt balance, the reserve of easily soluble salts in the soils or area varies little and the groundwater balance is regulated by slight subterranean outflow and transpiration.

Balance of salinisation In this case,the reserve of salts in the soil layer or area increases year by year. This type of salt balance is characteristic of sea deltas and of continental (dry) deltas, alluvial plains of the lower reaches of rivers and undrained depressions,where the groundwater balance is regulated mainly by evaporation,withtheresultthat salt accumulation processes prevail over processes of salt migration and removal. Bulance of de-salinisation This type of salt balance is characteristic ofterritorieswith natural or artificialdrainage,such as lifted ancient river terraces,watersheds and fore mountain plains. The groundwater balance in such landscapes is regulated mainly by subterranean run-off,as a result ofwhich de-salinisationprocesses prevail in the soils and throughout the whole of the area.

(a) Examples of the salt balance of irrigated soils The salt balance may be tested during the vegetativeperiod,throughout one year covering the entire seasonal cycle,or throughout a number of years,as a check on the effectiveness of amelioration measures. Table 2.5contains data for the easily soluble salts balance of the irrigated soils of the Pakhta-Aral State Farm,covering a three-yearperiod. The easily soluble salts reserve in the irrigated saline soils of this State Farm,fluctuate considerably from season to season and from year to year. According to the irrigation regime and the efficiency of farming and watering methods, either a salinisation or a de-salinisationtype of salt balance will be established. Thus for instance,on a medium saline sierozem (plot 6), for the period 1937-38, a de-salinisation type of salt balance was established during the first year of crop cultivationby means of leaching under alfalfa and carefulwatering.In 1939,the soil again became saline,since there was only one watering.As a result,the salt balance of the soil on plot 6 remained stable over a period of 3 years but with a tendency to gradual salinisation. The salt balance established on saline sierozems (plot 13) after alfalfa had been grown for three years was of

48

W A T E R A N D SALT B A L A N C E S a differenttype.Here,though the soilwas originally strongly saline,a de-salinisationsalt balance was obtained by the end of the period 1937-39 by careful watering of the alfalfa. On the solonchak patch (plot 7) during this same three-yearperiod a salinisation type salt balance with slight fluctuations set in,due to the fact that the soil surface was not levelled and waterings were insufficient. The mineralisation of the irrigation waters used at the Pakhta-Aral State Farm averages 0.3 g/l.When average waterings of 5000 maare applied,the amount of salt brought into the soil by irrigation is not more than 1-5t/ha.If we compare this amount with the data obtained for the salt balance on the three plots of irrigated soil investigated,we see that the groundwaters mainly determine the salt balance of these soils. It is particularly importantto analyse the saltbalance ofthe different soillayers,sincethe effects of amelioration measures depend to a great extent on the salinity ofthe arable horizon (0-25c m layer) and ofthe root zone (O-50,O-80 cm). The salt balance in these horizons fluctuatesvery much, since it depends more than the soilprofile does on atmospheric precipitations,waterings and evaporation;it may be of the opposite type to the salt balance of the whole soil profile (Tables 2.5 and 2.6). When the salt balance is stable and the total amount of salts in the soil horizons above the groundwater remains unchanged the salt balance ofthe arable and root zone,after undergoing seasonal and inter-watering fluctuationsin the course of the irrigation period and during the year,will operate,towards autumn, either in the direction of an increase of salts or else of a decrease (with autumn-winter leaching,for instance,or preventative watering).

(b) Salt balance offield or area Inirrigatedfarming,easily soluble salts are highly mobile, in both the vertical and thehorizontal plane. Water seepagein canals;the uneven distribution of water over the surface ofthe field during watering so that certain parts of the field are de-salinised;uneven evaporation from the field surface due to micro-reliefand the uneven thickness ofthe plant cover,may combined promote salt migration in various directions.Easily soluble salts leached out of the soil horizons in one place may accumulate during the same period in the soils of adjacent plots,so that the total salt balance ofthe irrigated area in questionmay remain stable.Thishappens Table 2.5 Salt balance of irrigated soils in the Hungry Steppe,Pakhta-Aral State Farm (in t/hain a layer of 3.5 m>

Number of plot and type of soil

Plot 6

Medium saline sierozem Plot 13 Saline sierozem Plot 7

1937 1938 1939 Spring Autumn Spring Autumn Compared with Spring Autumn Compared with 1937 1937 Spring Autumn Spring Autumn cotton 4th year

alfalfa 1st year

204 209 alfalfa 248 209 cotton plant 4th year

172 154 -32 alfalfa 2nd year 262 - +14 alfalfa 1st year

Solonchak spot on microhump Solonchakspot in 25 crn layer

alfalfa 2nd year seed -55

-

209 217 alfalfa 3rd year 202

129

+5

+8

-46

-119*

alfalfa 2nd year seed

307

282

286

351

-21

+69

289

295

-18

-13

65

112

81

151

i16

+39

52

94

-13

-18

* Compared with spring 1937

particularly often with rice growing,when the quantity ofeasily soluble salts pushed down by large quantities of irrigation water into the lower soil horizons and groundwaters is frequently balanced by the surplus of easily soluble salts accumulating in a wide strip (150-200m) round the edges of the rice fields. Hence the necessity for studying the fluctuations of the absolute reserve of easily soluble salts in the soil layer and groundwaters of an entire area, taking into account the quantities of salts able to migrate in a horizontal plane. This will enable us to draw up a salt balance for the irrigated field as well as for the area concerned.

49

IRRIGATION, DRAINAGE A N D SALINITY In order to differentiate between the inflow and disposal items,which determine the salt balance of a given area, it will be essential to establish the content,inflow and disposal of easily soluble salts in irrigation, overspill,seepage and drain waters. This is somewhat difficult,but with the minute water-balancereadingswhich are part ofthe regular routine of land improvementoperations,these data can be obtained. It is possible,when the balance ofthe irrigation water and groundwatersand also the data for the irrigation, overspill and drainage-collectornetwork are known,to determine the probable quantitative inflow and outflow of salts on the basis of the concentration of those waters. An interesting illustration of salt balance research is provided by the work done by ZAICHIKOV in the Hungry Steppe (Table 2.6). Investigations of the salt balance were made on special plots located on soils representing the main variations of salinity.To estimate the degree of soil salinity,special salt surveys were made on a scale 1 :1000in spring before the first irrigation,towardsthe end of May,in autumn,in the second half of October 1941 and in spring 1942. O n the basis ofsalinitymaps and offigures for volumetric weight,the saltcontentofthe soilwas calculated, firstly in the profile and in a layer of 2 m x 1 m2,then in the soil contours and lastly in fields grown with cotton plants, alfalfa and in virgin solonchaks each taken separately. The total volume of easily soluble saltsin solonchaksoils,in the top 2m layer,is 500-600 t/ha.The amount contained in the same layer of medium saline soils is 300-450 t/ha. Towards autumn, as a result of the seasonal salt accumulation,the salt content in the soil of the virgin solonchak increased to 95tons/ha;that ofthe soil ofthe alfalfa field to 127tons.The cottonfield,on the other hand,which had received a heavy watering in the autumn (about 4000ms/ha) in preparation for sowing of alfalfa amidst the growing cotton plants, showed virtually no increase in salt content.At the same time,the content of easily soluble salts in the plot as a whole, calculated on the basis of a 2 metre deep layer over an area of 106700 m2,increased from 3951-75tons to 4476.80 tons between spring and autumn, i.e. by about 525 tons. Table 2.6. Salt balance of an experimentalplot in the Zolotaya Orda (in tonsper 2 m layer)

Field Cotton

Alfalfa Fallow land Total in 2 m layer Total in 1 m layer

Salt content in spring 1941

Salt content autumn 1941

2166.96 1076.22 708.55 3951.75 1879.02

2176.07 1459.05 841.68 4476.80 1799.29

Balance

-

9.11

+382-83 + 133.13 +525.05

- 79.82

Analysis of the origins of the increase shows that it is due mainly to salt accumulation on the alfalfa field, where the salt content of 1076.22tons registered for May had increased to 1459.05 tons in autumn, i.e. an increase of 382.83 tons. There was also an increase in the salt content of the virgin solonchak,amounting to 133.13 tons (from 708.55 to 841.68 tons). O n the cotton field as a whole,there was virtually no increase in the absolute salt content (2166.96 tons in spring,2176.07tons in autumn). The above salt balance pattern is typical for a mixed farming area. The total salt balance of the plot is of increasing saline type but the salt balance of the cotton field,thanks to watering,is ofthe stable salt balance type. A more detailed analysis ofthe salt balance in the 0-100horizon shows that in this horizon there was a drop in the quantity of salts.O n the other hand,it was observed on virgin solonchaks that the maximum seasonal increase of easily soluble salts occurs in the 0-20 cm layer.Thus on virgin solonchak the salts contentin the 0-100cm layer increased from 288 tons/hain spring to 323 tons/hain autumn;the corresponding figures for the 0-40 cm layer are 123 tons/ha and 173 tons/ha,while in the 0-20 cm layer the salt content increased from 75 tons/ha in spring to 135 tons/ha in autumn. The balance of the total easily soluble salts content in the top metre layer of the whole plot, according to ZAICI-IIKOV’S data, remained stable throughout the vegetation period, thus differing from the balance of the 2 m layer. It was 1879.02 tons in spring against 1799.20tons in autumn-in other words,it even showed a slight drop. This was the result of the effects offarming and watering on the salt regime of the topsoils. 50

W A T E R A N D SALT B A L A N C E S

It is clear from this example that the stable balance for the territory as a whole representsthe outcome of contradictory types of balances established in individual sections of the plot. Table 2.7 contains data showing the effects of slight slopes on the local outflow of groundwater. This example is typical of irrigation projects located in marine deltas. The main features of the relief are flat, undrained depressions,intersected by low elevations and slopes. The water balance of the depressions is of a purely evaporation type, the whole of the inflow being compensated by evaporation loss. Accordingly, groundwaters and soils of the depressions are highly saline. At the same time,the groundwater regime and balance on the elevations and slopes (along which the irrigation canals usually run)is largely regulated by local subterraneandrainage.Inthese places,accordingto the data contained in Table 2.7, subterranean drainage constitutes up to 40% of the total groundwater expenditure;and since the waters ffowing away are alwaysmineralised, salinisation processes do not occur in soils and groundwaters. It is precisely for this reason that,on the slight slopes and elevations of the relief, very fertile soils occur, where large stable yields of cotton and alfalfa can be produced. Table 2.7. Ratio between inflow and expenditure items in the balance of an experiment plot (Chimbaiskregion,Amu-Darya delta)-(After KRYLOV)

Items of groundwater balance

Depression as a whole

inflow and consumptionof water in % Slope of depresCentre of sion along canal depression

InPow Items

Underground inflow Seepage of irrigationwater Seepage from canals Seepage of atmospheric precipitations

O

O 67 30

80

17 .3

3

100 O

60 40

"

Expenditure

Evaporation Subterranean outflow

100

O

(c) Pattern of the salt balance in the northern part of the Caspian lowland Sediments of the Caspian lowland have a high natural salinity,due to various transgressionsof the Caspian Sea.The territory is characterisedby large numbers ofsaltdomes which,when eroded,pass on large quantities ofsalts into the subterranean waters,sediments,rocks and soils.The northern part ofthe depression is wetted by the surface run-offof small and medium sized streams which never get as far as the Caspian Sea.At the same time,the depression has reasonably good natural drainage.The surface levels of the depression are of the order of +40, +20, fO m. The level of the Caspian Sea itselfis now about -25 to -27 metres, so that it constitutes a gigantic natural drain for the whole of the lowland. The arms of the Volga and Ural rivers cut deep into the surface of the Caspian depression. The Volga drains a strip of not less than 25-30 k m in width. All this means that large amounts of salts are removed by the river systems and subterranean waters into the Caspian Sea (Table 2.8). Table 2.8. Pattern of salt balance in the northern part of the Caspian lowland (tons)

Inflow of salts

1. Left in deposits of Caspian transgressions 2. Leached out every year from open salt domes 3. Brought in every year by the waters of the surface river flow

Removal of salts

2.5. 109 3.5.106

1. Removed by open river run-offby the Volga and Ural rivers 1-35.lo6 2. Removed by subter-

ranean flow into the Caspian Sea

26. loe

3.5. lo6

51

IRRIGATION, DRAINAGE A N D SALINITY The pattern of the total salt balance of the Caspian depression contains the key to a whole number of important problems. It explains,for instance,the exceptional salinity of the soil groundwaters and lakes of the Caspian lowland.It also explainsthe exceptionallylarge proportion ofchloridesin the compositionof the easily soluble salts which accumulate in the soils and waters of the depression.And finally,it explains the existence of patches of non-salinesoils with deep,fresh groundwater in some areas. (d) Pattern of tlze salt balance of the Mesilla valley (Rio Grande river) and of the Yumavalley (Colorado river) Research on the salt balance of the irrigation projects of the Rio Grande and Colorado rivers in the USA was carried out in the period 1929-38 by SCHOFIELD. The area ofthe Mesilla valley covered by this research ,measured 100000 acres, grown with cotton and alfalfa. The irrigated area is served by a system of open drains with a total length of 211 miles. On the basis of hydrometric data for the irrigation and drainage network and of a systematic study of mineralisation,a calculation was made of the amount of salts brought in with the irrigation waters and the amounts removed by the drainage network. The mean annual inflow of water into the irrigation system was 744380 acre-feet;the mean annual outflow 496 113 acre-feetor 66.3 %. O n the basis of the figures for mineralisation of the irrigation waters (0.9 g/l), the mean annual number of tons of salt brought into the irrigated oasis was then calculated,i.e.599369.The amount of salts removed by the drainagenetwork in a year was 608076tons.The results ofmany years ofresearch ofthe total salt balance of the Mesilla valley are given in Table 2.9. Table 2.9. Salt balance in the Mesilla valley (averagefor period 1931-37)-(After

SCHOFIELD)

Amount entering (in acre-feet)

Amount removed

744380

495 113

tons

tons

599 369

608076

101.5

(Na +K) (HC08 +CO,) (SO3 (CD (NO,)

80553 17557 101 985 93936 232419 71 306 1418

86.6 88.1 120.3 85.7 89.1 155.8 103.0

Concentration

0-805

69686 15468 122639 80540 206950 111 122 1461 Tons per acre-foot 1.226

Components Water Salts Salt components: (Ca) (Mg)

(in acre-feet)

in

%

66.6

152.3

Comparing the total influx of easily soluble salts with the total amount removed,we see that the latter is slightlylarger-101.5 %oftheinflow.However,somesalts are removed by the drain waters inlarger quantities than others: those removed in the largest quantities are chloride ofN a (155-120%ofthe inflow); nitrates are likewiseremoved in large quantities (103 % of the amount entering the system); carbonates and sulphates of C a are removed in far smaller quantities (85-89 % ofthe amount entering the system). The salt balance of the Mesilla valley is a de-salinisationone. Irrigation, however, causes qualitative changes in the composition of the salts circulating in the valley, owing to intensive removal of chlorides of N a and accumulation of carbonates and sulphates of Ca and Mg. The salt balance of the irrigated area of the Yuma valley, on the other hand, is of the salinisation type. This area measures approximately 50000 acres of irrigated land,located in the south-westerncorner of the state ofArizona,in the Colorado river basin. It is served by a drainage network.The annual irrigationwater inflow into the area is 259800 acre-feet,approximately 57000 acre-feetare discharged by the drainage network and about 260000 acre-feet are lost by evaporation and transpiration from the irrigated surface. During the ten-yearperiod covered by the investigations,the total accumulation of salts in the Y u m a valley was of the order of 390000 tons,i.e. 39000 tons per year (Table 2.10). Analysis of the 1938 balance shows that the sum of the salts removed from the irrigated area by the

52

WATER A N D SALT B A L A N C E S drainage network is only 45-8% of the sum of the salts coming into it,i.e. a very low proportion.It is clear that the drainage system has defects. Schofield did not reveal the reasons for this.However,as in the case of the Mesilla valley, the proportion of chlorides removed is very large (1 16%of those coming in). Thus,despite the factthatthe generalsaltbalance is unfavourable and conduciveto gradual accumulationof sulphates of Na, Mg, Ca, there is a progressive leaching of chlorides, leading to an improvement of the qualitative composition of the salts in the area. Table 2.10. Salt balance in the Y u m a valley during the year ending 30 September 1938 (irrigatedarea 49278 acres; sown area,36350 acres)-(Affer SCHOFIELD)

Components Water

Amount entering (in acre-feet)

Amount removed (in acre-feet)

in %

259917

57095

22.0

tons

tons

248428

113791

45.8

35 552 9113 34895 26303 113771 28024 770

11 517 4111 23 579 11 698 30256 32542 88 tons per acre-foot 1.993

32.4 45.1 67.6

0.956

I

.

44.5 26.6 116.1 11.4 208-5

3. Interrelation of groundwater and salt balances

The interrelation between the water and salt balances may be expressed as follows:

IC=Qo.Cd where

I=groundwater inflow C = concentration of the solutions feeding the groundwaters Qo= all forms ofgroundwater expenditure by outflow which is equalto Q -Qe,, i.e.totalexpenditure minus evapotranspiration Cd= concentration of groundwaters removed by natural or artificial drainage.

As seen from the above,highly simplified but nonetheless useful formula,the water and salt balances depend on each ofthe factors meationed but in extremely diverse and complex fashion.The most unfavourable combination will be when IC>Qo.Cdand when the salt accumulation in the groundwaters increases progressively. The best situation as regards maintaining irrigated soils in good condition will be when IC20 c m (b) medium solonetz soils,horizon A 5-20 c m (c) crusty solonetz soils,horizon A 0-5 c m By the combination of alkalinity and extremely unfavourable physical properties, solonetz alkali soils have exceptionally poor natural fertility. Even on soils with total alkalinity of about 0.07% HCOBand p H 8.7, many cultivated plants fail to develop normally; and with total alkalinity about 0.1 % HC03and p H 9.5, almost all cultivated plants die;a similar effect is observed when the E.S.P.reaches 15 to 30. In Table 3.5 data are given showing a connection between the level of natural fertility of soils and their alkalinity. Table 3.5. Connection between the alkalinity andfertility of soils

ESP. Total alkalinity of water extract in %of HCOBon weight of soil p H of water extract or paste of soil Relative fertility of soil %

78

5

10-15

25-30

50

0.02-0.04

0.05-0.06

0*07-0.08

0.1-0-2

7.5-8.4

8.5-990

9.0-9.5

9-5-10

1O0

60-75

20-30

0.00

SOILS-SALINITY,

IRRIGATION A N D DRAINAGE

T o sum up our knowledge of the geography,ofsolonetz and alkali soils-as yet far from complete-the following general observations may be drawn: It hasbeen established that there are,in every continent of the world,large areas of alkali and solonetz soils of various types. These soils are found in cold (permafrost), temperate, sub-tropicaland tropical belts, i.e.all the way from the sub-Arcticto the Equator and far south of it. Alkali soils and solonetzsmost often coincide with lake and river terraces,young and ancient alluvial plains, and deluvial and proluvial fore mountain plains in depressions and also, occasionally,on high mountain plateaux. Alkali soils and solonetzs occur in regions which have a continental or arid climate at least part of the year (monsoon regions,for instance), and where, therefore, evaporation exceeds run-off either permanently or at least some of the time. Alkali soils and solonetzsare associated as a rule with black humus soils:chestnutsoils and chernozems, prairie soils,meadow soils. They may, however, also occur among the podzol soils in the north, the brown soils ofthe warm belt,the red earths and regurs (black cotton soils) of the monsoon tropics,the tropics, the sierozem of the dry sub-tropicalbelts, and the savannahs of the dry equatorial zone.

REFERENCES USDI,Bureau of Reclamation,V, 2,53. DOMINY F.E.(1964), Irrigation developmentin the western United States as related to the great soil groups, Int. Congr. Soil Sci. Proc., 8th C o m m 6 (Bucharest, Rumania). KELLEY W.P. (1951), Aklali soils, New York. KOVDA V. A.(1937), Saline and alkali soils, Academy of Sciences USSR,Moscow (in Russian). KOVDA V. A. (1946), Chapter 2 (1947), Origin and regime of saline soils, Academy of Sciences, Moscow-

BUREAU OF RECLAMATION (1953), Land Classification Handbook,

Leningrad (in Russian).

KOVDA V.A.(1954), Geochemistry of desert of the USSR,Academy of Sciences USSR, Moscow (in Russian). LOBOVA E.V. (1960), Soils of arid zone of the USSR,Academy of Sciences USSR, Moscow (in Russian). MALETIC J. T.(1962), Principles involved in selecting lands for irrigation, Proceedings of International Seminar on Soil and Water Utilization, South Dakota State College, Brookings, South Dakota, 198-201. of project lands, International Conference on

MALETIC J. T.(1967), Irrigation,a selectivefunction-selection Water for Peace,7,513-24.

MALETIC J. T.and HUTCHINGS T. B. (1967),

Selection and classiJication of irrigable land, Chapter 10, Irrigation of Agricultural Lands, Agronomy Monograph No. 11, American Society of Agronomy, 135-73. ROZANOV A.N.(1951), The sierozems of Central Asia, Academy of Sciences USSR, Moscow (in Russian).

SHUVALOVS. A. and KOROVIN E.P. (1948), On the biogenic complexity of the soil-vegetativecover in the arid zone,Bull. Mosc. Ob. Ispyt. Prirody. T o m 53,Vip. I. (in Russian). SZABOLCSI. (1961), Effects of water regulation and of irrigations on the soil formation processes beyond the river Tisza,Budapest (in Hungarian with English summary). SZABOLCS I. (1965), Salt affected soils in Hungary,Agrokemia es Talajtan, 64,supp. 275. UNESCO (1960), The problem of soil salinity and water supplies, Reports by Soviet scientists presented to the U N E S C O symposium held in October 1958 at Teheran,Moscow. UNESCO (1961), Salinity problems in the arid zones, Proc. of the Teheran Symposium. WILLIS W.O.,PARKINSON H.L., CARLSON C.W . and HAAS H. J., (1964), Water table changes and soil moisture under frozen conditions,Soil Sci., 98,4 244-8.

79

4.Hydro-physicsof Arid and Irrigated Soils*

A. WATER IN SOIL 1. The soil as a three-phasesystem Soil is generally considered to be made up of three phases: solid,liquid (soil water or soil solution) and gaseous (soil air). The solid part of the soil is dispersed and consists in mineral soils mainly of particles of various minerals, the dimensions of which generally range from a few millimetres down to sub-micron.In organic (peaty) soils and organic horizons of mineral soils (forest floors and steppe covering), the solid part consists chiefly of vegetational remains at various stages of decomposition. The dispersed nature of the solid part of the soil accounts for two essential soil characteristics,the first being that the soil forms a porous body, and the second, that the solid part of the soil has an extensive surface. The liquid and gaseous phases occupy the pores between the solid particles in an inverse relationship. The porosity of the soil can be expressed in two main ways: either in relation to the total volume of the interstices,or in relation to their size. Total soil porosity,expressed as a percentage or a ratio of the total soil volume,is a dimensionless value which in mineral soils varies from 25 to 60%,but usually comes within the range of 40 to 50%.In peaty soils and forest floors,it may exceed 90%.The major variations in it are mainly due to the structure of the soil,which may be either loose or compact, and which in turn depends on the degree of soil aggregation. Taking as our model an ‘idealsoil’consisting of spherical particles of uniform size,the total porosity may vary from 48%when the soil is very loosely packed (cubic packing) to 26% when it is very tightly packed (hexagonal packing). W e might expect hexagonal packing to occur more frequently in nature. However, porosity equal or close to 26%is characteristic only of gley soil horizons,in which there is no aggregation and all particles exist independently,whereas in non-gleyhorizons possessing a micro- or macro-structure the porosity always exceeds 40%.The explanation is that soil consists of micro-and macro-aggregates.Ifthe particles in the micro-aggregates are extremely closely compacted,correspondingto a porosity of 26%,and if the micro-aggregatesthemselves are also densely packed,total porosity will amount to 45 %,i.e.the value that we most frequently find in the lower non-gleyhorizons of mineral soils. In the upper well-structured horizons of humic soils (for instance, in chernozems), aggregation may take place in three stages,i.e. the macro-aggregatesmay consist of micro-aggregates,which in turn consist of separateparticles. Ifthe packing is maximum in all three cases,total porosity will reach a value of 59 %.Hence the values for total porosity observed in nature do not belie the possibility of particles and aggregate packing at maximum density. The dimensions of the pores depend mainly on the size of the particles and micro- and macro-aggregates. In an ‘idealsoil’,the diameters of pores range from 0.41R to 0.73Rwhen the packing is cubic, and from 0.155R to 0.28811 when the packing is hexagonal,R being the radius of the particles or micro-aggregates. In natural soils consisting ofparticles of widely different diameter and shape,the diameters may vary from a few millimetres to thousandths of a micron, or even less. The pores between particles or aggregates are linked by narrower passages,which together form the porous * This chapter was edited by R. M.HAGAN from the manuscript submitted by W. H.GARDNER and S. L. RAWLINGS as and T.J. MARSHALL authors and by A.A. RODEas co-authorwith contributions of C.E.KELLOG

80

HYDRO-PHYSICS-ARID

A N D IRRIGATED SOILS

space of the soil.As a result,the soil ‘capillaries’take the form of chains linking up each pore, like a string of beads. Because organic matter and some clays expand when wetted and contractwhen dried, and because,during drying the retreating menisci draw solid particles into different configurations,the size and shape of pores depend upon the water content of the soil.O n the other hand, the water content of a soil at a given energy level is largely dependent upon the sizes and shapes of pores in the soil. Thus, it is often not practical to consider the three soil phases independently. 2. Water content of soil Water content of soil can be expressed in a variety of ways. The most common are mass of water per unit mass ofdry soil,mass ofwater per unit volume ofwet soil,and volume ofwater per unit volume of wet soil. When determined gravimetrically,the mass ofwater per unit mass ofdry soilis ordinarily obtained.However, it is possible to sample a known volume of soil so that mass per unit volume can be computed.In all cases, the accuracy of water-contentmeasurement depends upon the existence of an unequivocal dry state which can be defined and measured.Water is present as a component of the mineral lattice of soil particles (structural water) as well as an adsorbed component on the surfaces of these particles. Adsorbed water may be held to the soil with sufficient energy that its difficulty of removal is comparable to that for structural water. Thus, dehydration curves with temperature do not reach a definite plateau, making it difficult to define a state where the soil may be said to be dry. These dehydrationcurves are differentmineral substances.To have greatest practical usefulness,the drying W . H., temperature shouldbe chosenin a rangewhere weight change with temperatureis minimum (GARDNER 1965). For illite, montmorillonite,vermiculite, kaolinite,gibbsite, and chlorite,this range should be 165 to 175°C.N o good temperature range exists for allophanes. Definition of a dry state for organic matter is even more difficult than for a mineral soil.At temperatures where soilis usually dried (105-1 lO”C),organic matter will continue to lose weight for long periods oftimemuch of the loss being the result of organic matter oxidation. To reach reasonable equilibrium conditions without oxidation of organic matter, drying temperatures should not exceed 70°C.For high-organic-matter soils,consideration should be given to techniques such as vacuum drying with use of drying agents. With soilscontainingmoderate amounts of organic matter,the traditional temperature range of 105-110°C appears to be as good a compromise for drying as can be found.However,the weight-temperaturecurve for allophanes and montmorillonite is sufficiently steep in this range that close temperature control is required for precision drying. Without careful checking of drying temperatures and weight changes as the soil dries, water content is usually no more precise than 0.005g/cm3.If greater precision is desired, samples to be compared should be of equal size and should be dried at precisely the same temperature for the same length of time. (a) Methods of measuring water content Precision measurement of water content in soil (see GARDNER W.H., 1965) is possible by gravimetric techniques and, for relatively large masses of soil,by in situ neutron scattering techniques.Under special conditions, particularly in the laboratory, gamma ray and neutron attenuation measurements may be used to obtain precise values ofwater content.Also,electrical conductivity or capacity or heat conductivity measurements in porous blocks inserted in the soil, with appropriate calibration, may be used as low precision indicators of water content. (b) Calculations of water content Gravimetric water content (O , )on a dry mass basis is measured by weighing a wet mass of soil,oven drying and reweighing,determination of tare, and computation in one of the following ways:

B.,=---I x-t Y-t

(2) 81

IRRIGATION, D R A I N A G E A N D SALINITY

where x is the combined wet soil mass and tare,y is the combined dry soil mass and tare, and t is the tare mass. Or,if tare mass is balanced out,it is computed from: .. B,=--l .€ (3) Y

Multiplication by 100 puts these ratios in percentage form. The precision of gravimetric water content determinations,assuming ideal sampling conditions and uniform treatment of samples during drying so that only weighing errors are involved,is given in terms of the standard deviation as (GARDNER W.H., 1965)*

where UX,,,,~is the standard deviation for weighing, assumed to be the same for wet, dry and tare weighings, z is the dry mass of the sample, O is the water content in g of water/g of dry soil,and y1 is the number of times the determinationis made on replicate samples.For errorsin water content (308)to be lessthan0.005 g/g a balance with a standard deviation of rt 1 m g may be used on sampleswhere dry weight is equal to or greater than 2 g. A balance for which u is &10 m g requires samples whose dry masses are 15 g or more,a balance for which u is rt 50 m g requires samples whose dry masses are 75 g or more, and a balance for which u is +0-1g requires samples of 150 g dry mass. Conversion of water contents in a mass/mass basis to volume/volume basis requires multiplication by the ratio of the bulk density of the soil to the bulk density of water, thus:

-

Density changes in water with temperature are usually considered negligible in the range considered so that in the cgs system D,= 1. With this assumption,water contents in terms of mass/unit volume are the same numerically as volume/volume values. Wet mass water content is obtainable from dry mass water content by use of the following equation:

Determination of the volume of water per unit area contained in particular depths of soil is accomplished by multiplying the depth increment by the volume basis water content, thus:

O h = OVh

(7)

where h, the depth increment,is used as a subscript to O to indicate that the figure is on a depth of water basis. Definitions,formula and conversion factorspertaining to water content measurement are summarised in Table 4.1. (c) S o m e types of soil water The sorptioncapacity ofthe soil (or,more precisely,of the solid part ofthe soil) is the ability of soil particles to absorb moisture,in the form ofliquid or vapour,on their surface and convert it into a film whose characteristics differ from those of ordinary free water. This capacity arises mainly from the free energy possessed by solid soil particles and expressed by the appearance of sorption forces.The latter are mainly electricalin nature,and the sorption ofwater molecules by solid soilparticles is facilitated by the fact that these molecules are dipole.Two stages can be distinguished in the process ofwater sorption,the first being the direct adsorption forces of water molecules on the surface of the particles. The radius of action of adsorption forces is very small,and the film thus formed consists of no more than three to four layers of water molecules. This moisture is known as adsorbed or tightZy bound moisture, and has certain special characteristics distinguishing it from ordinary water such as: a high density (1.5-1.7), a very reduced (or even zero) ability to dissolve electrolytes,a very reduced (or even zero) electricalconductivity, reduced thermal capacity,modulus of displacement, etc. During its formation,a heat of wetting is released which represents the kinetic energy lost by the water molecules when they enter into the composition of the film. *A second order term (ez) has been omitted as being of negligible importance over ordinary soil water content ranges 82

AND I R R I G A T E D SOILS

HYDRO-PHYSICS-ARID

Table 4.1. Water content terminology,definitions,units and conversionfactors

Definition Formula

mass/mass wet mass-dry mass dry mass

1

Dimensions

vol/vol em

Db* DW 1

System of units

vol/area(total)

vol/area/depth

cated depth (e,) (total depth) (ev> (indiunit depth

0m D b

M/L3

L

1

Customary units

c.g.s.

s/gt

i.p.s. f.p.s.

lb/lbt lb/lbt

Conversion factors

mass/vol

cm3/cm3t in3/in3t ft”ft3t

1 g/cm3= 0.0361lb/ina 1 lb/ina=27.68g/cma 1 g/cma= 62.4lb/ft3 1 lb/ft3= 0.1 6 g/cma 1 lb/ina=1.728x lo4 lb/ft3 1 lb/ft3=5.79x lb/ina

Water content conversions-wet and dry mass basis

cm/indicateddepth in/indicateddepth ft/indicateddepth

cm in ft

g/cm3

lb/in3 lb/ftz 1 cm = 0.394in 1 in=2.54c m 1 cm=0.0328 ft 1 ft= 30.48 cm 1ft=12in 1 in=0.0833ft

-=,e

ed m

Ifedm

edm=-

@W*

l-ewm

* D6is the bulk density of the soil and Dwis the density of water.These must be in consistentunits -f Multiplication by 100 gives figures in percentages

The maximum amount ofmoisture which can adhere closely to the soil is known as the maximum moisture adsorption capacity,the value of which can be determined either by the heat of wetting or by the size of the ‘non-dissolvingvolume’, measured in the presence of highly concentrated solutions of electrolytes. The sorption ofmoisture by the soilis not restricted to the formationofa fdm offirmly adhering moisture. Since the water molecules are dipole and,when adsorbed by soil particles,assume a particular orientation in the film that is formed,the latter in turn acquires the ability to attract dipole molecules which form a second layer offilm (the second stagein the sorptionprocess). The moisture from which this layer is formed is known as ZooseZy bound moisture. Its characteristics do not differ appreciably from those of ordinary water except that it has a higher viscosity; its component molecules are likewise oriented,though not so rigidly as in the film formedby the firmly adhering moisture. Diffuseionic sphereswhich form onthe surface ofsolid particles may also take part in the formation of a film of loosely adhering water. When the ionic spheres of two neighbouring particles overlap, forces of repulsion arise under the influence of these spheres. In dilute cm,i.e.the number of layers of water electrolyticsolutionsthe width of diffused ionic layersmay reach molecules in the film may in these circumstances increase to several hundred. Some investigators consider that the outer edge of the layer of loosely bound moisture is sharply defined,while others believe it to be diffuse and not sharply defined,particularly when the concentration of electrolytes in the external solution is low. The largest quantity ofmoisture which the soil can adsorb from air saturated with water vapour is termed the m a x i m u m hygroscopicity of the soil. To determine this value over the surface of pure water,i.e. in an atmosphere thoroughly saturated with water vapour,where thep/povalue is equal to unity,is an extremely long process, on account of capillary condensation. It is therefore determined by satúrating samples of soil with moisture in desiccatorsover the surfacenot ofpure water but of a 3 %or 10 solution ofsulphuricacid or a saturated solution of potassium sulphate. These solutions guarantee the relative humidity of the air, i.e. the p/povalue amounts to approximately 94-98 %. The value of maximum hygroscopicity is considerably greater than that of maximum adsorption moisture capacity,which is normally just over one-halfof the value of maximum hygroscopicity.Consequently,when the air humidity corresponds to maximum hygroscopicity,the soil contains the maximum possible quantity of firmly bound moisture, plus a certain quantity of loosely bound moisture. Because of their hygroscopicity, the presence of salts, especially calcium and magnesium chlorides,

83

IRRIGATION, DRAINAGE A N D SALINITY producesa sharp increase intheValue ofmaximum hygroscopicity ascompared with soil ofsimilar mechanical composition but free from salts. Soil,when in contact with water in liquid form,is able to adsorb an additional small amount of moisture (of the loosely adhering type). To determine with accuracy the maximum humidity level at which moisture sorption ceases is obviously impossible,mainly because the’electrolytic content of soil solutions fluctuates, thereby causing the amount of adhering moisture to fluctuate also. In non-saline soils,the upper humidity limit at which moisture adhesion ceases is somewhat higher, apparently,than the level at which permanent wilting occurs. 3. Energy state of soil water Equally as importantas the amount of water in a soil is the energy with which that water is held.This energy at any given temperatureusually is measured with references to a flat surface ofpure water at some specified elevation and at a standard pressure. Pure water in a saturated soil sample at the same elevation,pressure and temperature as the reference has a total water potential of zero. As the water content of the sample is decreased,the proportion ofthe remainingwater held within the range ofthe adsorptiveforcefieldsemanating from the soilparticle surfacesincreases.Thus the drier the soil becomes,the more tightly the remaining water is held. Since energy must be added to this soil water to restore it to the reference state,its potential energy is said to be negative.Similarly,the water potential ofa soilat a lower elevation than the referenceis negative. Ifit is higher than the reference level its water potential can be positive. The same holds true for samples at different pressures than the reference. Solutes in the soil water also lower its potential energy. As defined by the soil physics committee on terminology for the International Society of Soil Science in 1963 (ASLYNG), the total potential of soil water is, ‘Theamount of work that must be done per unit quantity ofpure water in order to transport reversibly and isothermally an infinitesimal quantity ofwater from a pool of pure water at a specified elevation at atmosphericpressure to the soil water (at the point under consideration).’ This total water potential, #,can be divided into parts to distinguish between the action of different force fields indicated above. The algebraic sum of these parts, or component potentials,must always equal the total water potential. The four component potentials generally distinguished are: (1) the matric or capillary potential, #,,,which , results from the interaction of soil particle surfaceswith water;(2)the osmotic potential, #w, which results from the solutes dissolved in the soil water; (3) the gravitational potential, which results from elevation with respect to the reference level; and (4) the pressure potential, #p, which results from external pressure on the soil water. In unsaturated soils the pressure potential usually is considered zero,and in saturated soils the matric potential usually is considered zero.The osmotic potential is of considerable importancewith respect to plant growth,but for reasons to be discussed below it is of much less consequence where water movement is concerned. Ifthe quantity of water involved in the above definition of water potential is a mass, the water potential has units as joules/kg or ergs/gm.The quantity ofwater n be expressed in terms of a volume,in which case nsionally the same as pressure. Since lo8 ergs/cm3 thewater potentialhas units such as ergs/cm3,which is is numerically equal to a bar pressure,water potential can be expressed in bars if desired. To avoid the use ofnegative quantities for expression ofthe energy state of soil water,alternate systems of terminology are often used. One of these, the suction system, defines total suction as the negative gauge pressure,relative to the external gas pressure on the soil water, to which a pool of pure water must be subjected in order to be in equilibrium through a semi-permeablemembrane with the soil water at the same elevation. Total suction is the sum of osmotic suction and matric suction. Except for algebraic sign,these component suctions are identical to the same component water potentials defined above where unit volume of water is chosen as a basis for computation. Suction can be expressed in any pressure units. However, atmospheres or bars,or the equivalent height of a hanging water column,are the most popular. When dealing with the movement ofliquid water in unsaturated soil it usually is only necessary to consider the matric and gravitational components of the water potential. In this case,the term soil water tension has traditionally been used instead of matric suction. Soil water tension is defined identically to matric suction, being the negative gauge pressure relative to the external gas pressure on the soil water,to which a solution identicalin composition with the soilwater must be subjected in order to be in equilibrium through a porous permeable wall with the soil water. Soil water tension is that quantity measured by a tensiometer,the principle of operation of which is shown in Fig.4.1.In this figure,water held in a soil sample is in equilibrium with the negative pressure applied by a hanging water column.The openings in the porous wall are all small enough to remain water-filled zt the elevation of the soil above the free water surface,h. Water then moves into or

84

HYDRO-PHYSICS-ARID

A N D IRRIGATED SOILS

out of the soil until equilibrium is reached. (The direction of approach,i.e.from dry to wet or from wet to dry, makes a difference in the water content at equilibrium as will be discussed more fully later.) As the elevation,h, is changed the amount of water held in small pores is changed.

Fig. 4.1.Water in soil (a) and water in single pore (b) subjected to the pull of a ‘hanging’water column; capillary tubes of various sizes (c) raise water in a comparable way to pores in soil Water is held in soil pores in a comparable way to water held in capillary tubes as also is illustrated in Fig. 4.1.However, a capillary tube with its simple geometry illustrates only a part of the phenomenon. The actual pore space is much more complicated and a significant part of the water present, particularly at low water content,is better described as a water film on particle surfaces. None the less, the forces which hold water on the particle surfaces are the same as those causing water to rise in capillary tubes. In a dry soil,a tension or negative pressure,many times as large as the normal pressure of the atmosphere,may be needed to remove water held by adhesive forces on particle surfaces and in interstices between particles. In this circumstance,the simple picture of a hanging water column exerting such a íarge negative pressure loses physical reality. None the less, the terminology and units of measurement used still apply. Attention here should be focused upon the quantitative expression of the energy with which water is held in the soil rather than on the mechanics of measurement of this energy. When a column of soil, covered to prevent evaporation, is immersed partially in water, the hydrostatic pressure that will exist below the free water surface and the soil water tension that will exist above are shown for equilibrium conditions in Fig.4.2.Measured in terms of the length of a water column,the values are Pz

Elevation

Fig.4.2. Tensions and pressures at equilibrium in a soil column immersed in water equivalent to the elevation-positive if below the free water surface and negative if above. If the soil above the free water is either wetter or drier than should be the case at equilibrium,water will flow in a direction to restore equilibrium.The size of the tension head leading towards restoration is determined by deducting the soilwater tensionwhich would exist at equilibrium from the existing tension at the point under consideration. This is equivalent to adding the matric potential to the gravitational potential or the gravity head to the tension head (soil water tension measured in the same units). The gravity head is the elevation of the point under considerationmeasured from an arbitrary reference level,in this case the level ofthe free water surface. Ifthe tension head at point P, is 5 (arbitrary) units and the gravity head is +2 units (see Fig.4.2), then the net head is 3 units (5+2) and water movement will be upward at this position. If the tension head at

-

-

85

IRRIGATION, DRAINAGE A N D SALINITY

-

point Pzis 3 units and the gravity head is +5,then the net head is -f- 2units and water must flow downward at this position until equilibrium is restored. RODEdescribes sorption,by solid particles, of water either in vapour or in liquid phase as follows: The relationship between adsorbed moisture and relative vapour tension can be given in the form of adsorption isotherms.By way ofillustration,Fig.4.3shows the isothermsofwater vapour sorption by various Millimole/g

10.0

7.5

r

P

I 1

1

Relative vapour pressure p/p,

Fig. 4.3. Isotherms for the sorption of water vapour by various mechanical fractions taken from Versinhave clay (based on data by KURON) mechanical fractions of the same soil. It will be seen that the finer the particles the greater the quantity of moisture adsorbed. The explanation is that finer particles have a larger total surface area for the same weight. The relationship between the amount of moisture adsorbed and the relative humidity of the air can be described by different formulae for different sections of the sorption isotherms. In the initial stage of sorption, up to a relative atmospheric humidity of 0.38, the sorption process may be described by FREUNDLICH’Sformula,as follows: a=L

(5):

or,in logarithmic terms: log a=logL+-.1 log P n Po where a=the quantity of adsorbed moisture per unit mass of soil p =the equilibrium tension of water vapour po= the tension of water vapour saturating the space at the experimental temperature

P -=the relative water vapour tension Po L and n are constants In the relative water vapour pressure range of 0.40 to 0.80 the sorption process is well described by

SPERANSKIJ’S equation: -

(i) 2

a=ao+K

(9)

where a= the quantity of adsorbed moisture per unit mass of dry soil a,,and K are constants SOCVEVANOV proposed a formula which describes the sorption process for the full range ofrelative water vapour pressures from O to 0.95: 86

HYDRO-PHYSICS-ARID

A N D IRRIGATED SOILS

a=b J-log):-l( where a is the quantity of moisture adsorbed by the soil,and b is a constant. SOCVEVANOCdiscovered that the constantb depends on particle size and is inversely proportionate to their diameter.In four fractions of Versinhave clay,whose sorption isotherms are shown in Fig.4.1,he found that 0.0126, b=where dis the diameter of particles in mm.The sorption equations for these four fractions turned d out to be as follows: Os02 mm

a=0-417J-log

(1-f)

):-I(

(lob)

):-I(

In the PIP, range of O to 0.30,the water vapour sorption process can also be accurately described by the BET (BRUNATJER, EMMETT,TELLER) and HJ (HARKINS, JURA) equations. As can be seen from Fig. 4.3,the sorption isotherms take the form of a reverse S,and can thus be divided into three sections: the first is convex in relation to the axis of the abscissae;the second is practically a straight line;and the third is concave.Along the first section,a layer of iirmly adhering moisture is formed no more than 3-4water molecule layers inwidth.The second section correspondsto the formation of internal layers of film from loosely adhering moisture which, however, cannot reach its maximum size through the sorption of water vapour alone. Along the third section, which begins when the p/po value is equal to approximately 040,the phenomenon ofmoisture sorption is supplemented by that ofcapillary condensation, which increases as the p/povalue approaches unity. Methods of measuring water potential (1) Totalpotential The most satisfactory method for measuring total water potential is to measure the equilibrium relative humidity ofa soil sample.Total water potential, y?, is related to relativehumidity,p/p,(the ratio ofthe vapour pressure of water in the soil to the vapour pressure of pure water at the reference level and at the same temperature), by the relation

RTln P $=M Po

RT

or +=--In-

v

P Po

where R is the ideal gas constant,Tisthe absolute temperature,Mis the partial molar weight of water, and Y is the partial molar volume of water. Whether M or Vis used in the equation depends on whether water potential is to be expressed on a weight or a volume basis. The desirability ofthis method lies in the fact that the only assumption requiredin the derivation of the above equation is that water vapour behaves as an ideal gas.* In the vapour pressure range encountered in soil,this assumption never introduces significant error. The difficulty with this method arises from the fact that in the water potential range from O to -15 bars, the relative humidity differs only by about 0.01(see Table 4.2). *This equation can be simply derived from the known fact that in still air the variation of plp. with elevation, z, above a flat -RT

p

surface of pure water is gz=ln -where g is the acceleration of gravity and the water vapour is assumed to behave as an V Po ideal gas. Water in soil at a given elevation, z,must be in equilibrium (that is, it must have the same water potential) with the water vapour existing at that elevation, or water will evaporate or condense until equilibrium is established.Thus, the gravitational potential of the water vapour, gz,must equal the total water potential of the soil water, $,resulting in the equation desired (The minus sign arises from the fact that z was measured positively upward but g is a vector directed downward)

87

P

IRRIGATION, D R A I N A G E A N D SALINITY Table 4.2. Soil energy (typicalunits)*

Water potential (massbasis) (erss/gt>

O -i (X 104) 0.5 -1 (x106) -'2

-

-3

-4 -5 -6 -7 -8 -9 -1 (XlOB) -2 -3 -4 -5 -6 7 (x 106) -8 -9 -i (X 107) 1.1 -1.2

-

-1.3 -1.4 --11..56 -1.7 -1.8 --2.0 1.9 -2.5 -5.0

-1 (x 108') -1 ix 109 -1 (1010)

Water potential

(volume basis) (joules/kg) (bars) O -1 -5 -10 20 30 -40 -50 60 -70 -80 -90 -100 200 -300 -400 500 -600 700 -800 -900 -1000 -1100 -1200 -1300 -1400 -1500 1600 -1700 -1800 -1900 -2000 -2500 -5000

--

-

-

-104 -105 106

(cmwater)

O

O

-0.01 -0-05 -0.1 -0.2 -0.3 -0.4 0.5 -0.6 -0.7 -0.8 0.9 - -1.0 -2 -3 -4 -5 -6 -7.0 -8 -9 -10 -11 -12 -13

-

-14 --1156 -17 -18 --20 19 -25 -50 -100 -103 -104

-10.20 -51.00 102.0 -204.0 -306.0 -408.0 -510.0 -612.0 -714.0 -816.0 -918.0 -1020 -2040 -3060 -4080 -5100 -6120 -7140 -8160 -9180 -10222 -11220 -12240 -13260 -14280 -15300 -16320 -17340 -18360 -19380 -20400 -25500 -51000 -102000 -1020000 + 10200000

-

Suction or soil water Tension (salt free soil)

Vapour pressure at 20°C

(bars)

(mm Hg)

O 0.01 0.05 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1*o 2 3 4 5 6 7.0 8 9 10 11 12 13 14 15 16 17 18 19 20 25 50 100 103 104

(cm water)

O 10.20 51.00 102.0 204.0 306.0 408.0 510.0 612.0 7140 816.0 918.0 1020 2040 3060 4080 5100 6120 7140 8160 9180 1o222 11220 12240 13260 14280 15300 16320 17340 18360 19380 20400 25500 51000 102000 1020000 10200000

17.5350 17.5359 17.5344 17.5337 17.5324 17.5312 17.5299 175286 17.5273 17,5260 175247 17.5234 17.5222 17.5089 17.4961 17.4833 1734704 17-4572 17.4411 17.4316 17.4187 17.4059 17.3930 17.3802 17.3674 17.3546 17.3417 17.3222 17.3160 17-3032 17-2908 17.2780 17.2142 16.8990 16.2865 8,3773 0.0108618

Relative humidity at 20°C

1~00000 1*ooooo 0.99997 0.99993 0.99985 0.99978 0.99971 0.99964 0.99956 0.99949 0.99941 0.99934 0.99927 0.99851 0.99778 0.99705 0.99637 0.99556 0.99465 0.99410 0.99337 0.99264 0.99190 0.99117 0-99044 0.98971 0.98898 0.98786 0.98751 0.98678 0.98607 0.98534 0.98171 0.96373 0.91739 0.47774 0.06194

* Tabulated values are based on the assumption that the density of water is very nearly 1 g/cm3, g is 980 cm/secz,A is 8.3136 x lo7ergs/g,M and V (molecular weight and molecular volume of water) are 18 g/mo! and 18 cm3/mol, and that the potentials due to gas pressure,gravity and temperatureare zero 7 Values in this column are either potential energy per unit mass or per unit volume where the density ofwater is taken as unity

Relativehumidity measurements ofsoilsgenerally are made using a thermocouple psychrometer (RICHARDS and OGATA, 1958) which consists of a pair of thermocouple junctions in a closed chamber containing a sample of soil. One junction is kept wet by a small droplet of water held in a small ring soldered to the wet junction and the other is kept dry and at the temperature ofthe chamber.The temperatureat the wetjunction depends primarily upon the rate of evaporationinto the atmosphere adjacent to the soil.This atmosphere is considered to be in equilibrium with the soil.The instrument is calibrated with salt solutionscarried on filter paper in similar geometrical arrangement to the soil sample. Significant changes in geometry generally result (1966) has derived theoretical relationshipswith certain simplifying in calibration curve changes.RAWLINS assumptions for dealing with thermocouple psychrometry which are verified experimentally. The total water potential of a soil sample,which produces a particular e.m.f. at the wet junction, is considered to be equal to the osmotic potential that is computed for the salt solution giving the same e.m.f,

88

HYDRO-PHYSICS-ARID A N D IRRIGATED SOILS during calibration.Thermocouplessuch as Chromel-P-constantan made from 0.025 mm diameter wire are generally used. The thermocouple is usually inserted into a small cavity drilled into the soil sample.The soil container is placed in a constant temperature bath controlled to +O.OOl"C(usually with a double bathZOLLINGER and TAYLOR, within-a-bathdesign). Samples are best handled in a sample changer (CAMPBELL, 1966) which facilitates multiple measurements. For measurements of water potential to the nearest centibar over the plant growth range down to about 15 bars, e.m.f. must be measured with equipment having a sensitivity of about 0.05p V and a total range of about 10 pV. This ordinarily requires a sensitive d.c. amplifier. An alternative method for making the measurement involves the use of Peltier cooling (SPANNER,1951 ; MONTEITH and OWEN, 1958) to condense water on a thermocouple prior to measurement of temperature depression during subsequent evaporation. Until recently,neither this nor the first method described could be used with any precision in the field because of the close temperature control required.However,RAWLINS and DALTON (1967) have constructed a psychrometer for use in the field that measures water potential within + 0.5 bar in the presence oftemperaturefluctuationsgreater than 5°C per day.The psychrometer uses Peltier cooling to condense water on the wet junction. This makes it possible to correct for temperature gradients within the soil by measuring the e.m.f. of the thermocouple before it is wet and subtracting this from the e.m.f. obtained during evaporation ofthe water from the wetjunction.The thermocoupleis mounted inside a small porous ceramic bulb that can be buried in the soil and read at any desired time. A batteryoperated,d.c. amplifier that is commercially available provides an excellent portable readout instrument. Freezing-pointdepression measurements have been used to measure total water potential in the laboratory with somewhat limited success (CANNELL and GARDNER, 1959). The equation used to relate water potential to freezing-pointdepression (fpd) is

where gL is in dynes/cm2,p is the density of water in g/cm3,Tisthe absolute temperature and Lf is the latent heat of fusion of water in cal/g.Difficultiesbelieved to be associated with change in solution concentration that occurs as ice forms and poorly understood behaviour of colloidalparticles in the freezing solution,make it difficult to modify the equation or to determine a consistent corrective factor that would allow practical use of freezing-pointdepression measurements. Until methods are found to cope with these and possible other problems,freezing-pointdepression measurements are unlikely to be used extensively. (2) Osmotic potential (osmotic or solute suction)

Osmotic potential usually is measured by extracting solutionfrom the soil and determiningits total potential, either by vapour pressure measurements or by freezing-pointdepression. In a bulk solution, the matric potential is zero,so that the only remaining component of the potential is osmotic. Here,as in the case of measurement of the total water potential of soils,measurement of the equilibrium relative humidity is the more soundly based technique. However,freezing-pointdepression measurements of solutions do not suffer from the major errors indicated above for freezing-pointdepression measurements of water in soil. Often the osmotic potential of a soil solution is inferred from its electrical conductivity (USSLHandbook 60,1954). This procedure is rapid but requires some knowledge of the chemical composition of the solution,since the relationship between electrical conductivity and osmotic potential varies with chemical composition. A criticism of the technique of inferring osmotic potential from extracted solutions arises from the uncertainty of the changes that occur in the solution during the extraction process. In very wet soil,the error introduced by extractionis probably negligible, but as the soilsbecome drier a greater portion of the solution exists within the force fields emanating from the solid particles. This can give rise to negative adsorption, which causes the extracted solution to be non-representativeof the solution that exists within the soil. A salinity sensor that measures the electrical conductivity within a ceramic wafer buried in the soil has been (1966). The porosity of the ceramic is fine enough to remain water-filled at soil developed by RICHARDS water tensions exceeding 15 bars, ensuring that the electric current always passes through the same volume of solution.This instrument does not suffer from errors caused by extraction of solution from the soil,but must be calibrated for each different soil solution composition to yield osmotic potential. (3) Matric potential (matric suction or soil water tension)

As indicated above,matric potential can be measured in situ with tensiometers in the tension range up to 89

IRRIGATION, DRAINAGE A N D SALINITY about 0-8bars. A tensiometer is shown in Fig.4.4(a). This is comparable to the apparatus shown at the left in Fig. 4.2,except that it is constructed to be buried in the soil. Water flows into or out of the porous cup until equilibrium is reached between the soil water and the negative pressure exerted by the hanging water column in the tensiometer manometer. Mercury is often used as a manometer fluid to reduce the length of the manometer and to permit placement of the manometer at an elevation above the tensiometer cup. Dialtype vacuum gauges also can be used to register the tension of water in the tensiometer.

Fig. 4.4. Schematic drawings of devices for use in measurement of soil water tension: (a) tensiometerusing mercury manometer (b) porous plate arranged for establishment of a tension gradient by means of pressure at p1 (c) pressure-cookerapparatus (pl>pz) (d) pressure membrane apparatus (pa> p 1 by about 0.3 bar and p1>p2)and (e) differential mercury manometer for supplying pressures differing by 0.3 bar from a single pressure source If the water in a tensiometer could be maintained free of dissolved gases, there would be no reason why tensiometers should not be used at tensions up to and even exceeding 15 bars. However,because the porous cup is in contact with air in the soil,water in tensiometers does not remain air-free.As a result of this air coming out of solution,the water column breaks at tensions above about 0.8 bars allowing the water to be drawn into the soil. To overcome this difficulty,RICHARDS (1947) developed a technique for increasing the pressure on the soil rather than decreasing the pressure of water in the tensiometercup.In this way,water is not under negative pressure so that gas solubility does not become a problem. The principle of this method can be seen from the schematic drawing in Fig.4.4(b). Soil samples are placed on a porous membrane or plate that permits passage of water and solutes but not air. Theoretically the soil water tension of these samples at their initial water content could then be determined by increasing the gas pressure above the samplesuntil water from the samplesfirst appears below the membrane,where the gas pressure is atmospheric. At this time,the potential energy of the soil water has been raised from its initial negative value to zero.The increase in soilwater potential is brought about by the application ofgas pressure and isthereforenumerically equal to the pressure potential of the gas in the pressure chamber. If 15 bars pressure must be applied to a soil sample to raise its soil water potential to zero,before the pressure was applied its matric potential must have been 15 bars. It is important to note that we have equated potentials,not pressures. It is not appropriate to say that the soil waterpressure was increased 15 bars by the application of 15 bars gas pressure,and infer from this that initially the pressure of the water in the soil was 15 bars. The matric component of soil water potential results from adsorption forces which may or may not result in negative internal pressures in

-

-

90

HYDRO-PHYSICS-ARID

A N D IRRIGATED SOILS

the soil water. Since there is no method available for measuring internal pressure,it must alwaysbe combined into the term matric potential or soil water tension.Thispoints out once again that the term soilwater tension does not refer to the internal pressure condition ofthe soilwater,but to the pressure to which a bulk sample of water must be subjected to be in equilibrium through a porous wall with the soil water. In actual practice,it is necessary to saturate the porous membranes or plates of a pressure membrane or pressure plate apparatus in order to prevent air leakage through them.Thus,it is not practical to determine the pressure at which soil water first appears at the low pressure side.Rather than pressure apparatus being used to determine the soilwater tension ofsamples at theirinitial water content,the soil samples are saturated and brought to equilibrium with successively increasing pressures.At each pressure, the weight of the soil sample is determined,from which data a curve of soil water tension versus water content (desorption curve) is constructed.Soil water tension under experimental or field conditions is then inferred from water content measurements. For tensions below about 2 bars, ordinary pressure cookers are generally used (Fig. 4.4(c)). Here the porous plate is a circular ceramic plate with a concave recess formed around its edge. It is presoaked to assure that all pores are water-filledbefore use. A rubber or neoprene diaphragm is fastened securely over the bottom of the plate by means of wires that bind the rubber diaphragm into the concave grooves.The region between the bottom of the porous plate and the rubber membrane is vented through the porous plate and the wall of the cooker to the atmosphere outside.Usually several such plates are mounted on a rack inside the pressure cooker. Air pressure from a compressor or bottled nitrogen gas usually is supplied through a diaphragm-regulatedpressure reduction valve and read on an ordinary pressure gauge. Or, for precision work,more elaborate mercury manometer control systems are used (WILCOX, 1950). Soil samples generally are placed in 1 c m sections of bicycle inner tubing.Upwards of 20 samples may be placed on one plate in the usual form of the apparatus. For higher tensions a similar apparatus has been developed.It differs mainly in that the pressure cooker is replaced by a steel chamber that will withstand much higher pressures safely, and the porous plate used has been constructed so that all of the pores are sufficiently small to remain water-filled up to 15 bars, An earlier apparatus, still in common use,involves use of a porous membrane made of Visking cellulose (sausage casing). This membrane must be supported on a fine screen and soil samples must be held securely against the membrane. To accomplish this, a fine screen is placed on a plane circular steel plate with a tubular opening at the bottom (Fig. 4.4(d)). The cellulose membrane (presoaked prior to use) is placed over the screen.A steel cylinder with a pressure inlet tube and with shoulders on the outer edges for retention of ‘O’ring seals forms the chamber for the samples.As soil dries out,it is possible for it to shrink away from the smoothmembrane.Hence,the apparatusis provided also with a rubber or neoprene membrane arranged so that air pressure can be used to apply a smallmechanical forceto the tops ofthe soil samples,thus holding them tightly against the membrane. By use of a differentialmercury manometer (shown schematically in Fig.4.4(e)), it is possible to supply air or gas for both the soil chamber and diaphragm chamber at slightly different pressures from the same pressure source.The manometer is usually constructed so that the pressure P3exceeds P, by about 0-3bar. The steel plates are held together with bolts,with ‘O’ rings forming the seal. This type of equipment has been used for tensions considerably in excess of 15 bars,which is the usual upper limit for such measurements. Soil samples are placed on the porous membrane and allowed to equilibrate. Equilibrium time depends upon the porosity characteristics of the soil and the applied tension.At low tensions,equilibrium is reached overnight in some soils,but with other soils,particularly at higher tensions, equilibrium is not reached for many days. For some kinds of work,arbitrary time intervals of approximately two days may be used with sufficientprecision. For precise measurements,it is best to measure the outflow and tojudge equilibrium from this. Water may be removed from soil samples through use of inertial force in a centrifuge. This technique has been used primarily to determine the water content of a soilafter water held in largepores has been removedpresumably a water content value in the ‘fieldcapacity’range. This value,termed the moisture equivalent,is obtained by subjecting soil in small perforated cups to a force 1000 times gravity in a centrifuge.Thisresults in a soil water tension distribution in the sample ranging from zero at the surface at which outflow occurs to maximum tension on the opposite end of the soil sample. Because the moisture equivalent does not correspond to a unique tension,it has been largely replaced by pressure plate equipment measurements. It has been shown to correspond roughly to a soil water tension value of about -2bar. The moisture equivalent has been used as an index of soil texture as well as for characterising porosity. For some soils the moisture equivalent divided by the factor 1-84is an estimate of the wilting point. H

91

IRRIGATION, D R A I N A G E A N D SALINITY Matric potential is also sometimes inferred from the electrical conductivity of porous blocks, which previously have been calibrated in soil at known tensions in a pressure plate apparatus (Bou~oucosand MICK, 1940). Because their conductivity is also affected by solutes in the soil,porous blocks can only be used to indicate soil water tension in salt-freesoils.Even here, they yield low precision measurements because of calibrationshiftswith ageing.However,they do representessentiallythe only technique thathas been available for measuring the matric component of the soil water potential in situ below -1 bar. Since these blocks can only be used satisfactorily in salt-freesoils, psychrometric measurements of total water potential with the newly developed techniques described above could possibly yield the same information with greater accuracy.

4. Water content-soil

water potential relations

Curves of soil water content versus water potential are useful in characterising different soils.Such curves are shown in Fig. 4.5 for a desorption cycle (i.e. where equilibrium is approached from the wet side). The significance of these curves is best visualised by considering the meaning of the slope of the curve.A vertical section in the curve represents a situation where a soil contains a large number of pores of a particular size. When a tension is reached at which these pores can empty,it changes little until they are all empty so that a large water content change occurs with an extremely small tension change.A horizontal section in the curve represents a situation where no pores are available of a size that can drain over the tension range concerned. Straight-linesections at intermediate slopes represent the situation where pore size distribution results in uniform release ofwater per unit tension change.Thus,the shape ofthe desorption curve is indicative of the pore size and pore size distribution in a soil.

0.6

( -\ \

20

' .

10

Salkum

O

2

silty clay

I !

loam

4 6 8 10 12 14 Matric or soil water tension (suction) (bars)

16

c .-

18

Fig. 4.5. Typical desorption curves for soils of two textural classes.A sorption curve (dashed) is shown also for the Salkum silt loam. The figureson the axis at the right indicate the period of time remaining before plants would wilt,because of water shortage in the soil,when the matric or soil water tension (suction) is at the indicated value (Estimates are based upon a 64 cm rooting zone,0.6 cm/day evapotranspiration,and with wilting occurring at 15 bars)

92

1 O0 13

55 54 75 17

N4 36 34 16 5 9 12 13

Screened Sand R a m o n a Sand Indio L o a m Placentia CI. L o a m Greenfield L o a m Hanford F.S.L o a m Fresno L o a m Yo10 Clay L o a m Antioch Clay Olympic Clay Chino Silty Clay Chino Silty C1.L o a m

\ 80

M

60 --i

bD

FI *

1-

B

c.

c

8 40

20

Soil water tension (cm water)

Fig. 4.6. Soil water tension as a function of water content for the soils listed above (US Salinity Laboratory, unpublished) Differential water capacity cm3/cmz/ bar 1 0.5

o.1 0.05

\\

Salkum silty clay loam

0.01 0.00:

Warden fine sandy loam

0.00

O

2

4

6 8 10 Soil water tension (bars)

Fig.4.7. Differential water capacities for the two soils whose desorption curves are shown in Fig. 4.5 93

IRRIGATION, DRAINAGE A N D SALINITY

A slope on a curve of the type shown in Fig. 4.5 is referred to as the differential water capacity.This represents the amount of water released per unit change in soil water tension. Differential water capacity varies with water tension and is differentfor different soils as may be seen from Fig.4.7.Values for differential water capacity are plotted logarithmicallyto permit expansion of the scale at low values without loss of the higher values. Important differences in soil may be discerned from these curves.The coarse-texturedsoils are observed to release water rapidly over the low tension range,whereas the fine-texturedsoil releases water more gradually over the high tension range. The significance of desorption curves in soil-water-plant relations is discussed in detail elsewhere. However,it should be pointed out here that agricultural use and value of a soil are affected greatly by soil properties indicated by the desorption curve.It is evident in both Fig.4.5and Fig. 4.7 that water availablefor plant use at different soil water tensions varies greatly with the soil.To emphasise this point the number of days remaining before the soil will reach the wilting point, starting with any given soil water concentration or correspondingtension,is shown on the ordinate at the right in Fig. 4.5 for each of the soils.* Osmotic suction usually is not used as a soil characteristic in a comparable way to matric suction since it is generally a transient property. However, it is none the less important in considering the effectof water conditions on plant growth. The importance is illustrated in curves showing total suction as a function of water content for a normal soil and for the same soil with NaCl added at a number of different concentrations (Fig. 4.8). These curves were obtained by adding the osmotic suction value at each water content to the matric suction at the same water content for the soil at its original salt content and after addition of various quantities of NaCl. Totd suction (bars)

\

Matric tension

O

5

10

15 20 Water content (%)

Fig. 4.8. Total suction for a soil to which various quantities of NaCl have been added. The matric suction and AYERS, 1945) or soil water tension curve for the soil also is shown (Abridged from WADLEIGH Hysteresis The influence of previous wetting history on the water content-water tension relationship is evident in the dashed wetting curve for Salkum silty clay loam shown in Fig.4.5.This phenomena,known as hysteresis,in its simplest form may be illustrated by considering water in a short section of a small tube as shown in * These computations are based upon an arbitrary root zone of 64 cm, an average evapotranspiration rate of 0.64cm/day, and on the assumption that the average tension in the root zone at the wilting point is 15 bars. Although these conditions are arbitrary and do not talce into account the fact that plants can wilt with ample water in the soil if evapotranspirationrates are excessive, the general comparison of the soil is still meaningful

94

HYDRO-PHYSICS-ARID A N D IRRIGATED SOILS Fig. 4.9(a). Here it may be observed that two configurations of water distribution for the same quantity of water are possible. The solid curve indicates the condition where the tube has been completely water-filled and dried down to a reference water content. The dashed curve representsthe condition where the tube was dry and water was added until the same reference water content was reached. a

b

Fig. 4.9. Section of a capillary tube (a) showing equal amounts of water distribcted in two different geometrical arrangements, and air-water-solid interfaces (b) for a wetting situation at left and a drying situation at right illustrating hysteresis

The curvature of the water-water vapour interface* is radically different in the two cases and the water pressure is different.A second view ofhysteresis is presented in Fig.4.9(b) where a wetting situation is shown on the left-handside contrasted to a drying situation on the right-handside.There is evidence that an energy barrier may exist that prevents ready wetting of a dry surface and leads to the configuration at the left. By contrast,when a wet surface is dried,water molecules are held tenaciously giving rise to the water configuration shown on the right.Thisphenomenon also can give rise to differencesin water content at the same water tension or to diEerent tensions at the same water content. Whatever the cause, hysteresis is real and it is therefore impossible to infer water content accurately from tension or tension from water content without knowledge of previous wetting history. An additional hysteresis also is possible due to the swelling and shrinking of clays and organic materials as well as changes in soil particle configuration on wetting and drying. The resulting changes in porosity often occur slowly and are not completely reversible. Hence, permanent shifts of sorption and desorption curves are common; and, some hysteretic shifts, for many practical purposes, may be treated as permanent shifts.

B.

FLOW OF SOIL WATER

Where flow of water in the soil is not accompanied by flow of other forms ofmatter or by flow of energy, the driving force for water movement is the gradient of the water potential. The gradient of the water potential is the force per unit quantity ofwater.Ifthe water potential is expressed on a unit volume basis,the gradient ofthe water potential is the forceper unit volume ofwater.Ifit is expressed on a unit mass basis, the gradient of the water potential is the force per unit mass.Numerically they are the same ifthe density ofwater can be considered to be unity. An advantage of using the water potential expressed on a volume basis is that one bar is equivalent to 1000c m of water head, to an acceptable degree of accuracy. This simplifies adding the gravitational potential to the other components. When a temperature gradient exists within the soil,the flow of water is accompanied by a flow of heat, which may be linked with and contribute significantly to the observed water flow.In such cases,the gradient of the water potential (as water potential is defined above,it applies to isothermal conditions only) does not account for the entire driving force. Similarly,when a solute concentration gradient exists within the soil, and the solutes are not bound,the flowofwater is accompanied by a flow ofsolutes.Here again,the gradient ofthe total water potential does not give the correctdriving force for water flow.Ifthe solutes are completely free to move with the water,the correct driving force can be obtained by simply subtracting the gradient of the osmotic potential from the total potential gradient.However,if the solute flow is impeded by interaction * The interface is usually referred to as an 'air-water' interface. However, since the phenomenon can occur equally well in a chamber containing only the tube, water and water vapour, and since it is a water-water vapour equilibrium which is of interest, this terminology is preferable

95

IRRIGATION, DRAINAGE A N D SALINITY

with the soil,the osmotic potential gradient will influence the flow of water to an extent dependent upon the degree of binding of solutes to the soil matrix. A convenient theoretical frameworkfor handling the effect of osmotic potential and temperature gradients on the flow ofwater can be derived as a special casefrom the more general theory ofnon-equilibriumthermodynamics,This theory treats simultaneous flow of matter and energy in response to all the driving forces acting on a system.For simultaneous flow of solutes and water,the forcesacting on the system are: (1) that force which causes water to flow in the absence of solute gradients,which is the gradient of the total water potential minus the osmotic potential denoted by $,= $- $,, (2)that force that causes solutes to move in the absence of gradients of $,, which is the gradient of the osmotic potential, $,.The equation for steadymovement of water (no acceleration in the direction of flow) is

v = -(LW grad +,+L,, grad $,)

(13)

where v is the volume rate ofwater flow per unit area,L, is the transmissioncoefficientfor water Aow caused by grad $,, and L,, is the transmission coefficient for water flow caused by grad $,.A similar equation could be written for the flow of solute in the system as affected by the same two gradients. In this case,the coefficient which links flow of solute to flow of water would be Lwn.In many of those cases where the basic assumptions onwhich non-equilibriumthermodynamics is based are met,these so-calledlink coefficients are This condition is often used as an experimental check on the validity of these equal, i.e. L,,=L,,. assumptions. From equation 13 it is apparent that the influence of osmotic potential gradients on flow of water will depend upon the magnitude of L ,,. When the solutes are free to move with the water, L,, is zero. In this case,gradients of osmoticpotential have no effect on flow ofwater.When the solutes are completely impeded from moving with the soil water,L,, is equal to L,. In this case the full osmotic potential gradient must be added to the remainder of the water potential gradient to obtain the driving force for water flow. This is the case for flow of water through a semi-permeablemembrane. For flow of water through a leaky membrane or through soil where the solutes are not completely bound by the soil,the ratio of L,, to L, will vary between zero and unity.Theproportion ofthe osmoticpotential gradientthat must be added to the remainder of the water potential gradient to obtain the driving force for water movement is given by this ratio. Few experiments from which this ratio can be calculated have been carried out.The few that are available (LETEY and KEMPER, 1968) indicatethat for most soils,particularly at high water content,the ratio may be assumed to be zero.Since,until more data become available,it is not possible to take into accountthe effect of solute concentration gradients on water movement, the principles of water flow given in the remainder of this chapter are developed on the basis that either the gradient of osmotic potential or the link coefficientfor the effect of solute gradients on water flow is zero. The effectof temperature gradients on water flow can be treated by an equation similar to equation 13. By analogy,this equation is v=-(L,grad

$,+LTwgradT)

where L,, is the link coefficientfor flow ofwater caused by a temperature gradient,and Tisthe temperature. Here, as above, the magnitude of the water flow caused by a temperature gradient in the soil will depend upon the magnitude of LTw.Since in this case the two driving forces are not dimensionally the same,LTw is not dimensionally the same as L,. Thus,the ratio of L, to LTwwill depend upon the units used for the temperature and water potential gradients,and will not vary between zero and unity as in the case treated above.A discussion of flow of water and heat based upon non-equilibrium thermodynamics is presented by TAYLOR and CARY (1964),citing work such as that ofPRIGOGINE (1961), DE GROOT (1951)and DE GROOT and MAZUR (1962).

CARY(1965) measured water fluxthrough a loam soil induced by temperature gradients at five different tensions and compared this with the fluxinduced by a tension gradient. These data are shown in Fig. 4.10. Near saturation (at 0.07bars or 70cm of water tension), the water flow per OC/cmtemperature gradient was equivalent to that which would be caused by a tension gradient of only 4 cm of water per cm. As the soil dried to a water content equivalent to 0.46 bars tension,however,the water flow caused by a tension gradient decreased more than 100 times relative to that caused by a temperature gradient. If the soil were dried further, temperature gradients would become even more significant in moving water as liquid water data show,however, that in the tension range below movement is replaced by vapour movement. CARY’S 96

HYDRO-PHYSICS-ARID

A N D IRRIGATED SOILS

Water flux ("/day)

O

0.1

032 0.3 0.4 Soil water tension (bats)

Fig. 4.10.A comparison at 18°C of the various components of water transfer for Columbia loam soil. Curve a is that which flows due to a tension gradient of 5 cm water/cm,while b is the net thermal transfer of water from warm to cool under a temperature gradient ofO%'C/cm. Curves c and d are the components of curve b in the liquid and vapour phases respectively (Redrawn from CARY,1965)

-J-bar most of the temperature-inducedflow of water was in the liquid phase. H e postulates that this liquid flow is the result of increasing numbers of hydrogen bonds per water molecule in the direction of lower temperatures. The link coefficients for water flow induced by temperature gradients must be determined experimentally for each soil and for each set of boundary conditions imposed on the soil.Because of the lack of such data, the following sections will apply strictly to isothermal flow only, except in the case of vapour flow,which is the major component of water movement in dry soils. 1. Saturated flow (a) Principlesof.jlow In a saturated soil free of solute and temperature gradients,the components of the water potential active in the flow of water are the gravitational and the external pressure potentials. For steady flow, the sum of these two potential gradients and the force per unit quantity of water resisting flow of water through the soil must equal zero.By analogy with resisting forces of other types,it is probable that this resisting force is proportional to the velocity of flow. Writing this resisting force as (l/k)v, where v is the volume rate of flow per unit area or velocity of flow and k is a constant characterising the flow properties of the soil,the mathematical expression for the above statement is grad &+grad $,+(l/k)v=O

(15)

or

v=-kgrad(&-t-+,)=-kgrad J!,Z (16) This is the same as equation 13 where L,=k and only gradients of $exist in the soil. It is an equivalent equation to that proposed and verified experimentally by DARCY and is generally referred to as the DARCY equation in this as well as in other forms. If water potential is expressed on a unit volume of water basis, both &and +, can be expressed as a length by using the head system. If water potential is expressed on a unit mass basis, &is zg and # p is p/p, where z is the vertical distance from the reference level, g is the gravitational constant,p is the pressure of the water, and p is the density of the water. The equation giving velocity of flow as a function of the potential gradient is useful by itself under certain circumstances. However,this equation must be combined with the equation of continuity to have general utility.The equation ofcontinuity is a mathematical expression ofthe law ofconservation ofmass. In words this law is expressed as follows:the algebraic sum of mass entering a specifiedvolume and the mass leaving is equal to the change in mass within the volume. The equation is derived in books on physical mechanics or theoretical physics. In its most general form it may be expressed as

where p is the density and 0 the concentration ofthe fluid (O is less than unity for unsaturated Row conditions), t is the time,Y,, vy, and vz are the volume rates of flow per unit cross section in the coordinate directions,

97

IRRIGATION, DRAINAGE A N D SALINITY

x,y and z.Comparable expressions may be derived for other coordinate systems that often are of greater utility in solutionsto certain practical problems.If the density of water is taken to be constant and equal to unity then p may be moved in front of the derivative in the left-hand term in the equation. For saturated flow, the left-hand term becomes zero since O, the water concentration, is always considered to be unity. Writing the DARCY equation (equation 16) for three-dimensionalflow as

and substitutingin the equation of continuity for an incompressible fluid,equation 17 with the left-hand term zero,becomes

which is known as the LAPLACE equation. Solutions to this equation give the potential as a function of the position coordinates.Thisequation is a common equationin physics and thereare many solutionsforvarious boundary conditions already worked out in the literature of theoretical physics. Many of these solutions have practical applications in engineering and soils work dealing with saturated flow of water in porous materials. (b) Application of principles to Jieldproblems

A simple example of the solution to a saturated flow problem is a cylindrical well removing water from a layer of saturated gravel lying between impermeable clay layers. The solution to this problem is easiest if cylindrical coordinates are used. The equation of continuity in cylindrical coordinatesfor an incompressible fluid is

where r is the radius vector and x is the vectorial angle.For this problem where 0is constant and the velocity is invariant with angular position and elevation in thp,stratum z,equation 20 reduces to : 6 (rV,)=O 6r

Carrying out the differentiation indicated in equation 21 yields:

Integrating equation 22 over the limits r=r to r=rl and Y,=Y,to V r =V,,

or

(24)

At the surface of the permeable well casing rl, V,,is the well discharge Q divided by the surface area of the casing multiplied by the density of water,or -Q/(2mr1zp),with the negative sign indicatingthat the velocity is in the opposite direction to increasing radius. Equation 24 becomes

Q = -27r rzpV, Substituting the DARCY equation (16) into equation 25 gives

8

Q =2~ rzplc (p/p+zg) 8r 6P Q =2n rzk-, Br

98

(26)

A N D IRRIGATED SOILS

HYDRO-PHYSICS-ARID and,chancing to total derivative,

d-r -27rzk .dp r

(27)

Q

Integrating from rl to r and p to p1 and rearranging terms gives p - p l = L ln r/rl

27rzk

Thus,the pressure is a linear function of the logarithm of the distance from the wall. equation (c) The constant, k, in the DARCY equation encompasses the physical nature of the porous medium which The constant, k,in the DARCY affects flow,and, in some usages, includes the nature of the fluid. It also absorbs any dimensional inconsistencies that may exist because of the several different ways in which the moving force term and the velocity can be expressed.In the foregoing treatment velocity was treated as the volume rate of flow per unit crosssectional area (dimensions LIT),the force term was a potential gradient (dimension l/TZ) so that k had dimensions of T.If viscosity r] (dimensions MILT)is included explicitly in the equation (replacing k with k/q)then the dimensions of k are changed and it is referred to as the intrinsic permeability since it now characterises only the porous medium. Under these conditions k has the dimensions of MIL.The combinations of dimensions most frequently used are shown in Table 4.3. Table 4.3. Units and dimensions of terms in saturated flow equation

Permeabilityfactor

Driving Force

Name of k Dimensions Units of ofk k

Symbol

k

permeability

T

sec

V#

hydraulic

IC r)

-

intrinsic permeability

MIL

g/cm

grad. 4

potential gradient

k

permeability

L3TIA4

cm3sec/g

APIAL

pressure

Symbol

Name

Dimen-

LIT3

dynes/g

M/L2T2

dynes/cm3/ cm

gradient

k I

rl

K

Units

sions

= dynes/cm3

intrinsic permeability

L3

cmz

hydraulic conductivity

LIT

cm/sec

hlL AhlAL

hydraulic head gradient

L/L

cmlcm= unity

Velocity or flux: symbol V,V,;dimensions LIT;units cm3/sec/cm2=cm/sec

Flow velocities ordinarily computed are described as macroscopic velocities, referring to the average velocity over the entire cross-sectionof the porous medium. It is sometimes desirable to consider the microscopic velocity which is the velocity computed for the pore cross-sectionalone. The true velocity is variable, being maximum in the largest sections of the pores and minimum against the particle surfaces. Although the permeability factor is generally treated as a constant for a particular porous system in nature or in the laboratoiy,it is rarely constant.Even under conditions where porosity is thought to remain fixed with time, it is difficult to obtain constant values experimentally. Shrinking or swelling of soil colloids or growth of micro-organisnis,which are known to happen, have a marked effect upon porosity and hence on the permeability.Permeability would,of course,vary from stratum to stratum in the soil.It is also known to vary with direction,vertical permeability often being different from horizontal permeability. (d) Meusurement of permeability Permeability measurements in the laboratory are made ,ina variety of ways. Typical is a system where a i

99

IRRIGATION, DRAINAGE A N D SALINITY constant shallow head ofwater is maintained on the surface of a soil core contained in a short cylinder.Both disturbed and undisturbed cores are used. However,it is essential that porosity be close to what it was in the field.This means use of soil cores with field structure whenever they can be obtained. Loose materials such as sandy soils should be packed to approximately field bulk density. Cores are generally taken by pressing or driving a thin walled cylinder (15 c m or more in diameter and of similar length) into the soil and withdrawing the core. Often the cylinder itself is used as the container in which the permeability test is run. However,it is important to make sure that firm contact exists everywhere around the walls. The cylinder is supported vertically on a screen or bed of sand or spun glass in such a way that percolating water can be collected. Water of similar quality to that to be used in the field is passed through the core for a short time to remove most of the air from the system and the percolate is collected as a function of time. The volume of flow in unit time per unit cross-sectionalarea is divided by the hydraulic gradient (distance from water surface to the drip point divided by the length ofthe soil core) to give the hydraulic conductivity. Measurements should be made at about the temperature expected in the field or if deviation from such temperatureis appreciable a correction should be made for the viscosity of the water. Measurements should be made the same day samples are collected or the samples should be stored at low temperature to reduce growth of micro-organismsto a negligible level. It is not always easy to characterise a field permeability situation through laboratory measurements. For this reason several field methods have been developed. The most common of these involvesmeasurement of the rate of water rise in a piezometer hole extending below the water table and from which water has been and KIRKHAM, 1949). Small pipes (piezometers) are driven into the ground and soil augered pumped (LUTHIN out as the pipe moves down. At the desired depth in the soil,a small cavity is augered out below the pipe. Water is pumped out of the pipe with a suction pump and the rate of water rise in the pipe measured with a sounding device. The formula for computing hydraulic conductivity,k,in cm/hris

k=

-

2.303~R2 hl log

42-h) h2 where R is the radius of the pipe in cm, h, and h2 are the distances in arbitrary units of length from the water level in the pipe at time t, and t, measured in hours, and A is a geometric factor computed in cm which depends primarily upon the length and diameter of the cavity below the bottom of the pipe. Values (1949) as cavity diameter varies for a particular cavity of the A-functionas given by LUTHINand KIRKHAM length (10-2cm) and as cavity length varies for a particular cavity diameter (2.54 cm) are given in Fig. 4.11, ,

A-function (cm)

/ &itzEE

long)

gth (cavity 2.5 cm in diameter)

O

1 10 Size of cavity (a)

I

I

I

2

4

6

l

8

Fig. 4.11. A-functionfor various cavity sizes for use in measurement of hydraulic conductivity by the piezometer method described (Abridged from LUTHIN and KIRKHAM, 1949) and KIRKHAM (1949) Cavity length and diameter must be chosen so as to give the same A-function.LUTHIN have shown that if the diameter and length of the cavity are increased proportionately (so long as the cavity length does not exceed the depth from the water table to the start of the cavity,and so long as the depth from the bottom of the cavity to an impervious layer is greater than half of the cavity length) the A-function will remain practically the same.The position of the water table level must be determined by following the

100

HYDRO-PHYSICS-ARID

A N D IRRIGATED SOILS

rise of water as a function of time by use of a sounding device and noting the level at which equilibrium appears to be approached closely. This may require a few hours or a few days, depending upon the permeability of the soil.Hydraulic conductivity values can be obtained for various strata by carefully locating the cavity so that it is well within a particular stratum.LUTHINand KIRKHAM (1949)indicate that if the distance between a layer of different porosity and the cavity is not greater than the cavity length (and excluding any completely impermeable layer above the cavity), the effect of the layer, regardless of permeability,will be small. The double tube method of BOUWER(1961) can be used to measure the saturated permeability of soils in the absence of a water table. The method consists essentially of saturating a limited soil region below an auger hole in which two concentric tubes are placed. Permeability is calculated from measurements of the rate of change of the water level in the inner tube. The procedure separates the flow in the saturated soil between the two tubes,which results from the difference in water level within each,from the total flow into the soil that continues as the soil absorbs water. With a modified procedure (BOUWER,1964), it is possible to separate between the vertical and horizontal components of the permeability. 2. Unsaturated flow (a) Principles ofJEow in the liquidphase

Flow of water in unsaturated, isothermal, solute-freesoil may be approached from a point of view similar to flow in saturated soil. There must be a moving force and the flux depends upon the magnitude of this force and the size of the channel through which flow takes place. However,two important differences must be recognised. The first difference is that the moving force resulting from the gradient of the pressure in saturated flow, a pushing force,is replaced by a pulling force arising from within the porous material itself,the gradient of the matric potential. This results from the attraction of solid surfaces for water, adhesion,and the transmission ofthis forcethrough the water by virtue ofthe attraction ofwater molecules for each other,cohesion. The magnitude of this attractive force on a molecule of water decreases with distance from the solid surface. Therefore,water has a tendency to move from regions where the water films are thick to regions where water films are thin. As a result,in a homogeneous soil, water moves from regions of high water content to regions oflow water content.This moving force is zero in a uniformly wet or uniformly dry homogeneous soil and is large across the wetting-frontbetween wet and dry soil in soil undergoing wetting. The force of gravity acting upon a unit mass of water in a porous material is constant,but is small compared to the adhesive force between a solid and water close to the solid surface.However,as the thiclcness of water Nms increases,the relative effect of gravity increases.Hence,in wet soilthe effect of the downward pull of gravity is of importance whereas in a dry soil the effect is noticeable only over long periods of time. The second difference between saturated and unsaturated flow has to do with the reduced cross-section through which water flows when the pores only are partially filled with water. In saturated soil essentially the entire pore space carries water and the permeability factor in the flow equation depends primarily upon size and configuration of pores. Under unsaturated conditions,the water-filled cross-sectionparticipating in flow decreases as water content decreases and increases as the soil becomes wetter. As in the case of saturated flow,the permeability factor in the equation relating flux to moving force depends primarily upon the magnitude of this water-filled cross-section.Since this varies with water content,the permeability factor likewise must vary.At low water content,the permeability factor is small,and flux per unit moving force can become negligibly small;whereas,at high water content,flux for the same unit moving force can be large, approaching the flux which would be achieved under saturated conditions.Thus,the thicker water films are on soilparticles and through contacts between particles,the greater is the quantity ofwater that can be moved through the soil for a given moving force. As a consequence of these differences from saturated flow,water movement under unsaturated conditions differs appreciably from that which occurs when all ofthe pores are filled with water. Under saturated conditions, the highest permeability exists in materials that have the greatest number of large interconnecting pores, and least permeability occurs in materials that have few and small pores. By contrast,at low water content, a soil with large pores, such as a coarse sand,will have a negligibly small permeability and will transmit little or no water,even when the moving force is large.In fact,as is discussed later,a layer of coarse sand in a finer soil forms an effective barrier against water flow until such time as the water tension becomes

101

IRRIGATION, DRAINAGE A N D SALINITY quite low and the large pores between sand grains are able to fill.Under many conditions ofpractical importance,flow of water caused by unbalanced absorptive forces will be much greater in soil with finepores than in soil with coarse pores. Also,water retention in soil,usually a dynamic property rather than a static one, is frequently affected more by changes in porosity that result in reduced unsaturated permeability than by other factors. To study the migration of moisture in soil,RODEmentions that the water with which the soil is wetted may be conveniently labelled with a chlorine ion,which travels with moisture in liquid form but not with moisture in vapour form. The process of soil drying and the resultant water movement varying with the particle size and aggregate composition of the soil are illustrated in Figs. 4.12and 4.13. Figure 4.12 illustratesthe evaporation of moisture from sandy ground. It shows that while the upper layer of the soil mass dried out extensively as a result ofevaporation,the distribution ofthe chlorine ion remained unchanged.W e can deduce that water in sandy soil did not migrate to the evaporationsurface in liquid form, and that drying spread gradually from the top,the migration of the moisture proceeding quite obviously,in vapour form only. Figure 4.13 illustrates the behaviour of water in a light, dusty,loamy soil.It will be seen that the moisture content diminishes during the process of evaporation over the whole of the wetted soil mass, but only to a certain limit,which is reached after ten days. Continued evaporation for a further ten-dayperiod produces no furtherchange in moisture content (curves 5 and 6practically coincide). After the second ten-dayinterval, the depth of the uppermost and thoroughly dried-outlayer shows only a very minor increase.Examining the right-hand section of the diagram, we find that the chlorine ion accumulated in considerable quantities Depth (cm)

O

% moisture

50

100

C1 content (me/100g of sand)

Fig. 4.12. Distribution of moisture and chlorine ions in a sand column: I initial state;II after 20 days of evaporation;III after 80 days of evaporation (Based on ORESKINA) in the upper layer of the soil mass, during which time the width of this layer gradually increased from 1-2 c m for the first two days to 8-10 c m after twenty days. This shows, firstly,that the moisture migrated towards the evaporation surfacein liquid form,carrying the chlorinewith it,and secondly,that the.evaporation surface gradually shifted downwards. The water movement in liquid form towards the evaporation surface ceases after the moisture content in the wetted soil mass has descended to a particular point, which has been termed the moisture of rupture of capillary bond. This value obviously corresponds to an abrupt lowering of soil conductivity,since the value of the suctioii pressure gradient which causes migration (it can be estimated from the moisture gradient) remains high and is more or less constant. In our example, the moisture of rupture of capillary bond is equal to 11.5% of the dry soil weight, while field capacity amounts to 17% and maximum hygroscopicity to 4%. The moisture of rupture of capillary bond thus amounts to about 65% of the fieldcapacity and is three times the figurefor r.laximum hygroscopicity. 1 02

HYDRO-PHYSICS-ARID

A N D IRRIGATED SOILS

Depth (cm)

-

ó 1001

O

I I

'

' 5

I

I

'I

''

10 % moisture

I

'

I

' 15

I

"

I

I

' 20

11 day 2 2 days 3-.-.-._.3 days 4 5 days 5.-.-- 10 days 6-- 20days

---____ --

o

50 100 150 C1 content (me/lOOg of soil)

Fig.4.13. Distribution ofmoisture and chlorineionsinlight argillaceous soil:initially and at various periods after the beginning of evaporation (Based on ABRAMOVA) Migration of water towards the evaporation surface in liquid form likewise takes place in other soils having a clayey or loamy mechanical composition. It should be mentioned,in this connection,that in soils with a well-definedmacro-structure the cessation of movement on reaching a particular moisture content,i.e.after reaching that of rupture of capillary bond, is also very clearly marked, but this value normally comes closer to field capacity. O n the other hand, in soils having a heavy texture but which are structureless or have only a micro-structure,the approach of the moisture content to that ofrupture of capillary bond,at which the movement ofsuspended moisture towards the evaporation surface ceases,is less clearly marked, i.e. only a certain slowing down of the movement is observed,but not its total cessation. These differences in water migration when there is a moisture gradient (from which we estimate the gradient of suctionforce) are apparently explained by the fact that capillary and sorption forces both play a part in this migration (and likewise in the retention ofmoisture). The heavier the texture of the soil,the smaller is the role ofthe former and the larger the role ofthe latter.Where the texture is very heavy and there is not even a micro-structure,no moisture of rupture of capillary bond is to be observed at all and the deceleration in the rate of migration of suspended moisture is very gradual. The experiments we have described indicate that the behaviour of moisture in sandy soils and in loamy and clayey soils is fundamentally different.In the former,it exists in the form of isolated accumulations of contact moisture. In the latter,it possesses an inner cohesion at minimum moisture capacity which does not disappear or noticeably weaken until after the moisture content has decreased to the value of moisture of rupture of capillary bond. These differences indicate,among other things,that sand cannot be used as a universal model to study the behaviour of soil moisture and the hydrological characteristics of soils.The behaviour of moisture in sand is governed by capillary forces only,whereas in loamy and clayey soils sorption forces also play a part which increases with the heaviness of the texture of the soil. Quantitative expression of unsaturated flow for isothermal conditions involves an equation* resembling * The resulting equation is sometimes referred to as a 'modified DARCY equation', the use of such terminology being somewhat controversial.The point at issue is whether or not the conditions existing during saturated flow, which for low velocities permit the use of a simple proportional relationship between flux and applied force, are sufficiently analogous to those existing for unsaturated flow that, providing a variable permeability factor is used, a similar proportional relationship will hold. Although such a proportional relationship has proved adequate in the description of numerous unsaturated flow problems, evidence does exist that it is not exact and that under some conditions it may not be adequate. S WARTZENDRU~ER (1962) and others have also raised questions about the adequacy of the DARCY law

103

IRRIGATION, DRAINAGE A N D SALINITY

the DARCY equation but different in two important respects. The permeability factor is a variable that is extremely sensitive to the water content.And, the moving force term,given as the gradient of the sum of the gravitational potential and the pressure potential for saturated flow, now derives from the gravitational potential and the matric potential. The equation may be written as Y=

-kX grad4

(30)

4 is the sum of a matric potential, and a where Y is the water flux with dimensions L3/T/L2=L/T; gravity potential, h,with grad # having the dimensions of LIT2;h is a dimensionless variable* having the value of zero in a perfectly dry soil and unity in a saturated soil and is largely but not wholly a function of water content;and k is the saturated permeability appearing in the saturated flow equation and having the dimensions of T. The nature of the factors governing the water flux is best illustrated by considering a simple flow system like that shown at the bottom of Fig. 4.14. Here water flows horizontally and linearly from a source at small water tension into a dry soil.The force causing the flow is entirely due to the gradient of the matric potential. For this simple system the flow equation may be written

where the potential term 4,which includes a gravity component,is replaced by the matric or capillary potential #M which is a function of water content. The flux, Y, is obtained by multiplying kh by di,bM/dxat those positions in a wetting soil where such data are available. Some typical curves for kh and d#M/dX as a function ofposition in the soilcolumn shown at the bottom of the figure are presented in Fig.4.14.It should be observed that both kh and d#M/dXchange rapidly with position over at least part of their ranges and that the changes are in opposite directions.In regions where large potential gradients exist,such as near a wettingfront, the permeability factor,kh,becomes extremely small.And, despite the size of the moving force the flux is small.This is illustrated in Fig.4.15 where the flux as a function of position in the column is shown at four different times after initiation of flow. Unsaturated flowproblems are difficult to deal with quantitatively, even for simple flow systems,because of difficultiesin obtaining analytical or empirical functionsfor the two variables appearing on the right-hand (1964), in dealing with steady-stateflow problems,points out that side of the flow equation 31. GARDNER for some soils h is given to a good approximation by the empirical expression 1 (r/h)

-t- 1

where h is the value of the tension at which A=$, T is the tension, and n is a constant.The parameter h is of the order of 10 to 50 millibars tension and n ranges from about 2for clay soils to as high as 10 for sands. Where such an analytical expression can be used,the work involved in solving the flow equation is greatly reduced. Where analytic expressions are not available, as in the transient case considered here, graphical solutions usually are the simplest. Values for d+M/dx are obtained by graphical differentiationof the 31Kx curves shown in Fig.4.14.While it is possible to measure in place at values near zero by use of a tensiometer, until recently no method has been available for reliable,in-placemeasurements at values below about 1 bar (losergs/cm2). Even at values near zero,the size of the tensiometer cup and the slow approach to equilibrium are limitations to securing precise data, particularly in systems where water flow is rapid. Errors in measurements of #M are likely to be magnified in the inference of d$/dx.The z,bKx curves shown in Fig. 4.14 have been inferred from water content distribution curves shown in the same figure and a sorption curvet shown in Fig. 4.16.

-

* In m u c h of the literature about unsaturated flow a simple k replaces Ich as used here. Although it is often convenient to deal with a single variable, k,the two symbols are used here, first to preserve the generality of the equation and, second, to avoid confusion of such a variable unsaturated permeability term, IC, with the constant, IC, occurring in the saturated flow equation. BUCKINGHAM (1907)used A in a comparable way to k in contemporary use. The flow equation written as in equation (30)m a y be seen to apply to both saturated and unsaturated flowsince, at saturation, l. KIND(1965) refers to the quantity h as the relative permeability since it is the ratio of the permeability at any given water content to the permeability at complete saturation -1 This sorption curve has been obtained by extrapolationfrom a small number of data and by comparison with a more precise desorption curve

I04

HYDRO-PHYSICS-ARID

A N D IRRIGATED SOILS

1024min

O

5

16

10

15

20

30

144 400 Position of wetting-front at several different times

35 40 Distance,x (cm)

1024min

Fig.4.14. Water concentration,matric or capillary potential, and unsaturated permeability, k,as a function of position in the horizontal soil column undergoing wetting which is shown at the bottom of the figure

In a more general flowproblem where a soil is likely to have undergone both wetting and drying,it would be difficult to know the #,& relationship with any precision and the subsequent computations of dz,bM/dx would be even more subject to error. The difficulty associated with obtaining kh as a function of x is just as onerous.The nature ofthe kA-water content relationship is shown in Fig. 4.16and is plotted logarithmically as well as with linear coordinatesto illustrate the rapid decline to extremely small values at low water content. These curves were obtained by integration of successive empirically determined water content position curves (of the type shown in Fig. 4.14)to obtain water flux as a function of time and position.Then,with d+M/dxcurves such as those shown in Fig.4.14,the values of kh were computed.The resulting curves are multi-valued,depending upon the time or the position of the wetting-front. The multiple curves show that either i,hM or h is not a unique function of O. The cause for this nonuniqueness is a matter for speculationat this point; however,for many soils,the assumption of uniqueness probably causes little error. Although the foregoing treatment of a flow problem demonstrates the general nature of the components 105

IRRIGATION, D R A I N A G E AND SALINITY F I ~ (cm3/cd/min) X x10-2

I

0

5

I

I

1

I

15 20 25 Distance from source (cm) 10

I

30

Fig. 4.15. Flux as a function of position at various periods of time for the wetting column of soil shown in Fig. 4.14 Diffusivity (cm!/min)

A*,,,/As (ergs/g/cmycm’> 7 9 10’ ~

x lo6 3.6 I-

10

8 6 4 2

-I

I

(linear)

O

0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 Water content (cm’ /cm3)

Fig.4.16. Matric or capillary potential $M, A $/A O, and kh as functions of water concentration for the Salkum silt loam used in the soil column of Fig. 4.10. Values of kh are shown using logarithmic as well as rectangular coordinates to emphasise its minute size at low water content

0.35 0.40 0.45 0.50 Water content (cm3/cm3)

0.25 0.30

055

Fig.4.17. Diffusivity as a function ofwater content at various periods of time for the flow problem illustrated in Fig. 4.14

of the flow equation, it does not illustrate the simplest approach to the study of a flow problem. Under conditions where Ich and $M niay be regarded as unique functions of the water content, O, it is possible to apply the chain rule of calculus and to write equation 31 as

106

HYDRO-PHYSICS-ARID

A N D IRRIGATED SOILS Replacing kh dt+bM/dOby D(0)equation 30 then may be written in the form of a diffusion equation Y=

60 -

-D(B) sx

(33)

where the 'diffusivity term', D(B),is uniquely dependent upon water content. Although unsaturated flow is not a diffusion problem in the ordinary sense,the equation is comparable to FICK'S diffusion law and some of the mathematics used in the solution of diffusion problems become applicable.The replacement of two difficult water content dependent functions,kh and d7,bM/d0(shown in Fig.4.16for the porous system ofthe example discussed there), and the introduction ofa measurable concentration gradient as an index of the moving force makes the equation more tractable.However,the weakness of equation 31, as demonstrated in the multiple ICA curves of Fig. 4.16 still exists inasmuch as multiple D(0)curves are obtained where the same data are used for the diffusion-typeanalyses of the problem. Such curves are shown in Fig.4.17for the same flow system that is illustrated by the curves of Fig.4.14.However, diffusivity measurements have been obtained for some systems showing B(B)to be more nearly unique in a practical sense over substantial ranges of water contents. One of the many important problems of unsaturated flow is the prediction of water content distribution in a soil profile during infiltration as a function of position and time.If the equation of continuity for a one-dimensionalsystem,

is applied to equation 30 there results a differential equation containing 0,x and t:

60=-s [0(0) st

6x

$1

(3 5)

For a soil column such as that shown in Fig. 4.14,and where the water content at the source end of the column is held constant from the start of wetting and water flows into soil that is initially at uniform water content,this equation reduces to an ordinary differential equation:

transformation." where +( 0)= xdt,which is called the BOLTZMANN The diffusivity function,o(@,required in the equation, is difficult to obtain for natural soil conditions. GARDNER (1964)has proposed a method for measuring soil water diffusivity iz situ by applying water to the soil at regular intervals and observing the rate at which the waves of increasing soil water content move downward. Diffusivity is calculated in exact analogy to well-known techniques for the measurement of thermal diffusivity. He reports that preliminary experimentsindicate that the procedure is feasible,particularly in the tensiometer range of soil water tension. At lower water contents, removal of samples to the laboratory,where determinations so far must be made,without disturbance is difficult. However,approximations are possible from laboratory measurements made on soil samples (see GARDNER, 1956; MILLER and ELRICK, 1958; RIJTEMA, 1959;KUNZEand KIRKHAM, 1962;and BRUCEand KLUTE, 1963). Assuming that values of D(0)are available,equation 36 has useful solutions,which can be handled by iterative processes on a digital computer,thus permitting construction of curves describing the water content of a soil column as a function of position and time. Other types of problems have been handled similarly,but each change in boundary conditions requires a new analysis. The variable $(e) that replaces x and .\/t in equation 36 has physical significance in a wetting problem and may be used as a check upon the validity of the flow equations 31 and 35 or the assumptions involved. The locus of a point in the column at which $(O) has a particular value (or at which the water content is at a particular value) must advance linearly with l/t. Non-linear relationships have been observed by several and FENG,1949;NIELSEN et al., 1962;FERGUSONand GARDNER, 1963;RAWLINS investigators (KIRKHAM and GARDNER, 1963) which provides additional evidence that the flow equations or some of the assumptions made in their use are not completely valid. Despite this, they have still proved useful in many problems. * This has been carried out analytically by SWARTZENDRUBER (1966), thus establishing that the function, 4(O)=x4t, is a consequence of the equation and the boundary conditions I

107

IRRIGATION, D R A I N A G E A N D SALINITY Rigorous application of unsaturated flow theory to applied problems is mathematically complicated, particularly where boundary conditions are difficult. Moreover, some of the most interesting and important problems arise from the existence of stratification and temperature gradients in soil. Soil stratification and GARDNER, complicates the boundary conditions and makes mathematical solution more difficult (MILLER 1962). The presence of temperature gradients in soil and the addition of energy to the system in the form of heat further increases the complexity of flow through the introduction of an additional component of liquid flow and through the addition of temperature induced vapour flow, which often is of major importance. Problemswhere stratification existscan be handled analytically,but because of their complexity theyare often dealt with only qualitatively. For quantitative consideration of such problems, the reader should consult more specificliterature. (A review of this voluminous literature may be found in GARDNER, 1960.) Water flow in the vapour phase is dealt with in the next section and numerous implicatioiis of unsaturated flow theory for both the liquid and vapour phases are covered in subsequent sections. (b) Principles offlow in the vapour phase Under isothermal conditions,the vapour pressure gradient in soil with water content in the plant growth range is usually not large enough to cause significant vapour movement relative to water movement in the liquid phase. This is shown in Table 4.2,where at 20°C a water potential change from O to 100 bars is accompanied by a vapour pressure change of only 1.2mm Hg. Where temperature gradients exist within the soil,vapour pressure differences can become large. For example,the vapour pressure difference between water at 20°C and 19°C is 1.1 mm Hg, which is nearly as great as that given above for a decrease in water potential of 100 bars. As was shown above, temperature gradients also cause liquid flow of soil water. Using the chloride ion et al. (1952) showed that the major component of thermally induced as a tracer for liquid water flow,GURR soil water flow is in the vapour phase, and that the vapour transport exceeds that predicted by diffusion theory. These findings are coArmed by others using differenttechniques for separation ofliquid and vapour and DE VRIES (1957) questioned the validity of the assumption that flow was entirely in the flows. PHILIP vapour phase past the initial point of distillation and suggested that it was actually series-parallelflow through liquid ‘islands’located in a vapour continuum. Taking this into account, they derived an equation analogous to equation 14. This equation does not distinguish between liquid and vapour flow, but only between water driven by a potential gradient and that driven by a temperature gradient.For some purposes there may be no reason to distinguish between the phases,but the distinction could become important when the transfer of soluble salts under thermal gradients is studied. To handle such situations, PHILIP and DE VRIES (1957) suggestthe term ‘liquidtransfer’to apply to transfer exclusively in the liquid phase,and ‘vapour transfer’to apply to that in excess of the liquid transfer.Thus in the absence of liquid continuity,all transfer is vapour transfer, but so defined, one should not expect to calculate vapour transfer from the diffusion and DE VRIES (1957) coefficient of water and the vapour concentration gradient. Data reviewed by PHILIP show the ratio of the observed transfer to the predicted transfer to range from 3.6 to 18. JACKSON (1965) used the pressure dependence of the diffusion coefficientof water vapour in air to separate between liquid and vapour transfer in relatively dry Adelanto loam and Pachappa loam soils.Assuming that the diffusivity for liquid water was insensitive to pressure and that the vapour diffusivity varied inversely with pressure, by extrapolating measured diffusivities to infinite pressure where the vapour diffusivity would be zero, he obtained the liquid diffusivity. These data are shown in Fig. 4.18. For Pachappa,the liquid diffusivity was insignificant over the range of water contents used; but liquid diffusivity could not be neglected for Adelanto. In both cases, as was predicted by theory, a peak in the diffusivity occurred.The maxiinum water content was reported to correspond to a relative humidity of 0.97, which means that water potential was below -40 bars for all data reported.It is interesting to note that even in such dry soil,liquid flow along surfaces of particles was still significant in Adelanto soil.Similar curves (1966) to those of JACKSON (1965) but covering a wider range of water contents have been obtained by GEE for Palouse silt loam aggregates (Fig.4.19). Considerably more data are needed for soils in the plant growth range of water content to predict successully the relative importance of vapour flow in soil.

-

(c) Flow of water to afi.eezìngzone

A special feature ofwater migration,related to temperature conditionsin the soil,is the water movement into a freezing soil layer from an underlying non-frozenlayer. This movement takes place in different states. 108

HYDP.0-PHYSICS-ARID

A N D IRRIGATED SOILS

D x 105(cm2se~*) PACHAPPA

4t

Gravimetric water content

O

I *O10 -015 -020 425 Gravimetric water content

e005

Fig. 4.18.Diffusion coefficientsfor Adelanto loam and Pachappa loam as functions of water content at atmospheric pressure (730 mm Hg) and at 25°C.D,,, is the vapour diffusivity, DB is the liquid water diffusivity, and DB,is the combination of the two (From JACKSON, 1965) depending on the soil water content.Ifthe latter is close to or even above field capacity,migration in liquid form markedly predominates. In this case,the water content of the freezing layer rises very considerably to a point at which it can even exceed saturation. This excess value indicates that the accumulation of water (or,more accurately,of ice) causes an increase in the volume and hence the porosity of the frozen soil layer. The quantity of transferred water may amount to several centimetres in the course of a single winter. In this case,it is still not clear which forces bring about the migration of water in liquid form. Most scientists are inclined to believe that capillary forces may also be partly responsible for this phenomenon. It should be noted,incidentally,it is in soils ofloamy and clayey texture that this phenomenon is observed: in sandy soils, the migration is from the frozen to the non-frozenlayer. When the water content is low,water in loamy and clayey soils also moves from the non-frozen to the frozenlayer,but in vapour form,and the quantity oftransferred water is found to be considerablylower than where the water content is higher (10-15 mm in the course of one winter). D@m2/day)

/i

/-

.lo

.40 .50 Water content (cm’/C”) .20

-30

I e601I

Fig.4.19.Isothermal diffusivity, DO,and non-isothermal diffusivity, DT,as a function of water content. Dashed lines represent the vapour and liquid component diffusivities. Open circles represent DB values as measured by an evaporation experiment 1o9

IRRIGATION, D R A I N A G E A N D SALINITY

(d) Infiltration orid redistribution Water from rainfall or irrigation applied to dry soil increases the inatric or capillary potential at the surface creating a potential gradient which causes water to infiltrate.The rate of infiltrationat the surface and the rate offlow and advance of a wetting-frontwithin the soil depend primarily upon the porosity characteristics and the nature ofthe mineral and organic substancesforming the pores.Ifparticle surfacesdo not wet readily, as is possible when certain kinds of organic residues are present, infiltration can be seriously retarded. If particle surfaces wet easily the resulting flow is to a large degree dependent only upon pore size and size distribution-including, particularly, whatever stratification may exist. Gravity is a factor over long periods of time but in the early stages of infiltrationit inay often be neglected. For uniform soil conditions it is possible to derive an infiltrationequation from the diffusion form of the unsaturated flow equation.The resulting equation is a power series in ?/t but for most practical considerations only the first two terms are important,and often times only the first,thus, Q = St*+At

(37)

where Q is the cumulative infiltration,t is the elapsed time from start of infiltration,and A and S are parameters. PHILIP (1957a) has suggested that the first coefficient,s,be called the sorptivity.For small values of t only the first term is important and for long times where gravity becomes more important,only the last. The distance of advance of the wetting-frontis a similar function of 2/t with different parameters. The infiltration rate at the soil surface is a function of l/dt.The exponent of t in equation 37 is 0.5 as a consequence of the analyses.However,there is experimental evidence that this exponent may not be exactly 0.5 but is a variable depending upon soil conditions and differs from 0.5 by as much as 0.15. Empirical data show that infiltration into uniform materials for periods of severalhours appears to be well described by the equation

Q=AtB

(38)

which is convenient since,when data are plotted on log-log coordinate paper, the parameters A and B are easily obtained,A being the Q interceptat t= 1 and B being the slope ofthe line.The advance ofthe wettingfront is described by an equation similar to (38) and the infiltration rate at the surface is

The linear portions of the curves in Fig.4.20are fitted by equations 38 and 39. Infiltration rate

Infiltration rate

(cm3/cmz/s) 0.02

(cn~’/cm’/s)

Advance

r

(cm) 20 10 6 4

2

1 0.6

0*0004

0.0002 I

1

R I

I

I

I

2

4 6 10 20 Time (min)

0.4 IlaJO.2 4060 100

Fig.4.20.Infiltrationrates and advance ofwettingfront plotted logarithmically for water flow into soil columns containing sand and clay lenses (Arrows indicate time at which wetting-front strikes layer) 110

0.6 1

2

4 6 10 20 40 GO 100 Time (min)

Fig. 4.21.Infiltration rates plotted logarithmically for upward, downward, and horizontal flow of water into a uniform soil column (MILLER and GARDNER,1962)

HYDRO-PHYSICS-ARID

A N D IRRIGATED SOILS

For short periods of time following wetting, flow of water into relatively dry soil is only slightly affected by the gravitational field,as can be seen in the rate curves of Fig.4.21 for downward,upward,and horizontal infiltration.The same thing may be observed for flow in two dimensions in Fig. 4.19 where for 1.5 hours following water application at zero head on a point in a silt loam soil the wetting-frontadvances almost an equal distance in all directions;after 5hours,however,downward movement is slightly greater than upward, indicating the action of the gradient of the gravitational potential. The pattern of wetting-frontadvance is affected by soil porosity,being greater in the vertical direction for coarser materials with larger pores. Depth ofwater abovethe soil surfaceis a factor but a variation ofonly a few centimetreshas an appreciable effect only at the start of infiltration.The initial or antecedent water content of the soil is a larger factor. The greater the initial water content the slower the infiltration.PHILIP (1957b) has computed a curve for sorptivity (the S in equation 37) as a €unction of water content which shows a near linear decrease up to about 35%water content on a volume basis for a light clay soil.Sorptivity is about half the value of that for a dry soil at this point and decreases more rapidly thereafter. The A value is not affected greatly up to this point but increases thereafter,showing the greater influence of gravity at high water content. The net effect remains the same,with initial water content becoming less important at increasingly long times. For downward flow of water into a uniform soil, after sufficient time, the rate of infiltration becomes constant,as is seen from equation 37. At such time,if the water table is sufficiently deep,it turns out that the soil water content and the tension are virtually uniform with depth almost down to the water table with the only water-movingforce being the gradient of the gravitational potential. If a soil profile has been wet in such a manner and then the source of water is taken away,water continues moving downward under the influence of gravity,draining the upper part of the profile. As the profile drains,its water content,and thus its diffusivity,decreases with time,causing the rate of drainage to decrease.Ifthe drainage proceeds from all portions of the profile at the same rate,the water content, and thus soil water diffusivity,will be uniform throughout the proHe and will vary only with time.The mathematics for such a problem have been worked out by GARDNER (1962) and have been applied to the measurement of diffusivity in small soil samples in the laboratory by DOERING (1965). The equation for drainage rate is dW/dt=

-D(IVWy)a2 4L2

where W is the total water content of the profile, W,is the final equilibrium water content to which the profile will ultimately drain,and D is the soil water diffusivity corresponding to the water content, W,and L is the height of theprofile. IfD is known as a function of O, and therefore W,equation40 can be integrated to yield the water content of the profile at any time. Infiltration and rate of advance of water into soil are greatly affected by stratification or non-uniform conditions-so much that the utility of equations describing infiltration into homogeneous soil is severely limited inasmuch as most natural soils are generally non-uniform.The effect of certain kinds of stratification in infiltration at the surface is often spectacular as may be observed in Fig. 4.20.Here,infiltration rate is severely reduced when a wetting-frontreaches sand or clay layers of the types illustrated in Figs. 4.23 and 4.24. The effect of porosity changes in soil in checking the advance of a wetting-frontis clearly shown in these figures.Any kind ofporosity change will have an effect upon water movement in the profile,as may be observed in Fig. 4.25when soil aggregates or granules,formed from the same soil,check water flow. This is furtherillustrated in Fig. 4.26 where layers of coarse aggregates (soil conditioner treated and untreated) are placed on top of a finer soil. The large pores give rise to rapid infiltration but as soon as the finer soil is reached infiltration is drastically reduced.The importance of porosity in the surface layer is thus illustrated. If a surface crust were formed with low porosity the effect would be comparable to what is illustrated in Figs.4.20 and 4.24 but at reduced scale,where fine materials overlying coarse are observed to slow water entry and the advance of a wetting-front. The effects of some kinds of stratification on water content profiles are shown in Fig.4.27.Typical water contentprofiles during I-dimensionalinfiltrationintouniform dry soilare showninFig.4.27(a). Here saturated conditions exist in the surface and move down for a few centimetres into the soil (depending upon length of time since water was applied). Below the saturated zone the water content remains high, decreasing only gradually with depth until the wetting-frontis approached. Here the water content reduction to that of the 111

Fig. 4.22.

Fig. 4.23.

Fig. 4.24.

Fig. 4.25. Fig. 4.22. Wetting pattern under unsaturated conditions for 2-dimensionalflow of water from a source beneath the soil surface at the position of the white spot Fig. 4.23 Wetting pattern for 2-dimensionalRow of water from a furrow into soil overlying a sand layer for 0.3,1.5 and 5 hours Fig. 4.24.Wetting pattern for 2-dimensionalflow of water from a furrow into soil overlying a clay layer for 0.6and 6 hours Fig. 4.25.Wetting pattern for 2-dimensionalflow of water from a furrow into soil containing a layer of coarse soil aggregates made from the same soil at 0.5 and 1.5 hours

112

HYDRO-PHYSICS-ARID

A N D IRRIGATED SOILS

soil in its initial condition is rapid. The position where the water content decrease is the most rapid corresponds to the visual wetting-front.The profile in 2-and 3-dimensionalflow is comparable except that the position variable becomes the radius of an approximate circle or hemisphere centred at the water source. The downward pattern differs slightly from the horizontal pattern due to gravity with the effect becoming increasingly important at long times. Both liquid and vapour phase flow are involved but, in the absence of a temperature gradient,both are in the same direction and usually are not considered separately.

0.002

-

0~001 6 A

i

0.1

1

I

10

l

l

1

I

100 Time (seconds)

I

I

I

,

lo00

Fig.4.26. Infiltrationrate and advance of a wetting-frontas a function of time for l-dimensionalwetting of a soil overlain with 3 cm of coarse aggregates made from the same soil and for the uniform soil.One curve is for unstabilised aggregates and a second curve is for comparable aggregates stabilised with soil conditioner (Arrows indicate time at which the wetting-frontreaches the fine soil) Where the wetting-frontreaches a water table the water content profile is nearly uniform with depth,the soil approaching saturation as infiltration continues. The water content depends upon the matric potential existing at the point in question,but since the total depth of soil in the root zone rarely exceeds a few metres, this potential is near zero and the water content is high. In some respects water content distribution in soil overlying strata of coarse or fine materials which are initially dry resembles the distribution existing after a wetting-front reaches wet soil above a water table. In the case where a pronounced sand or gravel layer is encountered,wetting is abruptly checked at the layer until the entire profileiswet enough for the water tension at the interface to be sufficientlylow to permit water and BUNGER,1963). This is evident in the water content distribution curves of Fig. 4.27 (b). entry (MILLER The wetting of the sand or gravel below is usually erratic because of the variability of pore sizes and the fact that the smallerpores take water first.In the case of clay layers (Fig.4.27(c)) water moves in immediately on contact. However,wetting is slow and the profile above wets up in a similar fashion to that which occurs with a sand layer. However,the clay layer constitutes a permanent barrier to rapid water flow rather than a ‘checkvalve’ as in the case with sand or gravel. Saturated conditions easily develop and the water table can extend up to the surface if water application continues long enough. 113

IRRIGATION, D R A I N A G E A N D SALINITY

0.10

-

0.20

O 0.10 Water content (g/g)

0.30

0.20

0.30

0.40

Fig. 4.27.Water content profiles during infiltration (solid curves) into uniform soil and redistribution (dashed curves), without surface evaporation, of water in uniform soil and soil containing sand,clay and aggregate layers Sand and gravel layers on the one hand and tight clay layers on the other,although common in nature, constitute extreme conditionsfor water flow.Any porosity change whatsoever encountered by a wetting-front alters the flow and water retention to some degree. Uniform conditions are the exception and smooth curves for water content distribution, easily predicted from analyses,are rarely found.An intermediate situation is illustrated in Fig. 4.25where soil aggregates,made from soil like that above and below them so that only the porosity is different, are observed to check the advance of a wetting-front.Water does move into the aggregate layer but only through the contact points between aggregates. Thus it is seen that for non-uniform or stratified soil profiles,the water content to which the upper portion of a soil profile drains following wetting depends to a very large degree upon the nature of the lower strata. For this reason it should be obvious that no laboratory measurement on a sample ofsurface soil can be used to determine water retention of a profile.

114

HYDRO-PHYSICS-ARID

A N D IRRIGATED SOILS

(e) Water retention in soil and its use by plants The maximum quantity ofwater available in soilfor plant growth between periods ofheavy rain or irrigation has been traditionally determined by deducting the quantity of water in the rooting zone when plants wilt permanently from ‘fieldcapacity’.Field capacity has been considered to be the quantity of water thought to remain in the rooting zone if uptake by plants and evaporation from the soil surface are prevented. That a unique value does not exist for field capacity is recognised in early definitions of the quantity.These discuss a ‘fieldcapacity range’which is reached ‘oneor two days’after a heavy rain or irrigation and after downward drainage below the root zone becomes negligibly small.Application ofthe concepthas been restricted to soils which were considered to be well drained. Modern usage of the terms ‘maximum available water’, ‘wilting point water’, and ‘field capacity’has continued along original lines. But the utility of these concepts as originally defined is open to question. A more precise and reproducible ‘fieldcapacity’ appears to be needed. And, ‘maximum available water’ has lost its earlier utility as it has come to be recognised that optimum plant growth and crop quality factors depend more on the energy state of water than upon the total quantity present between field capacity and wilting point. Furthermore,plant water status often is affected as much by external environment as by soil water. None the less,the time between irrigation or the period oftime over which a given supply ofwater will last are quantity factors requiring volumetric measurement. The central issue in both situations involves a knowledge ofhow much water is available for evapotranspirationin an interval oftime between water application and thetime at which plant growth or some plant quality factor becomes affected significantly by low water potential (a high negative value) in the root zone. What constitutes a significant reduction in plant growth or reduction in quality are subjective matters and will depend upon plant variety and economic factors. Furthermore,the quantity of water in the soil at which plants begin to suffer depends upon water conduction properties of the soil,upon the salinity status and upon external environmental factors.Additionally,the quantity of water lost to deep percolation will vary with water potential gradientswhich may be significantly affected through removal of water by plants. Hence, a simple answer to the question is not possible.

(1) Field capacity

At the present time there is no consensus regarding the best method to deal with field capacity.However, numerous investigators believe that field capacity should be defined as a dynamic property of the soil profile without regard to the many additional factors which enter into computations of such things as maximum available water. Taking this point of view,field capacity may be defined as the quantity of water remaining in a specified depth of soil,wetted excessively by rainfall or irrigation and then covered to prevent evaporation, when the rate of water flow out of the soil is reduced to a specified level. Thus it is the maximum quantity of water which can be held in a designated soil depth without exceeding a specified rate of water loss through deep percolation. Amounts of rainfall or irrigation in excess to that needed to bring the soil to this water content,but including sufficient water to supply evapotranspirationlosses during applications ofwater and its penetration to the designated depth,is presumed to be wasted. Three arbitrary factors exist in such a definition: the depth of soil considered,the depth of wetting to be regarded as excessive,and the rate of water movement out of the specified depth at which time field capacity is considered to exist. The first factor, depth of soil considered, depends upon the use to be made of field capacity data. In agriculture,soil depth usually would be the estimated rooting depth for the crop grown. However, this depth often cannot be defined unequivocally since the depth of penetration of roots varies-particularly during the early stages of growth of plants. When not otherwise defined,the depth of rooting of mature plants generally is meant but often it is an assumed rather than a measured depth. The second factor,wetting depth to be regarded as excessive,is not arbitrary ifthe amount ofwater added is sufficientthat additional quantities do not change the water retention pattern. However,in some deep soils such a requirement may be unrealistic and other criteria may be found more useful. Hence a degree of arbitrariness exists. The third factor,a consequence ofthe dynamic nature ofthe water retentionprocess,is entirely subjective. Where evapotranspirationis ofthe order of, say,0.6 cm per day a loss of0.25c m per day may be reasonable for many soils.However,in salt-affectedsoils a larger value may be chosen because of the need for moving salts below the root zone.The number of days following water addition at which time such a rate is reached may vary widely,from one day or less to as long as two weeks.Hence,the popular view that field capacity is reached in about two days may be seen to lack general applicability.

115

IRRIGATION, DRAINAGE A N D SALINITY Field capacity figures often are reported on the basis of depth of water per unit depth of soil (i.e. cm/cm or in/ft,etc.). While this is a convenientway to report values for a uniform soil where the variation in water content with depth is small,it can be misleading as may be seen from data in Figs.4.28,4.29 and 4.30. Depth 0-

10

-

20

-

Depth (an)

(mi)

O-

10 .

Day Days Days Days

20 ’

db 30 0.02

I

0.06

1

I

1

1

0.10 0.14 0.18 Water content in g/g

30

0.22 I

40

Fig. 4.28. Water content as a function of depth at various time intervals after addition of 2.3cm3/cm2 of water

I

I

I

I

Fig. 4.29. Water content as a function of depth at various timeintervals after addition of4.7 cm3/cm2 of water

Depth (cm) O

10

Water added 11.7 cm

30

40

50

6C

70

I

0.02

0.06

I

0.14 Water content (g/g) 0.10

I

0.18

Fig.4.30. Water content as a function of depth at various time intervals after addition of 11.7 cms/cmzof water

116

HYDRO-PHYSICS-ARID A N D IRRIGATED SOILS Under irrigation conditions it has been observed (RICHARDS, GARDNER, and OGATA,1956) that profile water content following irrigation may often be approximated by the equation

W =ALt- ’ (41) where L is the depth of the profile considered, t is time and A and B are parameters. If the logarithms of W and t are taken the equation becomes: log W=lOg AL-B log t,

(42) and it is obvious that a straight line would result if log W were plotted against log t. The parameter A is the value of W/Lat t= 1, the parameter B is the slope of the line.Under such circumstances the rate of water movement from the designated part of the profile may be obtained by differentiating equation 41 with respect to time.

Placing dW/dt=0.25cm/day,or some other appropriate value and putting the depth of profile to be considered in place of L permits determination of the time at which profile water loss is 0.25 cm/day. Then, reference to equation 42 or the graph of the function permits determination of’a profile water content which is to be called ‘field capacity’. The process of determining fieldcapacity is illustrated in Figs. 4.28-31. Water distribution as a function of depth on the profile,with time as the parameter,is shown in Figs.4.28-30 for three different quantities of water that have infiltrated into a uniform silt loam soil initially air dry. The soil was covered to prevent evaporation.Using data from these distributions,water content for three differentprofile depths,O-12,O-20, and 0-40 cm,have been computed and are plotted logarithmically as a function of time in Fig.4.31. Water content in profile depth indicated (cm’/cmZ’r Profile water content (cm3./cmz) 10

-a-.MOcm 0-20cm

-

0-12cm 2

I

I

-.c

I l l ,

0-20cm 0-12cm

Water added 11.7 cm3/cm* 0-20 cm

O 1

2

3

4

8 10

Days

2 4 6 8 10 12 14 Water added initially (cm3/cm2)

15 20 40 60

Fig. 4.31. Water content in 0-12 cm,0-20 c m and 0-40 cm depth intervals as a function of time since addition of the quantity of water indicated

Fig. 4.32. Soil profile water content for 0-12 cm, 0-20 cm and 0-40 cm depth intervals as a function of quantity of water added

Parameters A and B of equation 41 have been computed from these curves and the value of the water content, W,for each of the profiles at the time when profile loss became 0.25cm/day has been determined. These values given on a cm3water per cm2area basis,are plotted as a function of quantity of water added for three profile depths in Fig. 4.32. It is to be noted that the water content of each of the two shallowest profiles increases at a diminishing rate as quantity of water applied increases,so that after the profile has been wet to considerable depth water content values are less affected by depth of wetting. Thus,the most unequivocal value to assign as field capacity is the value for the greatest addition of water. None the less, under some circumstances values for smaller water additions might have greater practical meaning. Field capacity for the 0-40 cm depth interval, determined from equation 43 for dW/dt=0*25cm/day 117

IRRIGATION, DRAINAGE A N D SALINITY Soil depth (cm)

Profile water content (cm3/cm2 ) 0-40 cm profile F.C.-9.2cm3/cm2

O

10

30

20

40

50

Days

I

0.1

Fig. 4.33. Profile water content as a function of time for a 0-40 c m profile after the addition of 11.7 cm3/cm2 of water. The tangent line to the curve is for a rate of water content change of 0.25 cm/day

I

I

I

0.2 0.3 0.4 0.5 Water content in cm3/cmz

Fig.4.34. Water content as a function ofsoil depth after addition of the indicated quantities of water and an interval of time of 1 week (Abridged from NIELSON, BIGGARand MILLER, 1967)

and from Fig. 4.31,is about 9.2cm3/cm2. The 0-40 c m profilewater content is plotted as a function of tinie in Fig. 4.33 and a straight line with slope 0.25 cm3/cm2/dayis fitted tangent to the curve. The point of tangency is at about three days,the time interval computed from equation 43. Where a water table or soil wetted nearly to saturation is present near the soil depth for which field capacities are to be determined, the water content distribution curves will be different from those shown in Figs. 4.28-30. Where soil comes to equilibrium with a water table the water content at each elevation will correspond to the water content at a matric potential equal to the gravity potential measured from the elevation of the water table surface. Such curves, at least for uniform soil, show water content decreasing with elevation as contrasted to the situation where water moves into drier soil as considered above.However, as this equilibrium is approached the same criteria as described above may be used for designating field capacity,i.e.the water content ofa designated profile when drainage out of the profileis reduced to a specified arbitrary value. Distribution curves after a period of one week for a profile where drainage is into wet soil BIGGARand MILLER, 1967). Here different quantities of water have been are shown in Fig. 4.34 (NIELSON, applied. It is to be noted that for a 120 c m profile the water content is essentially stable after addition of 30 cm of water. And for a 150 c m profile the water content is essentially stable only after addition of 45 c m of water.

(2) Effect of plunt tcse of water on proJile water loss One of the complicating factors in using field capacity values to compute available water is that evapotranspiration is a factor in determining water potential gradients in a soil profile. Such use reduces the gradients that cause downward flow. Downward water loss from the profile thus is reduced below that for a bare soil with evaporation prevented. Also, any water used during the period prior to the time computed for designating field capacity must be considered to be available for evapotranspiration. These factors plus other environmental or plant factors must be considered to determine available water from field capacity measurements. Thus,available water, using wilting point or some other choice for the lower limit (e.g.the 118

HYDRO-PHYSICS-ARID A N D IRRIGATED SOILS water content at 2 bars total suction), may be obtained from field capacity values using a formula such as : Available water = f.c.-water content at arbitrary condition such as wilting point correction factor, (44) where the correction term takes into account all factors affecting available water, apart from those directly associated with water retention properties of the soil profile itself. Although determination of a correction factorfor various practical circumstances can be an involved process,such an equation,none the less,must be used rather than an equation such as Maximum available water=f.c. -wilting point

(45)

(which,at best,fits only limited field conditions) if accurate predictions are desired.

REFERENCES ABRAMOVA M.M., BOLSHAKOV A. F., ORESHKINA N.S. and RODEA.A.(1956), Evaporation of suspended moisture, Soil Science,2 (in Russian).

ASLYNGH.C. (1963), Soil physics terminology,Int. Soc.Soil Sci. Bull.,231, 2-5 BABCOCKK. L. and OVERSTREET R. (1955), Thermodynamics of soil moisture: a new application, Soil Science,60,4.

BAVERL. D.(1956), Soil Physics,3rd ed,New York. BODMANG.B.and COLMAN E.A.(1943), Moisture and energy conditions during downward entry of water into soils, Soil Sci.Soc.Am. Proc.,8, 116-22.

BOLSHAKOV A.F.(1961), Water regime ofdeep black-earthsoils ofthe Central Russian uplands,Transactions of the Soil Institute (in Russian). BOLTG.H. and MILLER R. D. (1958), Calculation of total and component potentials of water in soils, Tu.Am. Geophys.,30, 5. BOUWERH.(1961), A double tube method for measuring hydraulic conductivity of soil in situ above a water table, Soil Sci.Soc.Am.,25, 334-9. BOUWER H.(1964), Measuring horizontal and vertical hydraulic conductivity of soil with the double-tube method, Soil Sci.Soc.Am.,28, 19-23. Bowoucos G. J. and MICK A. H.(1940), A n electrical resistance method for the continuousmeasurement of soil moisture under field conditions,Mich.Agr. Exp. Sta. Tech. Bull.,172. BRUCER. R. and KLUTEA.(1963), Measurements of soil moisture diffusivity from tension plate outflow data, Soil Sci.Soc.Am. Proc.,27,18-21. BUCKINGHAM E.(1907), Studies on the movement of soil moisture, US.Dept.Agr. Bur. SoiEs Bull.,38. CAMPBELL G.S.,ZOLLINGER W . D.and TAYLOR S. A.(1966), Sample changer for thermocouple psychrometers :Construction and some applications,Agron.Jour., 58, 315-18. CANNELL G.H.and GARDNER W.H.(1959), Freezing-pointdepressionsin stabilized soil aggregates,synthetic soil,and quartz sand,Soil Sci.Soc. Am. Proc.,23,418-22. CARY J. W.(1965), Water flux in moist soil: Thermal versus suction gradients,Soil Science,3100, 168-75. CHILDS E.C. and COLLIS-GEORGE N.(1950), The permeability of porous materials,Proc.Roy,Soc.A.,201. COLMAN E.A. (1944), The dependence of field capacity upon the depth of wetting offield soils,Soil Science, 58, 1. DERJAGIN B. V.,MELNIKOVA M.K.and NERPIN S. V. (1956), Theory of equilibrium and migration of soil moisture at various degrees ofwetting,Report to the 6th International Congress of Soil Science,Commission I-Soil Physics M.(in Russian). DOERING E.J. (1965), Soil water diffusivity by the one-stepmethod, Soil Science,99,322-6. DOLGOV S. I. (1948), Studies on the mobility of soil moisture and its availability to plants (in Russian). EDLEFSON N. E.and ANDERSON A.B. C. (1943), Thermodynamics of soil moisture, liilgardia,15, 31-298. FERGUSON HAYDEN and GARDNER W.H.(1963), Diffusion theory applied to water flow data obtained using gamma ray absorption, Soil Sci.Soc.Am. Proc.,27,243-6. FUKUDAH . (1956), Diffusion of water vapour and its exchange between condensation and evaporation in soil,Soil Science,81, 2.

119

IRRIGATION, DRAINAGE A N D SALINITY

GARDNER W.H.(1965), Water content, 82-127, in BLACK C.A.et al.,Methods of soil analyses,1,Agronomy 9,Amer. Soc. Agron. GARDNER W.R. (1956), Calculation of capillary conductivity from pressure plate outflow data, Soil Sci. Soc.Am. Proc.,20,317-30. GARDNER W.R. (1960), Soil water relations in arid and semi-arid conditions, in Unesco publications. Plant-water relationships in arid and semi-arid conditions:review of research,Arid Zone. Research,15, 37-61.

GARDNER W . R.(1962), Approximate solution of a non-steady-statedrainage problem,Soil Sci.Soc. Ain. Proc.,26, 129-32. GARDNER W . R. (in press), Water movement below the root zone, VIZZ Int’l.Soil Sci.Soc. Cong. Trans. GEEG.W.(1966), Water movement in soils as in.uenced by temperature gradients,P1i.D.thesis,Washington State University.

GROOT S. R.DE (1951), Thermodynamicsof irreversible processes, North Holland Publications,Amsterdam, 94-7, 124.

GROOT S. R. DE and MAZURP. (1962), Non-equilibrium thermodynamics,North Holland Publications, Amsterdam.

GURRC. G.,MARSHALL T.J. and HUTTON J. T. (1952), Movement of water in soil due to a temperature gradient, Soil Science,74,5. HALLAIRE M.(1958), Soil water movement in the film and vapor phase under the influence ofevapotranspiration,Highway Research Board,Spec.Rep. 40. HUTCHEON W . L. (1958), Moisture flow induced by thermal gradients within unsaturated soils, Highway Research Board,Rep,40. JACKSON R. D.(1965), Water vapor diffusionin relatively dry soil: IV.Temperature and pressure effects on sorption diffusion coefficients,Soil Sci. Soc. Am. Proc.,29, 144-8. JOUNGS E.G.(1958), Redistribution of moisture in porous materials after infiltration,Soil Science,86,3 and 4.

KINGL. G.(1965), Description of soil characteristics for partially saturated flow,Soil Sci. Soc. Am. Proc., 29, 359-62.

KIRKHAM DONand FENGC.L.(1949), Some tests of the diffusion theory, and laws of the capillary flow in soils,Soil Science,67,29-39. KLUTE A.,BRUCER.R.and PIASSELL M.E.(1956), The application ofthe diffusivityconcept to soil moisture movement, Tr.of the 6th Inst. Congr. of S.S.,13. KOTSJAKOV A.N.(1960), Reclamation principles, M.(in Russian). KUNZE R. J. and KIRKHAM DON(1949), Simplified accountingfor membrane impedancein capillary conductivity determinations,Soil Sci.Soc. Am. Proc., 26, 421-6. KUZMAK J. M.and SERDAP. J. (1958), On the mechanism by which water moves through a porous material subjected to a temperature gradient,Highway Res.Board.Spec. Rep. 40. LEBEDEV A.F.(1936),Soil andground waters,4th edition.M.,L.(in Russian). LETEYJ., KEMPER W.D.and NOONAN L. (1968), The effect of osmotic pressure gradients on water movements in unsaturated soils,S.S.SAP (in press). LUTHIN J. N.and KIRKHAM DON(1949), A piezometer method for measuring permeability of soil in situ below a water table, Soil Science,68,349-58. MARSHALL T. J. (1958), Relations between water and soil,Techn. Comm.56,Commonwealth Bureau of Soils. MILLER D.E.and BUNGERWM. C.(1963), Moisture retention by soil with coarse layers in the profile,Soil Sci.Soc. Am. Proc.,27, 586-9. MILLER E.E.and ELRICK D.E.(1958), Dynamic determination of capillary conductivity extended for nonnegligible membrane impedance,Soil Sci.Soc. Am. Proc., 22, 483-6. MILLER D . E.and GARDNER W . H.(1962), Water infiltration into stratified soil,Soil. Sci.Soc. Am.Proc., 26, 115-19.

MONTEITH J. L.and OWENP. C.(1958),A thermocouplemethod for measuring relativehumidity in the range of 95-loo%, J. Sci.Inst., 34,443-6. MOORE R.E.(1939), Water conduction from shallow water tables,Hilgardia, 12,383-426. NIELSEN D.R.,BIGGARJ. W.and DAVIDSON J. M.(1962), Experimental consideration of diffusion analyses in unsaturated flow problems, Soil Sci.Soc. Am. Proc.,26, 107-1 1. 120

HYDRO-PHYSICS-ARID

A N D IRRIGATED SOILS

NIELSEN D.R., BIGGARJ. W.and MILLER R.J.

(1967), Field observations of infiltration and soil-water redistribution, Trans. Am. Soc. Agr. Eng.,10, 382-7. ORCHISTON H.D.,Adsorption of water vapour, Soil Science,76, 6 (1952); 78, 6 (1954); 79, 1 (1955). PENMANH.L.(1956), The movement and availability of soil water, Soils and Fertil.,19, 3. PHILIP J. R., The theory of infiltration,Soil Science,83, 5 and G (1957); 84, 1, 3 and 4 (1957,a and b); 85, 5 and 6 (1958). PHILIP J. R.and DE VRIES D.A. (1957), Moisture movement in porous materials under temperature gradients, Trans. Am.Geophys. Union,38, 222-32. PRIGOGINE I. (1961), Thermodynamicsof irreversible processes, John Wiley, N e w York. PROSKURNIKOV S. M.(1948), Results of experimental studies of the movement of gravitational film water in uniform sands, Transactions of the Hydrological Institute, 8 L (in Russian). RAWLINS, S. L. (1966), Theory for thermocouple psychrometers used to measure water potential in soil and plant samples,Agric. Meteor, 3, 293-3 10. RAWLINS S. L.and GARDNER W . H.(1963), A test of the validity of the diffusion equation for unsaturated flow of water in soil,Soil Sci.Soc.Am. Proc.,27, 507-1 1. RAWLINS S. L.and DALTON F. N.(1967). Measurement of soil water potential without precise temperature control,Soil Sci. Soc.Am. Proc.,31, 279-301. RAZUMOVA L.A. (1950), Basic results of agro-hydrologicalresearch. Hydro-Meteorological Service. Trunsactions of the Central Institute of Weather Forecasting,18 (in Russian). RICHARDS L.A.(1947), Pressure-membraneapparatus, construction and use,Agric. Engr.,28, 451-4. RICHARDS L.A.(1966), A soil salinity sensor of improved design,Soil Sci. Soc. Am. Proc.,30, 333-7. RICHARDS L.A.,GARDNER W . R.and OGATA G.(1956), Physical processes determiningwater loss from soil, Soil Sci. Soc. Am. Proc.,20, 310-14. RICHARDS L.A. and OGATA G.(1958), Thermocouple for vapor pressure measurement in biological and soil systems at high humidity, Soil Sci.Soc.Am. Proc.,128, 1089-90. RICHARDS S. J. (1965), Soil suction measurement with tensiometers in ‘Methodsof soil analysis’, Soil Sci. Soc.Am. RIJTEMA P. E. (1959), Calculation of capillary conductivity from pressure plate outflow data with nonnegligible membrane impedance,Neth. Jour. Agr. Sci.,7,209-15. RODEA.A. (1952), Soil Moisture M.,L (in Russian). RODEA.A. (1956), Types and forms of soil moisture and water characteristics of soils. Report to the 6th International Congress of Soil Science, Commission I, Soil Physics M.M., L (in Russian). RODEA.A.(1961), Problem of the ‘hydrophysicalconstants’of soil,Soil Science,6 (in Russian). SCHOFIELD R.K.(1955), The PF of the water in soil,Trans.of the 3rd hit. Congr. Soil Sc.,2, 37-48. SPANNERD.C. (1951), The Peltier effect and its use in the measurement of suction pressure, Jour. Exp. Botuny,2, 145-68. SPERANSKIJ and KRASHENINNIKOV (1907), Hygroscopic water in the soil and underground dew,Journal Op. Agr., 3, St. Petersburg (in Russian). SWARTZENDRUBER D.(1962), Modification of Darcy’s law for the flow of water in soils, Soil Science,93, 22-9.

SWARTZENDRUBER D . (1966), Variable-separablesolution of the horizontal flow equation with nonconstant diffusivity,Soil Sci.Soc.Am. Proc.,30, 7-11. TAYLOR S. A.and CARY J. W.(1964),Linear equations for the simultaneous flow of matter and energy in a continuous soil system,Soil Sci.Soc. Am. Proc.,28, 167-72. TROHMOV A.V. (1927), Film water in soil,ScientiJicAgricultural Journal, 4 (in Russian). us SALINITYLABORATORY (1954), Diagnosis and improvement of saline and alkali soils,Agriculture Handbook, 60, USDA.

VASILEV I. S. (1958), Water regime in sod-podsolic soils in grass-arablecrop rotation, Collection,Fertility of sod-podsolic soils M (in Russian). WADLEIGH C. H.and AYERS A.D.(1945), Growth and biochemicalcomposition of bean plants as conditioned by soil moisture tension and salt concentration,Plant Phys.,20, 160-232.

WILCOX L.V. (1950), Pressure-controlunit for use with the pressure-plateapparatus for measuring moisture sorption and transmission by soils,Soil Sci.,70,427-30.

121

5. Chemistry of Saline and Alkali Soils of Arid Zones* A. INTRODUCTION

ALLTYPES and groups of soils have a specific level of natural fertility. Under identical climatic conditions, this levelis determined by many factors: the character of the parent rock,the physical,physico-chemicaland biochemical properties of the soil and, particularly, the presence of reserves of mobile elements of mineral nutrition for plants. Most soils may retain a certain minimum of nutritive substances against leaching. The natural fertility of a given type of soil can be increased by influencing one of the above listed factors, i.e.by changing the physical, physico-chemicalor biological properties of the soil,and taking steps to transform the elements ofthe mineral nutrition ofplants into a more accessibleform.Another measure that can be taken is to increase the reserve of nutritive substances and moisture in the soil. Among the major nutrients,the substancesfound in the smallest quantity in most arable soils,especially in arid zones,are compounds of nitrogen. The second scarcest ofthe major essentialelements of the mineral nutrition ofplants is usually phosphorus. The shortage of phosphorus supply for plants may be due not merely to the fact that the actual content is small,but also to the transformationofphosphorus intoforms with low solubility.Inparticular,the carbonate character ofarid soils (high content ofcalcium carbonate) leads to the formation of calcium phosphates with low solubility.In some meadow hydromorphic soils,with active anaerobic processes,phosphorus may also be bound with bivalent iron.To prevent this,there must be better soil aeration so as to promote the transformation of ferrous oxide combinations of iron,which are soluble,into acid ones,which are insoluble. Potassium fertilisers which are widely used in non-arid zones are of little effect in arid zone irrigation farming. The presence of large quantities of tiny fragments of little-weatheredprimary minerals (micas, feldspars,and so on) provides a constantreserve of accessible potassium in soils.Only if applied in addition to large quantities of nitric and phosphorus fertilisers does the application of potassium have any effect. Most arid soils do not suffer from a lack of micro-elements.The most common exception to this general rule is a shortage of iodine in areas where there are no marine sedimentary rocks. The use of mineral fertilisers does not cancel out the usefulness of organic fertiliserswhich provide the soil with a whole series ofother elementsessentialto the mineral nutrition ofplants.Inaddition,organic substances promote the creation of conditions for the activity of useful micro-organisms,improve the buffering of the soil against harmful reactions, accelerate the process of obtaining nutritive elements from compounds not easily available, etc. The decomposition of organic remains is accompanied by the production of carbon dioxide.Saturation ofthe soil solutionsby carbonic acid increases the solubilityofthe carbonates ofcalcium, so balancing the composition of soil solutions and absorbed bases,and facilitating the process of calcium assimilation by plants ; moreover, some organic compounds exercise a stimulating effect on plants. Contrary to ordinary soils of humid climate the saline soils of arid zones which either have poor fertility or are completely barren,do not lack nutritive elements (though this may sometimesbe the case) but have a strong surplus of easily soluble salts.The question of how to utilise these soils for farming in many countries constitutes one of the most pressing problems of present-dayworld agriculture. Unless they are desalinised,we cannot reckon to be able to improve their fertility to any marked extent by use of fertilisers. In Chapter 3 the saline soils of the arid zone have been characterised in general. Here we have to consider especially the chemistry of saline and irrigated soils.

B. MAIN SOLUBLE SALTS

IN SOILS AND WATERS OF ARID ZONES

Nearly all known acids form salts which are to be found in varying quantities in the soils. Let us take a look at the main types of salts which lead to the formation of saline soils. * This chapter was edited by V. A. KOVDAfrom the manuscripts submitted by K.L. RATKOCK(mainly on Parts E and F). V. V. EGOROVand V. A. KOVDA as authors with contributions by G.PASCAUD and P. MINART 122

SALINE A N D A L K A L I SOILS CHEMISTRY (ARID ZONES) 1. Carbonates

Salts of carbonic acid are widely found in soils,subsoils and groundwaters of deserts, semi-deserts,steppes and even forest-steppes.The role of these salts depends very largely on the character of the compounds, as do also the amounts of salts which are accumulated in the soil,their degree of solubility and toxicity for plants and their effects on the water economy. (a) Calcium carbonate Calcium carbonateis a salt with extremely low solubility (0.0131g/l). In the presence of carbonic caid,however,as aresult ofthe formationofcalciumbicarbonatesthrough the reaction:CaC0,+H2C0,= Ca(HCO,),; the solubility of calcium carbonate increases sharply to 0.06-0-14g/l.The solutions of calcium carbonate, being salts of a strong base and a weak acid, are highly alkaline in the absence of accessible free carbonic acid (pH 10to 10-2).But the presence of carbonic acid in quantities found in the soil air reduces the alkalinity of the calcium bicarbonate solution to a fairly low level (pH 7.5 to 8.5). A large quantity of carbonic acid increases the solubility of the CaCO, from 0.4 to 1.0 g/l,but this is accompanied by a sharp drop in the p H of the solution from 6-8to 6.1. In view of its low solubility, the presence of calcium carbonate in soils is not harmful for the majority ofagriculturalplants;althoughplants which are suitable for growing in acid subtropical and tropical soils (such as cocoa,coffee,bananas,tea,citrus fruits,tung trees) do not thrive on soils with a high calcium carbonate content,therefore reducing both the quantity and the quality of their crops. Many fresh rivers and undergroundwaters containlarge quantities ofdissolved calcium bicarbonate.Thisis why deposits brought by river, lake or sea waters nearly always have a high content of calcium carbonate (from 7to 15%). Large quantities ofcalciumcarbonate pass into the soilhorizons from the calcareous groundwaters, when the table lies close to the surface and when these waters are exposed to transpiration and evaporation.The soils of steppes and deserts invariably contain carbonate.The quantity of CaCO, in steppe and desert soils and loesses may be as high as 20%or even,in some cases,80%. Horizons of this type are strongly cemented and impenetrable both for the roots of plants and for irrigation waters. Sometimes the accumulationoflimefrom the groundwaters and the cementationprocess ofthe soil may occur after a period of 5 to 7years ofirrigation,so killing valuable fruit trees.In soilswhere down-flowingsolutions predominate, the calcium carbonate is gradually leached out. Calcium carbonates are easily detected, morphologically speaking, owing to the whitish colour of the carbonate horizons, the white veins (mycelium), and the mealy accumulations and concretions; also by making a qualitative analysis with hydrochloric acid,which gives a violent reaction of escaping CO,. (b) Magnesium carbonate Magnesium carbonate has much greater solubility than calcium carbonate.In the presence of carbonic acid, its solubility increases sharply,owing to the formation of bicarbonates of magnesium. Since magnesium carbonate is a salt of a strong base and weak acid, during the process of alkaline hydrolysis,its solution is highly alkaline (pH of up to 10). This alkalinity may cause stress in plants. Therefore,the presence of free magnesium carbonate in soils may be regarded as a negative factor,causing a lowering of the fertility.However, the accumulation of magnesium carbonate in soils in free form is rare. This is due to widespread adsorption ofmagnesium by clays;and also to the formation in the soils ofdeserts and steppes,of a virtually insoluble compound of magnesium-dolomite (CaMg(CO,), and of dolomitised concretions containing up to 2.5 % of magnesium carbonate. (c) Sodium carbonate (soda) The salts of carbonic acid and sodium are commonly found in nature and sometimes accumulate in soils in considerable quantities;they exist in soils and groundwaters in several forms. Normal sodium carbonate Na,CO, is the salt of carbonic acid and sodium hydroxide. In soils,this compound crystallises out with varying quantities of water (Nazco,. 10 H,O,Nazco3.H20). Na,CO, is highly soluble (178 g/l at 2GOC). As a result of hydrolysis,it provokes acute alkalisation of the milieu, up to p H 12.Owing to its high alkalinity and solubility,it is extremely toxic for the majority of plants. The presence of Nazco, in soil solutionscauses peptisation of soil colloids,soil disaggregation and reduces the natural fertility, low water permeability. Its existence in soils,even in quantitiesof0.05 to 0.1%, due to alkalinity and lack of structure. Bicarbonate of sodium is both less alkaline and less toxic than normal soda. This is due to the fact that K

123

IRRIGATION, D R A I N A G E A N D SALINITY bicarbonate of sodium is partially neutralised by carbonic acid.The formationofbicarbonate ofsodium occurs during the reaction of soda with free carbonic acid,according to: Nazco,+H,O +CO,-+2NaHCO3 The tendency of normal soda to turn into bircarbonate of sodium is accentuated with the increase of the content of carbonic acid in the soil air and soil solution,i.e.in the conditions of intensive decomposition of organic matter and by low temperature.On the other hand,with a moderate content of carbonic acid in the soil air, i.e.with weak activity of micro-organisms and small content of organic matter, or when the soil solution is warmed up, bicarbonate is easily tranformed into normal carbonate according to:

2 NaHC0,-->Na,CO,+

H,O +CO,

Under evaporation of groundwaters containing carbonates and bicarbonates of sodium,there accumulates from the soil solution crystals of double salt-tronas Na,CO, .NaHCO, .2H,O, or of pure NaHCO,. The majority of natural waters (river, spring,underground,lacustrine) with a concentration of0.5 to 3.0 g/1 (dissolved solids) contain substantial quantities ofcarbonatesand biocarbonates of sodium.Depositsand soils formed with the participation and under the influenceofsuch waters will always be alkaline,highly colloidal if containing montmorillonite but not very permeable and compact. It is comparatively rare for considerable quantities of soda to accumulate in soils. This is due to the fact that in most desert and semi-desertsoils,there is intense accumulation of gypsum in the presence of which soda turns into calcium carbonate according to : Nazco,+ CaSO,->CaCO,

4-Na2S0,

Hence it is only in sediments containing no gypsum that free soda and bicarbonate of sodium can accumulate in substantial quantities. These conditions sometimes occur in the prairies of the USA and Canada,in the black earth and forest-stepperegions ofManchuria,Siberia,the Russian plain and the Hungarian depression. In these areas, too, there are found soda lakes and soils with a high soda content (soda solonchaks and soda solontsy). The soda content in soda solonchaksmay attain 5 %.Soils with soda salinity are widespread in the arid monsoon areas of Pakistan, India,in the savannahs of Africa, in the low lying river plains of Argentina, Chile, Morocco, Western China, Mongolia and Armenia. The solubility of soda drops sharply at temperatureslower than 8°C.Owing to this factor and to the low permeability of such soils the amount of soda which can be leached during the wet,cold seasons ofthe year is very small. Carbonates and bicarbonates once accumulated in soils tend to be conserved there and are little affected by external influences,natural or due to man’s activities. Soils containing considerable quantities of carbonates and bicarbonates of sodium are alkaline saline soils with poor natural fertility and their development requires radical chemical reclamation measures coupled in some cases with drainage and summer leaching,combined with rice irrigation.

(d) Potassium carbonate Potassium carbonate (potash) is found in soils much more rarely than sodium carbonate. In regard to its high solubility,its alkaline hydrolysis,giving the solution a high alkalinity, its toxicity for plants and its disaggregating effect on soil structure, potassium carbonate is practically identical to sodium carbonate. Considerable quantities of potash may accumulate in the ash of some agricultural plants (sunflower); some halophytes also contain potash in large quantities (e.g.haloxylon). The ash ofsuch plants is used as raw material for obtaining potash. The increase ofpotash,combined with the fallen leaves ofthe haloxylon on the soil,causes an increase of alkalinity. 2. Sulphates

Sulphuric acid salts are found in varying quantities in almost all types ofsoils.In the soils and groundwaters of steppes and deserts, sulphates sometimes accumulate in very considerable quantities. The significance of sulphuric acid salts for agronomy and reclamation varies very much with their chemical composition. (a) Calcium sulphate Calcium sulphate(gypsum) is a saltphysiologicallyharmless to plants;this is due to its low solubility(1.9g/l).

124

S A L I N E A N D A L K A L I SOILS CHEMISTRY (ARID ZONES)

The soils and soil-formingrocks of semi-desertsand deserts very frequently contain large quantities of gypsum,accumulated during the evaporation of lacustine waters or saline groundwaters,lying close to the surface.There are often found,in deserts,gypsiferous soils of ancient origin which no longer contain groundwaters. In the particularly dry climate of deserts,such as in Chile or the Sahara,the gypsum (CaSO, .2H,O) becomes dehydrated and turnsinto a mealy mass ofsemi-hydrate-CaSO, .1/2H,O. In some areas ofCentral Asia, the Caucasus, North Africa, and Argentina, soils contain gypsum in quantities of up to 50% and sometimes 90%. Gypsum crystallises,in soils,in a great variety of forms:from thin transparent or mealy crystals to large nodules; concretions or regular shaped slabs. In soils with a very high gypsum content the gypsum forms a compact,spongy,porous mass,causing cementation of the entire horizon. Such compact accumulations of gypsums in soilsmakes them mechanically impenetrable for roots,water and air,hampering the development of plants and sometimes killing arboreal and palm plantations. Gypsum is widely used for the reclamation ofalkali soilswhich contain soda and adsorbed sodium.Gypsum is found together with chlorides and other sulphates which accumulate in saline soils. (b) Magnesium sulphate Magnesium sulphate(epsomiteMgSO,. 7H,O)is a typicalcomponentofsalinesoils,accumulatinginquantities of several per cent. It is also often present in saline groundwaters and in many saline lakes and types ofmud. Owing to its high solubility (262g/l), it is one of the most toxic and harmful salts to plants. Magnesium sulphate never accumulates in soils in pure form,but always in combination with other easily soluble salts;in such cases,radical measures for reclamationby leaching are necessary. (c) Sodium sulphate Sodium sulphate is a typical component ofsaline soils,saline groundwater,lakes and saline muds. Its toxicity is two or three times less than that of magnesium sulphate.Its solubility depends very much on temperature conditions,increasing considerably when the temperature rises. In view of this fact,the status of sodium sulphate in soils is extremely complex. In the warm seasons of the year, sodium sulphate rises to the soil surface together with the most soluble salts (magnesium sulphate,magnesium and sodium chlorides). In the cold season,on the other hand,sodium sulphate,owing to the lowering ofits solubility,is not leached.When the temperature rises, the mirabilite (Na,S04 .10H,O) easily dehydrates, turning into a whitish powder of thenardite (Na,SO,). When the temperature drops,the precipitated sodium sulphate forms large transparent crystals ofmirabilite. At times sodium sulphate crystallises together with calcium sulphate,forming so-called glauberite-CaSO, .Na,SO,. In particularly arid deserts (such as in Chile, the Sahara, Ust-Urt), sodium sulphate is always found in the form of thenardite. Thenardite, accumulating in the top horizon of solonchak soils,participates, together with gypsum and calcium sulphate,in the formation of the powdery, ‘fluffy’salt horizon containing up to 30% of sodium sulphate. Reclamation of soils with sodium sulphate necessitates leaching by large quantities of water; when possible, this operation should be carried out during the warm season of the year (combined with irrigation of rice).

(d) Potassium sulphate Potassium sulphate does not accumulate in soils in large quantities. Its properties are similar to those of sodium sulphate,but is toxicity is considerably lower. The presence of large quantities ofpotassium sulphate has been ascertained in some saline formations,where it is extracted for use as fertiliser. 3. Chlorides

Chlorides, together with sulphates,are the main compounds responsible for the formation of saline soils. All chloridesare characterised by a high solubility and,consequently,a high toxicity.The higher the degree of salinity of soils,groundwaters and lakes,the greater,generally speaking,is their chloride content. (a) Calcium chloride Calcium chloride is seldom present in soils,reacting with sodium sulphate and sodium carbonate,it easily changes into calcium sulphate and calcium carbonate and passes out of the solution. As a result,calcium 125

IRRIGATION, D R A I N A G E A N D SALINITY chloride is found in soils, soil solutions and waters of saline lakes (natural brine) only when there is a very high salinity (400to 500 g/l). Calcium chloride may also exist (as an ephemeroid) in the upper solonchak-likehorizons as a result of exchange reactions of the rising solutions of sodium chloride with exchangeable calcium according to : Na+ X=Ca+ +2NaC1 fs X --t-CaCI, Na+ +

Considerable quantities of calcium chloride have been found in deep-lying interstratum groundwaters connected in some cases with oil deposits. These interstratum waters, rising through cracks to the surface, may introduce calcium chloride into soils. Calcium chloride is toxic for plants, but to a lesser degree than either magnesium chloride or sodium chloride.

(b) Magnesium chloride Magnesium chloride is much more common in saIine soils,groundwaters and lakes than calcium chloride. However, the accumulation of large quantities of magnesium chloride occurs only with extremely high salinity.Owing to its high solubility (353 g/l) magnesium chlorideis exceptionallytoxic,and is one ofthe salts most harmful for plants. Magnesium chloride is also found in large quantities in interstratum groundwaters,i.e.where it may pass through cracks into soils and soil solutions. Like calcium chloride,it may appear in the upper horizons of solonchaks as a result ofexchange reactions between the ascending solution of sodium chloride and the adsorbed magnesium of the soil. Magnesium chloride and calcium chloride are extremely hygroscopic salts,which absorb vaporised water from the atmosphere at low air temperatures.When this occurs, their crystal sediments rapidly liquefy and turn into a concentrated solution.Hence solonchakswhich contain calcium chloride and magnesium chloride on their surface remain humid for a long time after rain or dew. Solonchaks containing a large quantity of magnesium chloride are difficultto reclaim,and need intensive leaching and drainage in order to remove this harmful salt. (c) Sodium chloride Sodium chloride, together with sodium and magnesium sulphate, is the most common and widespread component of saline soils. The toxicity of sodium chloride for plants is exceptionally high as well as its solubility (264 g/l). Even with a content of about 0.1% of NaCl, plants suffer and fail to develop normally. Many saline soils contain 2 to 5% of NaCl, and are totally barren. Such saline soils can only be improved by leaching of these salts. Washing out of sodium chloride solonchaks is very easy if the soil contains gypsum,as is very often the case. If chloride solonchaks do not contain gypsum, washing out is much more difficult because sodium assumes an exchangeable form,the alkalinity is high and the soil is peptised. (d) Potassium chloride

As regards its chemical properties, potassium chloride is, generally speaking,similar to sodium chloride. However,it is not at all widespread,since potassium is consumed by organisms and is subject to irreversible adsorption by clays.When potassium chloride is present in the soil in large quantities,its toxicity is as high as that of sodium chloride. However,as a rule,its content,even in saline soils,is not large. Owing to the great importance ofpotassium in the mineral nutrition ofplants,deposits ofthis saltare ofgreatvalue as a source of fertiliser. The use of a potassium fertiliser in the form of potassium chloride or carnallite (MgCl,. KC1 . 6H,O) increases the fertility of non-salinesoils;but in saline soils,its use is not recommended.

4. Nitrates Nitric acid salts constitute a chemical compound which is of great importance in soils.These salts do not accumulate in large quantities in soils. In most cases, the nitrate content is not more than 0.05% NO,. Nitrates constitute a highly important element in the mineral nutrition of plants, providing a source of nitrogen,without which plants cannot live. But, in exceptionally arid deserts, such as those in Chile,Peru, India,Central Asia and Arabia, sodium and potassium nitrates-in the same way as sodium and potassium

126

SALINE A N D A L K A L I SOILS CHEMISTRY (ARID ZONES) chlorides, and together with them-cause acute salinisation and make soils barren. In the soils of these regions,the quantity of nitrates attains several per cent or even, in some cases up to 50%. Nitrates in soils in amounts of over 0.07 to 0.1%NO,are much more harmful to agricultural plants than chlorides.In view of this,nitrate or nitrate-chloride solonchaks are just as difficult to reclaim as all other types of soIonchaksand need leaching. Owing to their high solubility,nitrates can be easily washed out of soils.When there is insufficient oxygen (e.g. in swamps or as a result of over-irrigation)nitrates are reduced by denitrifying micro-organisms in ammonia or elementary nitrogen.In swampy soils,this process of denitrification reduces the supply of an important element of plant nutrition. 5. Borates

It is even rarer to find in soils an excess of boric acid salts.When this occurs,it is usually in the proximity of volcanoes (the case of Latin America).

C. ORIGINS

AND SOURCES OF SALTS ON CONTINENTS

The formation and accumulation of salts in soils is due to a large number of geochemical processes taking place in the upper strata of the earth‘s crust. With the weathering of the various rocks, the former links between the chemical elements are broken, giving place to new combinations,which may be in the form of either secondary clay minerals or various kinds of oxides,or simpler compounds. These may also include simple salts. The main elements,combinations of which give rise to the formation of saline soils,are Ca, Mg, Na, K, CI, S,C,N,B and I.Apart from these, Cu, Z n and B often accumulate,in micro quantities,in saline soils. The main salts in the form of which these elements migrate and accumulate in saline soils have been mentioned above. Elements from which solublesalts can be formed are some ofthe most common found in the earth’scrust;‘!’ they are,in fact,among the first fifteen elements (Table 5.1). Table 5.1. Content of the most common elements in the earth’s crzM in N a m e of element Oxygen” Silicon Aluminium Iron Calcium” Sodium” Magnesium”

%

49-13 26-00 7.45 4.20 3-25 240 2.35

N a m e of element Potassium” Hydrogen“ Titanium

Carbon” Chlorine“ Phosphorus

Sulphur” Manganese

%(CLARK)? %

2.35 1.00 0.61 0.35 0.20 0.12 0.10 0.10

(The sign x is used in Table 5.1 to denote the elements which participate in the formation of the easily soluble salts leading to soil salinity. These elements, with few exceptions, play a limited or even extremely limited role in the various biological processes and therefore do not accumulate in organic residues)

In the course of geological time, the continuing magma-forming processes and the volcanic and postvolcanic phenomena occurring in the earth’scrust facilitatedthe accumulation ofvast quantities ofchlorides, sulphates and borates both in the solutions circulating on the earth and in the oceans and in continental and marine deposits. * The term ‘earth’scrust’ is commonly applied to the top cover of the earth, down to a depth of 16.5 km (10miles) t All the remaining elements, aver 70 in number, constitute only 0.39% by weight of the total, 127

IRRIGATION, DRAINAGE AND SALINITY

Every stage of volcanic activity has resulted in the rising to the surface of new masses of easily soluble salts of magmatic origin. To this day, volcanic activity continues to bring up to the surface of the earth further quantities ofcompoundsofchloride,sulphur,boron,and so on.But the original chemical composition of the ocean salts,which was predominantly ammonium chloride,has changed in the course of the geological and geochemical history of the earth due to the effects of weathering,biogenesis and soil formation on the continents and in the oceans of biochemical and geochemical transformation processes. The present composition of ocean salts is the result of the constantinflow,from the continents,of sulphate and carbonate-Na, KyCa and M g of the formation of the chlorides ofthese elements,and ofthe biological destruction of the ammonium salts formerly predominating in the oceans,accompanied by the accumulation of nitrogen in the atmosphere. The weathering of the crystalline rocks forming the continents was considered,at one time,as the main source of easily soluble salts entering into natural waters, sediments and soils.GLINKA, GEDROITZ, POLYNOV and FERSMAN in the USSR and HILGARD, HARRIS and KELLEYin the USA took the view that weathering processes were mainly responsible for the appearance and accumulation of these salts in natural waters, deposits and soils on the continents. The chemical composition of the salt solutions formed by the weathering ofigneous rocks depends in very large measure on the mineral composition of the rocks. Waters flowing through acid magmatic rocks (e.g. granite, porphyry, gneiss) are the least mineralised, and contain principally carbonates,chlorides,silicates and sulphates of alkali; while waters flowing through alkali magmatic rocks (basalt, diabase) are more highly mineralised, and contain mainly carbonates-Mg, Ca-some Fe,and their sulphates and silicates. During weathering, all the chlorine and a large part of the sulphur turn into acids,which are subsequently transformed into salts of alkali and alkaliearth. Chlorides, on land,migrate faster than sulphates,which is due,according to Harris,to the formation of sulphides and to the fact that sulphatesin general have a lower solubility than chlorides. According to FERSMAN, the geochemistry of salts on land is based on the solution and extraction of the ions from minerals in the course of weathering, followed by their precipitation and accumulation under specific physico-geographicalconditions.The sequence of extraction ofthe ions,their speed ofmigration and the capacity of some ions to accumulate in endoreic inland depressionsin the form of salt masses are proportional to the coefficient of energy of the ions,(C.E.),the ionic radius,the valendy, and the stability of the crystal network of the compound (Table 5.2). Table 5.2. Sequence of ìon-extraction during weathering (according to FERSMAN)

Sequence of extraction

CI’, NO3

Ions

I

Ions

so4

CO3

Br

C.E. Ions 0.23 N a 0.18 K 0.66 Ca 0-78 M g

II

C.E. Ions 0.45 SiO, 0.36 1.75 2-10

III

C.E. Ions 2.75

Fe Al

IT!

C.E. 5.15 4.25

The mobility of the compounds formed and the amounts in which they accumulate in the form of salts increases with the decrease of the coefficient of energy of the ions and salts,and the decrease of the ionic radius and valency. It follows that chlorides,nitrates,sulphates and carbonates of alkalis and,in some degree,of alkali-earths must inevitably be the main salts which form in the weathering crust and salinise endoreic areas. The processes of precipitation and accumulation of the salts occur in reverse order.The longest to remain in solutions (of marine, ground and underground waters) are the ions with the lowest coeficient of energy (i.e.monovalent and bivalent cations and monovalent anions). It is these components which play the main role in the formation of salt accumulations and solonchaks. POLYNOVand KOVDA divided the elements into the categories shown in Table 5.3, according to their mobility during Weathering and their migration capacity. Both the absolute and the relative participation of elements in the formation of saline soils and natural waters will be greater, the higher the migration category of these elements. Elements of the fourth and fifth

128

SALINE AND ALKALI SOILS CHEMISTRY (ARID ZONES) Table 5.3. Migration categories (mobility)of elements ~~

1. Virtually non-leachable 2. Slightly leachable 3. Leachable 4. Highly leachable 5. Very highly leachable

Si of quartz. . . . . . . . Fe,Al,Si . . . . . . . . Si, P, M n . . . . . . . . Ca,Na,K,Mg,Cu,Co,Zn CI,Br,I,S,CyB

migration categories will constitute the main compounds contributing to contemporary salt accumulation: NaC1, Na,SO,, MgCI,, MgSO,, CaSO,, Na,CO,, NaHCO,, CaCO,, MgCO,. In inland depressions, arid, endoreic lake regions with a dry climate,marine deltas and in the oceans,it is the elements of the fourth and fifthcategories,that are bound to accumulate in the largest quantities. Throughout the whole geological history of the earth's crust,a process of exchange has occurred between land and ocean with a sort of rotation of easily soluble salts.The balance of this circulation,on the whole, is in favour ofthe ocean where the total saltsreserve has,consequently,grown.This rotation of salts between the oceans and the continents has been accompanied by a process of differentiation of salts according to degree of solubility,as a result of which sulphates and carbonates predominate in the salts accumulated on land,chlorides in those accumulated in the oceans. The oceans, which cover three-quarters of the earth's surface, have always been regarded by scientists as constituting the source,both direct and indirect, of the salt accumulations found on the continents.Now, as in the past,salts blown by the wind from the face of the seas and oceans are reckoned to be one of the main elements contributing to the accumulation of the salts on land. According to the figures given by scientists of various countries,the amount of salts of different chemical compositionswhich is deposited on land together with dust and atmosphericprecipitations is between 20 and 500 kg/ha per year. This permanently operating geochemical factor constantly replenishes the reserve of salts in surface and underground waters. The deposit of salts in marshes and lagoons along the seashore is the second most importantmeans whereby salts from the oceans pass on to land.When we consider that the area ofland covered by the transgressionsof the ancient seas runs into millions of square kilometres,we shall have some idea of the colossal amounts of salt which have passed from the oceans on to dry land since the time when the oceans were first formed. Lastly, the most important transference of salts from the oceans on to dry land, as regards geochemical effects,has been due to the salt masses drawn into the strata of sedimentary rocks ofmarine origin.Of special importance in this context are the marine salts entering into the composition of the sedimentary rocks of geosynclinic regions now characterised by mountain formations,such as the Tien Shan,Altai,Pamirs and Himalayas,Caucasus,Carpathians,Atlas and Pyrenees. During the geological existence of the earth,both the process of salt formation due to the formation of magma, weathering and chemicalerosion and also the movement and accumulztion of salts in inland regions with no run-offhave been accentuated during periods of orogenesis and periods characterised by the uplifting ofthe continents.In the history ofthe earth there have been,as we know,about 20such major orogenic periods. The periods of the most active upheavals of the earth's crust have been marked by intensified volcanic activity,sharperdifferentiationofthe climatic zones on the continents,and intensification ofdesert conditions in the inland depressions oflarge continentalmasses-all ofthese factors lead,in turn,to the intensification of processes of salt accumulation. Salt in the form of chemical deposits accumulates in the furthermost areas reached by salt migration, in inland depressions of steppes and deserts, in lakes and in oceans. Inthe course oftheir migration,in underground and surfacewaters,from thealluvial area to thefinal main salts reservoir-oceans-salts are retained and accumulate along the way in deposits,soils and groundwaters of the low lands. Salts through accumulating in sea conditions (sea-bottom deposits, swamps, deltas, lagoons) or on the continents,being covered up by deposits of various kinds,undergoing various forms of metamorphosis, and,lastly,sinking down to various depths of the earth's crust,were cut out of the processes occurring on the earth's surface for periods equivalent to whole geological eras. Then,under the influence of upheavals in the earth's crust and of erosion,the buried salt accumulation passed into a second migration cycle,again accumulating in deposits and soils. 129

I R R I G A T I O N , D R A I N A G E A N D SALINITY The formationofsalinesoilsin the contemporaryperiod is part ofthese geochemicalprocesses,which have been occurring on the earth since very ancient timesand which consistin the exchange and differentiationof salts as between magma, the oceans,the crust of the earth and the surface of the continents.

THE SOLUBILITY OF THE MOST IMPORTANT SALTS

D.

When studyingthe processes of soil salinisation,it is extremely importantto talceinto account the differences of solubility of salts. The essential data on this point are given in Table 5.4 and onward. Table 5.4. Maxiinum solubility ìn water of some saltsfound in soils at differenttemperatures(saturatedsohtion)

In g per 100 g of solution

In g per litre of solution

Temperature in "C

Temperaturein "C

Salt O

Nazco3* NaHCO, 1\;la2S0,t NaCl IVlgSO, MgCL

CaCl,::

NaN03 KN03 &COB KHC03 K2S0.5

KC1

10

20

30

40

50

O

10

20

30

40

50

122 80 90 317

213 93 185 317

441 121 430 318

429 137 415 319

-

-

371 107 373 317

6.5 10.9 6.5 7.5 8.3 4-3 26.3 26.3 18.0 22.0 38.8 39.8 37-3 39.4

17.9 8.7 16.1 26.4 25.2 41.0 42-7

28.4 32-4 10.0 11.3 29.0 32.6 26-5 26.7 28.0 30.8 48.6 51.8 50.7 53.4

32.1 12-7 31.8 26.9 33.4 54.5 56.0

70 68 45 318

42.1 11.6 51-7 18.4 6.7 21.9

46.7 24.0 52.6 25.2 10.0 25.6

49.0 31.5 53-2 28.5 11.5 27.2

51.2 39.0 53.9 32.2 12.9 28.7

53.3 46.1 54.7 36.0 14.2 3.01

570 125 814

607 194 823

686 279 829

686 384 839

724 498 852

762 614 867

71 253

91 277

108 301

125 322

142 341

157 359

44.4 17.5 52.2 21.5 8.5 23.8

-

-

-

-

-

-

--

I

* U p to 30°C,the precipitate contains decahydratesalt;above that temperature,it is anhydrous U p to 30"C,the precipitate containsdecahydratesalt; above that temperature,rhombic salt 3 U p to 30°C,the precipitatecontains hexahydratesalt; above that temperature,tetrahydrate Note: The last six salts listed are not found in soils in theform of saturated solutions

In complex solutions,the solubility of most salts changes. As a rule, the presence in a solutiofi of salts having a common ion,causesthe solubilityof these salts to drop. For example the existencein a solution of large quantities of magnesium chloride provokes a very sharp decrease in the solubility of sodium chloride (Table 5.5) and causes it to be precipitated. Table 5.5. Solubility of the salts in the system. NaCl-MgCl2-HzO

in

%by weight.

Temperature

NaCl

MClz

Solidphase

25"

26.45 20.5 15.2 10.5 6.5 3.3 1.1 0.3 O

O 5.0 10.0 15.0 20.0 25.0 30.0 35.55 35.6

NaCl NaCl NaCl NaCl NaCl NaCl NaCl NaCl+MgCl, .6H20 MgCl,+ 6Hz0

130

SALINE AND ALKALI SOILS CHEMISTRY (ARID ZONES)

A similar change occurs in the solubility of salts in the presence of increasing concentration of NaCl. Analogous phenomena are also observed in combinations of sulphates. In the case ofmixtures ofsalts with dissimilar ions,the solubility ofthe componenthaving the lower level of solubility increases.This is due to the dissociationofsalts.An example ofthis is the change in the solubility of gypsum when combined with chlorides (Tables 5.6 and 5.7). The actual solubility increaseis due to the effect of N a + and C1- in lowering the activitycoefficientsofCa+ and So,- - in solution so that the concentrations of the ions must increase to maintain the constancy of the solubility product. The solubility ofcarbonates is greatly influenced by the content ofcarbon dioxide gas dissolved in the water (Tables 5.8-5.10). +

Table 5.6. Solubility of calcium sulphate in water in the presence of varying quantities of sodium chloride at a temperature of 20°C (ing per 100 g of water)

NaCl

CaSO,

O 17.2 20.0 24.4 29.3 35.8

0.204 0.787 0.823 0.820 0.614 0.709

Table 5.7. Solubility of calcium sulphate in water in the presence of calcium chloride (in g per I00 g of water)

CaCI,

CaSO,

O 5.2 9.9 18.9 40.8

0.204 0.103 0.086 0.077 0.035

Table 5.8. Solubility of calcium carbonate according to the quality of COzin the air at a temperature of 16°C

Content of COzin the %(by volume)

air in

Content in grammes in 1 litre of water According to SCHLESING According to WEIGNER 0.0131 0.0634 0-1334 0.2029 0.4700 1.0986

0.00

0.03 0.3 1 .o 10 100

PH

0.0131 0.0627 0.1380 0.2106 0.4889 1 .O577

10.23 8-48 7-81 7-47 6.80 6-13

Table 5.9. Solubility of calcium carbonate, with partialpressure of COz,equal to 0.00032 atm (ing per litre)

Temperature in "C O CaCO,content

0.081

5

10

15

20

25

30

0.075

0.070

0.065

0.060

0.056

0.052

13t

IRRIGATION, DRAINAGE A N D SALINITY Table 5.10. Solubility of mapmium carbonate in water at temperature of 18°Cfor diflerentpartial pressures of CO, ~~

p. CO,,in atm.

0.005

0.001

0.01

O.1

1.0

5.0

10.0

MgCOBcontent (in 41)

2.51

3.11

6.04

12.2

25.8

46.0

59.2

In highly concentrated solutions, some easily soluble salts form new complex binary and ternary salts which are frequently more soluble than each salt when taken separately.There are,however,cases when the opposite occurs.The solubility of some salts may also vary to some extent according to the forms of salt in solid phase (for instance,according to the number of molecules of constituent water). It is importantto take into account the solubilitieswhen leaching saline soils.The order in which they are to be leached must be carefully planned beforehand, especially when mixtures are present;this is difficult to do without a thorough knowledge of chemical laws. In this connection it is important to know the strong decrease of the solubility of sodium carbonate and sulphate at temperatures below 12°C (Fig. 5.1). The amelioration of soda and sulphate solonchaks by leaching is therefore difficult during the cold season.

-40

-20

O

3-20

1-40

+60

$80 to

Fig. 5.1. Salt solubility depending on temperature

E. CLAY MINERALS

IN ALKALI AND SALINE SOILS”

The clay minerals occurring in alkalit soils are a function ofthe mineral composition at the time of salinisation and the length of time the soil has been salinised or has remained at high pH. Obviously,if a soil has been recently salinised by the incursions of sea water or by the rise of a saline water table,the structure of the minerals in the colloidal fraction will not be immediately affected, although the composition of the exchange complex may be profoundly altered. In such soils,any of the commonly occurring clay minerals may be expected and in the same proportion as that prior to salinisation. The examination of a wide variety of saline soils reveals that the dominant clay mineral is usually montmorillonite (30 %or more) although,at least,small quantities ofillite and kaolinite are nearly always present. Very commonly they contain colloidal calcium carbonate,and they are also characteristically high in quartz (5to 30%)and in colloidal feldspars.Certain alkali soils are high in analcite,which has the unusual property * This text is partly based on the unpublished work of I. BARSHAD and on the work of W.P.KELLEY The term alkali is used here in the sense recommended by W.P. KELLEY to mean soils high in salt, exchangeable sodium or both

132

S A L I N E A N D A L K A L I SOILS C H E M I S T R Y (ARID ZONES)

that it containsNa+ which will exchange with K+ and partially with NH,+but not with Ca++, and there has been one report of a desert soil containing attapulgite. The zeolites may be expected to be formed in soils which have been salinised for a considerable period of time and which have remained at high pH. Probably most alkali soils contain amorphous silica. Solonetzic soils appear to be generally lower in montmorillonite and higher in amphorous colloidal materials. Perhaps few colloidal crystalline clay minerals are stable under low salt, high p H conditions and over long periods of time.

F.

EXCHANGE PHENOMENA IN ALKALI SOILS

1. Cation exchange equations

Cation exchange phenomena play an important,if not dominant, role in the chemistry of alkali soils. The equilibrium between Ca+ and N a + is especially important, since for many practical purposes, M g + can be considered to behave much like Ca+ +. However, the distinction between Ca+ and M g + may become very important in cases where the exchangeable Mg+ becomes high enough to induce Ca+ deficiency in crops growing on the soil.However,for present purposes it will be convenient to group Ca+ and M g together. Through a study of cation exchange phenomena, it is hoped to gain knowledge of the relative energies with which soil colloids adsorb N a + and Ca++.When this is known,it becomes possible to predict changes in relative amounts of these exchangeable ions during reclamation or upon the use of irrigation water of a specific quality. Such studies are therefore fundamental for the chemistry of alkali soils. One approach to the problem ofrelative energies ofadsorption is throughthe determination ofan exchange isothem for N a and Ca for a particular soil.Samples ofthe soil are brought to equilibrium with solutions containing N a and Ca+ at various compositions and the corresponding equilibrium exchangeable ions are measured.When solutions have been equilibrated with soils,the problem of determining what proportion ofthe known total amount ofa given cation is in the exchangeable form and what proportion is in the soluble form,always exists.W e may determine unambiguously the composition of a solution which has been equilibrated with soil across a dialysis membrane, or the composition of the supernatent of a centrifuged soil system or of a liquid extracted in some other way. But to calculate the amounts of the exchangeable ions, some assumption about the distribution of the ions in the soil itself must be made. In soils high in Na', the nature of this assumption may be very significant for the calculation of the exchangeable ions. Generally speaking,there have been two quite different approaches to the question of calculating the exchangeable ions.Both approaches assume a particular model for the distribution of ions in the soil system and compute the exchangeable ions on the basis of the assumed distribution. These models will be briefly presented in turn. The first model is known as the diffuse-layertheory. It assumes that there are continuous concentration gradients ofthe ions in the liquid phase near negatively charged clay particles such that the cation concentration decreases with increasing distance from the particle surface while the anion concentration increases. O n the basis of a number ofassumptions and the application of certain physical laws,gradients such as those shown in Fig. 5.4 are predicted for a single monovalent electrolyte (Cois the electrolyte concentration far from the surface). The theory thus predicts that anions will be excluded from a region near the surface of the particle. It follows that the concentration of anions in an equilibrium filtrate will be higher than it is in the soil.Where this effect is observed it is known as negative adsorption,and it may be very significant in alkali soils.For act is made of a soil exhibiting negative adsorption and the amount of chloride per unit lculated from the chloride concentration in the extract,the result will be erroneously high. The error is known to increase with the total amount of salt in the soil. The diffuse-layertheory can be quantitatively applied to a mixture ofmonovalent and divalent cationsto deduce the exchange isotherm.The result has been widely used in the form: +

+

+

+

+

+

+

+ +

+

+ -

+ +

+

133

IRRIGATION, D R A I N A G E A N D SALINITY

r*

Pdß

arc sin h

r

r-4vc2/C2 Concei@ation

II

Distance from surface

Fig.5.2. Ion concentrations according to diffuse-layertheory

In this equation, Plis the adsorbed monovalent ion and P is the surface charge density of the clay in me/cm2,,ßis a constant, C, is the divalent ion concentration in the equilibrium solution in molesllitre,and v c is a particle interaction parameter usually taken as unity. The quantity r is called the reduced ratio and is defined,

where Clis the monovalent ion concentration in the equilibrium solution. Thus,with the aid of equation 1, the amounts of adsorbed monovalent ions can be found as a function of the reduced ratio and the surface charge density. A second model for the distribution ofions in soil system assumes that the exchangeableions are adsorbed on clay surfaces essentially in a non-rigidmonolayer at particular adsorption sites. On the basis ofthis model it is meaningful to represent an exchange reaction for N a and Ca+ as follows: +

+

Ca" +ad+ 2Na +-+2Na+

ad

+Ca .+.+

Here, the subscript ad denotes an adsorbed ion while the absence of a subscript denotes an ion in the equilibrium solution. By thermodynamic argumentswhich parallel those for a chemicalreactionit is possible to deduce the so-calledmass-action relationship: (Ca+ +I (Na+ (Ca+ +ad) (Na+),

(3)

Here, parentheses denote activities and K is the equilibrium constant. The equation may be rewritten, (Na (Na"), -K (Ca"" ad) (Ca +) +

(4)

+

The activity ratio (Na+)2/(Ca+ +) for the ions in solution is thermodynamically well defined and can be closely estimated under favourable circumstances. The ratio (Na+.J2/(Caf .t ad) is less obvious. One assumption is that the activity of adsorbed ions is equal to their mole fraction in the exchange complex.W e then have, N2Na+3d=K-(Na+), (5) NCa+ a& (Ca +) +

134

+

SALINE A N D A L K A L I SOILS CHEMISTRY (ARID ZONES)

It then remainsfor experimentto determinethe value ofK and its constancy.An approach through statistical thermodynamics yields similar results. It will be noted that equations 1 and 5 are based on quite different models and differ in several respects. the reduced Equation 1 predicts that the exchangeable ion composition will depend primarily on CI/dCz, concentration ratio, while equation 5 predicts that it will depend on (Na+)/d(Ca+ +) (taking the square root of square 5 and recalling that the sum ofthe mole fractions is unity). Moreover,the functionalrelationship between these quantities and the exchangeableionsis differentfor the two cases.However,ifthe exchange isotherm is plotted for the two equations,the results do not differ greatly in appearance.This result, while disappointing from a theoretical view, may mean that either equation can often be used in the solution of practical problems. Both of the equations discussed predicts a principle, widely observed in alkali soils,called the 'cation dilution effect'. According to this principle, as the soil is diluted, the exchange reaction, 2Na+ +CaC +-+-2Na

+

+Ca+

+

ad

is shifted to the right. Thus, as the water content increases soluble Ca+ replaces adsorbed N a + and the amount of soluble Na+ increases. Since this effect may be very marked, the amount of adsorbed N a + found in a soil is a function ofthe water content used to determinethe solubleions.Moreover,adsorbed N a + is also replaced with H+ (hydrolysis) upon dilution that so the p H always increases with water content. This in turn may result in appreciable increases in soluble CO,= or HC0,-. It also followsfrom the cation dilution effectthat as a soil is dried,the exchangeableNa+ may be considerably higher at water contents in the field moisture range than it is in saturated soils where it is customarily estimated. +

2. Specific effects

There are a variety of ways in which either of the above approaches to the Na+-Ca+ exchange will fail. Under these conditions,the specific behaviour of a particular soil must be investigated. First,neither equation can be expected to work if the soil is composed of a mixture of clay minerals with properties which differ considerably from one another. If a mixture is present with considerably different charge densities or different values of K,this will be the case. In fact,as stated above,alkali soils characteristically contain mixtures ofclay minerals and this forms an important limitationon a quantitative approach. Second,there is increasing evidence that in many soils which have been at high p H and salt concentration for long time periods, minerals of a zeolitic character may be formed.Insuch soils adsorbed N a + is found in forms which exchange only slightly with Ca++, but substantially with K + and NH,+. The Na+-Ca+ exchange relationshipsin such soils would presumably not follow theory. Many alkali soils are known to contain forms of N a + which dissolve in reagents such as NH,acetate which are used to determine the extractable Na+ but not in water. In such cases,the exchangeable N a + found as the difference between the extractable and water soluble Na+will be erroneous. It may then be necessary to determine exchangeable N a by other procedures, such as the isotopic dilution technique, but this problem is not one of principle. +

+

+

3. Anion effects

As indicated above,the anion concentration in a solution in equilibriumwith a soil system may be higher than the average or bulk concentration of the anions in the soil.Thiseffect is called negative adsorption and it has been most widely interpreted in terms of the diffuse layer theory.From this theory it is possible to show that an approximateequation for the negative adsorption in a system containing a single symmetrical electrolyte is,

Here, r is the negatively adsorbed salt, in me/cmz,Z is the valence, Cois the equilibrium solution salt concentration and the other symbols are as before. This equation may be used to estimate the negative adsorption in soils of known surface charge density,or alternatively if the negative adsorption is measured, 135

IRRIGATION, DRAINAGE A N D SALINITY rniay be estimated from the equation.The value of rfound in this way often agrees with the value found by other methods although in one recent report,agreement with the ethylene glycol method was poor. Other interpretationsof negative adsorption are based on the assumption of non-solvencyof ‘bound’water on the clay surface,or on the Donnan equilibrium. In systems exhibiting negative adsorption,it is to be expected that when extracts are made from a soil by the application of pressure,such as in making saturation extracts with a pressure membrane apparatus,the initial fraction of the extract will have a higher concentration than the mean concentration of the solution remaining in the soil.As the extraction proceeds,the concentration would then be expected to decline. However, the situation is actually rendered more complicated by an effect called ‘saltsieving’.The actual solution which immerges is lower than the equilibrium solutionbecause the soilretainssalt.As the extraction proceeds, the concentration rises through a maximum and then declines. In suspensions of clay particles,the effect is most prominent at low salt concentrations (i.e. 10 me/l).Therefore,in assessing the significance of extracts obtained by pressure filtration,one must consider both negative adsorption and salt sieving. Efforts to deal with these phenomena on the basis of double-layertheory have met with the same success,although their general significance is not yet known. In any event,when negative adsorption is present, an estimate of the soluble salts made from an extract will be too high and the corresponding calculated values for the exchangeable ions will be too low. In fact,negative values for exchangeable ions have not been found in this way. Evidently the soluble salts must be determined at high dilution. 4. CaCO, systems

Probably most alkali soils contain CaCO, in some form. Most of the studies to date indicate that while the ultimately stable form may be calcite, the solubility product of calcium carbonate is not strictly obeyed. However,in any solution in which the dissolved CO,,HCO,-and CO,= are in equilibrium,whether or not the solution is in equilibrium with the air, the following relations must hold:

The parentheses denote activities.In dilute solutionsthe activity coefficients ofthe ionic species is given by the Debye-Huckel theory according to, log yi = -Z2(0.511) p1” (9) in which p = 1/2E Zi2Miis the ionic strength and yi is the activity coefficient.DefiningpX= -log X where X is either an equilibrium constant or the hydrogen ion activity,one obtains from these relations,

[CO, =I =p H -10.33+2.04p1’2 log[HC0, -1 Here, the brackets denote concentrations. These relations are often useful. For example,if we select severe conditions of p H and salinity,say p H = 9 and p=l.O, and a CO,concentration representing equilibrium with the atmosphere (1 x M),we find from equation (10)that the corresponding HC0,-concentration is approximately 0.014.It may be concluded that HCO,will not contribute very much to the total salinity when the salinity and p H are very high and the soil is in equilibrium with the air. Actually,the calculation overestimates the HCO,-concentration,since at p = 1.0,the Debye-Huckel equation greatly overestimates the effect of the ionic strength.It should be noted, however, that the ionic strength term is nearly always important. When alkali soils containing CaCO, are diluted,the cation dilution effect described above will operate, except that as Ca+ in solutionexchangesfor N a + ,CaCO, will dissolve tending to replacethe solution Ca+ +. Experimental studies have shown that under these conditions,the equations relating the Naadand the N a + in solution is a Langmuir isotherm: +

136

SALINE A N D A L K A L I SOILS CHEMISTRY (ARID ZONES)

k,k,(Na +> Naad= 1 +k,(Na +)

It can be shown on theoreticalgrounds that the resultis to be expected from equation 5 provided the Ca+ concentration is essentially constant.Moreover,if we write the solubility product of CaCO,, =Ksp (Ca) (CO,=) and use equations 9 and 10 to eliminate the carbonate we find,

+

(13)

will be constant if(H*) is constant. Thus,in equilibrium with the air at fixedCO,concentration,the (Ca+ +) Therefore,it may be concluded that when an alkali soil containing CaCO, is diluted,very little adsorbed N a + is replaced by I-I+.Rather,it is replaced by C a from CaCO, so that the p H and the calcium concentration remain constant and this results in the Langmuir isotherm. In concentrated solutions,this conclusion may fail. +

+

5. Effect of the presence of plants

The presence of a growing crop has a profound influence on the distribution ofsalt and exchangeable sodium which results from the use of irrigation water of a given composition.First,the removal of water primarily by transpiration rather than evaporation means that as the soil is dried out,there is little tendency for the salts which have entered the soilto move upward.Each successiveirrigation leaches salts downward with the result that strong downward gradients ofsaltconcentrationare produced.Where waters containing significant amounts of sodium are used,this in turn means that the levels of exchangeable sodium increase with depth owing to the reversal of the cation dilution effect.Thus the distribution of salt and exchangeable ions may be very different from that to be expected in the absence of plants. A second effect of plants is to excrete CO,in the soil and thus tend to keep CaCO, in solution. Although pot experiments suggest that such excretion of CO,is not very effective in dissolving pre-existingCaCO, and thus effecting the replacement of exchangeable sodium,lysimeter experiments,on the other hand, have clearly shown that plants may preventtheprecipitation of CaCO,. Moreover,in one five-yearlysimeter experiment the loss of HCO,from the soil without a concomitant precipitation of CaCO,, taken with other evidence,suggested that the reaction,

HCO,+H,O+Ca+ =(CaOH) +H,CO, +

+

may be significant.It is assumed that the (CaOH)+ formed by this reaction becomes adsorbed. Finally,where waters of low salinity are applied,plants may significantlyaffect the composition of the soil solution by ionabsorption.For example,in a long-termlysimeter experiment conducted by W.P.KELLEY and co-workers,it was shown that the sulphates in the soil solution can be increased as much as 57.8 times over the level in the applied irrigation water,while chloridesincreased only 8.9 times,the crop being sudan grass. This is the result of differential uptake of SO, and CI by the crop.These same experiments revealed considerable differences among crop species in their removal of salts. It is also well known that, in general, the grasses remove relatively more sodium in relation to calcium than do other plant species.

G.

ALKALINE SODA SALINE SOILS

Ample geographical data have now been accumulated showing that alkaline soda saline soils are extremely widespread throughoutthe various continents of the world. In view ofthe high toxicity ofcarbonic alkali and of the very unfavourablephysical properties imparted by these salts to both soils and soil-formingsediments, alkaline soda-salinesoils have very low natural fertility and can only with difficulty be used for farming. 137

IRRIGATION, DRAINAGE A N D SALINITY The geographical particularities, as far as is known, have been summed up in Chapter 3 of this book. The chemical properties of alkaline soda-salinesoils vary very widely. The total amount of easily soluble salts contained in water extracts is not large-usually between 0.3 and 0.5%though it may sometimes be as much as 1.0%,and on very rare occasions,even between 2 and 3 %.In soils with a high groundwater table (1.5 to 3.0m)there is a definitepattern inthe distribution of the easily soluble salts,with the maximum on the surface or in the B-horizonjust below the surface;and a gradual decrease down towards the groundwater table.In soilswith a deep groundwater table (5-10 in),the maximum amount of easily solublesalts (including carbonates and bicarbonates of sodium) is found in the lower part of the B-horizon and in the C-horizon. As regardssalinity,these soils are not,relatively speaking,very strongly saline.As regardscomposition,analysis of the easily soluble salts contained in a water extract taken from alkaline soils shows that carbonatesand that of HCO,of the order bicarbonates of alkali head the list:the CO,content is between 0.05and 0.07%, of 0.1 to 0.2%. In soils with maximum soda salinity,the content of these ions may be as much as as 1 %. The presence of free carbonates and bicarbonates of alkali, being accompanied by hydrolysis, causes a strongly alkaline reaction;with the result that the p H in soda-salinesoils is above 8.5 (between 9 and 11). Strong alkalinity is in fact one of the main features of the chemistry of soils of this group. There are a large number oftransitional stages between dark alkaline soda-salinesoils and slightly alkaline dark-coloured soils and cemented black soils of various types: meadow soils,terrace soils,prairie, valley smonitzes, chernozems, black cotton soils, etc. Soils with fairly low alkalinity (pH 8.5 to 8-8) will have a specific plant cover adapted to these particular conditions; whereas strongly alkaline soils (pH 9 to 11) have,as a rule,practically no plant cover. Soilswith strong soda salinity-1.5 to 3 %-lose their dark colouring owing to the coagulating effect of the soluble salts,acquire a false,friable structure and become, albeit only temporarily,permeable. Bicarbonates and carbonates of alkali may be found in alkaline soils either in practically pure form or mixed with chlorides or sulphates.For this reason,a distinction must be drawn between two types of alkaline soils:soda-sulphateand soda-chloride. Both solutions of alkaline soils and alkaline groundwaters almost always contain much dissolved silica (60-100mg/l,SiO,). These solutions are partly colloidal.The total quantity of mobile SiO,in a water extract obtained from alkaline soil may sometimes be as much as 0-1%,and in soils it increases with the general alkalinity and pH. There have been cases where the formation ofmobile silica in the profile of soda soils has been maximum near the surface. A regular feature of solutions and water extracts from alkaline soils is the presence of mobile forms of organic substance (alkalihumates). Indeed,it is these humates that give water extracts the dark coffee colour so characteristic ofthese soils.And lastly,under extremely alkaline conditions,water extracts and solutionsof soda-salinesoils contain sometimes anions of aluminium (from aluminates of alkali). It is clear from the foregoing that alkaline-typesaline soils contain,in addition to carbonates and bicarbonates of alkali,also silicates,humates and aluminates of the same alkali. As regardsabsolute and relativecontent ofsoilcolloids,soda-salinesoils are higher than othertypes ofsoils. Highly dispersed fractions(particle-diameterless than 0.2microns) may constitute,in soda-salinesoils,up to 60%ofthe total weight;such large quantitiesof dispersed matter have not been found in other types of soils. As regardsdistribution ofhighly dispersed fractionsin the profile ofalkaline soils,thereis a marked maximum in the surface horizons, which are most strongly alkaline. In cases where there is no A-horizon,and the horizon of maximum alkalinity lies on the soil surface,then the maximum of soil-colloidcontent will also be on the soil surface.O n the other hand,in the case of alkaline soils having both an A-and a B-horizon, the maximum soil-colloid content will be found in the B-horizon.As regards mineral and chemical composition,the highly dispersed fractions of alkali-saline soils are characterised by a wide ratio of compounds of silica to sesquioxides.It has been shown by severalexamples for conditions existing in the USSR that the ratio SiO,:R,O,is of the order of 4:l.Colloidal fractions ofalkali-saline soils contain,in addition to crystallised minerals, a considerable quantity of amorphous minerals and organic compounds. Among the crystallised compounds,the most important is montmorillonite, which was almost invariably found to predominate in the mineral composition of highly dispersed fractions of alkaline soda-salinesoils.Montmorillonite and the organic colloids give alkaline soils their very large adsorbing capacity,attaining 45 milliequivalents per 100g. Montmorillonite is also responsible for the shrinkage of alkaline soils when dry with the formation of deep fissures;and for the swelling of these soils when wet-the crystal lattice of montmorillonite then expands considerably.The dark hue of montmorillonite (especially its ferrous forms,formed in hydromorphic conditions) further intensifies the dark colour of alkaline soils. Then again, an abundance of montmorillonite 138

SALINE A N D A L K A L I SOILS CHEMISTRY (ARID ZONES) mineral facilitates the formation of a prismoid-columnarsoil structure with a tendency to hexahedron formation-montmorillonite, as we know,is one of the tabular minerals,hexahedron in shape.In this respect, the columnar-prismoidalhorizons of alkaline soils constitute,in a sense,paracrystals of montmorillonite. The result of the constant presence of sodium carbonates and bicarbonates in the soil solutions of alkaline soils is that the adsorbing complex of these soils is almost saturated with exchangeable sodium. There is a direct dependence between alkalinityand water extractsofthese soilsand the quantity ofexchangeablesodium. The higher the total alkalinity and p H of soda-salinesoils,the higher is the exchangeable sodium percentage. 1. Processes of formation and accumulation of soda in soils The formation,migration and accumulation of compounds of carbonates and bicarbonates of alkali constitutes one of the oldest geochemical processes and one which is still taking place all over the world. As we know,aluminosilicateminerals represent by far the most important component in the mineral composition of the earth's crust,constituting about 85 %of the rocks.Some of these minerals are highly stable;while others have little stability and are easily weathered in the presence of water,particularly when it contains carbonic acid.The chemicalweathering ofsodium and potassium aluminosilicatemineral-more particularly those like feldspars and feldspathoids-is accompanied by the formation of solutions of bicarbonates and carbonates of alkali,also ofionic and colloidalforms of silicasand aluminium oxides.The universal predominance offeldspathic minerals in the earth's crust in all climatic zones leads to the continuous formation,everywhere,of solutions of silicates,bicarbonates and carbonates of alkali,and alkali-earths.It is for this reason that,when we analyse the chemical composition of natural waters in different climatic zones we invariably find that the lower their mineralisation,the greater is the relative quantity of dissolved silica,bicarbonates and carbonates of metal they contain. The formation of bicarbonates and carbonates of alkali is particularly intensive in the case ofrocks and minerals ofrecent volcanic origin.Basalts,volcanic lava,volcanic ash and volcanic tufa, when exposed to the action of natural waters and carbonic acid,undergo intensive disintegration, forming large quantities of mobile silica, alumina and free bicarbonates and carbonates of alkali.The experiments which were carried out in 1935 by KOVDA and BYSTROVshowed that the hydrolysis which nephelinic syenites undergo, especially in the pressnce of carbonic acid,is so intensive that a simple water extract taken from pulverised syenite removes large quantities of mobile silica, and bicarbonates and carbonates of alkail are formed.The same thingwas established by STEVENS and CARRON in 1948,by observing the hydrolysis reaction ofaluminosilicatessubjectedto intensefrictioninwater (Table 5.11). Similarphenomena arealso observed in nature: spring,ground and subsoil waters, also lakes fed by streams in areas of recent volcanic deposits contain large quantities ofcarbonates and bicarbonates ofalkali and ofmobile silica.In an arid climate zone, new volcanic areas will always be characterised by the formation of waters and soils containing soda (East African graben, alkaline soils of Chile and Argentina, and the soda solonchaks of Transcaucasia and in particular Armenia, soda lakes and soda accumulationsin the USA). Table 5.11. Hydrolysis of minerals in water

Mineral Name Amphiboles Carbonates Clay mineral and Al oxides Feldspars

Feldspathoids Micas Olivine

Pyroxenes Quartz

p H when abraded in water

10-10 8-10 5-7 8-1 O 10-11 7-9 10-1 1 9-1 1 6-7

Hilgard's theory of soda formation,which is accepted by very many scientists,presupposes the coming together of limestones,marls and carbonate deposits (loess) with solutions of chlorides and sulphates of sodium. L

139

IRRIGATION, DRAINAGE A N D SALINITY

In itself,the system CaCO,+2NaCI (Na,SO,) = Na,CO, +CaCI, (CaSO,), if these compounds are merely juxtaposed,cannot give rise to the formation of soda to any marked extent.The reaction will proceed in the direction of the least soluble compound-CaCO,. Under usual conditions,therefore,this process produces only insignificant quantities of soda.All the same,it would be a mistake to underestimate the part which this system may play in the geochemicalprocesses,in view ofthe innumerable differentconditionswhich may arise in the weathering crust. Evaporation,low temperatures,the presence of other salts-all these may cause soda to separateout and be precipitated in soils.Gedroitz established in 1912that the risingmovement ofweak solutions of sulphate and chloride of sodium,alternating with descending streams,causes accumulation of exchangeable sodium in the adsorbing complex ofthe soil.In presence of dissolved carbonic acid,the hydrolysis of sodium clays is accompanied by the formation of bicarbonates and carbonates of alkali. The exchange reactions between the sodium-saturatedsoil on one hand and the carbonic acid or calcium bicarbonate on the other hand are expressed by the following scheme,worked out by GEDROITZ, SIGMOND, and KELLEY: (adsorbing complex)-2Na+

+H,CO,=(adsorbing complex)-2H +Na,CO,

(adsorbing complex)-2Na

+Caco,= (adsorbing complex)-Ca -t. -I. +Na,CO,

+

or +

These reactions may be repeated indefinitely with the same result-i.e. the formation of soda. There are,finally,grounds for believing that the mineralisation of the organic substance of certain plants may lead to the formation ofcarbonates and bicarbonates ofalkali.Examples are the high potassium carbonate content of the sunflower and the high alkali carbonate content of halophytes like Haloxylow,Anabasis, Bassia, Artennisia and Elymus. Alkalis, apparently, exist in these plants in the form of salts of organic acids or absorption compounds which, when mineralised and brought into contact with carbonic acid, form carbonates. In an anaerobic milieu,processes of desulphurisation and denitrification of sulphates and nitrates occur. The hydroxides of alkaline and alkali-earthmetals which remain after desulphurisation thrdugh interaction with the carbonic acid of solutions,produce carbonates and bicarbonates. Na,SO, +2C=Na,S +2C0, Na,S +CO,+H,O=Na,CO, +H,S

This reaction is always accompanied by the formation of sulphides of hydrogen and of heavy (FeS, Fes,) and light (Cas,Na,S) metals.The main conditions for this reaction are the lack of oxygen and the presence of organic matter and of deoxidising micro-organisms,e.g.in swampy soils,on the bottom of shallow lakes,seas and coastalbays and in lagoons,estuaries and stagnant deltaicponds.Similar conditions also occur inenclosed deposits of such minerals as coal,lignite,oil and bitumen. A detailed study has been made,in particular,of the disappearance of sulphates and the accompanying accumulation of carbonates of alkali in the petroleum waters reduced in the course of the formation of oil deposits. However,the preservation,in these circumstances,of the bicarbonates and carbonates and in particular the soda formed,is possible only on condition that there is absolutely no inflow of oxygen. Oxidation, in particular of ferrous sulphide, will cause the formationoflarge quantities offree sulphuric acid,leading to intense acidisation ofthe milieu and destruction of the accumulated carbonates.It is in this way that free-sulphurousand sulphuric acids, ferric oxides,sulphates, alum, etc.,are formed during the working of petroleum deposits and during operations for drying out and aerating deposits on the bottom oflakes and bogs,deltas,lagoons,and estuaries.The above oxidation of sulphides may be expressed by the reactions: 2FeS,4-2H,O+70,=2FeSO,+2H,S04 FeSO,+ 2H,O= Fe(OH), +H2S04 Na2C0,1- H2S04=Na,SO, +H,O+CO, Oxidation of sulphides may occur in coal mines,in drying organo-mineraldeposits of lake bottoms,and in saline and waterlogged coastal areas after reclamation or decrease of waterlogging. The resulting acute acidity not only disintegrates the calcium carbonates and completely neutralises the carbonates and bicarbonates of alkali,but also affectsthe biological properties and fertility of the soil.Most naturally formed soils,including bogs,lake-bottomdeposits and delta soils,are subject to an alternating oxidising-reduction 140

SALINE A N D A L K A L I SOILS CHEMISTRY (ARID ZONES)

regime;with the results that,as a rule, the soil and groundwaters of these areas are characterised by acute acidity rather than by alkalinity.Processes of reduction cannot,therefore,be considered to play a significant part in the formation offree bicarbonates and carbonatesofalkali on the land surfacein ordinary atmospheric conditions. It is only in closed,underground strata where no oxygen can penetrate that it is possible for desulphurisationand elimination of sulphuric compoundsfor subterraneanwaters,with accumulationinthose waters of free bicarbonates and carbonates of alkali to occur and continue over a long geological period. The discharge of these deoxidised reduced alkaline waters in depressions, low-lyingareas and river-valley terraces causes the formation of soda-salinesoils which cannot be attributed to present conditions on the earth's surface. It is possible that this is the explanation for the presence of soda accumulations and sodasaline soils in Western Siberia,the Yakutsk depression, Moldavia and North Africa.

2. Conditions of existence and accumulation of free soda

One of the most important factors limiting the solubility of bicarbonates and carbonates of alkali is the presence of dissolved salts of calcium-such as calcium sulphate or chloride:both these compounds when reacting with sodium bicarbonates and carbonates,form calcium carbonate-as well as sodium chloride and sulphate.The presence of calcium sulphate in landscapes-which is characteristic,for instance,of the territories of Soviet Central Asia,where there are almost always deposits of gypsum in Quaternary and Tertiary sedimentary rocks, and also in soils-causes the constant neutralisation of bicarbonates and carbonates of alkali,and their transformation into calcium carbonates. This is probably the reason why soda-salinesoils are virtually unknown in Uzbekistan,Southern Kazakhstan,Tadjikistan and Turkmenia. Free carbonates and bicarbonates of alkali cannot accumulate in substantial quantities until the whole of the calcium sulphate and chloride reserves in any given territory have been transformed into carbonates. A similar effect,though to a much lesser degree,is exercised by the presence in soils,soil-formingrocks and aquiferous horizons, of adsorbed exchangeable calcium. Alkaline solutions,passing deposits saturated with exchangeable calcium,cause physico-chemicalreactions whereby the exchangeable calcium is replaced by sodium,or by potassium from bicarbonates or carbonates. The result of these reactions will be saturation with sodium of the sedimentary rock or soil,and a deposit of calcium carbonate.After this has happened, the circulating solutions will remain alkaline and give rise to sodium carbonate accumulation. In areas consisting mainly of loess and loess-type rocks, which as a rule contain large quantities of exchangeable calcium,the reaction described above may be of immense geochemical significance.The sample applies to river and lake alluvium which is likewise,in most cases,saturated with exchangeable calcium.The continued sedimentation of loess-likematerial from aeolian deposits,or of alluvium from river and lacustrian waters, will bring a constant inflow of exchangeable calcium into the sphere of reaction,thus neutralising the free carbonates and bicarbonates of alkali which are formed by the weathering or mineralisation of organic matter. A similar effect may be exercised by the appearance of calcium ions of biogenic origin-i.e. formed by the mineralisation of the roots or surface organic residues of plants whose tissues contain calcium in large quantities. The chief plants of this kind aïe cereals and pulses,the ashes of which are particularly rich in calcium compounds. Biogenic calcium sulphate in soil solutions produced by decay of the proteins will be a very effective factor in reducing alkalinity. A vital part ill regulating the presence or absence of free bicarbonates and carbonates of alkali in natural waters, sedimentary rocks and soils is, apparently, played by the process of formation of secondary clay minerals. As we know,natural waters always contain a small quantity of dissolved silica and sesquioxides. The interaction ofthese componentsduring the drying up of the solutionsis accompanied by the appearance of products of reciprocal coagulation, such as allophanoids and crystallised clay mineral of various types (and especially montmorillonite.)The formation of the crystal lattice of these minerals may be accompanied by non-exchangeablesorption of alkalis and their elimination from the solution. with residual formation of carbonic acid and water. This process is of special importancein the case of potassium,which becomes fixed in non-exchangeableform in clay minerals ofthe mica group.However,some sodium also passes out into the crystallattice ofa number ofclay minerals.Increasing quantities ofdata are now being accumulated to suggest that even feldspars are capable ofsecondary formationin ordinary surfacethermodynamicconditions.Ifthis is so,then it may be assumed that secondary mineral formationplays a vital part in the removal ofpotassium

141

IRRIGATION, D R A I N A G E A N D SALINITY. and sodium ionsfrom solutionsofnatural waters and,thereby,in restricting the presence and accumulation of large concentrations of bicarbonates and cc?.rbonatesof alkali. In what geographical conditions do we find small concentrationsof calcium combined with low intensity of formation of clay mineral? One of the main processes responsible for reducing the calcium salts in the milieu to a small quantity is their leaching by natural water. This happens,for instance,in humid marine climates,in mountainous and northern climates, and in humid monsoon tropics. In such conditions,both weathering and soil-forming products and also natural water will be characterised by a low calcium-saltscontent. However,the natural waters of such regions will carry very weak solutions of carbonates and bicarbonates which, on entering areas with an evaporation regime,may cause the formation of alkaline soda-salinesoils,or else of alkaline lakes. Conditions such as these evidently exist in California,Argentina,Transcaucasia, Equatorial Africa, the Northern Ukraine and Hungary, In absolute desert regions such as,for instance,the deserts of Western China or Latin America, chemical and mechanical weathering is so slight that the necessary prerequisites for the formation of secondary clays are lacking,Another point is that gypsum and calcium chloride in large quantities are non-existenthereowing to the low rate of chemical and biological weathering. According to EGOROV'S data, the absence of clay-formationin the desert of Western China, and the slowness of chemical weathering are particularly conducive to the accumulation of free soda in exceptionally large concentrations.Lastly,in high mountain regions and areas with a cold climate such as that of Northern Siberia and Canada, conditions likewise promote the loss of calcium compounds and the preservation, in solutions, of sinall concentrations of carbonates and bicarbonates of alkali. In cold winter conditions,compounds of carbonic alkaliswill accumulate in soils because the low tempzrature greatly reduces their solubility, and they are able to precipitate. The most favourable conditions for the formation of soda,and the preservation and accumulation of free soda in soils and solutions,will be a combination of the following:(a) young volcanic areas enclosed by new pyrogenous deposits (from which the geochemicaldischarge derives) ;(b) depressions with insufficientnatural drainage (towards which the geochemicaldischarge is directed) ;(c) a climate suchthat evaporation preclominates over run-offin the water balance. This combination of circumstances is fairly common: examples are South and Central America,California,Transcaucasia, and the East African graben. 3. Physico-chemicalproperties of sodium carbonates

The easily soluble salts which cause salinity in soils are mainly the following: soda (Nazco,. 10 H20) forming a mealy-fibrous mass;thermonatrite (Nazco3.1 0.H20) having a different crystal form,accumulating in mealy granules in soils and sedimentaryrocks;trona (Nazco, .NaHC0, .2H,O)usually present in soda-saline soils and along the shores of soda-alkaline lakes in pure salt deposits; and lastly,nahcolite (NaI-ICO,) present together with the three above components,being found in solutions as well as in deposits in both soils and sedimentary rocks. The solubility of carbonates and bicarbonates of alkali depends to a very great degree on surrounding conditions, and varies within very wide limits.Most astonishing is the extent to which the solubility of sodium carbonates and bicarbonates depends on the temperature: at temperaturesofzero and below,the solubility of soda and sodium bicarbonate,also that of sodium sulphate (but not that of sodium chloride) drops to as little as 3 to 5 g/l.At temperatures around 30°C,the solubility of soda is as high as that of sodium chloride,i.e.approximately 350 g/l;while at high temperature its solubility is nearly 530 g/l,i.e.higher than that ofsodium chloride (Fig.5.1). The behaviour ofsoda in relation to temperatureis similar to that of sodium sulphate.The solubility of sodium bicarbonate is likewiseincreasing with temperatures, though it varies less than soda. In the study of soil geochemistry,this factor is of outstanding importance.It means soda,at atmospheric temperatures (15°C to O" and below), will precipitate into deposits in lakes and soils together with sodium sulphate:while chloride solutionswill be carried away by ground,subsoil and surface waters. It is thus to be expected that,in regions with a cold climate and severe winters,saline soils will be characterised mainly by accumulation of sulphates and carbonates of alkali,In a dry,hot climate,on the other hand,owing to the fact that soda will then be moxe soluble than chlorides,separation may occur and soda may accumulate in the areas of final evaporation of the solutions.Combined migration and accumulation of soda and sodium chloride will be comparatively rare,occurring mostly in places where there are deep,reduced underground

142

SALINE A N D A L K A L I SOILS CHEMISTRY (ARID ZONES)

petroleum waters rising to the surface.The solubility of sodium carbonates depends also in great measure on the salts contained in solution. In view ofthe decreased solubility and migration czpacity of sodium carbonates,natural brines will contain only small quantities of sodium carbonates and bicarbonates. A sharp distinction can thus be drawn,in thegeochemistry ofsalts on land,between regions ofaccumulation of carbonates of alkali and regions of accumulation of chlorides and sulphates: the potential areas of accumulation of bicarbonates and carbonates of alkali being much larger than those of chlorides or even of sulphates.Soda and sodium bicarbonates will tend to accumulate in regions where the general salt content is fairly small,i.e. mainly in those at the initial stages of salt accumulation. A high soda concentrationin solutions,in its turn,exercises a stronginfluenceon the solubility and mobility of calcium carbonate,which is an extremely important compound in soil chemistry. Figure 5.3 shows that there is a very sharp drop in the concentration of calcium even in solutions with relatively low alkalinity. Inthe presence of sodium bicarbonate,calcium bicarbonate is still a little soluble;but the soda,even when the total alkalinity drops to 0.1 g/l,of HC03, virtually eliminates the calcium from solutions.This explains why thegroundwaters,in regions of alkaline salt accumulation,contain practically no calcium despite the presence of calcium carbonate in the rocks and soil horizons. When it comes to compounds of dica, aluminium and organic matter, it will be seen that normal and bicarbonate alkalis influence their solubility and mobility in precisely the opposite direction (Fig. 5.4):the greater the alkalinity of the solution,i.e. the greater the concentration of soda in it, the more compounds of silica, alumina and humus in the form of true, molecular or colloidal solutions it will contain. The same applies to soils:the higher their alkalinity,the greater amount of mobile forms of silica and organic matter their solutions or water extracts will contain (Fig. 5.5). It is a well-knownfact that rain puddles, surface streams and small brooks flowing through soda-saline soil areas, are strongly tinted by organic matter. This capacity of soda solutionsto turn compoundsofsilica,alumina and organic substances into mobile form has very far-reachingconsequences in soil science and geochemistry. Soda-salinesoilswillbe characterisednot only by the accumulation ofcarbonates ofalkali and by exchangeable sodium:there will also occur at the same time-though the process may not be visible-accumulation of compounds of silica,alumina and organic substances brought into the area with the alkaline surface or underground waters.Theconcentrationofthese components inwater is lowbut theseprocesses,continuing throughout geological time, result in the accumulation of considerable quantities of silica, alumina and organic substances,and the emergence of a highly specifictype of soil-formingsubstrata.This property-the capacity to transport silica and alumina compounds into areas of soda-saltaccumulation-is, in our opinion,responsible for the formation in these areas of sedimentary rocks rich in secondary amorphous minerals with marked predominance of silica over alumina and,more particularly, responsible for the formation,in these

% Ca of the solid residue

a

. ... 0

% : $ = : : & & : : .." P f 5 0

0.10

0.20

..f

0.30

.

CI

* f. * 0

**

0.40

0.50

.5--L6*

e-

0.60

0.70

0.80

*

*

1

0

0.90

-I 1.0

%I-ICO,

Fig. 5.3. Relationship between content of water-soluble Ca and the total alkalinity regions of montmorillonite-type clays containing a constant admixture of highly dispersed organic matter. Even kaolinite-typeclay deposits,under the influence of alkaline groundwaters,containing dissolved silica, are inevitably transformed into montmorillonite-type clays. As a result of all these factors combined,the

143

IRRIGATION, D R A I N A G E A N D SALINITY sedimentary soil-formingrocks of soda-salineregions will be dark in colour,be characterisedby high dispersion,have a large swelling capacity when damp, and shrink and crack when dry. In other words,they will possess all the properties which distinguish soda-salinesoilsfrom the other soils.The main reasonfor the black colour ofmeadow soda-salinesoils is ofcourse the accumulationof humus in hydromorphoussoil-formation conditions. Millimoles /I 1 1 7 10

-

98-

7-

0

6543-

21-

I 0

1 2 3 4 5 6 7 8 9 1 0 1 1

PH

Fig. 5.4 The solubility of silica and alumina as a function of p H Einfuhrung in die Mineralogie After CORRENS' SiOzmg/100'g of soil 50 7 40

7

-

* * O

0.1

Of!

0-3 0.4

0.5

.al 0.6

** I

0.7 0.8 % of HCOl

Fig. 5.5 Alkalinity of water extract of soil.Relationship between concentration of water-soluble SiO, and the total alkalinity of soils 4. Soda accumulation-the

first stage in the process of salinisation of soils and waters of arid zone

The results ofstudies carried out over a period ofmany years,areportonwhich was published at various times (KOVDA, 1946,1947,1959), show that there exist specificrelations between the quantity of the salts in natural waters and soils and their composition.In the process of the water mineralisation increase,the following quantitative and qualitative stages occur: 1. Silicious waters-completely fresh waters of the tropics and of northern forest regions, containing silica and organic substances;total concentration 0.01 to 0.1 g/I 2. Fresh hydrocarbonate-calcium waters with total concentrations of 0.2to 0.3 g/1

144

SALINE A N D A L K A L I SOILS CHEMISTRY (ARID ZONES) 3. Hydrocarbonate-sodium waters with total concentrations of 0.5 to 0.7 g/1 4. Hydrocarbonateand carbonate sodium waters with salts concentrationsof0-5to 3 g/l,containingsulphates and,less often,chlorides 5. Chlorido-sulphatewaters with concentrations of 2.5 to 5 g/í,containing soda 6. Sulphato-chloridewaters with concentrations of 20 to 50 g/l,usually not containing soda in substantial quantities 7. Chloride waters-brine-with concentration of 100 to 300 g/1

The foregoing general pattern for the existence of hydrocarbonate-sodium underground waters with low mineralisation (0.3 to 5 g/l) was established on the basis of a large quantity of geographical and hydrogeochemical data. The West Siberian depression, including the Baraba and Kulunda plains, is a typical example ofthis phenomenon.In this immense soda-salineregion it is established that about 85%ofthe cases, the first aquiferous horizon,and also the groundwaters of the Quaternary,Tertiary and Cretaceous strata, when the concentrations are between 0-5and 5 g/l,are hydrocarbonate-sodium in type,with some sulphates or chlorides.The similar hydrochemical pattern is established for the Hwang-Ho and Sungari basin in China, where, as mentioned earlier, there are large areas of soda-salinesoils. The same chemical composition is found in the groundwaters of the Ararat Valley in Armenia,the Karabakh steppe in Azerbaijan in the subterranean waters of North Africa, California, Argentina, and so on. There are few known examples of deviations from this general pattern for the formation of alkaline hydrocarbonate-sodium waters. The most striking example of the complete absence of hydrocarbonates and carbonates of alkali in groundwaters is thatof the Ferghana Valley inUzbekistan:here the rocks formingthe valley basin contain abundant gypsum; and both sedimentary rocks and soils are supersaturated with this substance. In these conditions, it is impossible for bicarbonates or carbonates ofalkali to form large concentrations or accumulatein solutions.Due to this,the Ferghana Valley constitutes a classicalexample of the absence of soda accumulations and of the predominance of processes of accumulation of sulphate and, in particular, calcium sulphate. This is also characteristic,though to a lesser extent, of the Hungry Steppe and the Bokhara region in Uzbekistan,the Vaksh river valley in Tadjikistan and the lower reaches and delta of the Amu-Darya. A diametrically opposed anomaly-that of intensive accumulation of bicarbonates and carbonates of alkali in highly concentrated solutions-is observed on the alluvial plains of the deserts of Western China, where,along the terraces of the Tarim,Aksu and other river valleys,extensive meadow soda solonchaksare found.Generally speaking,the groundwaters in this area also have a relatively low mineral content (from 0.5 to 5 g/l) and are alkaline. Research done by EGOROVand his colleagues in 1962 showed,however, that there are also,in this area,groundwaters which have concentrations up to 30 g/land contain carbonatesand bicarbonates of sodium.EGOROVattributes this anomalously large accumulationof carbonates and bicarbonates of alkali in the deserts of Central Asia,to the fact that,in this region,clays saturated with exchangeable calcium are practically non-existent.As demonstrated above,clay-formationand in particular reactions with exchangeable calcium may cause sodium to be separated out of solutions,thus limiting the accumulation of free soda. The absence of clay,with a constant inflow of fresh weathering products may,together with the evaporation and concentration of alkaline underground waters, lead to exceptionally large accumulations of carbonates and bicarbonates ofalkali in solutionsand groundwaters.Another extremely importantfactor,in the conditions of Western China,is the virtually total lack of gypsum. The existence ofsuch geochemicalanomalies ofone kind or another merely underlines the need for carrying out detailed research on the chemical composition of the soluble salts contained in the underground waters and soils of strongly,medium and in particular weakly saline areas. At the same time,the existence of such anomalies does not detract from the significance of the general laws enunciated above.

H. RELATIONSHIPS BETWEEN SALINITY A N D ALKALINITY OF SOILS KOVDA, working in the basin ofthe Nile,Indus,Hwang-Ho and Sungari rivers, has analysed the relationship Author’s note: The term ‘alkaline soda-saline soils’ always means soils which contain large quantities of carbonate and bicarbonate salts of sodium and potassium as well as considerable amounts of adsorbed sodium,i.e. soda solonchaks, soda solonetzs,etc.

145

IRRIGATION, DRAINAGE A N D SALINITY between alkalinity and salts contents of groundwaters.In Fig. 5.6 the total salts concentrations in groundwaters and in soils are compared with the corresponding p H readings. In groundwater,concentrations below 4g/1go together with a p H reading of about 7.8to 8.0,with a fairly wide range of variation (7.2to 8.9). It should be remembered that this water was collected and analysed without any measures being taken to protect it from the atmospheric carbon dioxide,so that the p H readings were in fact slightly higher than the figures obtained.There is a marked tendency for the alkalinity ofgroundwaters to decrease as their mineralisation increases. There are some exceptions but, generally speaking,it may be said that,with concentrations of above 5 g/l,the groundwaters’p H of the Hwang-Ho and Sungari river basin do not rise above 7-5.Groundwaterswith a salts content of 30 to 50 g/1 are, as a rule,neutral. A similar relation between alkalinity and salt content is found in soils as well. Alkaline soda-salinesoilsformed under the influence ofslightlymineralised groundwaters are in themselves only slightly saline;the total salts content in such soils rarely attains 1 %; more frequently,and measured in a water extract,it is not more than 0.7”/, at the surface,and considerably less deeper clown.It is only in exceptional cases that mineral soda solonchaks,with totalsalts content (measured in a water extract) ofseveral per cent,are formed.This may occur,for instance,along the shores oflakes,where thereis a chemical precipitation of salts from saturated solutions, with accumulation of trona. Figure 5.6 illustrates the relation between the p H reading and the soil salinity in the Hwang-Ho and Sungari river basin. Generally speaking, the soil p H are considerably higher than those of groundwaters. It is as though the soda were separated out by the soil.The soda,after entering the soilhorizons from the groundwaters,remainsthere,both because of the low water-permeabilityof alkaline soils and of the low solubility of soda in cold waters. The general pattern is the same as for groundwaters: the lower the total salts content in the soil horizons, the more likelihood there is of the soil being strongly alkaline.Thus,with a salts content of 0.6% and less, the p H averages 8.7with variations from 8 to 9.6.With high salts contents in the lower part of thc profile (1 to 3 %) soil alkalinityprogressively decreases,approachingpH 8.The same diagram shows that the alkalinity depends on the salinity ofthe top soil horizon. Soilswith exceptionally high p H (9to 10) exist only with a salts content of less than 2%.Where there is a salt content of2-5to 4%, the p H reading in the top horizons of the saline soils of China are never very high (approximately 8). All this indicates that there is a very close connection between the process offormation ofalkaline soda-salinesoils and the chemical composition ofgroundwaters. At the initial stages of salt-accumulation, when the groundwaters themselves have a low salts content (Q 0.5 g/l), there are formed,in hydromorphic conditions,humus-rich,dark-colouredmeadow soils which are completely noa-saline,have low alkalinity and possess a high natural fertility. Subsequently, as the salts-concentrationin the groundwaters increases owing to the accumulation of the carbonates and bicarbonates of alkali,the p H rises to 9 to 10 on account of the intensive accumulation of free sodium carbonates and bicarbonates.A n increasein the salts-concentrationofthe groundwatersis accompanied by accumulation ofsulphateand chloride,including salts containing calcium.There then begins a gradualprocess of ‘gypsumisation’ of the salinising soils; the soda and sodium bicarbonates accumulated at an earlier stage become neutralised and their strong alkalinity disappears. As a result, strongly saline soils are formed under the influence of highly mineralised groundwaters;but they are weakly alkaline or even, in some cases,have a neutral reaction. Figure 5.7,drawn up on the basis of a large quantity of analytical data,shows that alkaline soils-i.e. soils in which bicarbonates and carbonates ofsodium predominate in the salt composition,both in absolute and in relative terms-exist in places where the total salts content in a soil prism of 1 m3does not exceed 7 kg.When the total salts content in the soil profile approaches 10 kg,then in the conditionsprevailing in Western Siberia,a marked gypsum accumulation begins,while that ofcarbonic alkalis decreases until they finally disappear altogether transformed,under the influence of gypsum,into calcium carbonate. At the same time,the sodium sulphate content rises sharply,and the soil, slightly or medium saline, becomes strongly saline, but loses its alkalinity, so entering the category of neutral sulphato-chloride saline soils. also for the plains of Central Azerbaijan. The fact that This phenomenon was confirmed by MURATOVA alkaline soda-salinesoils coincidewith the minimal salinityfigures explainswhy they are difficult to recognise. There is no doubt,however, that many compact,impermeable,unstructured,fissured,dark-coloured steppe, savannah and meadow soils (so-called vertisols) are in fact soda-soils which have not been identified as such. and MINART.If the samples analysed for pH, salinity Similar phenomena have been found by PASCAUD and alkalisation are plotted on a semi-logarithmic coordinate system (Fig. 5.8), it is found that a smooth curve can be drawn between the two zones with the alkali soils zone above it and the non-alkalisoils zone below it.

146

SALINE A N D A L K A L I SOILS CHEMISTRY (ARID) ZONES DH

in

1

7L 10 -I

ll 9 8

O 1 O

1

20 I 0.66

t O

I O

40 I

I 16

I

I

I

I

I 32

I

I

I 48

I

I

I

120 .Total meq

I

I

2.64

1500

I

100

80 1

1.98

1000 l

I

I

1.32

500 l

60

I

3-30 3.96 Total soluble salts %

I 2000

I

I 64

I

I 2500

l

J

5000 Total m e d l I

80

1

96 gr/l

Fig. 5.6 Relationship between salinity and the p H value I: in top soil horizons II: in lower soil horizons III: in groundwaters

The example given here refers to saline soil surveyswhich were carried out in the Abadla Plain in Southern Algeria. The soils considered are recent and present alluvia,rich in lime and lacking organic matter.Salinity is of continental origin. Practically the same curve has also been found to apply to the saline soils in the low-lyingplains of the Quantity of individual Salts (in kg.m-2) 5r

Total quantity of easily soluble salts in the soil above ground water table (inkg. m-2)

Fig. 5.7. Salt accumulation in soils (Western Siberia)

147

IRRIGATION, DRAINAGE A N D SALINITY pH (H2O)

EC conductivity of saturation extract (in mmhos/cm at 25OC)

Fig. 5.8. Relationship between pH, salinity,and exchangeable sodium percentage (ESP) (Plaine d’Abadla,Algeria) Lower Rhône and Languedoc regions in the South of France in recent alluvial lime soils containing varying amounts of organic matter (nocorrelation could be established if the organic matter rate was high). Salinity is of marine origin. Several empirical formulaewere tried out in a mathematical study in order to find a suitable expression for data of Fig. 5.8. The best results were obtained with an equation of the following form:

Y =U, +a,xi+ a2x2 +~ 3 ~ 1 + x ~2 ~ 4 x 3 where: Y=ESP (exchangeable sodium percentage) x, = p H of the soil x2=logarithm of the conductivity of the saturation extract, xa=clay content. The values of coefficients a,, u,, a3,and a4 and the residual quadratic error 60 were determined by the method of least squares. The values found in this case were: ao= -52.9 al= + 6.68 a, = 88.85

a3= -k 12.75 a4= 0.12

-

-

6,=2.996 (residual quadratic error) 62= 3.587 (degree of scatter)

Other more general investigations involving various types ofsalinity show that correlations of this kind are valid for soils of a comparable nature. They cannot,of course,be generalised to apply also to soils with too widely differing pedological characteristics,especially as regards organic matter content,particle size,lime content and soil. In particular, attempts to find such correlations for sandy, peaty or humiferous soils have all so far failed to produce any satisfactory results.

I. DYNAMICS

OF SALTS IN THE SOILS OF ARID ZONES I

l. Salts in soils The presence in soils of surplus amounts ofeasily soluble salts may derive from solutions formed at an earlier stage (sea waier, for instance,water from salt springs,and so on). More frequently,however,contemporary

148

SALINE A N D A L K A L I SOILS CHEMISTRY (ARID ZONES) soil salinityis due to processes which liberate the requisite elements and salts in the course ofthe weathering and chemical denudation of rocks.Salinity ofthis type cannot occur in the immediate vicinity ofweathering, but only laterwhen the saltshave concentrated over a smaller area or in smallervolume. Hence salinity occurs after the newly formed salts have been leached by natural waters for a considerable period oftime.The most active factor responsible for leaching and for the subsequenttransportation ofthe dissolved compounds into the concentration areas is the groundwater,reacting with large quantities of rock.The chemical composition of the leaching waters is characterised by the predominance ofcarbonate salts-hydrocarbonates of calcium, magnesium and,in lesser degree, of sodium.These waters as a rule contain only small quantities of chloride and sulphatecompounds,unless they are affected by the proximity ofactive volcanoes or deep groundwaters. Inregionscontaininglarge quantities ofgranite or especially alkalinerocks,weathering may be accompanied by the appearance oflarge quantities of sodium silicate,which subsequently turns into sodium carbonateand sulphate. These leaching waters always have a low salt content (approximately 1 g/l). When the rocks are porous and permit a certain flowage so that these waters sink to a great depth (otherwise,they quickly fill up all the spare spacein the rocks), they may retain a low mineralisation over long periods,even in arid zones.When they approach the surface ofthe earth (in low-lyingareas) their compositionbegins to change as a result ofheating and evaporation. Soils usually play an important part in changing the composition of these waters.

2. Changes of the salt composition in natural waters solutions

Rapid evaporation of surface waters is common in arid zones and constant evaporation of the groundwaters after capillary rise also occurs,although, when they lie deep,the amount of evaporation is not very large. When,on the other hand,the groundwater table approaches the surface,and the capillary fringe penetrates into the root zone,the evaporation increases sharply. Groundwater consumption,in this case, constantly increasesas a result of the action of certain wild plants. This causes accumulation of salts both in part of the soilsolution and also in the rocks,in solid phase. Some salts also accumulateinplant organs.The sequence of salt concentration and precipitation is governed by specific laws.Among the first salts to saturate groundwaters or soil solutions are carbonate of calcium and magnesium,which may begin to precipitate even in the aquiferous stratum or in the capillary fringe.This precipitation is facilitated by the temperature rise which occurs as the solutions approach the surface. Bicarbonates turn into carbonates of lower solubility. This reaction occurs as follows: f H,O Ca(HCO,),-+CaCO, +H,CO, L CO,

In some soils,as a result ofthis reaction,a solid layer of carbonates may be formed;this is known as meadow marl (shokh,hardpan). Continuing evaporation of the solution results in its being saturated with gypsum (provided it does not contain soda,which preventsgypsum-formation). In the soil profile,as the rising capillary solutionsevaporate, gypsum begins to precipitate on top ofthe carbonates,thus forming the next salt horizon.Subsequentlythere emerge,on the soil surface,solutions relatively rich in the most soluble compounds,This process occurs even in areas where the solutions (groundwaters) as they proceed from higher-lyingparts towards the depressions become steadily poorer firstin little-solublesalts and then in medium-solublesalts.The solutionswhich reach the lowest-lyingareas (wherethe groundwaters are stagnant)contain large quantitiesofchloride compounds. 3. Exchange of salts between soil and groundwaters

In nature,few processes are one-way.In the case of soil salinity,the process of salinisation often alternates with periods of desalinisation.The salts accumulating during the dry period of the year move downwards during the rainy periods, according to their respective solubilities. Those which move down fastest are chloride,whereas sulphates,less soluble,are retained in the soil.This process, repeated from year to year, results in a fairly large accumulation of sulphate in the soil,with an accumulation,in the groundwaters,of

149

IRRIGATION, DRAINAGE A N D SALINITY chloride (KOVDA). This only occurs,however, after completion of the first phases of salinisation,when salt accumulation ceases and is supplemented,increasingly by the removal of part of the salts from the soil. During the first stages of this process the salt predominant in the top strata of the soil remains chloride. The possibility of premature accumulation of more soluble salts in the soil depends on the depth of the groundwaters:when the groundwater table lies very close to the surface so that these waters moisten the top soilhorizon,even when the capillary pores are very wide,the saltsdo not have timeto divideup into categories according to their solubility.Moreover,salts accumulating under these conditions remain almost constantly in solution,so that they can easily return into the groundwaters. One means whereby they can move down into the groundwaters is by downward flow of the more highly concentrated solutions (Processof convective flow, according to MOROZOV). Towards the end of the hot season,the concentration of the soil solutions in the top horizons of solonchaks is often as high as 300 to 400 g/l.At the same time their specific gravity increases considerably,and the more concentrated solutions sink downwards,their place being taken by the more diluted solutions rising up from below. With rather deeper groundwater tables,the soil surface is only moistened when the capillary pores are narrow,in which case part of the salts from the evaporating solution pass into deposit,and thus cease to participate in the downward gravitational flow.The overall speed of the salinisation process is thereupon reduced but the salinity of the topsoil horizons eventually becomes higher than in places where the groundwaters are shallower. On the other hand, the salinity of comparatively deep groundwaters is weaker. This phenomenon is also observed in very arid deserts where extremely rapid evaporation causes the whole ofthe salts to precipitate on the surface,forming a salt crust30 to 60c m thick. Since these salts are then irrevocably fixed in the soil,they do not accumulate to any extent in the groundwaters, which,therefore,remain relatively non-saline.

4. Salt accumulation in soil solutions When defining the total salinity of soils,by whatever means, data are obtained for the total sum of salts to be removed by leachingin order to make the soilfertile.Since,however,the toxicity of salts for plants depends on the concentration of the solution,it is very important to study also the composition of soil solutions. The rate and the causes of soil salinisation are thereby explained In the top horizons of arid zone solonchaks,soil solutionshave usually reached saturationpoint in regard to at least one (predominant)soluble salt,at any rate during the dry part of the year. The higher the concentration of the groundwater causing soil salinity,the greater the depth to which the soil solution will be saturated, and the slower the concentration decrease of the solution deeper down will be. In the case of comparatively little-mineralised groundwaters,such as,for instance,those of meadow solonchaks,the soil solution will only be saturated in the top stratum,whereas lower down its concentration will drop quickly (Table 5.12). Permanent or ‘periodical’(in the vegetation season) saturation of soil solutionsin relation to one or several easily soluble salts is a specific characteristic of solonchaks.This is not found in other saline soils. In highly concentrated soil solutions,the salt composition differs markedly from the general salt composition ofthe soil as a whole. For instance,the calcium content in the soil solution is always much lower than that in the soil and the sulphate considerably lower. On the other hand,the quantity of chloride,sodium and magnesium contained in soil solutions is comparatively large, frequently as large as that contained in the soil itself. Table 5.13 gives approximatefigures for the quantity of salts precipitated when the groundwater turns into a soil solution,according to its concentration and stage reached in its rise towards the surface.Calculations were made on the assumption that the whole of the chloride contained in the groundwater remains in the soil solution. It is clear from the above that considerable quantities of CaSO, MgSO, and Na,SO, and also a certain quantity of carbonate-Ca and Mg-precipitates in concentrated soil solution. The composition ofthe soil solutions varies a great deal from time to time.During the rainy season,part of the salts in solid phase pass into solution,thus levclling out the total concentration.On the other hand, this causes precipitation of those compounds which become less soluble in cold weather. A predominant place in the solutionis then occupied by those salts whose solubility is little affected by changes of temperature-such as,for instance,sodium chloride.The variations in the composition of the soil solution have an effect on the growth of plants,the properties of the soils. 150

SALINE A N D A L K A L I SOILS CHEMISTRY (ARID ZONES) Table 5.12. Data obtained from analysis of soil solutions

Depth of

In milli-equivalentsper 1 litre of solution

Total salt content

sample

(cm>

(fm

HC03

CI’

No. 1 0-2 2-8 8-17 17-33 33-51 51-80 80-120 120-145 145-180 Groundwater No. 20-10 10-17 17-31 31-57 57-90 90-127 127-150 150-170

256.5 151.4 112.7 107.8 105.1 113.6 88.3 86.1 91.9 430 383 156 72 30 11 8 5

2.0 2.2 2.6 2.6 3.8 4-5 6.6 6.6 6.1 5.6 6.1 3.4 4.3 5.1 4.0 3.8 3*4

4303 2470 1827 1730 1675 1840 1382 1366 1365 4789 3805 1875 900 361 106 58 58

SO“, 104 126 106 144 134 135 134 132 235 2083 2126 428 164 97 53 24 24

Ca”

Mg”

65 80 60 74 77 75 74 67 72 106 117 55 32 26 23 22 22

426 177 163 291 233 339 321 328 430 2032 1010 567 206 1O0 30 18 12

Na‘-K’ calculated 3918 2341 1712 1502 1496 1565 1120 1109 1104 4740 4011 1685 830 338 110 62 52

No. 1. Solonchak with extremelyhighly mineralised water-Kura-Axes depression No. 2.Solonchakwith slightly mineralised water-Southern Sinkiang

Table 5.13. Total ion deficit in soil solutions of a chlorido-sulphatesolonchak (compared with groundwater) as a result of the evaporation of the initial (soil)solution and the precipitation out of the salts into solidphase (data by GRABOVSKAYA)

Number

Depth

tion and date 5 Aupst 1947

M-2

Soil

Increase

0-5 5-15 15-30 30-45 45-60 75-90

ture

in %

-2877 -1294 -700 -592 -378 -106

-2346 -1412 -987 -528 -327 -124

-2474 -983 -673 -342 -219 -17

17.7 23.8 18.7 20.9 24.4 24.4

29.7

43.6

74.2

9.36

O

-7396 -3354 -2231 -1297 -886 -215

49.5

90.4

98.2 45.1 32.2 21.4 14.2 5.0

-740 -335 -243 -156 -100 -32.6 7.7

Na+

Ca+

HC03+ C1+

O

Mg+

so4=

CI

Mineralisation of groundwater in milli-equivalents

mois-

Deficit inmilli-equivalents/litre

of concentration

of sec-

O

o o O o

+

+

per litre

5. Cementation of soils

The phenomenon ofcementation ofsoilsis common in arid zones. Cemented or compacted soils (smolnitzes, regurs,tirs,certain black tropical soils,takyrs,solonetz) occur in heavy clayey soils,relatively rich in montmorillonite,which are subjected to a regular cycle of over-wettingfollowed by drying up.This can be considerably reinforced in the presence offree soda,or under the influence ofadsorbed sodium.A n ‘amalgamating’effect on cemented soils also tends to be exercised by active fornis of alkaline humus.Cemented soils, as theylosetheirfractionalmicro-aggregatestructure,become compact.When swollen,their volume increases considerably,and the resulting pressure binds the particles more closely together. During the late stages of 151

IRRIGATION, DRAINAGE A N D SALINITY

the process,particles of the soil are squeezed out by the pressure,from hillocky micro-relief.Then when the soils dry up,their volume shrinks substantially,a whole network of large fissures and large clods are formed. Irrigation and leaching of such soils is a complex problem. Specific measures of mechanical and chemical reclamation are then required.

CHEMICAL ANALYSIS OF SALINE SOILS

J.

To chemically characterise an alkali soil it is desirable to obtain the following data: water soluble CO,=, HC03-, SO,=,CI-,and B;soluble Ca++,Mg++,N a f and K+; exchangeable Ca++,M g + + ,Na+,and K ;conductivity of the saturation extract;cation exchange capacity.Under special circumstances,analysis for M o or Se may be indicated. Extensive reference works for the determination of these quantities are available. Many different methods have been widely used for the study of saline soils.Both for thorough investigation of saline soils and for the interpretation of the data obtained,precise information on all soluble salts existing in the soil has to be obtained. For this purpose,a combination of water and acid extracts must be used.Water extracts with soil-water ratio,by weight, 1 :5 afford a means of determining the total content of easily soluble salts in the soil.Gypsum content is determined by means of a hydrochloric acid extract or else by dissolving the whole of the gypsum content in a large quantity of water. In cases where the content of difficult soluble carbonates in the soil has to be determined,a simplified method consisting of destruction of carbonate salts by acid and determination of the volume of carbonic acid gas obtained will also have to be used. For analysing the composition of the water extract, the same method is used as for analysis of natural waters,with determination of all the ions which form the easily soluble salts found in soils.The most usual composition of these components and the form they take are the same as in the analyses of soil solutions described earlier. In the case of saline soils containing soda it will be advisable also to determine the p H reading of a water extract or suspension.A p H reading higher than 8.2 indicatesthe probable presence of free soda;while with soda salinity,the p H reading will be more than 10.The appearance of soda in soil can likewise be ascertained by means of a water extract. If the total alkalinity figure (expressed in HCO,)exceeds 047% or 1 milliequivalent, per 100 g,there are grounds for assuming that the soil contains a certain quantity of sodium bicarbonate. The extraction of soil solutionsis effected either by squeezing out on a specialpress (from samples collected in steel cups) or else by treatment with alcohol in narrow tubes.When extracting the soil solution from soil by the alcohol treatment some quartz sand (washed-outacid and water) should be added for greater efficiency. Sometimes it is desirable to determine the water soluble constituentsin an extract made from saturated soils. This gives a determination at a water content which is related to the range of water contents which may be expected under field conditions,thus allowing for the fact that the toxicity of a given amount of salt in soil dependsupon soiltexture.A largebody ofinformation on the effect ofsoluble salts in soilshas been correlated with the concentration in the saturationextract.Similarly,the conductivity ofthe saturationextract correlates very well with the total salt concentration-one millimho corresponds to approximately 0.08 % of soluble salts. In fact, if more than a moderate deviation from the rough rule CT=lOX (wherc CT is the total salt concentration in me/l and h is the conductivity in mmho/cm) is found, it is usually well to look for an analytical error. The single drawbackwith analysis ofthe saturationextract is the error due to negative adsorption.The error suggest determining increases with salinity and in very saline soils it may be very large.BOWER and HATCI-IER the total chloride content by exhaustive leaching and then using the ratio: C1 content by leaching/Cl content by saturation extract,to correct the other water soluble constituents. The great multiplicity of methods for the determination of the exchangeable ions in soil is indicative that none is wholly satisfactory. Ammonium acetate is a widely used reagent both for the extraction of exchangeable ions and for the It has been used because it makes possible both determinations on one determination ofthe exchange-capacity. +

152

SALINE A N D A L K A L I SOILS CHEMISTRY (ARID ZONES)

sample.However,its use has serious drawbacks.First,CaCO, is quite soluble in NH4Acand leads to high values for extractable Ca and low values for the exchange capacity.Second,while NH,will not remove all of the Na+ from zeolites,it does remove some. At present, it does not appear that N a derived from zeolites should be included in calculating the E.S.P.for alkali soils.Finally,there may be non-exchangeableN a + in alkali soils which dissolves in NH,Ac but not in water leading to high values for the extractable sodium. In view of the complexities,it will be clear that the best method for extracting alkali soils will be some appears compromise method. At the time of writing,the procedure recommended by BOWER and HATCHER to be most satisfactory.This involves the determination of the exchange capacity by saturating the soil with N a + at p H 8.2,washing out the excess salt with alcohol,and extracting the N a with Mg(Ac),. The extractable N a is determined in a separate Mg(Ac), extraction ofthe soil,and the exchangeable Na' is calculated from this by subtracting the water soluble sodium which has been corrected for negative adsorption. It is believed that the use of Mg(Ac), in these procedures minimises the errors discussed above. For many years N a + analyses have been run with the use of a flame photometer. More recently electrodes have become availablewhich are sensitive to Na+,only slightly so to K + and H + , and insensitive to Ca++ and Mg++. Such an electrode may be used in place of the glass electrodein a p H meter assemble to measure N a + ion activities.Ifthe activity coefficientof Na+ can be estimated,the concentration ofN a + can be found. Alternatively,procedures can be devised in which the measurements are made in an excess of some standard reagent and the concentration read from an appropriate standard curve. Methods for the determination of exchangeable N a and the exchange capacity employing this electrode have been devised (BOWER, 1960). +

+

REFERENCES USSR,Collection of articles, Editors: I.V. Tiurin,I. N.Antipov-Karataev,M.G.Chydgevski,Moscow (in Russian). ACADEMY OF SCIENCESUSSR (1958), Amelioration qf alkali soils, Collections of articles,Editor: I.N.AntipovKarataev,Moscow (in Russian). ACADEMY OF SCIENCES USSR (1956), Takyrs of west Turkmenia and their development, Collection of articles, Moscow (in Russian). BABCOCKL. K.,CARLSON R. M.,SCHULTZ R. K., and OVERSTREET, R. (1959), A study of the effect of irrigation water composition on soil properties,Hilgardia. 29, 155-64. BIGGARJ. W., and NIELSEND.R. (1963), Miscible displacement. V. Exchange Processes, BOLT G.H.(1955), Ion adsorption by clays,Soil Sciences, 79, 267-76. BOLTG.H. (1961), The pressure filtrationof colloidal suspensions,II.Kolloid Z.,175, 144-50. BLACKC.A. (Ed.) (1965), Methods of soils analysis,ASA Monograplz, Madison,Wisconsin. BOWER C. A. (1959), Prediction of the effects of irrigation waters on soils, Proc. UNESCO Arid Zone Symposium, Salinity Problems in the Arid Zones,Tehran,Iran,215-22. BOWER C. A.,and HATCHER J. T. (1962), Characterisation of salt-affected soils with respect to sodium, Soil Sciences, 93,275-80. BOWER C.A.(1960), Sodium electrode and its use for salinityinvestigations,7th Intern. Congress of SoilSci.,

ACADEMY OF SCIENCES USSR (1953),Ameloriation of Alkali Soils of the

II, 16-21. BROOKSR.H., GOERTZEN J. O.,and BOWER C. A. (1965), Prediction of changes in the compositions of the dissolved and exchangeable cations in soils upon irrigation with high-sodiumwaters,Proc. Soil Sci.Soc. Amer., 22, 122-4. (1967), Laboratory Procedures, Series 510, Land ClassiJication Techniques and Standards,Part 517, 165. DUTTGORDONR.,and DONEEN LLOYDD.(1963), Predicting the solute composition of the saturation extract from soil undergoing salinization,Proc. Soil Sci. Amer., 27, 627-32. DYER KENNETH L.(1965), Unsaturated flow phenomena in Panochesandy clay loam as indicated by leaching of chloride and nitrate ions,Proc. Soil Sci. Soc. Amer., 29, 121-6. EGOROV V.V. (1954), Suline soils and their development,Academy of Sciences USSR, Moscow (in Russian). GEDROYZ K.K.(1917), Saline soils and their amelioration, M u g . Opytnuju Agronomya, Book 2-4 (in Russian).

BUREAU OF RECLAMATION

153

IRRIGATION, DRAINAGE A N D SALINITY

GEDROYZ K.K.(1928),Alkali soils,their origin property and amelioration,Nosovskaju s. h. op. st. vipusk,46 (in Russian). KELLER N.D.(1957), The IJrincQJles of chemical weathering, Lucas, Columbia,Missouri. KELLEY W.P. (1951), Alkali Soils, Reinhold, New York. KELLEY W.P.,CHAPMAN H.D., and PRATTP.F.(1961). Effect of plant growth on salts of irrigated soils, Soil Sciences,91, 103-12.

KOVDA V.A. (1 965), Alkaline sodu-salinesoils, Symposium on sodic soils,Budapest,T o m 14,Supplementum. KOVDA V. A.(1946), Chapter 2 (1947), Origin and regime of saline soils, Moscow-Leningrad (in Russian). KOVDA V. A. (1947), Saline and alkali soils,Academy of Sciences USSR,Moscow-Leningrad (in Russian). KOVDA V.A. (1959), Sketches OJZnature undsoilsof China,Academy of SciencesUSSR, Moscow (in Russian). KRISHNAMOORTHY C.,and R.OVERSTREET (1950), A n experimentalevaluation of ion-exchangerelationships, Soil Science,69,41-53.

LAGEILWERFT J. V. (1964), Extraction of clay-watersystems,Proc. Soil Sci. Soc. Amer., 28, 502-6. MOLEN W.H.VAN DER (1956), Desalinisation of saline soils as a column process, Soil Science, 81, 19-27. SCHULZ R. K.,OVERSTREE R.,and BARSHAD I (1964), Some unusual ionic exchange properties of sodium in certain salt-affectedsoils,Soil Science, 99,161-5. us SALINITYLABORATORY (1954), Diagnosis and improvement of saline and alkali soils,Ag. Handbook, 60, U S D A , Washington, DC.

us SALINITY LABORATORY (1958), The exchangeablecations in soilsflooded with sea water,Thesis,Wageningen. VILENSKY D.G.(1924), Saline soils, their origin, composition and improvement,Novaja Derevnia, Moscow (in Russian). WHITTIG L.D,, and JANITZKY P.(1963), Mechanisms of formation of sodium carbonate in soils,I.Jour. Soil Sci.,14,322-33.

154

6.Landscapes in Relation to Irrigation,Drainage and Salirnitu" Analysis ofthe pattern of the migration,differentiation and accumulation of salts in natural waters and soils shows that irrigated territories in arid zones differ very widely as regards geochemical and hydrological features. Since the long-term consequences of irrigation depend directly on these features,it is a mistake to approach problems relating to the construction and operation of irrigation systems from a routine, topographical angle. The irrigation of territories-especially large ones-involves not merely providing and distributing water on the fields. It is a complex operation,calling for the solution of technical problems of hydrology and geochemistry,in addition to biological questions relating to the production ofa high agricultural yield. Thorough preliminary examination of the territory is essential in order lo forecast what changes in the salt and water regime of the soils may occur after the beginning of irrigation, and to decide what kind of land improvement and prevention measures to use in order to ensure good fertility of irrigated soils. An all-roundsurvey of territory selected for irrigation,combined with an estimate of amelioration prospects, provides a basis for the selection of areas offering the best physico-geographicalconditions,which can be developed most cheaply without installing drainage, or with the lowest drainage and leaching costs. It is possible to classify landscapes in relation to the salinity hazard and the drainage requirement as a preventive measure.The criteria for such a classification is given below. It is mainly based on investigations carried out in the USSR.

A. THE ROLE OF NATURAL CONDITIONS IN THE FORMATION OF

SALINE SOILS Saline soils are formed in places where the amount of salts accumulating is greater than the amount removed. Since the migration of salts in the weathering crust and in soils occurs mainly through salt solutions, the processes of salt accumulation are governed first and foremostby the water balance of each particular area. The main condition ofsaltaccumulationin soils and solonchakformationis thepreponderance ofevaporation over drainage.Hence all conditions and factors which increasethe evaporation of groundwaters having little drainage will facilitatethis process. Both the water balance of the area and the ratio of evaporation to drainage depend on climatic, geomorphological,topographical,hydrogeological and biological (vegetation) conditions. Moreover, a vital part is played by the economic activities of men. 1. The influence of climate

Ifwe put the areas of saline soils on a map showing climatic features,we seethat saline soils most often occur in regions with a pronounced hot and dry climate,where the evaporation of surface and, in particular, of groundwaters exceedsthe run-offeither all the year round or,at any rate,during certain seasons,The water balance in these areas is controlled mainly by evaporation from the soil surface.The drier the climate,the greater the importance of evaporation in the water balance of the region and the smaller the importance of run-offand drainage. The areas where such climatic conditions are most pronounced are the deserts of Central Asia, Iran,the Caspian basin, Arabia,North Africa,Chile and Peru. As indicated by the figures given in Table 6.1,the characteristic features of such areas are aridity,high temperature,low humidity level and high evaporation capacity for a large part of the year. Solonchak soils are most widespread in tropical and sub-tropicaldesert regions, where the potential evaporation may attain 3000 mm per year; although semi-desert and steppe regions where the annual evaporation attains 1000-2500 mm may also be characterisedby pronounced salinisationprocesses.However, solonchack soils may be formed even in the forest steppes of Siberia and Manchuria, owing to the fact that *This chapter was edited by V. A. KOVDA from his o w n manuscript with contributions by V. V.EGOROV and N.S. HILU M

155

IRRIGATION, DRAINAGE AND SALINITY the total mean annual evaporation may assume large proportions-from 500 to 1000mm;and also in the monsoon tropics ofPakistan and India,where the dry winter season may sometimes be conducive to a strong soil salinisation.

Table 6.1. Main climaticfeatures of the saline soil regions of the USSR

Temperaturein "C Landscape mean annual July

January

period

Annual atmos.

(dayslyear)

precipi-

Frost-free

tations (in mm)

Relative

Annual

humidity of the air during the

evapor-

ability (in mm)

day,

measured over 2 months of the dry period Desert

Semi-desert Steppes Forest steppes

15-1 8 10-12 5-10 3-5

26-30 24-26 20-25 20-22

+51-1 -1O -51-1- 10 -5/-/- 15 -51-1 -16

200-240 180-200 150-180 120-150

80-100 200-300 300-450 350-500

20 and less 2000-2500 2&30 1000-1500 35-45 800-1000 40-45 500-800

The most pronounced cases of salinisation occur in the excessively dry and arid deserts of Western China, Arabia and Chile,where the soil salinity may attain 75%.These soils are covered by a friable or crystallised salt crust 30 to 50 cm thick or even more,These salts are mainly chlorides, sulphates and,very frequently, nitrates. The soils of deserts in Asia which have a cold (winter) climate-Kara Kum, Kizyl Kum, Southern Turkmenistan-also have a high salinity. Here, underneath the solonchaks,soil and groundwaters have a salt content of 350 g/l;the top soil horizon contains 15 to 25% of salts,including,as a rule, a large proportion of chlorides and sulphates of magnesium and sodium and also, occasionally,sodium nitrates. The solonchak soils of the semi-desertregion of the Aralo-Caspian depression are characterised by a lower salinity. Here the mineralisation of the groundwaters may occasionally be as high as 150g/l,but is more often ofabout 15to 20 g/l;and the salinityofthe top horizonsdoes not exceed 8%. The saltcomposition in this case does not include sodium nitrates;sulphates usually predominate. In the steppes of the USSR and the People's Republic of China solonchak soils occur only in patches, though they exist in Western and Eastern Siberia,in Manchuria and in the forest-stepperegionsofthe Volga area and of the Ukraine.However,the salinity is lower. The salt content of the groundwaters does not as a rule exceed 3 g/l,though it may on rare occasions rise to 30 g/l.The salt content in the top horizons of these solonchaksis between 0-5%and 1 %, rising occasionallyto 2%. Thepredominant elementsin the salt composition are soda and sodium bicarbonate,chlorides and sulphates playing a secondary role. This type of solonchak soil is sometimes arso found in the pushta of Hungary,in the pampas of the Argentine, in the cactus, xerophytic forests of north-westBrazil and north-eastArgentine, and in the savannahs of Africa. In northern forest zones and in the pluvial forests of the tropics,the formation of saline soils is possible only along the sea shores and in the tidal zone.In humid tropical conditions with a high groundwater table, salt accumulation is replaced by the accuinulation of sesquioxides,iron phosphates, silica and secondary clays,formed from solutions of silica and sesquioxides. The part played by climatic factors in the occurrence and location of contemporary salt accumulation processes is shown in Table 6.2. As the climate becomes less arid and continental,passing from Central Asia northwardsand down towards the coastal monsoon regions,so the processes of salt accumulation are attenuated. As we pass from the super arid desert areas towards the damp,temperate zones ofwestern,northern and eastern Eurasia, a regular succession of changes in the type of salt accumulation occurs :nitrate-sulphatechloride gives way to sulphato-chloride; this is then replaced by chlorido-sulphate,which in its turn gives way to soda-sulphateand pure soda accumulation,combined with silicates of alkali. 156

DRAINAGE A N D SALINITY

LANDSCAPES-IRRIGATION,

Table 6.2. Characteristics of accumulation processes in Eurasia, in relation to nutural conditions ~~~

~~~

~~

Max. quan- Typical

Salinisation

tity of

Conditions

Residual Maximum mineralisation of salinisation waters (in g/l) of sedimentary rocks river ground lake

of irrigated soils

Desert

common

25-75

20-90

200-350

350-400

com-

soluble pounds salts in top horizons of solonchaks (in %> NaCl KNOs NaNO,

widespread

MgCl2

MgSO4 CaSO, CaCl, Semi-desert frequent

Steppes

rare

10-30

100-150

300-350

5-8

NaCl NazSOc CaSO, M~SOI

often found

3-7

50-100

100-250

2-3

Na2SO4 NaCl

rarely found

Nazco3 NaHCO,

Forest steppes

none

0.5-1’0

1-3

10-100

0-5-1.0

Forests

none

0.1-0.2

0-1

none

none

NaHC03 Na,CO, Na2SO, Na,SiO,

unknown

RZO,

none

sioz

2. The influence of geomorphological,hydrological aiid topographical conditions

The existence of a hot,dry climate is not in itself enough to set up salt accumulation and cause the formation of saline soils.When the groundwaters lie deep down,the moisture evaporated from the soil does not exceed the total of the atmospheric precipitations. With a deep groundwater table (more than 10m) salinisation does not occur in the soil despite the dryness ofthe climate.In each of the climatic zones outlined above,salt accumulationis possible only under specificgeomorphologicaland reliefconditions.There is in the orography of all deserts which constitute regions of contemporary salt accumulation a very important specific feature: these deserts lie in vast deep depressions,wholly or partially encircled by mountain chains or uplands,at a level, usually, hundreds or even thousands of metres below them. These features are characteristic of the solonchak deserts of Chile and Peru and,to some extent, of the Acatan desert;also of the saline desert of Argentina-the ‘Salina Grande’-and the ‘Dashti-i-Kavir’ in Iran. The deserts of Central Asia (AraloCaspian depression,the Turufan depression,Lop Nor, Takla Makan) are also typical in this respect. From the encircling mountains an ancient underground water flow down into the continental or coastal depression,carries dissolved salts dating back to the geologicalformation of the mountains and depression. The groundwaters of the depression itself have been subjected,throughout long geological eras, to strong hydrodynamic pressure and geochemical influencefrom the deep underground waters. Some deep ancient underground waters are highly mineralised, while others are fresh. In large, deep hollows,valleys, river deltas and in regions marked by tectonic cracks,rising underground waters,wedging out or drawing near to the surface become a very important factor in salinising the waters,residual deposits and soils in depressions. 1 57

IRRIGATION, D R A I N A G E A N D SALINITY

The influence of these ancient underground waters is all the greater when they are under high pressure,so Corining a continuous rising flow to the surface.This flow is neutralised to a large extent both by the low permeability of the horizons and also by evaporation;but it is uninterrupted, and so exercises an enormous influence. Thus for instance,according to calculations,‘artesian’supplementary feed of this kind under the conditions in Uzbekistan may amount,inside the Hungry Steppe depression,to as much as 100 maper square kilometre per year. As a result of continuous evaporation, the rising waters become strongly mineralised in the top (30 to 50 ni) of the aquiferous stratum and salinise the residual rocks through which they pass. Figures 6.1 and 6.2show data illustrating these phenomena for the Ferghana depression and the Hungry Steppe in Uzbekistan.In both cases,mineralised waters give place to fresh,artesian waters below a depth of 50 to 70m. O

10

Solid residue g/l 12 14 16

Depth into groundwater stratum in metres

Fig.6.1.Mineralisation ofthe subterraneanwaters of the Hungry Steppe (after RESHETKINA)

Fig.6.2. Mineralisation ofthe subterraneanwaters of the top layer of the Ferghana basin (after

RESHETKIN A) 1. saline water, solid residue more than 5 g/l 2. brackish water, solid residue up to 5 g/l 3. fresh water, solid residue up to 2g/l

In the Azerbaijan depression,on the other hand,ancient Subterranean waters have a concentration between 70to 100 g/1 and extend to a depth of 300to 500m.A thin layer of fresh alluvial and irrigation water floats, as it were,on top ofthe underlying ancientbrines which,in tectonic areas,frequentlyspillout on to the surface in the form of ‘squirts’(grifony) or mud volcanoes. Thus, the most general and essential condition for the contemporary formation of active (as opposed to relic) solonchak soilsin dry,hot climatesis the existenceofvast depressionshaving,withintheircircumference, deltas,low river, marine or lake terraces, or some kind of watercourse or trough to act as centres of salt accumulation. It is importantto mention another generallaw-namely thetendency ofareas ofcontemporary saltaccumulation and large stretches of saline soils to concentrate along the left banks of vast ancient alluvial plains, within the limits of their first, second and third terraces,This phenomenon is observed, for example,along the valleys of the Dnieper,Don,Volga, Syr-Darya,Amu-Darya and Hwang-Ho rivers, amongst others,at various sections along their course. This tendency is a general one, it is observed also in other continents. There are also,of course,quite a number of cases of saline soils being formed under other conditions,but the salinity is never as high as along left-bankriver regions.

158

DRAINAGE AND SALINITY Inside depressions, deltas and terraces, salinisation processes are concentrated in secondary hollows. A relatively small difference in altitude (1 to 2 in)is sufficient to make the soil salt content completely difFerent from adjacent land. It is in fact in mesorelief hollows like this that solonchaks and solonchak-likesoils are most often formed;whereas the soil on the higher parts of the macro-and mesoreliefis invariably less saline. The effect ofmacro-and mesoreliefis thatthe groundwatersin the lower-lyingparis are closer to the surface and therefore more liable to evaporation. The efiects ofmicrorelief (differences< 30 cm)in the formationof solonchak soilsvary widely.Evaporation of groundwaters is always more intense on the raised parts of the microrelief than in the dips,owing to the factthatthey receiveless moisture from atmosphericprecipitations,wann up more easily,are better ventilated and dry out more quickly. In the dips,rain, snow and irrigation water accumulate and the soil warms up less easily. It is therefore as a rule in the higher-lyingparts that salinisation processes begin. LANDSCAPES-IRRIGATION,

Table 6.3. Elements of the water-salt balance and solid discharge of the Aralo-Caspian depression

Solid discharge

Water

discharge Elements of the

(km3)

% in lo6

balance

Chemical discharge

%

in loa

%

tons

tons

%to solid

Harmful salts in loG tons

%

discharge Annual total discharge Including the following amounts from the rivers: Volga

Amu-Darya

%to

solid discharge

355

100

326

100

86

100

26

21

100

6

256 42

72 12

32 168

10 51

50

58 21

156 11

9 5

43 24

28

18

3

An example of the immense geochemical significance of chemicalriver discharge is provided by the rivers of the Aralo-Caspiandepression,which forms part of the vast Eurasian inland drainage basin. Now, as in former geological eras,the formation of continental residual deposits, groundwaters and soils within this depression is governed by the action of the main rivers there: the Volga, Amu-Darya,Syr-Darya,Kura, Ural, Terek (cf. Table 6.3). As shown by Table 6.3 the formation of sedimentary contiiiental deposits is due mainly to the discharge ofthe Volga and Amu-Darya.The chemical discharge of the Volga,which drains a vast area of the Russian plain, salinises the deposits and soils of large regions of Central Asia infhenced by the transgression of the Caspian Sea.Thetotal annual solid and chemical discharge in the Aralo-Caspiandepression is approximately 412 million tons which,for a volumetric weight of 1.4,gives 0.3 km3 of deposits. The ratio of the chemical to the solid discharge gives an indication of the mean salts which may be found in flood deltoid deposits. It may amount to 156 %in the deposits of the Volga and 11 %in those of the bu-Darya.After deduction of calcium carbonate and calcium sulphate,this gives the salt content of the alluvium of the Volga as 28 %, that of the Amu-Daryaas 3 %. In practice, soils and ground with such a high salinity are found only in small areas of the Aralo-Caspian depression. Indeed,the surprising fact is not that salt accumulation occurs in deserts of inland drainage basins,but is that the salinity in the deposits and soils of deserts is not nearly as high as it might be. The main reason for this lies in the separationofthe mechanical and chemical deposits brought down by contemporary rivers into the Aral and Caspian Seas. A large proportion of the easily soluble salts carried by the river waters separate from the mechanical deposits accumulatingin flood plains and deltas and pass into lakes and seas where they raise the mineralisation.From lacustrine and maritime basins,a large proportion ofthe saltspass into shallow bays and lagoons where they salinise the coastal area. In deltas, on the other hand, on newly formed dry land, most of these salts accumulate in the groundwaters. This explains why,despite the large quantities of salts migrating continuously into the continental depressions of the Central Asian deserts,there are nevertheless large areas of fertile non-saline or slightly saline soils there. 159

IRRIGATION, DRAINAGE A N D SALINITY In absolute deserts,special importance is attached to the process of the transportation and redistribution of salts together with deluvial waters with little local discharge. After sporadic downpours there appear, temporarily, surface and underground waters which wash down the residual,saliferous rocks on the slopes and carry along easily soluble salts,together with the silt.The silt deposited by these waters, after the solutions themselves have evaporated,is enriched in salts. Over long geological periods, this continuous inflow and evaporation of deluvial solutions causes the formation of solonchaks on the low-lying parts of talus deposits,or on the alluvial fans of local proluvium (Kazakhstan,Kirghizia,Azerbaijan). The inflow of solutions with low salt concentration often provokes only solonetzcharacteristics,so that it looks as if development into solonchaks can be avoided. Salinisation of deluvial slopes is common in the deserts of Western China and Chile,where the rocks which form the mountains are highly saline,and the climate is exceptionally dry. 3. The significance of hydrogeological conditions

The above-describedclimatic,geomorphological and topographical conditions are,however, in themselves not sufficientto set up a solonchakprocess and cause the formation of saline soils.Contemporarysalinisation occurs only when the aforesaid climatic and topographical features are found in combination with a high groundwater table, and where the groundwaters are stagnant and have little natural drainage. When the groundwaters lie close to the surface,but have a good drainage,salinisation cannot take place. Among the factors of the origin and development of active solonchakprocesses it is essential,therefore,to include:mobility, depth and balance of the groundwaters.In deserts,however,we frequently come across ‘dry solonchaks’,i.e. saline soils with a very deep-lying groundwater table (between 20 and 100 m). This merely indicatesthat the groundwaters which,in the past,were responsible for salt accumulation have either flowed away or dried up and the soil salinity has been conserved due to the desert conditions. Regions with natural drainage,i.e. high plains with subjacent sandy gravel horizons and areas intersected by a hydrographical network,are characterisedby deep-lying,freely circulating groundwaterswith the result that solonchak processes do not occur. Stagnant groundwaters lying at a depth of 1 to 3 m (depending on the mechanical composition of the ground) are subject,in a hot dry climate,to intensive evaporation leading, in turn,to the development of solonchak processes. In such cases, a constant lateral inflow occurs. The groundwater level above which there will be intensive evaporation through the soil surface leading to solonchak formation is known,at POLYNOV’S suggestion,as the ‘criticaldepth‘.When groundwatersremain for a long time at this depth,they also undergo a gradual increase in salinity.The critical depth will be greater,the drier and hotter the climate and the higher the salinity of the groundwater. The salt accumulation process in groundwaters is not confined to the top strata of these waters, but extends down into the depth of the aquiferous horizon, as far as 80m.Contemporary salt accumulation processes constitute one of the most important factors contributing to the creation of highly mineralised groundwaters extending tens and even hundreds of metres down into the aquiferous horizon. The fact that the salts spread into the depth ofthe aquiferous horizon is due to convection,diffusion,and to the downward movement of the heavy salt solutions. In the conditions of Western Siberia,intensive salt accumulation in soils begins when the groundwater levelis between 170 and 200 c m ;in Central Asia,at groundwaterlevels of between 3 and 3.5 m. The amount of salt accumulation in soils increases as the climate becomes drier and the mean annual temperature rises. The accumulation of easily soluble salts is accompanied by even more intensive accumulation of gypsum (Figs. 6.3and 6.4). The soil salinity for various layers and for the same depth of the groundwater table,increases with the mineralisation of the water (Fig. 6.5). When the absolute accumulation of salts in the soil is small,the predominating elements are bicarbonates and carbonates of alkali;this constitutes the alkaline phase of slightly mineralised groundwaters and occurs most often in relatively humid saline areas (savannahs,monsoon, black earth and forest-stepperegions, the prairies of Manchuria and the USA, the Hungarian pushta, the pampas of Argentina). As the soil salinity increases,intensive gypsum accumulation occurs in the soil,accompanied by the neutralisation of alkali carbonates and bicarbonates. When this happens,the soil stratum is characterised mainly by sulphate salinity,and a high content ofgypsum and calcium carbonate.When the salinity is higher,and when intensive 160

LANDSCAPES-IRRIGATION,

D R A I N A G E A N D SALINITY Salt content in layer of soil 3 m thick tons/hectare 6500 r

concentration of grwndwoter g/L

-----

_.......-.

-....- A m u Darya (delta)

Casplan depresslon Kalonda (WSlberld ---___.-_-___Hungrysteppe _.___._ Vaksh Barabo

Fig. 6.4. Relation between soil salinity and the groundwater depth

Fig.6.3. Relation between the depth of the groundwater table and the mineralisation of groundwaters Salt content kg per 2m x Im2prism of soil

'4

'Ø '8

I

I 8

I

/* Above groundwater

, '

30

Ø Ø

25

-

I /

/

_.-.-.-.-.-.-.0-20 c m

O

I

200

400

I

I

I

600 800 1000 Mineralization of groundwater m.e./l

I 1200

Fig. 6.5. Relation between groundwater mineralisation and salt accumulation in the soil 161

IRRIGATION, DRAINAGE A N D SALINITY and continuous salt accumulation occurs,sulphates recede and sodium chlorides begin to assume the leading position (Fig. 6.6).

I Content of salts kg per "prism of soil .CaSO, -**>Na2SO4

Reserve of salts in stratum above groundwater

II

Content of ions

10 15 20 25 30 35 Reserve of salts, kg per m 3

III Content of ions kg per 3.5 m x Im2prism of soil 50

-

40

-

CI /

J

/

O Reserve of salts, kg per 3.5 m x 1 m2prism of soil

Fig. 6.6. Salt accumulation in soils: I.initial (Baraba-West Siberia); II. moderate (Ferghana); III. strong (Hungry Steppe)

4. The role of plant cover

A large proportion of easily soluble compounds do not appear in the surface and underground waters until after they have passed through the biological absorption system of both primitive and highly developed plant organisms and the tissues of these organisms have become mineralised. VERNADSKY'S calculations showed that living substance is conducive to bringing about a deep transformation in the products of weathering and soil formation and also in the natural waters: some elements are absorbed by organisms in large quantities such as Ca, K,P,S,and other elements are little absorbed such as Na,M g and CI.The effect of living substanceis,in the end,to separate these two groups of elements: the fkst are retained,accumulatingin eluvial regions,in residual rock strata and in soilhorizons,while those of the second group form easily soluble compounds and pass with water streams,into the oceans or into undrained continental depressions where they accumulate as a result of the evaporation. Plant cover plays a very important part in the salt regime of soils and the formation of solonchaks;its effects are both complicated and contradictory.A thick cover of herbaceous plants reduces the evaporation of groundwaters from the soil surface because the roots absorb water through transpiration.This reduces the salinisation of the top soil horizon. After dying,the root system of plants enriches the soil in humus,which improves the soil structure and increasesits non-capillaryporosity. This in turn lessens the capillary conductivity and so decreases both the surface evaporation and the salinisation of the topsoil horizons: a good cover of mixed meadow grasses grown on fiood plains and deltas,i.e.in areas tending to natural salinisation,delays the salt accumulationfor a long time and prevents solonchaks from forming on meadows. Destruction of the plant cover changes this regime.The surface evaporation increases and solonchak soils are quicklyformed.Thisprocess can be observed onthe solonchakmeadows ofWestern Siberia,the meadows along the lower reaches of the Volga and generally in all deltas in arid zones. In areas used for grazing and in strips along the side ofroads which have lost their natural meadow plant cover,the sod and top horizon soon (within 15 to 20 years) become saline and turn into solonchaks. Cultivated plants in irrigated agriculture and more particularly perennial grasses used with correct crop rotation have the same qualities as a natural vegetation with respect to improvement of the structure and salt-water balance of the soil;the case is especially marked with alfalfa. However,saltaccumulationunderneaththe plant cover not only does not stop,but may even be intensified. The amount of groundwater consumed by plant cover may attain 2000"/year, and the plants absorb water, whereas almost all the salts are left in the soil solution and groundwaters.Salt accumulation by this means,however,occurs not in the very top horizons (as it does in the absence ofplant cover) but throughout 162

LANDSCAPES-IRRIGATION,

D R A I N A G E A N D SALINITY

the whole ofthe root zone and in the top strata ofthe groundwaters.For this reason,the rate of soil salinisation under a grass cover will always be much slower than when the soil surface is bare. After a certain point, the quantity of saltsin the groundwatersreaches suchproportions that it kills meadow plants;direct evaporation of the groundwaters takes place and the soil salinisation is speeded. Another point to be taken into consideration is the way in which vegetation affects the migration of salts by absorbing them and freeing them again when dying. Both the chemical composition and the amount of the mineral substances which are drawn through plants into the biological cycle vary widely. Northern meadow plants contain between 2 and 3% of ash; while the grasses growing on the Russian steppes,have an ash content of 5 to 8%. As regards the chemical composition of the ash, compounds of silica,sulphur,calcium,potassium and phosphorus predominate. Halophytic plants growing on semi-desertsand more particularly on solonchaks are characterised by a high ash content (up to 50%) with a relatively large accumulation of compounds of sodium and chloride, matched by a relatively small proportion of compounds of sulphur, silica, phosphorus, calcium and potassium. In

% of ash I

A

SiO,

Ash content in %

Fig. 6.7.Changes in the content of the mineral componentsin the ash of herbaceous plants in the USSR Figure 6.7shows that plants growing on steppes retain silica,sulphur,phosphorus and potassium sifting them out,so to speak,from the more mobile substances such hs chloridesand sulphates of sodium included in the composition of the ash of halophytes. Since the amount of chemical substancespassing from the continentsinto the oceansevery year is 26tons per km2,the mean annual amount of mineral substances absorbed and returned by plant organisms is ofthe same magnitude (about 30 tonnes per km2per year). Thus the chemical composition of river, lacustrine and groundwaters is governed,to a great extent,by the activity of plant organisms in their catchment basins. The influence of biogenic inflow of mineral substances into the soil horizons cannot,however, compete with that of tlie abundant quantities of salts which flow in from the groundwaters when the groundwater table lies close to the surface.When the groundwater depth is from 2.5 to 3 m , evaporation and transpiration may total 10000 to 20000 m3/ha per year and as a result the soil may receive from 100to 200tons/ha of chemical deposits a year while the biogenic inflow of salts amounts, even for halophytes,to no more than 1000kg/ha/year. Thus in soils with capillary activity both the accumulation rate and the chemicalcomposition of the easily soluble salts will be controlled,in the main, by the evaporation of the groundwaters, Plants cannot themselves salinise the soil as a whole;however,in automorphic soil formation (i.e. without groundwaters delivering mineral substance to the topsoil), they play a great role.Minerals absorbed by their roots in the deep layers are later liberated in the upper horizons and very valuable elements are therefore moved: potassium, calcium, sulphur, and phosphorus. At the same time, radical changes occur in the physico-chemicaland chemicalproperties ofthe soil.Underthe influence ofplant cover,alkaline,unstructured and infertilesoils formed after solonchak soils at early stages ofdesalinisation (takyrs,solontsy,solonetz-like soils) become non-alkalineand adsorbed sodium and magnesium are exchanged for calcium and potassium. 163

IRRIGATION, D R A I N A G E A N D SALINITY They are enriched by chemically fertile elements,acquire structure and favourable hydro-physical properties. This process is known as the transformation of solonetz into steppe-soils (‘steppification’) or autoamelioration.

B.

LAWS OF MIGRATION AND DISTRIBUTION OF SALTS O N CONTINENTS

In accordance with the nature of the processes of migration and accumulation of salts described earlier,a distinction has to be drawn between primary cycles,connected with the inflow of saltsfrom weathered strata of crystalline volcanic and plutonic rocks,and secondary cycles connected with the redistribution and transportation of salts in the sediments previously accumulated.The two processes differ not merely in regard to the form they take and the way in which they occur,but also in regard to the specific types of saline soils to which they give rise.Thus for instance,the formation of soda,borate and nitrate solonchaks is usually due to primary salt migration cycles in particularly arid climatic conditions. The formation of solonchaks is closely connected with the formation of sedimentary deposits of various types. Almost all kinds of deposits both on land (deluvium, proluvium,river and lacustrine alluvium) and in the sea (shore and sea-bottom)are,under certain conditions, characterised by salinity. The formation of saline soils,mineralised lacustrine reservoirs and new saliferous deposits in deserts and steppes is governed by contemporary geochemical processes; although these processes occur on the basis of earlier migration cycles of easily soluble salts.The migration process extends through mountain formations and the depressions encircled by them-which normally lie within the area of tectonic movement-and into the residual strata to a depth of hundreds and thousands of metres. Owing to the uplifting of mountain formations,the mechanical and chemical erosion, and the flow of surface and subsurface waters down into the depressions,the process ofthe geochemicalflow of salt solutions from those parts of the earth‘s crust is continued over periods of thousands and millions of years. Among the most important factors which-determine contemporary salt accumulation processes are the geochemical mobility of various types of salts,the degree of aridity of the climate and the hydrological and geomorphologicalfeatures of the particular region concerned.

1. Zones (areah)* of accumulation of chemical deposits in soils The size of geographical ‘areals’of contemporary accumulation of chemical salts will be greater (in wet as well as in dry regions) the greater the biological significance of the chemicals for organisms,the lower their solubility and the earlier they reach saturation point in surface and groundwater. The most extensiveincluding even humid regions in the north and the tropics-is the accumulation ‘areal’of secondary compounds of silica, aluminium, iron and secondary clays. The accumulation ‘areal’or calcium carbonate, though less extensive,neverthelessincludes dry savannahs,steppes,semi-desertsand deserts. Smallest of all, and confined exclusively to acutely arid regions having no natural drainage,is the accumulationzone of such highly soluble salts as KNO,,NaNO,, NaCI, MgCl,, MgSO,, CaCI,. Special mention should be made of alkali carbonates whose accumulation zones tend to be concentrated in non-gypsiferous forest-steppeand steppe regions and in regions with a tropic monsoon climate (Table 6.4). The chemical deposits, in the regions of accumulation,are formed in a certain order, according to the solubility of the various salts,and their accumulation ‘areal’. Each salt participating in the accumulation processes begins, after the saturation point is reached, to precipitate into a deposit which enriches both sediments and soils.At this moment, the outer limit of the accumulationzone of that particular compound is fixed.Subsequently, however,as each other more soluble salt reaches saturationpoint, accumulation of salts will be provoked not only by these new compounds,but also by the total of all the compounds which have already reached saturation point earlier. In every new zone into which chemical deposits are precipitated there will therefore be an accumulation of several compounds. * The word ‘areal’is used in Russian to denote a clearly defined large territory

164

DRAINAGE AND SALINITY According to both the horizontal and the vertical direction of migration and evaporation ofgroundwaters (i.e. both up through the soil profile and along the slope of the region), the following zones of precipitation of chemical deposits are formed :secondary clay minerals,SiO,and R,O,, CaCO, and CaSO,, easily soluble salts. LANDSCAPES-IRRIGATION,

Table 6.4. Areals of accumulation of products of weathering and soilformation on continents

Very extensive Extensive Small Very small

RzO3,Sioz,secondary clays

Regions of deluvium, proluvium, valley, lacustrine and deltaic alluvium,in all climates CaMg(C03)2, Nazco,, Regions of deluvium, proluvium, valley and deltaic CaCO,, CaSO, alluvium, depressions, bogs and lakes of savannahs, monsoon tropics, forest-steppesand steppes Na,S04,MgS04,NaCl Regions of valley and deltaic alluvium, depressions of dry steppes and deserts NaNO,, KN03,CaCI,, MgClz Central, driest parts of depressions in absolute deserts (sometimes Nazco,)

2. Migration and accumulation of salts on ap ‘ideal’inland endoreic area H o w then are the accumulation ‘areals’of the easily soluble products of weathering distributed over the continents?The answer to this important questionis,on general lines,as follows:Let us take a vast continent with an endoreic (inland) depression (Fig.6.8);the crystallinerock bed ofthis ‘idealinland region’is covered with residual deposits and weathering products, as a result of a lengthy geological process. The raised plateaux and mountain formations along the edges of the continent are covered by ancient eluvium comprising the most mature products of weathering,bound together by silica,alkali-earthsand alkalis (in other words,by more mobile elements) and enriched in some degree,by slightly mobile, inert compounds.

Fig. 6.8.Diagram of differentiation of compounds during salt accumulation on continents

As we know,an ancient weathering crust of this type consists,mineralogically speaking, of kaolinitic or alitic rock, enriched by residual quartz and sesquioxides. Ancient continents such as this have already undergone several weathering cycles and several accumulations of mechanical and chemical deposits; SO that the contemporary soil formation process is based predominantly on residual rocks belonging mainly to the Quaternary period. As a result of a lengthy process of weathering and soil formation,the inner,endoreic part of the ‘ideal continent’will be covered by accumulationsofvarious types of compounds.The differentiation and distribution of these compounds will be governed,generally speaking,by the following laws. The mobile compounds of silica and part of the sesquioxides carried out of the eluvium region soon attain saturation point in solution and form a large area of accumulation of secondary compounds of silica and secondary clays (chiefly montmorillonite in type). Lower down the slopes of the continent there forms an 165

IRRIGATION, D R A I N A G E A N D SALINITY extensive area of accumulations ofcompounds of carbonates of calcium and carbonates of sodium.This area is easily recognisable on account ofits allcali soils,loess-likeloams,limecrusts and deposits of meadow marl. Inserted,as it were, into the area of carbonate accumulations is an area of accumulation of gypsums and sodium sulphate,forming the famous sulphate deposits found in Transcaucasia, Central Asia, Argentina and Chile. The existence of these deposits is due to precipitation of Sulphates from solutions which reach szturationpoint by migration and evaporation. This is connected,lithologically,with sediments of diluvium and alluvium in dry deltas and with river and lacustrine alluvium. The lowest and least-drainedparts ofinland depressionsare covered with accumulationsofthe most mobile and highly soluble compounds-sodium chlorides and nitrates,potassium,magnesium,calcium and magnesium sulphates.Chemical deposits of these compounds are found in the lakes of dried-updelta plains, lake terraces and inundated areas-at the point where evaporation of the underground and surface waters ceases. At the same time,the area of accumulation of nitrates will be the smallest and most sporadic,inserted,as it were, into the chloride accumulation area. As a result of the geochemicalflow of weathering products, areas of salt accumulation are thus formed according to the solubility: towards the centre of the depression the most soluble salts are found. The above scheme of the geochemical zones of Contemporary salt accumulation was worked out for the conditions in the Aralo-Caspian depression in the USSR.Subsequent observations,however,in the steppes and deserts of Central Asia, the Sahara,Chile,Argentina and Australia confirmed the validity ofthis scheme which may be assumed to be universally applicable.The most clearly pronounced geochemical salt accumulation zones have been observed in Chile where,over a distance of approximately 700km,from the slopes of the Andes down to the endoreic depression,there stretches a series of zones,one after another,in the following order from east to west: silicates and carbonates of calcium;sulphates (includingthick strata ofthenardite) ;chlorides;and in the lowest lying parts ofthe desert without natural drainage accumulationsofnitrates, which are used for producing fertilisersand saltpetre.In practice,ofcourse,this schememay prove incomplete or be subjectto substantial changes.Thusfor instance,in regionswhich slope down towards the ocean,some of the geochemical zones may be eliminated entirely,owing to the existence of a free flow.The same effect may be produced when the climate is very moist and there is an extensiveriver system facilitatingthe removal of the salts.This is precisely what happens,for instance,in India.The run-offfrom the Himalayas leads to the formationin Western India oftwo distinct geochemicalzones ofcontemporary salt accumulation:a large zone of secondary silicate and calcium carbonate (known as Kankar) and a zone of accumulation of carbonates,bicarbonates and sulphates of sodium.This second zone extends over a vast surface of alluvial plains, formed by the deposits of the Ganges,the Indus and innumerable mountain streams.But farther south and at lower altitudes,owing to the fact that the Indian sub-continentlies within the humid tropical belt, salt accumulation is not very marked and such easily soluble salts as chlorides and nitrates pass into the oceans. In the delta of the Ganges,round Calcutta,not even calcium carbonates accumulate.It is only in the delta of the Indus,lying in the arid tropical climate belt, that salt accumulation follows the zonal pattern completely and chlorido-sulphatesalinity is widespread. In the Amazon delta, only distinct accumulations of sesquioxides and deposits of secondary silicate are found and in the delta of the River Plate in Argentina only accumulationsofcarbonates and bicarbonates of sodium. In the Nile delta, on the other hand, and the region between the Tigris and Euphrates rivers, the geochemical salt zoning corresponds fully to the ‘ideal’scheme. A perfect example illustrating the laws governing the distribution of salts on continents is Australia,where conditions approximate very closely to those of the ‘ideal’endoreic region.

C.

BASIC CRITERIA FOR ASSESSMENT A N D CLASSI FI CATION OF TER RITO RI ES

The criteriafor assessing territoriesin relation to amelioration are many and diverse,and they differ according to practical conditions in each case. W e shall confine ourselves to listing a few of the most important and .decisive. 166

L A N D S CAP ES -IR R I G A T I ON , D R A INA G E A N D S A L INI T Y 1. Aridity of the climate

The main climatic features which have to be taken into accomtwhen assessing the danger of salinity and the need for leaching and drainage of irrigated soils are drought,temperature,the annual evaporation and the quantity and distribution of atmospheric precipitations. It is essential to distinguish between at least three main types of climate. (a) Hot, dry (ubsolute) deserts There are practically no atmosphericprecipitations;evaporation predominates and irrigation constitutes the only means ofcropping.In such territories,the soil-formingrocks,soils and groundwaters are aIways saline, often extremely (deserts of Chile and Peru, North Africa, Central Asia). Irrigation,in these conditions, causes radical changes in the geochemistry and hydrology of the soils;as a result,prevention of salinity is almost always necessary. (b) Semi-deserts,dry steppes und suvunnahs Examples of these regions are the steppes of the southern Volga area and Kazakhstan,the sierozem plains of Uzbekistan, the shores of the Mediterranean, the southern transitional regions of the Sahara, Central Chile and the north-westernparts of Argentina. These regions have an annual rainfall of 150 to 300mm. Where natural drainage exists, this leads to desalinising processes; irrigation plays an important part as regards the hydrological regime, often intensifying natural desalinisation. Where natural drainage is poor, however, saline soils and groundwaters will also exist and strong secondary salinity may set in. (c) Steppes,pushtu,pampas, suvunnuhs und prairies These regions are subjected to periodical drought,but have a fairly large atmosphericprecipitation (500 to 700mm per year). Such territories are generally characterised by non-salineor, occasionally,slightly saline soils and groundwaters; although in this zone also small patches of saline or alkali soils are sometimes found. Irrigation, in this case, constitutes an addition to the atmospheric precipitations and, as a rule, intensifies the soil desalinisation. There may sometimes be, however, secondary waterlogging of irrigated lands and alkalinity may increase in some places.

2. Geomorphology, relief and lithology

The second set of data for assessment of a territory with regard to the salinity hazard and the need for drainage,relatesto the geomorphology,reliefand lithology ofthearea.Itis essentialto investigate the genesis and trend of developmentboth ofthe landscape as a whole and of its main sections.This provides the basis for evaluating the natural drainage and forecasting the effects ofprolonged irrigation on groundwaters and on the geochemistry of salts. The value of geomorphological study,as opposed to the purely topographical investigations,which are, of course,essential for any programme of irrigation or drainage,lies in the insight it gives into the possible distribution of soil and young sediments both in depth and regionally. It may assist the soi1 surveyor by suggesting the ruisons d'être for the distribution of soil types and shallow geological profiles, as for example in relation to prior streams in an alluvial region of anastomosing streams which may account for the depths and types of gravel,sand and silt found beneath the surface and having a profound effect on irrigation and drainage problems; it permits the ready integration of ideas as to the regional picture and thus affords an invaluable preliminary basis for planning. It is indeed perhaps in the earlier stages of a programme rather than the laterthatthe geomorphologist is most helpful.For example,the determination ofthe depth to which soil profiles will be excavated during the soil survey may best be gauged after a reconnaissance in which the geomorphologistwould play a major role.Where this is not done,some fundamental problem may have to remain unsolved because the profiles taken were too shallow in criticalareas or provision for shallow drilling was not made. Although some action might be takenpost hoc,it has proved difficult to achieve once the field parties and organisation have been dispersed. It may justly be said of geomorphology that,whereas the engineer must regard each site,whether for a dam,channel alignment or drain, as unique, and therefore requiring a carefully planned pattern of testing by drilling or other means,geomorphology permits somevalid generalisations causing similitudesto be made. 167

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which in turn allows a better statistical approach to the testing programme,e.g. spacing and alignment of drill-holes; closeness of net of stations for topographic work;preliminary notions as to suitability of damsites, level of flooding,location of possible geological faults and so forth. Irrigation systems usually have rivers as the source of water supply and the installation and operation of drainage systems are cheaper in flat areas.This explains why most ancient irrigation systems and many new ones have been installed or are to be installed on large alluvialand delta-alluvialplains,marine and lake deltas, continental (dry) deltas,alluvial terraces of various levels and deluvial-proluvialfans of mountain streams. Although the relief,in all these cases,is more or less flat,there are great differences as regards the lithology of the underlying rocks,conditions of natural drainage and groundwater circulation,and also the processes of salt accumulation.Marine deltas,especially in places where there are strong tides,are the leastfavourable from the point ofview ofthe development ofnatural salinisation and intensive artificial drainage is therefore necessary. Conditionsin lake deltas,continentaldeltas,lower river terraces,round the edges ofdry deltas and alluvial fans are somewhat more favourable;but they too are usually subject to intense natural salinisation in arid climates and require intensive draining,when irrigated,in order to guarantee free circulation ofgroundwater flow. Ancient alluvial-deltaicplains and also high river terraces which have been raised to a considerable height by the neotectonic activity may have deep-lyinggroundwaters. In an arid climate,however, such landscapes are always characterised by residual soil salinity and mineralised groundwaters.Ifthe underlying rocks consist of pebbles or porous sands,irrigation will often intensify the desalinisation and the irrigated soils will be very fertile.Artificial drainage is of course not necessary in such landscapes. In ancient alluvial plains and ancient deltaic plains (in a hot,desert arid climate) formed oflittle permeable loams and clays,on the other hand,irrigationwill very frequentlycause the saline groundwaters to approach criticallevel,leading to strong secondary soil salinity (and even waterlogging). For the development of such territories,provision will therefore have to be made for drainage at somejuncture or else at a certain fixed interval after the beginning of irrigation. With the progress ofindustrialisation,the technical improvement ofirrigationequipment and the development of high dams and pumping plants, it has become possible to supply irrigation water to geomorphological regions where this was not possible in the past (high ancientterraces,fore mountain plains,mountain slopes and high watershed plains). The following general rule must be borne in mind:the higher the territory (or section of territory) to be irrigated,the more ancient it probably is and the more likely the underlying rocks contain pebbles and sand.This also means that it will be more strongly dissected by the hydrographical network, will have better natural drainage conditions and will,therefore,be less liable to secondary salinity when irrigated. There can be, and of course are,exceptions attributable to the geological and geochemical history of the landscape. As a rule,however,high watershed and fore mountain plains,high ancient terraces,mountain slopes and diluvial plains have natural drainage. 3. Hydrological and soil conditions

The hydrological conditions of the landscape constitute an extremely important criterion for assessment of a landscape in relation to salinity hazard. It is especially worthwhile to investigate the interrelations ofsurfaceand subterraneanwaters (groundwaters). It is essentialto determinewhether the existing network of rivers, streams,ravines,etc.,constitutes a drainage system or whether,on the contrary,the hydrographical network feeds the groundwaters through seepage from the water courses.In the first case,the groundwater balance is regulated by natural drainage and it will be possible to irrigate the whole of the area,or part of it, without drainage system.In the second case,the groundwaters of the territory are thrust up and their balance is regulated by evaporation and transpiration (in deltas, for example) :natural soil salinisation processes prevail, so that provision has to be made for drainage from the outset, using deep drains. There may, in absolute ancient deserts, be a third case,when the groundwater table is very deep (50 to 75 m), seepage from rivers does not affect the groundwater feed and the dry subsoils constitute an almost unlimited reservoir able to soak up the seepage waters from the irrigation system without any danger of salinity or waterlogging. (Examples of this phenomenon are found in the deserts of Latin America,North

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Africa and Central Asia.) In such cases,irrigation without drainage systems may continue 20 to 40 years without any signs of secondary salinity-which may in fact not occur for a hundred years (especially if the canals are watertight). However,if the loss by seepage is large and the internal drainage ofthe irrigation project small,the highly saline groundwaters will end by rising to the surface and the installation of drainage will become necessary. All the above comments refer to normal irrigation waters,with mineralisation of 0.2to 1.0g/l. If,in dry climates and particularly in hot deserts,the waters are mineralised (4to 5 g/l)irrigation without drainage will not be possible exceptin highly permeable sandy and stony soils.It will be essential to installan extensive network of drains simultaneously with the irrigation system and to apply a leaching regime to remove salts resulting from the evaporation of irrigation water. A no less important factor,when assessing territories with a view to irrigation, is the character of the underground waters ; their origin (deep artesian,lateral inflow,local seepage) ; chemical composition (fresh, up to 2 g/l;weakly mineralised,up to 6g/l;mineralised,up to 15 g/l;brines, up to 50 g/l;strong brines, up to 300g/l);type of circulation and balance (regulated by evaporation,by evaporation and transpiration, by transpiration and run-off,by run-off). The main object of irrigation,amelioration measures and agricultural work is the soil. Hence particulai importance attaches to the soil cover and to the processes of natural salt accumulation occuring therein (including not only changes in the quantity of salts,but their qualitative composition as well). It is essential to determine the general character of the prevailing salt balance of the soils and the area, general contemporary salinisation,ancient residual salinity (relict), partial local salinityagainsta background of desalinisation,stable salinitywith a transitional type of salt balance,deep ancient desalinisation and so on. When a territory is irrigated,it is essential to determine whether salinityis natural,existing prior to irrigation or whether,on the contrary,it is secondary appearing after irrigation. All the above factors are interconnected in complex but absolutely regular fashion,which simplifies the task of assessing landscapes with a view to selecting the most effective and economic locations for irrigation. Applying the above criteria,landscapes may be classified,in a very general way,into several types according to natural drainage,suitability for development and stability of soil fertility. W e give below a brief survey of the main irrigated landscapes in arid zones where salinity is a constant feature.

D. DESCRIPTION OF THE MAIN TYPES

OF LANDSCAPES

IN IRRIGATED TERRITORIES 1. Dry steppe, savannah and pampas zones (a) Territories with natural drainage This group includes fore mountain and high watershed plains dissected by a network of ravines and rivers. These may comprise either ancientalluvial plains of the glacial or post-glacialepoch,transformedby tectonic and erosion processes (Ukraine,Argentina); or again, ancient deluvial plains formed at the foot of gentle slopes of mountain formations (Southern Ural area, Morainic area of the Trans-Volga region, Southern Kazakhstan). This group includes also ancient alluvial and deluvial plains which have subsequently undergone epeirogenic uplift and dissection (Western Caucasian foothills,Mnaych plains,Stavropol uplands). In most cases the soil cover ofthese territoriesconsists of evolved zonalsoils: brown soils,dark sierozems, chernozems, dark chestnut and chestnut. The soils are as a rule not subject to contemporary salinisation processes; only the subsoil horizons at a depth between 50 and 150c m contain small quantities of residual salts.In the southern chernozems and chestnut soils,solonetz patches are fairly common.These landscapes, generally speaking,are undergoing at the present time,intensedesalinisation.The fresh groundwaters usually occur at a considerable depth-of the order of 10 to 30 m;only in a few cases are the groundwaters highly mineralised (Trans-Volgaarea,Black Sea region). Irrigation which can,as a rule,be used without drainage speeds the desalinisation. It does,however,lead to a rise in the groundwater table and intensification of the groundwater drainage into various kinds of depressions,ravines and on to the river terraces ofvalleys. As a result the groundwaters,in these places,may 169

IRRIGATION, DRAINAGE A N D SALINITY intersect the soil surface causing local waterlogging and salinity. In order to prevent this, the canals can be lined,ravines must be cleaned and the drainage intensified.

(b) Territories with inudequate natural drainage (1) Pluins and delwessions Areas ofthis type are usually ancient alluvial plains,deltaic and marine plains and marine depressions which have not undergone epeirogenic uplift and subsequent dissection by erosion (the Hungarian depression,for instance,the Salina Grade plains in Argentina, the Caspian depression, the Southern Ukraine, the West Siberian depression,the Sivash depression in the Crimea). Such territories,in the comparatively recent past, underwent intensive salinisation which is still continuing in the low-lyingparts of the relief.The subsoil and the groundwaters of these regions contain large salt reserves. The groundwaters have little outflow and are relatively highly mineralised.At the present time,these areas have entered into a period of desalinisation,but of a preliminary character only.The groundwaters which occur at a depth of 5 to 8 m may rise within two to three years after irrigation and cause salinisation to recommence. The soil cover of such territories consists of meadows and meadow-chernozems,residual solonchak-type and solonetz-typechestnut soils in combination with solonchak-typesolontsy.In the local depressions of the macrorelief,solonchak soils and high, mineralised groundwaters are usually found.Irrigation of large areas causes groundwatersto rise and as a result,preliminary desalinisation may give way to salinisationprocesses. Salinity will fist affect all the low-lying parts of the mesorelief,where the groundwaters will approach the surface at an earlier stage. However,neither the intensity nor the extent of the salinisation will be as great here as in irrigated oases in desert zones,since evaporation rarely exceeds 600mm;the amount of irrigation water used will not be very large since the influence of seasonal atmospheric precipitations is fairly high. There have been, nevertheless,in the history of irrigation,cases of secondary salinisation occurring in such territories.When use is made of large,unlined irrigation canals from which large amounts of water are lost by seepage,an intense salinisation of the adjacent territoriesmay occur.It willtherefore be essential,in many cases,to take measures both to prevent secondary soil salinity and also to eliminate residual soil salinity, i.e. by installing deep drainage and lining of the canals. At the same time,steps will have to be taken for the a m oration of solontsy. In soils which contain gypsum fairly close to the surface-also in some cases in soils which contain carbonates of calcium-it will be possible to use,for this purpose, the deep ploughing method proposed by KOVDA and BOLSHAKOV (1937), ANTIPOV-KARATAEV et al. (2) River terraces In the arid parts of the steppe zone of the Ukraine,Crimea, Caucasus,Volga basin and Kazakhstan, local run-offwaters are widely used for feeding small reservoirs to irrigate river terraces.Small irrigation systems are found on the terraces ofthe small rivers of Chile and Argentina. River terraces,being of younger formation than watershed plains,are characterised by a greater residualsalinity.But on the other hand part ofthese terraces, consisting ofhigh,steep slopes running down to the river,have good natural drainage.The second and third terraces of the small streams of the Volga basin are of this type. It must be said,though,that the soil in those parts lying at some distance from the river often has a high residual subsoil salinity,a large porportion of solonetzpatches and mineralised groundwaters (10 to 20 g/1with a groundwater table occurring at 6 to 10m). Residual salinity is especially high on the second river terraces. In all cases the soil cover and development prospects will be more favourable on third river terraces than on second terraces, where the groundwaters are usually higher and the residual salt reserves larger. The installation of dams and reservoirs on small rivers usually causes feeding of the groundwaters upstream and also local salinisation,This phenomenon is very common in places where irrigation is done with water from local run-off.In places where large,unlined canals are installed on river terraces and the water intake is not properly controlled,the groundwater table on the terraceswill rise and salinityand waterlogging will develop. Undesirable phenomena of this kind are found in sinal1 irrigation systems in the Volga region. The places where irrigated farming flourishes are those where the second and more particularly the third terraces have natural drainage.In such places the groundwaters,after rising slightly,are stabilised by increase of the flow. This favourable situation arose,for instance,in a large experimental irrigated area near the town of Malouzensk. Secondary salinity did not occur. It may be reckoned that conditions for soil improvement are always more favourable on second and third river terraces below the dam than above. In such places, only measures for dealing with solontsy may be required;whereas in order to develop the entire surface of the river terraces,it will be necessary to apply a

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whole series ofpreventive measures against salinity.O n the second and first terraces,where the groundwaters are more stagnant and more mineralised,local drainage may also be necessary. (c) Landscapes witlzout natural drainage Deltas and terraces at river mouths,river flood plains,and also the lower benches of the first terraces above the flood plain in dry steppe and savannah zones are subject to some form or other of contemporary salt accumulation.As a rale,salinisation occurs here in dark meadow and meadow-bog soils and the salts consist mainly of sulphates,bicarbonates or a mixture ofthe two.The groundwaters are little mineralised (4to 7g/l) and occur at a depth of2to 5 m.Saline soils usually take the form ofmeadow soda solonchaks,soda-sulphate patches and, less often, chloride-sulphate solonchaks. Examples of landscapes of this type are the flood plains and lower terraces of the Volga and Ural rivers and the deltas and lower terraces of the rivers Don, Kuban, Danube,Tisza and Sacramento (California). In the best parts of these areas,there are dark,highly fertile meadow soils without any traces of salinity. On the other hand,depressions of all types,ancient silted-upwater courses,dried-upmarshes and so on will often be saline,owing to the complete lack of natural drainage. In order to reclaim territories of this kind,a series of measures have to be taken: flood control,drainage and leaching,and prevention of alkalinity. Since these measures are fairly costly, it may be advisable to exclude such areas in the land selected for irrigation. It is clear,from a general survey of development possibilities of steppe landscapes,that,in these zones, natural and secondary salinity are of little and above all of local importance.The main stress in the management of irrigated farming must be on correct agrotechnical methods and measures for prevention of waterlogging, though attention must also be paid to prevention of alkalinity, and dealing with solontsy and solonetzic soils.At the local level,drainage will have to be used to control salinity. 2. Semi-desert and desert zones (a) Landscapes having natural drainage (1) Well-drained,high loamy and loess plains on pebbles and sands (Fore mountain plains alongside the mountain chains of the Caucasus, Central Asia and the Andes.) The soil cover of these landscapes mostly comprises brown steppe soils and sierozems with slight traces of residual salinity. There is no risk at all of irrigation causing soil salinity. Necessary land improvement measures may include prevention of subsidence,irrigation erosion and excessive seepage losses. (2) Ancient proluvial-alluvialfore mountain plains and upper river terraces (Samarkand and Tashkent oases,upper terraces ofthe Ferghana valley.) The groundwaters lie at a considerable depth and have good general drainage.When irrigation is applied,the water table may rise slightly,but it usually remains at a depth such as to exclude any interaction between soils and groundwaters.The salt balance of such landscapes,both after and before irrigation,is of a strongly desalinisation type. Flagrantly inefficient water use may sometimes provoke waterlogging and sometimes slight salinity in the lower-lying parts of the territory. (3) Lower flood terraces und terraces above thefloodplain along the middle and upper reaches of rivers (Mainly Eastern Ferghana,the lower terraces of the mountain rivers Chirchik,Zeravshan,Syrkhan-Darya, etc.) In these areas,the water table occurs at a high level and intersects the soil surface at a number of places. At the same time,the sandy-pebblynature oÎ the subsoil and the existence of good general outflow down the slopes into the river beds exclude all danger of an increase of groundwater mineralisation or substantial soil salinisation.The groundwater balance is regulated mainly by duainage,with transpiration and evaporation playing only a very minor part and the salt balance is thus favourable. The small quantities of easily soluble salts accumulating in soils and groundwaters are carried away by the general outflow.There occiirs only a partial increase in the soils,of little soluble compounds,principally calcium carbonate and some gypsum.When irrigation is applied seepage losses,combined with other factors upsetting the water balance, may cause waterlogging in some places,but soil salinisation rarely occurs. The main land protection measureswill be seepage control and efficient water use. Water management will depend on the level of fresh groundwaters which can be used by plants.Tube-wells,shallow drains and collectors may be recommended as a means of preventing waterlogging. Deep drainage is not recommended in such cases,since it may cause the soils to dry up,in which case more water will be needed for irrigation. N

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(b) Landscapes huviizg inadequate natural drainage (1) Higher terraces (second and third) of rivers, ancient alluvial and ancient deluvial plains This category includes the second and third terraces of rivers and the corresponding alluvial and proluvial plains (Hungry Steppe, Milslc Steppe, terraces of the Vakhsh, Araxes, Tigris, Euphrates rivers, etc.). The groundwatershere,before irrigation,occur at a depth of 5 to 10in,occasionally 15 m;and in low-lyingparts of the mesorelief often not more than 2 m deep. They have a mineralisation of 10 to 15 g/l.They have a slight general outflow into the natural drains constituted in most cases by the rivers,in addition to a certain amount of local run-oirinto depressions. The groundwater balance is regulated,under natural conditions, in the raised parts of the relief by slight drainage and transpiration and in the low-lying parts mainly by transpiration and surface evaporation. The salt balance of the landscape, before irrigation,may operate in the direction of gradual, overall desalinisation with local salinisation of the undrained depressions of the mesorelief. Soils, subsoils and groundwaters are characterised by substantial residual salinity. Surface irrigation leads to a rapid rise in the groundwater level almost always,and since there are normal residual salt reserves in groundwaters and subsoils, strong secondary salinisation occurs first alongside the canals and extends subsequentlyto the rest ofthe territory.In the undrained,flat depressions,the process ofsalinisation is usually progressive,eventually attaining considerable proportions. The slight natural drainage is not large enough to prevent salinisation, but the phenomenon is less acute than in oases where there is no groundwater run-off. Experience has shown that measures to ensure a good water application and efficiency do not solve the problem of salinity,and deep drainage has to be installed. (2) Lower terraces and alluviaIplains along the middle and lower reaches of rivers Landscapes ofthis type are located either on young flood plains or else on ancient lake depositsleft when the river valley is formed (Bokhara oases, Chardzhou-Faraboases, Central Ferghana,etc.). Since these territories are fairly low lying,they are subjectto the indirect influence of flooding,as the groundwaters are able to obtain replenishment,throughout most of the year,from the river.Hence the groundwater table here lies close up to the surface (1 to 3 m). The positive side ofthe water balance is made up by seepage from the river and the irrigation network;the negative side, by evaporation, transpiration and slight flow into the lowlying parts of the area.In some instances,there is periodical drainage of groundwaters by the river when the level of the river is low. In such landscapes,a slow general salt accumulation will occur.In this connection,however,the following point must be noted: the existence of a slight,permanent natural flow may create a situation such that the inflow ofchlorides arepractically identical.In such cases,no increase ofthe reserveofchloridesin the territory as a whole is possible,except in isolated cut-offdepressions.In regard to sulphates,there is a slight general increase in reserves of these salts within the landscape as a whole. Although the water table is high, the water is not, as a rule, strongly mineralised. Salinisation occurs chieflyin the topsoil layers and is mainly sulphatic in type. River banks and elevated areas are often nonsaline or only slightly saline.Strong salt accumulation occurs in depressions remote from rivers and round the outside edges ofthe oases,to which the salt is pushed by water seeping from the rivers and the irrigation network.The rise in the water table due to irrigation may increasetheir outflow,but it increasestheir evaporation even more, with the result that when irrigation is used without drainage,the salt balance as a whole further deteriorates. At the same time, irrigation accentuates both the natural desalinisation trend of river banks and elevated sectors,and the natural salinity increase in depressions and peripheral zones. The experience both of the Chardzhou oasis on the Amu-Darya and of a number of other irrigation projects shows that a system of collectors and deep drains have to be installed whenever a large proportion of the land is to be developed for farming,Other very important steps are to reduce seepage losses from the irrigation system and to make extensive use of what is known as biological drainage,which is possible here. As regards specific measures,steps must be taken to protect the projects against flood waters. (c) Landscapes not having adequate natural drainage Regions of this type include dry,lacustrine and marine deltas. The last in their turn may be sub-dividedinto deltas of tideless inland reservoirs and deltas subject to the influence of marine tides.

(1) Dry deltas (Bokhara, Karakul, Kashka-Darya,Shirobad, Sokh, Isfara, Murgab and Tedjhen oases in Central Asia; Geok-Chai,Bolgar-Chaioases,etc.,in Transcaucasia.) The upper parts of dry deltas consist right from the surface or from close below it, of coarse,lumpy sub-

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stance,often gravel. In the central part,sandy sedimentsmay predominate and in the lower parts (along the edges) loams and clays.At the same time,regular changes occur in the depth and composition ofthe groundwaters.In the gravel sections,the groundwaters are freely replenished from the river and equally freely flow away down the slope;as a result,their level is often at a considerable depth while their mineralisation is roughly the same as that ofthe river water.When the gravel gives way to finer-textureddeposits,the groundwaters rise towards the surface and often spurt out in the form of fresh or very slightly mineralised springs. Lower down the alluvial fan the groundwaters spread over the whole broadening expanse of the fan and sink down again. O n the peripheral sections,owing to the presence of loamy deposits,the groundwater flow is stemmed and practically wholly absorbed by evaporation and transpiration.The groundwaters in these areas are the most mineralised of all (80 to 100g/I). As the groundwaters moving towards the periphery are consumed by evaporation and transpiration, a complex process of differentiation takes place in their salt composition. In the central part of dry deltas, mainly sulphates will accumulate in groundwaters and subsoils; in the lower,peripheral sections,mainly chlorides. The upper parts of alluvial fans and dry deltas do not, as a rule,undergo salinisation. Soil improvement methods should differ widely between one part of a dry delta and another. Such deltas consist of three distinct zones:the upper zone,which is non-saline;the middle zone with slight or less often medium salinity,mainly sulphatein type (sometimes soda-sulphate) and also,in some cases,swampy spots; and lastly the lower peripheralzonewhich is as a rule subjectto strong salinity,difficult to remedy on account of the unfavourable properties of the subsoils. It might be thought that amelioration of each of these zones could be done independently,for instancein bringing a soil cover on the pebbles in the upper zone;by installing drainage in saline patches in the central zone,at the same time reusing fresh groundwater for irrigation;and by intensive drainage and leaching ic the lower zone. However,all zones of a dry delta have a common source of groundwater feed from a river or from canals (mainly in the upper part of the delta, where the groundwaters lie deep). As a result of the movement of the groundwaters towards the periphery ofthe dry delta and of evaporation and transpiration, the composition of the groundwaters will undergo a gradual, regular change,necessitating corresponding adjustments in the amelioration methods applied in each zone. It is obvious that any measures which change the natural groundwater balance in the higher parts of a dry delta (such as intercepting the groundwaters in the central part of the alluvial fan,for instance) will automatically create more favourable conditions in the lower parts of the delta. This being so,it is essential when applying any improvement measures even of a partial character,to take account of the effects they will have either on adjacent areas or on the region as a whole. Soil improvement measures in dry deltas must be applied as part of a coordinated plan. These measures include:all kinds ofmeans to reduce seepage losses from river and irrigation system throughout the whole of the delta and more particularly in the upper pebbly section;and the intercepting offresh and slightly mineralised groundwaters in the central part of the delta for irrigation of additional areas. This last measure, besides eliminating waterlogging,is designed to bring about a gradual lowering of the groundwater table in the lower-lyingparts of the delta. Then lastly,steps will have to be taken to prevent the saline areas from spreading and to eliminatethe solonchaks.Thiswill be done by leaching and by installing an extensive deep drainage system. (2) Marine deltas (a) M o i n e deltas of tideless basins (deltas of the Terek,Amu-Darya,Syr-Darya,Kura, etc.) Here, as in dry deltas, conditions in regard to land improvement differ, according to a regular pattern between the upper part of the delta and the part close to the coast. In this case, though,there is less stability and distinctions between different parts of the delta are less marked. This is due to the principles governing their formation:marine deltas are constantly shifting in the direction of the basin,forming more and more new land;and the river beds in deltas undergo frequent and considerable changes of position. Marine deltas are the scene of constant accumulation of sand, silt, clay and easily soluble salts. The water table is universally high (1 to 3 m); the groundwaters are fed all year round from the river, delta lakes and also from the sea,there is no general groundwater outflow,and their consumption is mainly through evaporation and transpiration with slight local redistribution within the mesorelief. Groundwater mineralisation is usually patchy, but on the whole fairly strong (up to 80g/l). A basic feature of the landscape of every marine delta is the elevated river beds (present or former) formed of sandy loam and deposits, and the flat depressions between river beds formed of loamy clay or,

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IRRIGATION, DRAINAGE A N D SALINITY sometimes,of extremely heavy salt-bearingclays. In accordance with the size of the rivers and deltas and also of the delta rivers and streams,the size of the mesorelief elevations and flat depressions may show substantial variations (from hundreds of square metres up to many hectares). Sandy-loamriver-bed deposits have deeper-lying,invariably less mineralised groundwaters and non-salineor only slightly saline soils. In the flat depressions between the river beds,the groundwaters are higher and as a rule more strongly mineralised. For amelioration purposes, the best parts of a delta landscape are the raised ancient water courses,where irrigated land with good, stable fertility is usually found.The depressions between river beds are difficult to devclop since they are strongly saline and easily subject to secondary salinity. Takyrs and solonchaks frequently form in these depressions.Depressions in the flooded parts of deltas are often occupied by marshes or lakes (ilmeni) or else by salt creeks and lagoons cutting deep in from the sea. The salt balance is of a general salinisation type.At the same time,it has certain specific features due to the constant formation of new areas of delta. Partly on these grounds and partly on the basis of the behaviour of the groundwaters,marine deltas may also be divided,for soil improvementpurposes, into a number of zones. Youngpartsof deltas are characterised,as a rule,by the absence ofsalinity,but are subjectto waterlogging; they are often formed by sandy river-mouthdeposits which have not yet passed through the stage of warping by flood water deposits.In agriculture,such territory is generally used for natural pasture land and may also serve for growing coarse green fodder (young rushes). Centralparts of tlze delta. These parts of the delta are usually strongly affected by the flow of mineralised groundwaters moving into the young parts ofthe delta and have the most saline soils and groundwaters. W e often find,in these parts, overall soil salinity,which is most difficult to deal with. Where there is no drainage, irrigation was practised in the past on very small plots. Such farming caused intensification ofthe salinity of the unused areas. In order to eliminate salinity and increase the proportion of farmed land,major land improvement measures will have to be applied: installation of an extensive network of deep collectors and drains and repeated leaching of solonchak soils with high water depths. Drainage must extend more or less evenly over the whole ofthe central part of the delta. The main drains should run along the low-lyingparts of the mesorelief-this is particularly important in cases where the territory has already been developed without drainage and the main salt reserves from the elevated parts of the delta have been pushed down by the irrigation network into the mesodepressions.Irrigated rice is an effective method of reclamation of areas of this type. Upper parts of deltas and cleltaic plains.When the marine delta plain is sufficiently wide,the nucleus of the mineralised groundwater stream (common to the whole ofthe deltaic-alluvialplain) may in the central part of the delta be of mixed composition,due both to natural causes and also to the effects of irrigation,which in this case operate in the same direction as natural phenomena. In the upper parts, as in the case of dry deltas,younger links ofthis stream are usually to be found.As a result,the groundwaters in the upper parts of the delta may be less mineralised than in the central part. There may also be differences of chemical composition :the upper parts are often more sulphatic than those in the central part of the delta. For development purposes,therefore,a less dense collector and drainage system can be used in the upper parts and smaller leaching norms; although the general principles on which amelioration measures are based will be the same in both cases. It is also essential to take steps to prevent seepage and loss ofirrigation water and to protect against flood waters. Development of marine deltas without drainage was always done by means of using only a very small porportion of the land for farming purposes. Not counting the swampy patches,this proportion was lowest of all in the central part of the delta (0.2-0-3);and slightly higher in the upper parts (0-3-0-4). (b) Marine deltas subject to the effects of tides (deltas of rivers flowing out into seas linked with the world oceans such as those of the Nile, Tigris-Euphrates,Liao Ho,Hwang-Ho,etc.) As regards geomorphological structure,hydrogeology and soil cover,marine deltas subject to the effects of tides have much in common with the deltas of rivers flowing into inland tideless seas and lakes. Hence the amelioration analysis and general assessment principles we laid down for marine deltas may be applied,by and large,to deltas subject to the effects oftides.In view,however,of the specificfeatures ofthese deltas,we are bound to draw attention, once again, to the fact that they possess hydrogeological and geochemical featureswhich are most unfavourable from the point of view of irrigated farming. The waters ofmany inland reservoirs,such as the Caspian,Aral and so on,are much less mineralised than those of the ocean and ofthe seas directly linked therewith.Hence the soils of deltas subject to the effects of

174

LANDSCAPES-IRRIGATION,

D R A I N A G E A N D SALINITY

tides are formed under the influence of more highly mineralised solutions;with the result that the content of easily soluble salts-among which sodium chloride predominates-is much larger in these soils. Since the growth of all deltas is due to filling up by fine marine deposits left by the tides,the delta-marine deposits in marine deltas subject to the effects of tides will contain,deep down, buried marine solutions.In the deltas of inland water reservoirs,the salts from the reservoir only penetrate on to the land as a result of the action of the winds,which is limited;in the case of deltas subject to the effect of sea tides,on the other hand, the tides constantly carry the seawater deep onto the land,especially in the depressions between river arms and along the beds of the delta streams.In Eastern China the direct effect of the tides,especially where combined with typhoons, is evident to a distance of 10 to 15 km from the coast. In addition,the indirect influence of the tides, through the upward thrust of the groundwaters,extends over a further 20 to 30 km, so promoting the growth of general salt accumulation. Coastal land subject to the effects of tides may be divided into four salt accumulation zones. The strip nearest to the sea is affected daily by the tides: soil formation is prevented by their flooding and washing action, with the result that this strip consists mostly of solonchaks without any vegetation. Next comes a strip where the direct effects of the sea are felt not daily,but only during exceptionally high tides.This strip also may be several kilometres in width;the groundwaters here have usually the highest mineralisation (30 to 70g/l); the soils consist of meadow solonchak-typeand solonchaks. Beyond this is a third strip,where the tides exercise an indirect influence through the river channels and creeks. This too is characterised by meadow solonchak-typesoils with highly mineralised groundwaters (25to 35 g/l). And lastly,farther still from the coast,comes a wide zone which is indirectly influenced by the sea tides, through the upward thrust of the groundwaters. In this zone,there are solonchak and weakly saline soils with groundwaters having mineralisation of the order of 15 to 20 g/l.Inland from this zone is a delta-alluvial plain with meadow-solonchakand meadow soils.The main feature of such landscapes is the constant alternation of mounds formed of ancient sanded-upriverbeds having non-salinesoils and groundwaters with flat depressions containing loamy-clayeysolonchak-likeand meadow solonchak-likesoils and mineralised groundwaters. It is clear from the foregoing that conditions for development oftidal deltas will improve in proportion to the distance from the coast. All recommendations for amelioration of deltas of inland water reservoirs are applicable here;but it will be essential to take,in this case,certain additional preventive and improvement measures of a somewhat specific character. As shown by the experience of China it will be necessary when reclaiming land in the immediate vicinity of the tidal zone to construct along the sea coast large dikes to protect the land from the tides. Since the drainage network,in marine deltas,flows into the sea or into streams subject to the influence of the tides, steps will have to be taken to protect the main collectorsfrom the inflow of sea water at high tide.The same system has been used for a long time in the Netherlands where at low tides water was drained off to the sea. However, as a result of soil shrinkage on one hand and transgression on the other,the method had to be replaced in most cases by artificial pumping of drainage water. In view of the fact that the tides influence the groundwaters throughout the whole of the wide coastal plain, drainage has to be intensified in deltas of this type in order to prevent the daily rise of the salt groundwaters. All marine deltas are subjectto constant danger of catastrophic flooding due to the fact that the river beds and banks in such deltas are as a rule considerablyhigher than the rest ofthe land.Thisis particularly marked in the deltas ofthe rivers ofEastern China.Hencethe most importantmeans of establishing stable conditions for agriculture both in the deltas ofinland seas and in marine deltas affected by tides is the elimination ofthe danger of flooding.This may require construction of dikes along the river and its tributaries in addition to dams along the coast.Along severalrivers like the Amu-Darya,Hwang-Hoand many others,dikes of several metres high have been built over distances of hundreds of kilometres.Construction of pumping plants may further be needed to get rid of drainage and eventual flood water. The soil-formingprocesses and soil development conditionsin marine deltas subject to the effects oftides have not been thoroughlyinvestigated,despite the fact that deltas ofthis type are ofthe greatest importance. The theory of the soil formation process in deltas and the methods for developing these areas constitute one of the main scientificproblems for the future.

I75

IRRIGATION, DRAINAGE A N D SALINITY

REFERENCES BERTLE FREDERICK A. (1966), Effect of snow compaction on runoff from rain or snow, US Bureau of Reclamation Monograph 35,43 pages.

EGOROV V. V. (1951), History of the formation,natural properties and economic development prospects of the Kura delta,Problems of physical geography,X W (in Russian). GIRINSKIJ N.K. (1955), Seepage of subterranean strongly mineralised waters and brines out of the sea, Works of the Jiyclrogeologicalproblemslaboratory of the USSR Academy of Sciences, M yXII (in Russian). KOVDA V.A. (1947), Origins and regime of saline soils,II,USSR Academy of Sciences,Moscow (in Russian). KOVDA V. A. (1947), Soil-amelioration bases for prevention of soil salinity in irrigated farming, Moscow Hydro-ameliorationInstitute,Xm,33 (in Russian). KOVDA V. A. (1954), Increasing soil fertility and improvement of soils in irrigated areas,Pochvovedenie,7 (in Russian).

KOVDA V.A. and BOLSHAKOV A. F.(1937), Water and saltregime ofthe soilsin the centralpart ofthe Caspian plain, Transactions of Conference on Soil Science and Plant Physiology, USSR Academy of Sciences, 1 (in Russian).

KOVDA V.A. and EGOROV V.V. (1953), Some laws governing soil formationin marine deltas,Pochvovedenie, 9 (in Russian).

KOVDA V. A., EGOROV V. V.,MOROZOV A. T.and LEBEDEV YU.P. (1954), Laws governing the process of salt accumulation in the deserts of the Aralo-Caspian depressions, WorJcs of the Soil Institute, USSR Academy of Sciences,XIV (in Russian).

176

7. Quality of Irrigation Water* A. SOLUBLE COMPONENTS IN IRRIGATION WATER Irrespective of its source,all irrigation water contains dissolved salts,the type and quantity ofwhich depend on its origin and also on its course before use. Some properties of water from different sources are given in Table 7.1. Table 7.1. Types of water

Properties of water Origin of water

Silt

Rains Springs

traces traces

Groundwater (incl.artesian water)

traces

Rivers

2-5 811

Mine and indus- traces trial waste

water Lakes

little

Drainage

little

Sewage water

little

Estuary and gulf water Sea water

little

Chemistry

matter

seeds,spores, Fertility bacteria

6-7 6-9

traces traces

traces traces

poor poor to rich

7-9

traces

traces

poor to rich

7-9

medium

medium

rich

4-9

traces

traces

poor

7-9

little

much

rich

brackish saline fresh S1.brackish brackish

7-8

little

much

rich

7-8

much

7-8

little

very much medium

very rich toxic

highly saline

7-8

little

little

-

fresh fresh alkaline saline fresh alkaline salted fresh alkaline acid fresh saline fresh alkaline

pH

Organic

saline

little

There is a general tendency to use mineralised waters for irrigation. As a result, salinisation of irrigated land occurs in many cases;hence the appraisal of irrigation water quality becomes of universal importance. Two main groups of salts in irrigation water can be distinguished: 1. The major constituents,determining the specific character of the water 2. The minor constituents,to which attention has to be paid in special cases 1. Major constituents of irrigation water as a function of its origin (a) Rainwater Rainwater has the lowest salt content of all types of water used for irrigation.This water contains dissolved gases (N2, Ar, Oz,COz)and dissolved salts originating from terrestrial and marine sources. Generally,the amount of ions in rainwater (NH,, C1,Na) varies widely and is dependent on the distance from the sea and the areas of aeolic deflation(2-20tons per Ism2).For example,Table 7.2illustratesthe composition of rainwater collected close to Israel'scoastline,and Fig. 7.1 shows the changes in chloride content ofrainwater in Germany over a distance of 1300 km from the sea coast. *This chapter was edited by V. A.KOVDAfrom his own manuscript with B. YARONand Y.SHALHEV~Tas co-authors and with contributions by I. SZABOLESand K.DARAB 177

I R R I G A T I O N , DRAINAGE A N D SALINITY Table 7.2. The influence of distance froin the sea and salt content of sea water on the chemical composition of rain water (YAALON, 1961) -~

~~

~~

Location

Sea con-

cerned

Distance from

IICQ, mg/l

SO,

c1

mg/l

mg/l

Ca mg/l

M g mg/l

Na

K

Conduc-

mg/l

mg/l

tivily

at 25°C

pmho/cm

sea km

Jerusalem

at 25°C

Mediter-

ranean

52.5

Waifa

Mediter-

Eilat Sedom

ranean 1.2 RedSea 0.1 DeadSea 0.1

The Mg/Ca, K/Na and

34.77

30.35

43.69 33.09 53-64 21.95 85.79 118-70

Cl

13.06

1.87

18.81

13.84 26.98 13.05

2.19 1-22 9.95

25.84 26.00 49-20

7.66

1.02

170

8.02 9.06 18.62

1.64 3.01 8.13

200 232 467

ratios (ions being expressed in me/l)for rainwater near the sea

are similar to those for seawater and differ greatly as the distance from the sea increases (SCHOELLER, 1962). Na and C1 content meIl M Chloride

e---+ Sodium

O Distance from sea (km)

Fig.7.1 The relationship of sodium and chloride content of rainwater to distance from the sea (According to RIEHMand QUELLMOLZ, 1959)

(b) Surface water The salt content of surface water is a function of the rocks prevalent at the water’s source,of the climatic zone, of the nature of the soil over which the water flows and of eventual pollutions by human activities. Surface waters can be classified into two groups:flowing water (rivers) and stagnant water (e.g.lakes). From the main composition of river waters throughout the world (Table 7.3) it is seen that the predominant anions are HC8,-and SO,=,and the main cations are Ca+ and Na+ [or Ca+ i- and Mg+ onan equivalent basis). +

+

Table 7.3. Mean composition of river water of the world in ppm (LIVINGSTONE, 1961)

HC0,-

so&=

North America South

68.0

20.0

America

31.0 95.0

4.3 24.0 8.4 13.5 2.6 11.2

Europc Asia Africa Australia World

79.0 42.0 31.6 58.4

c1-

Na+

K+

Sum

5.0

9.0

1*4

142

1.5 5.6 5.6 3.8 2.7 4.1

4..0 5.5

2.0 1.7

69 182 142 121 59 120

NO,-

Ca++

Mg++

8.0

1.o

21.0

4.9 6.9 8a7 12.1 10.0 7.3

0.7 3.7 0.7 0.8 0.05 1.o

7*2 31.1 13.4 12.5 3.9 15.0

11.0 2.9 6.3

9.3

1.4 2.3

Stagnant lakes found in arid and semi-aridregions are generally characterised by high salt content.

178

QUALITY OF I R R I G A T I O N WATER (c) Groundwater The salt coilient of groundwater is dependent on the source of the water and on the course over which it fiows.Mineralisation of groundwater is in accordance with the law of dissolution,based on the contact between the water and the water-bearing strata. Changes in the salt content of groundwater in the recharge process result from reduction,base exchange,transpiration,evaporation and precipitation. The reduction processes,mainly of a biochemical nature,infiuence the concentration of SO,in the groundwater. Y7hile the groundwater is flowing the soil acts as an ion exchanger and cations in the water reach equilibrium with the soil cations.The generalsalt content increase is due to evapotranspirationor dissolution, and is mostly affected by climate.There is a zonal distribution of water salt content,determined by geology, climate and depth to the water table.

(d) Seawater Seawater is a complex solution containing a large number of elements (ions,gases,organic matter, microfauna,-flora,etc.). Among the chemical elements are: chloride (predominatingwith 55%),sodium (30%), sulphate (7%), magnesium (3.7%),potassium (1.1 %). PICKARD(1964.)noted that the range of surfacewater salinity values in the open ocean is 33-37 g/l.Higher values occur in regions with high evaporation such as the eastern Mediterranean (39 g/l) and the Red Sea (4.1g/l). Seawater can be used for irrigation purposes only after undergoing an industrial desalinisation process,Such treatment produces water having a composition differing greatly from the original water.

2. Minor constituents of irrigation water

Not all of the minor elements are found in every source of irrigation water. They appear sporadically,singly or in groups,in different water sources.Bromine,fluorineand iodinemay be found when chlorideis present. Most fresh water contains less than 1 ppm fluorine,0-01ppm bromine and 0.2 ppm iodine. Irrigation water may also be found to contain Li,Rb,Cs,Be,Sr,Ba,Ra,etc.Due to the minute quantities of these elements, they mostly have no influence on the water quality. Other minor elements which may appear are selenium, arsenium,antimony,bismuth group and different metals as Cu, Co,Ni,Zn,Ti,Zr,Vn, Cr,and Mo. A micro-elementfound in most irrigation water at low concentration is boron (Table 7.4). Table 7.4. Boron content of different lakes and rivers-(After

Locations of water River Tone,Japan Watarasa River, Japan Agatsumo,Japan Okuresawa,Japan Great Salt Lake,Utah,USA Florida streams Greek River, Uganda City water supplies,USA River waters of USSR

Mean of samples 10 3 4 4 6

24

LMNGSTONE, 1964) B (PPd 0.345 0.197 1.970 1.305 43.500 0.019 0.386 1.000-0.010 0.013

The highest boron concentration in the world in flowing water occurs in Japan (1-3-1-9ppm). Boron is essential for plant growth;however,it is harmful only slightly in excess of optimum,particularly to citrus and walnuts, and is toxic to most crops and plants at concentrations only six or eight times the optimum. Lithium,found in some waters in California,causes tip and marginal burn and defoliation of citrus leaves at a concentration of less than 0.1 part per mission in the irrigation water. Selenium,molybdenum and fluorine are found in some soils and irrigation waters and are absorbed by plants withwt apparent damage.However,they are harmful to animal life at a relatively low critical level. Animals may obtain these elements from water or from feeds and forage (FIREMAN). 179

IRRIGATION,DRAINAGE AND SALINITY 3. Seasonal variation in the composition of irrigation water The salt composition of irrigation water is not static,but is continuously changing. Thus, the evaluation of an irrigation water must be based on knowledge of seasonalvariation in the salt content.The composition of flowing water changes under the influence of precipitation in the area (e.g.Fig. 7.2). BRYSINE noted that the chloride content of Oued O u m er Rbia in Morocco varies during the irrigation season from 200 to 1500 mg/l.The lack of rainfall and a high evaporation rate during dry seasons contribute to the rise in salt concentration of lakes. While in large,open lakes (e.g.Lake Tiberias) the salt rise due to such climatic condition is no greater than 20%, there may be an increase in salt content of up to 100% in small marshes. The contact between the soil and the irrigation water can bring about a sharp increasein the salt concentration,especially under conditions offlooding.The increase ofsalt content in water standing in differentrice paddies in the Danube Valley is presented in Table 7.5. While deep groundwater tables as a rule do not have seasonalfluctuationsin saltcontent,the salt content of high groundwater tables changes as a function of evaporation, rainfall, drainage and irrigation practices. Drainage water may be used for irrigation. Under conditions of high.groundwater during an irrigation season,the process ofpumping-filtration through the soil-recharge contributes to the salt content changes in thewater.For example,groundwater was used to supplementsurfacewater forirrigationat CanyonCounty, Idaho,and the salt content and composition of the irrigation water was consequently changed due to the addition of Ca,Mg and SO,which resulted from the leaching of the soil (STEVENS,1962). Table 7.5. Salt concentration of paddy water measured at differentlocations in the same vice field (OBREJANU et al., 1958)

Water

Irrigation water Stagnant water Paddy water Paddy water Paddy water Paddy water Paddy water Paddy water Paddy water Paddy water Paddy water

Conductivity at 25°C pmho/cm

CI meIl

Na

Mg

Ca

me/l

me/l

meIl

330 384 611 752 831 1129 1442 1632 2258 3710 4329

0.49 0.79 2.16 2.04 3-71 5.00 11.80 10.86 20.56 29.45 49.33

0.56 0.95 3.69 5.00 5.65 5.73 8.17 7-60 16.08 2465 27-39

0.98 1-28 1.28 1 a72 2.13 2.30 9.45 5.59 4.65 8.53 1267

2.65 2.79 2.98 1 -47 2.79 4.20 4.11 6-20 5.31 1-18 12-79

mm 30 I-

q,, i

I

,

I

,

a

s

o

,

,

O

j

f

m

a

m

j j Months

n

d

Fig. 7.2.The average depth of monthly rainfall (A)and the monthly fluctuations of average total dissolved salts (B) of Tigris river at Baghdad (DIELEMAN et al., 1963) 180

QUALITY OF IRRIGATION WATER

B.

SUSPENDED INORGANIC AND ORGANIC MATERIAL IN IRRIGATION WATERS

All the river waters contain some suspended material. The fertility of river silt depends largely on the mineralogy and chemistry of the transported particles. In each new irrigation project therefore questions relatingto quantity and quality ofsuspended silt should be given a specialstudy.Themore quartz ormagnetite there is in river silt the lower is its fertility.The more minerals such as feldspar,mica,clay aggregates, fresh organic matter and humus the silt contains the greater is its natural fertility. It is known from ancient Egypt how extremely fertile the Nile silt is. Research carried out in the Amu-Darya delta showed that one hectare ofirrigated land annually received the following components through silt sedimentation (Table 7.6.). Table 7.6. Annual deposits of silt in the irrigated land of the Amu-Darya delta-(After KOVDA)

Total silt Total humus Total nitrogen Total K20 Available K20 Total PzO,

nearly 40 tons per hectare 250 kg per hectare 20 kg per hectare lo00 kg per hectare 50 kg per hectate 50 kg per hectare 60 kg per hectare 4500 kg per hectare 40 kg per hectare 130 kg per hectare

Caso4

Total Ca (including the above) Total Mno Total TiO,

To this must be added the tremendous amount offresh and semi-destructedorganicmatter,simultaneously deposited. The same may be said concerning water of other rivers utilised for irrigation. According to HAMDI the irrigated soils of Egypt are formed from the suspended material carried by the Nile. This material is the product of the weathering of rocks of the Nile Basin and varies seasonally as well as from year to year (Table 7.7.). Table 7.7. Solid material carried by the Nile past Cairo in solution and suspension-(After

HAMDI)

Solid material

In solution (tons)

In suspension (tons)

Average annual total Average total for the 4 flood months Average total for the remaining 8 months Daily average during flood months Daily average for the remaining months of the year

10700O00 7230O00

56 890000 55200000

3470000 59 O00

1690000 452000

14300

6500

The chemical analysis of mud from the Nile shows its richness (Table 7.8.). Table 7.8. Chemical analysis of the clay fraction of the suspended material in the Nile, flood season 1954-(after HAMDI)

Constituents SiO, Fez03 Ca0 MgO KzO Na,O SiO2 :Alzo3 Org. C. Org. N

C/N Carbonates

Per cent 44.94 14.81 13.99 3.98 1.60 1.77 1.38 3.21 1.14 0.09 14.50 0.99

181

I R R I G A T I O N , DRAINAGE AND SALINITY

MOLODZOV has reviewed the annual accumulation of nutritive elements in soils irrigated with water from the three largest rivers of Central Asia: Zeravshan,Amu-Darya and Syr-Darya(Table 7.9.). Table 7.9. Annual acciiniiilation of nutrition in soils of CentralAsia owing to deposits from irrigation waters-(After MOLODZOV)

Rivers

Total

Humus

Zeravshan Amu-Darya

2000 4000

250

70 20

Syr-dar ya

Ca

K20

2350 4500 1300

485

43 50 7

119

The continuous deposit of nutritious components in this sediment maintains stability of the fertility of the irrigated soils.Experiments made in tanks by KOVDA showed that the fertility of the best irrigated soils of Central Asia could be considerably increased through the addition of Amu-Daryasilt (Table 7.10). Table 7.10. Naturalfertility of the silt of the Amu-Darya river

Wheat yield in Scheme of the experiments Soil plus NP Soil plus NP plus 10%silt Soilplus NP plus 30%silt Soil plus NP plus 50%silt Soil plus NP plus 70% silt Silt 100%plus N p

g

7-2rfr. 0.0 13*6+1.1 14.1+0.9 10.3rfr. 1.1 10.5k 0.7 8.2+0.0

% 100 190 196 143 146 114

It can be seen that Amu-Daryariver siltin combinationwithfertilisers can greatly increase the productivity of irrigated soils. From the point of view of protection of soil fertility it is thereforepreferable to transport river silt to the irrigated fields rather than to let it accumulate in the reservoirs. There are many other aspects of the same problem, connected with silt deposits in irrigation systems. Progressive year-by-yearsedimentation of river alluvium on the fields results in permanent elevation of the level of the land improving the natural drainage conditions,and reducing the flow of capillary rise of saline groundwater. As far as possible uniform distribution ofalluvium onirrigated fieldsshould be aimed at,so that separation of different silt fractions and formation of cup-likerelief are prevented. Among scientistsand engineers working in irrigation,many prefer to retain sediments in specially designed reservoirs :sediment-ladenwater makes irrigation difficult,particularly where corrugation and furrowmethods are used. Specific cultivation may be involved to obtain adequate water penetration. In some cases the sediment also interferes with germination;where the suspended sediments carry high loads of exchangeable sodium,loss of productivity may be experienced and the topography of the irrigated fields is not preserved. It is obviousthat clear pure water does not improvefertility.Moreover,seepage ofclean irrigationwater is much greater than seepage of muddy water owing to the lack of silting ofthe canals.The smaller the particles suspended the more they penetrate the canal walls and bottom (auto-colmatage); thus the canalpermeability decreases and seepagesare reduced.It has been observed in ancient canals ofthe Central Asian SovietRepublics that after 30 or 40 years’ exploitation of big canals,the soil forming their walls becomes impregnated to a dcpth of 20 to 25 c m by fine colloidal particles and compounds of calcium carbonate. In the Soviet Union it is therefore recommended that during cleaning of sedimentary mud from canals this colmated stratum should be left undisturbed. From the standpoint of soil fertility and canal seepage muddy waters are therefore preferable to clean water. In the design ofirrigatingprojects it should be rememberedthat eventualcanal seepage will also depend on the phenomenon of ‘auto-colmatage’. 182

,

QUALITY OF IRRIGATION W A T E R

C.

WATER ANALYSIS

The analysis of irrigation water must include the total salt content,pdl, anion and cation composition and content of minor elements of particular importance to the crop involved (US Salinity Laboratory,1954; CHAPMANand PRATT, 1961; SKOVGSTADand FISHMAN, 1963). A sample form for recording irrigation water and analysis data is outlined in Table 7.11. 1. Total salt content This is determined either by measuring the specific electrical conductivity (expressed in micromhos/cm at 25°C) or by weight, the total salt content is then expressed in mg/l or ppm. 2. Cations

The cations generally determined are Ca++,M g + +, N a and K+, and occasionally also Cu++,Fe++and Li+. Ca plus M g can be determined by titration with different complex organic compounds (EDTAor CDTA), using Eriochrome Black T or calmagite indicator, then Ca+ alone is determined by using ammonium perpurate indicator.M g + is obtained by subtraction.Thesetwo elements may also be determined by using the flame photometer method.Na+ may be obtained by the uranyl zinc acetate method,by using the flame photometer method or by the sodium electrode method. IC+ is determined by the sodium cobaltinitrite method or with the flame photometer. Cu+ is obtained by the dithiazone carbamatemethod;Fe+ by the ferrous sulphate zinc soda method; and Li+ by ion exchange or flaDe spectro-photometermethods. + +

+ +

+

+

+

+

3. Anions

The anions generally determined are CO,=, HC0,-, SO,=,C1-and NO3-, and in special cases F-.CO,= and HC0,- are obtained by titration with H,SQ, or HQ,using phenolphthalein and methyl orange indicators;SO,' by precipitation as barium sulphate or by cation exchange;CI- by silver nitrate or mercurium nitrate titration or potentiometrically using a silver chloride electrode. For micro-determinationof chloride, a colorimetric method using diphenylcarbazone is recommended. NO, is determined by the sulphanilic acid alpha naphthylamine method,and F by the acid-zirconiumalizarin method or spectro-photometrically after prior separation of fluorine by anion exchange.

-

-

4. Minor elements

Boron and silica in the irrigation water are also determined occasionally.Boron is usually obtained by the mannitol titration method by carine colorimetricprocedure or by electro-titametric.Silica is determined by the molybdenum-blueprocedure.

D.

CHANGES IN SOILS BY IRRIGATION WATER

The long-termaction of irrigation water on different types of soil depends on the properties of the soil itself, and especially on drainage conditions and on the balance of subsoil water and salts. As is well known,the granularity,permeability, salinity,alkalinity and acidity of soils vary greatly according to the area (desert, humid tropics,temperate areas). The consequences of long-termirrigation depend also on annual rainfall, seasonal rain distribution, and annual evaporation,which is sometimes considerable. For more than 2000 years the peasants of North Africa and Arabia have utilised highly saline subsoil

183

IRRIGATION, DRAINAGE AND SALINITY Table 7.11. Description and analysis of irrigation water (Modijicationof USDA Handbook 60)

Collector’s description

Report of analysis

sample number Sample number 1. Date sample received 1. Location 2. Date sample analysed 2. Origin:spring,stream,lake,well* 3. Description on:Temperature ...Odour. ..Gas ... 3. Conductivity EC x loG at 25°C 4. Dissolved solids:g/1......kg/m3...... Colour.Specificcharacteristics 4.Depth of water on sampling date . . .Information 5. Dissolved organic matter: g/l......kg/m3.... 6. Total dissolved salts: gl/......kg/m3...... on seasonal depth fluctuations 5. Use :Irrigation,Municipal,Individual,Stock,Dom- 7. p H 8. Boron mg/l......Silica(Sioz)mg/l estic* 9. 6.Type of soil being irrigated

7.Observations on the water’s influence on the soil

Anions

/ I - me/l

.............................................. 8. Irrigation method employed : flooding, sprinkling,

furrow*

Ca++

I

I

I

9.Groundwater: Mean height . . . Maximum and Mg++ minimum level Naf IO. Crops irrigated (express yield in kg/ha)and amounts K+ of water applied 11. Are there any observed salinity effects? .............................................. Sums .............................................. ..............................................

mg 1

bo,-HC&-

so,-C1-

-

NO3-

10. Special determinations.......................... ..................................................

13. Name, address and profession of collector .................................................. .......................................... .... 1 1. Analysis by Reported by 14. Sampling date Signature Signature 15. Date sample forwarded to laboratory

Signature * Circle the proper word

water (containing up to 7g/l) for the irrigation ofsandy oaseswithout causing salinisation or deterioration of the soils. In Israel, water with 2300 mg/l total salt content (100 mg/l chloride) has been used to irrigate tolerant crops on sandy soils (SHALHEVETand REINIGER, 1964). Water, containing between 1800 and 10000 mg/l total salts,is also used in Italy in the Pouilles area (BOTTINI, 1961). In Texas,water with more than 4000mg/l total saltsis occasionally used (LONGENECKER and LYERLY, 1959). In contrast,fresh river water in Mesopotamia, Northern India,Pakistan,Iran and Central Asia induces strong secondary salinisation after a relatively shortperiod ofirrigation,leading to abandonment ofthe land. Excellent plantations ofdate and coconut palms are to be seen in salt lands bordering the sea or salt marshes and lakes.On the other hand,many bare spots ofland may be observed in irrigated cotton and alfalfafieldsof the Nile delta,in spite of the fact that the salinity of the irrigation water is very low. Theoretically there exists many potential combinations and long-termconsequenceswhich are difficult to predict,in the interrelationshipof water,soil and vegetation.Nevertheless an irrigation project can only be successfulon the basis of a thorough understanding ofpossible interactionstried under different conditions. 1. Direct chemical influenceof water on irrigated soils

It has been shown above how variable the qualities of irrigationwater are and that all waters are,as a rule, more or less mineralised.The final chemical action of such waters on soil depends on whether during and after irrigationsthe water is fully evaporated or partly removed through natural or artificial drainage;in the first case all dissolved compounds accumulate in the soil. The use of water with a relatively high salt content results in a marked increase in soil salinity within a relatively short period of time (less than 20 years). An example is given in Table 7.12.

184

QUALITY OF IRRIGATION WATER Table 7.12. Increase of soil salinity by using differentqualities of irrigation water in Texas (LONGENECKER and LYERLY, 1959) ~

~~

Location

~

Conductivity water used in irrigation pmho/cmat

Years under irrigation

Depth in cm

25°C

Labo flats area Soil:reddishbrown clay loams low permeable

Conductivity of saturation extract pmho/cm at 25°C Non-irrigatcd

Irrigated

454

5-7

0-25 25-60 60-90 90-120

650 1430 2700 3445

740 1300 2960 4000

Wild horse area Soil:reddishbrown sandy loams and loams

1960

5-7

0-25 25-60 60-90 90-120

480 500 730 1140

1190 1650 1780 1690

Pecos area Soil:grey silts and silt loam

4390

15-20

0-25 25-60 60-90 90-120

1915 2190 2770 3285

5525 5595 5580 5165

The soilsalinitymay increaseconsiderably after only one season of irrigation with saline water.Due to the lack of rainfall during the growing season,all the salts brought to the root zone by the irrigation water in a 'normal' irrigation programme without leaching,remain there. The accumulation is directly correlated with the amount of salt in the irrigation water and with the amount of water applied. In Israel,the use of water with an electrical conductivity ranging from 700 to 4000 micromhos/cm has increased the conductivity of the saturated soil extract from 200 to 2500 micromhos/cm after one irrigation season. The chloride content of a soil following prolonged irrigation is a result of the chloride content of the irrigationwater (CI,, and ,)the mean annualrainfall (Rm). Irrigationwith water having a high concentration of chloride generally results in an increasein the chloride content ofthe soilprofile,together with a total salinity increase. A broad Salinity Survey is being conducted in Israel. The survey was started in 1963. One of the main objectives of the survey is to determine the effect of irrigation waters on the sòil salinity and the possibility of perennial salt accumulation at the root zone of different plants. The Salinity Survey comprised,in 1968,about 360 plots (one half of which were citrus groves). The plots are located on about ten different soil types.The irrigationwaters contain between 2 and 15 me/l chlorides. The amounts of irrigation water applied to the citrus groves are between 500 and 700 mm per irrigation season and the annual rainfall in the winter season is between 250 and 700 mm. The collected data,pertaining to water-salt-soil-rain relationships,in each plot consists of:

I. Twice a year,in the spring and in the autumn, soils samples(twenty samples per plot) are taken for soil salinity testing (six different tests per sample) 2. Water salinities of every irrigation 3. Seasonal water applications 4.Rainfall data from every plot vicinity 5. Characteristic soil properties of the plot

It was found that in most cases the salts which are introduced with the irrigation water during the irrigation season accumulate in the 0-150 c m soil profile and during the winter season the soluble salts are leached down the soil profile. In light soils the entire sampIed profile (0-150cm) is being fully leached while in heavy soils only the upper 185

IRRIGATION, DRAINAGE A N D SALINITY 0-60, 0-90 c m profile is being leached.There are many cases where a perennial salt accumulation occurs in the 90-150c m layer of the heavier soils. It was found that in the spring,the Concentration of the chlorides in the 0-90 cm profile can be given by the following multiple regression equation:

R =0.798

n=135

where: y=Average concentration of CI in the saturated soil paste extract in the spring,to a depth of 0-30 cm,in me/l X,=Average concentration of C1 in the saturated soil paste extract in the autumn, to a depth of 0-90 cm,in me/l X,=Total amount of rain during the winter season,in mm X,= Saturation percentage of the soil paste,in % R = Multiple correlation coefficient n=Number of samples per variable (a) Dilution of soil solutions When the soil moisture is separated from the soil by means of a special pressing apparatus or by replacing through alcohol we obtainthe so-called‘soilsolution’.Many studies by KOVDA in the USSR have shown that the salt concentration of such soil solutions is, as a rule,much higher than the concentration of any type of waters used in irrigation. Even the best non-salineirrigated soils of Central Asia and Transcaucasia have a concentration of soil solution of 4to 8 g/l. Solutions of slightly and medium saline soils have a considerably greater concentration (20 to 30 g/l). In strongly saline soils,including the solonchaks,the concentrations in the top horizons reach 1CO to 300 g/l.Typical concentrations of the best river water vary between 0.2 and 0.5 g/l. Irrigation with such waters causes very strong dilution of concentrated soil solutions. Even brackish irrigation water with 2 to 5 g/l is several timesless concentrated than the solution of saline soils.Therefore each application ofwater is followed by a strong temporary decrease ofthe salinity ofthe soil’stop horizons.Even the leaching of strong solonchakswith seawater (35 g/l) would be followed by a considerable decrease of the extremely high concentration ofthe soil solution.On thisphenomenon surprising cases of effective utilisation of mineralised brackish waters for irrigation in deserts are based. The harmful effects of mineralised water are the result of cumulative enrichment of soluble salts after a large number ofirrigations without sufficient drainage and leaching of salts.After each irrigation the diluted soilsolutionis transpiredby plants or evaporated and again concentrated.If the dilution is great enoughand if the downward flow ofirrigationwater is drained,even a relativelyhigh concentrationof saltsin applied water remainsharmless. Physiologicallythe fatallevel of the cmcentrationofthe soilsolution according to Mov~n’s studies is about 12 to 15 g/l;normal plants such as cotton and alfalfa cannot exist if this level is exceeded. Itmeans that any irrigationwater with a saltcontent below this limit,for example 3 to 7 g/l,can be successfully used for the improvement and irrigation of saline soils. The only but most important condition is to maintain adequate leaching and drainage. (b) Dissolution and precipitation of some chemical components in soils

In any soil there is a sort of mobile equilibrium between components dissolved in the soil solution and the components deposited in the form of crystallised salts. The less soltible compounds are calcium carbonate, calcium sulphate and to some extent sodium sulphate;a considerable part of them is usl-lallypresent in the soil in the form of crystallised deposits.Irrigation water can play the role of a very strong solvent:the fresher the water, the stronger is its capacity to dissolve these deposits. Irrigation water, very fresh and even acid owing to the presence of CO,,can dissolve calcium carbonate, transforming it into calcium bicarbonate. The same occurs with gypsum and sodium sulphate. So the content of these components in the soil solution after each irrigation will be very different from their content in irrigation water. Even irrigation water of greater concentration (5 to 7 g/l), particularly if it contains NaCI,has a great capacity for dissolving calcium carbonate and calcium sulphate deposits in the soil. This phenomenon may have very strong positive eReck

186

QUALITY O F IRRIGATION W A T E R on the physiological salt toxicity,decreasing the harmful influence of the non-equilibrated solutions.On the other hand,many soluble componentspresent in applied water can precipitate after irrigation (for instance dissolved sodium carbonate or bicarbonate if the soils contain some amount of gypsum or calcium chloride). As soon as the irrigation is terminated and evaporation starts,the less soluble componentswill subsequently precipitate in the soil.

(c) Temporary soil alkalinity increase after irrigation One importantphenomenon should be particularly stressed:the temporary,sometimes very sharp rise of the alkalinity ofsoil solutionafter each routinewatering. KOVDA has shown that the greater the concentration of soil solution before each irrigation the more immediate will be the alkalinity increase after irrigation.So the p H ofthe soil solutionincreased from 7to 8 up to 9 or 10,which is very harmful for cotton, alfalfa and many other irrigated plants (Figs. 3 and 4). This alkalinity increasecould persist for three or four days, depending on the temperatureand biochemistry ofthe environment,then it gradually disappears.This phenomenon has been experimentally studied by SHAVRIGIN with solutions especially extracted from strongly saline soils and diluted with increasing doses of distilled water. The result of the experiment is given in Table 7.13 and confirmsthe phenomenon observed under field conditions:both p H and titrated alkalinity rose considerablywith dilution.However,the alkalinity increase was replaced by a gradual decrease towards the original alkalinity level when dilution is as great as 1000.This temporary increase in irrigated soilsis widely known in Asia and Transcaucasia because it is sometimesfollowed by the parching ofirrigated cotton,alfalfa or rice.To prevent this, one must apply a very great depth of irrigation water in order to dilute as much as possible the soil solution and to leach the products of alkalinity.But it is much better to ameliorate the saline soil in advance in order to prevent even the possibility of such an alkalinity increase.

*or+< o

2

40

4

/*

I



60 80

;/

100

Depth in cm

Fig.7.3. Dynamicsofthealkalinity of irrigatedsoih ofthe Ferghana valley (insoil solution)-(After

KOVDA)

1. CO,' before watering, 7 July 1940 2. CO,' after watering, 10 July 1940 3. HC0,- before watering, 7 July 1940 4.HC0,- after watering, 10 July 1940 Table 7.13. Changes of p H of soil solution of solonchaksfollowing dilution (initialconcentration was 325.8g/l;after KOVDA and SHAVRIGIN) Water added (in cm3)to 15cm3of soil solution

O 5 10 30 130 260 410 870 1210 1710 2210 O

PH 7.98 8.93 9.18 9.18 9.18 9.18 8.92 8.60 8-24 8-02 8.02

187

IRRIGATION, D R A I N A G E A N D SALINITY D

C1, me/l saturation extract 24

-

20

E

-

4 2

5

O

4 8 CI,me/l

12

Q-9 o

10 15 20 25 30 35 Concentration of soil solution before watering g/1

Fig.7.4. Alkalinity increase of soil solutions in relation to their concentration-(After

KOVDA)

(d) Changes ii2 adsorbed cations Chemical compounds of any type in irrigation water are very strong reagents,influencing the ratio,quantity and quality of the cations adsorbed previously by clay minerals and by organic soil colloids. The long-term chemical effect of irrigation water on adsorbed cations of soils is one of the most important questions of the problem here discussed.These consequencescan be either very positive or negative,i.e. increase or decrease of the natural soilfertility.Of specialimportance is the forecasting ofthe hazard of secondary alkalinity of soils after a long period of irrigation. Alkalinity of irrigated soils considerably reduces their productivity.Table 7.14shows the influence of two types of water with different N a content on the same soil after seven consecutive years of irrigation:prior to irrigation the exchangeable sodium percentage was low,and after seven years the soils were alkaline. Table 7.14. Changes in the exchangeable sodiunipercentage (ESP)afer seven years’ irrigation with sodic water (RAVIKOVITCH and MURAVSKY, 1959)

N a in irrigation water (in me/l)

5.7

Depth N a (in me/100g) in cm and ESP in soil Na 0-30 30-60 60-90

3.7 4.1 5.0

18.4

After

Before

ESP

Na

ESP

Na

14.2 14-5 16.8

4.2 5.0 7.9

16.0 18.9 26.1

3e 4 4.4 5-1

After

Before ESP

Na

ESP

12.9 14.6 16.1

7.2 7.6 8.9

22.0 28.9 29.0

Soil alkalisation is more pronounced when sodium carbonate and bicarbonate are present in the irrigated soil.Ifwater supplied contains more COs-- and HC0,-than Ca4 and M g + +, then after evaporation and plant uptake calcium and magnesium precipitate as carbonates,the residue of COs=being paired with Na+. In this manner,soda alkali soils were formed.The examination ofNile and Tigris water over a long period of time (Table 7.15) showsthat the Nile water produced alkali soil due to the appearance of sodium carbonate. while the Tigris water did not have such an effect. +

188

QUALITY OF IRRIGATION W A T E R Table 7.15. Chemical composifbn of the Nile and Tigris water (EATON, 1950) Cations (nie/l) Source

Anions (me/l)

Residual sodium

Years

CafMg

Na

HCOs

so4

CI

bicarbonate

Flood : Nile, 4 months Tigris, 3 months

1906-36 1925-28

1.21 3.24

0-50 0.38

1.44 2.75

0-16 0-49

0.14 0.39

0.23

Low: Nile,8 months Tigris, 9 months

1906-36 1925-28

1.59 4.42

2.04 0.71

3.03 3.56

0-23 0.94

0-46 0.63

1 -44 0.00

0.00

Research carried out by Russian,Indian,American and British scientistshas shownthat the productivity of alkali soils is correlated with their exchangeable sodium percentage, alkalinity and p H (see Chapter 3). The sodium in irrigationwater also influences the physical properties of the soil,particularly permeability, by afiecting the swelling and dispersion of the clay.Ifthe ratio Na/totalcationsis high in the irrigation water and initially low in the soil, the increase of the ESP causes a reduction in the permeability. The sodium concentration causing a decrease in initial permeability of 10 to 15 %is designated as the threshold concentration (QUIRK and SCOFIELD,1955). O n the other hand a general increase in electrolyte concentration in the applied water leads to an increase in the permeability of alkali soils (Fig. 7.5). The same was observed in 1939 by FIREMAN and BODMAN. Electrolyte concentration me/l

5m Decreasing permeability

O

10 20 30 40 Exchangeable sodium Percentage

50

Fig. 7.5. Electrolyte concentration required to maintain soil permeability with various degrees of sodium

saturation (QUIRK, 1957) Recently MCNEAL and COLEMAN (1966)have provided detailed information on the effect ofa wide range of salt solutions on the hydraulic conductivity of several well-characterisedsoils from California. The soil commonly demonstrated a rather pronounced decrease in hydraulic conductivityin the exchangeable sodium percentage range of 25 to 35 and a salt concentration of 3 to 50 me/l. YARON and THOMAS (1968) have found that the soil hydraulic conductivity became constant only when a constant exchangeable sodium percentage was achieved along the soil profile.Under field conditions,where a non-sodicsoil is irrigated with sodic water,a constant exchangeable content along the soil profile is difficult to achieve.Therefore,the transitional series of values determined by the volume of specific water passed through a specific soil profile must be taken into consideration.The continuous change in the flow rate of the solution during the period between the first addition of sodic water and the achievement of equilibrium suggeststhat hydraulic conductivity measurements made at equilibrium are not particularly valuable for field prediction. The mean exchangeable sodium percentage over a depth of soil controlsthe hydraulic conductivityin that 189

IRRIGATION, DRAINAGE A N D SALINITY soil.The ESP,in turn,is a function of the cation exchange coefficient,which is a measure of relative sodium afinityandis controlledlargelyby mineralogy ofthe soil.The mean ESP is also a function ofthe total amount ofsalt eluted through the soil until equilibrium is attained.By knowingthe soilproperties and the composition and amount of irrigation water applied to the soil the hydraulic conductivity may be estimated.Good agreeand THOMAS (1968) between the calculated and experimental value for four ment was obtained by YARON Texas soils. FIREMAN’S investigations showed rapid alterations in the base status of soil by saline water. The most striking effects were the increasesin exchangeable sodium: the ESP,initially 2.3%and 1.2%,increased after application of 1500to 2250 mm ofhigh sodium water,to 20% corresponding to average ratio of exchangeable calcium to sodium of about 2.8. Continued percolation with water of the same quality,however,produced a higher exchangeable calcium content and at equilibrium with the high sodium water both soils would have a calcium/sodium ratio of approximately 4.During certain stages in the alteration magnesium may have been displaced from the upper to the lower soil layers.No definite relation could be established between the velocity of wave flow through the soil and the alterations in base status. The fact that high electrolyte concentration can influence soil permeability has suggested that leaching of alkali soils should be facilitated or started with water of a sufficiently high electrolyte content to increase soil permeability (REEVE and BOWER, 1960). Normal (i.e. non-salineand non-alkaline)water (concentration 0.2 to 0-5g/l) has usually an ameliorating effect ifemployed in the irrigation ofalkali soils.Calcium usually dominates among the cations ofsuch water. After several years or decades of irrigation by fresh calcium-bearing water alkaline soils have a lower pH and their physical,chemical and biological properties are greatly improved. This is the result of very simple physiochemical reactions: /Na+ Soil +Ca (HCO,),= Soil=Ca +2NaHC0, \Na Na,CO, +CaSO,= CaCO, +Na,S04

--

+

The rate and degree of this reaction depend on the quantity of calcium in the irrigation water and on the alkalinity of the irrigated soil. Slightly alkaline water (containing sodium bicarbonate and carbonate) from springs,lakes or underground, having as a rule a total salt concentration of 0.7 to 1-5g/l,can after several years of irrigation cause a very strong alkalisation of soils,including total disaggregation,accumulationof adsorbed sodium and magnesium and of free sodium carbonate and bicarbonate. Alkalisation of this type can reduce or even fully destroy the natural fertility of the irrigated soil. Irrigation practice has shown several examples of such difficulties connected with high alkalinity of irrigation waters in the USA, in the basin of the Nile and its delta,in some areas of Transcaucasia, India,Pakistan and China. Excess of free sodium carbonate and bicarbonate in irrigation water is extremely dangerous.The intensity of the sodium adsorption increases proportionately to the concentration of soda in water (Fig. 7.6). But even when soda content is very low (1 or 2 me/& the irrigation water causes strong alkalinity after repeated use. ANTIPOV-KARATAEV (1960) has found the relation between the adsorption of sodium and the concentration of soda to be:

Y =Ki +Kz l0gC where

Y=exchangeable sodium (in me/100g) in soil

C = soda content of the water (in me/l) Kland K,=Coefficients (KI= 12,Kz=92) Fortunately,the top horizons ofmany saline soils are very high in gypsum,and there is no danger in using alkaline waters for their irrigation. The use of alkaline waters in gypsum-free soils,however,is inevitably followed by a secondary alkalisation. Alkaline soils can be ameliorated and secondary alkalisation prevented by adding gypsum and organic manure producing CO,.It should be borne in mind that after decades ofirrigationwith alkaline water the natural gypsum resources of the soil may be exhausted and the soil then needs the addition of gypsum. Mineralised irrigation water with a salt content of 5 to 10 g/1,saturated with calcium bicarbonate and calcium sulphate,even with high sodium/caIcium ratio, never leads to increased alkalinity of the soil. The

190

QUALITY OF IRRIGATION WATER accumulation of gypsum and calcium carbonatein the soil prevents and even improves alkalinity.Under field conditions moreover CO,production, leading to formation of calcium bicarbonate, will tend to decrease alkalinity. Although the changesin adsorbed cationsunder field conditions may differ widely from those in laboratory experiments this does not affect the conclusions about the potential danger of alkalisation by sodacontaining irrigation water on soils without gypsum. The influence of continued irrigation on adsorbed cations is particularly complicated. The influence of different types of water on soils is indicated in Table 7.16.

:I

Sorption of N a (m.e./lOOg) 100

40 20 O

0.20

0.30

0-48

0.70

Soda content (me/l)- (log. soda)

100

Fig. 7.6.Relation between the intensity of adsorption of sodium and the concentration of soda in solutions and KADER) -(After ANTIPOV-KARATAEV Table 7.16. Results of irrigation as influencedby water and soil Type of water

Type of soil

Results of long-termirrigation

'Normal'

(a) Normal

Positive,stable fertility

neutral reaction; Ca predominating

(b) Alkaline

Positive,gradual dealkalinisation

Alkaline salt cont.=050-0.6 g/1 high quantity of sodium carbonate and bicarbonate

(a) Normal (b) Alkaline (c) Acid (d) Gypsoferous

Negative:alkalinisation Strongly negative: increase of alkalinity Neutralisation Positive:no alkalinisation

Acid,with presence of H,CO,,H,S04, etc.

(a) Normal (b) Alkaline (c) Acid

Slight acidity increase Positive:reclamation Strong acidity increase

Brackish salt cont. 3 to 7 g/1

(a) Normal (b) Alkaline (c) Acid

Depending on drainage Positive:neutralisation Positive:neutraIisation

salt cont.= 0.25440 g/l

2. Other influences of irrigation water on soils

(a) Inflow of dissolved nutrients Many componentsdissolved in irrigationwater are very usefulfor plants as long as their concentration is low. Irrigation water usually contains some quantity of dissolved compounds of N,P,KyMg,C a and trace elements (Ni,Co,Cu) which play a very importantrolein plant nutrition.River,lake and subterranean waters 191

IRRIGATION, DRAINAGE A N D SALINITY are richer in these nutritious elements than rainwater or river water in the humid tropics or northern forest areas.Uusually the irrigation waters enrich the soil with nutritious elements.As an example in the A m u Darya basin the irrigated soils received per hectare and per year in dissolved form: 10 kg of P,05,60 kgofIC, 100 kg o€ Mg, and 500 to 600 kg of Ca, together with other nutritious elements in silt sediments (Table 7.6). These compoundsform a permanent basis for a stable fertility of irrigated soils of the delta.Complementary use of fertilisers and manure can certainly increase the soil fertility much more. (b) Specific chemical and biochenzical action of water

FIREMAN pointed out some specificaspects ofthe directinfluenceofwater on the biochemistry ofirrigated soils. The use of brackish irrigation waters depresses nitrogen fixation,nitrification and ammonification in soils. It has been recognised recently that irrigation water quality may have an important bearing on certain diseases or disease-likeproblems ofcrop plants.Certainwilting has been caused in California by a decrease of soilpermeability as a result ofhigh sodium-adsorptionratio,and low total saltcontent ofthe irrigation water. Blossom-endcot of tomato has the appearance of a disease but is a nutritional problem caused by irrigation with high calcium water which depresses potassium absorption by the plant. Water quality is probably an important factor in such problems,as (FIREMAN): 1. the formation or cementation of claypans, hardpans and limepans (caliche) 2. the corrosion of metal pipes (by acid and ‘corrosive’waters) 3. the disintegration of certain cements (by high sulphate waters) 4.the harmful deposition ofcalcium carbonate scale on some deep-wellpumps (by high calcium bicarbonate waters)

E. FACTORS AFFECTING SUITABILITY OF WATER FOR IRRIGATION Five factors must be taken into consideration before the suitability for irrigation of a water of a given total salt content is determined.These are:

1. chemical water composition 2. crop to be irrigated 3. soils to be irrigated 4. climate 5. management of irrigation and drainage

The interaction of these five factors constitutes a water classification. 1. Chemical composition of irrigation water The quality of the water is determined by the total salt content and by its ionic composition.The total salt content (expressed g/l,me/l,ppm) or the electrical conductivity may give a general indication of the water’s quality. Also important is the determination of the main cations and anions usually expressed in me/l,as several ion ratios influence water suitability.This will be discussed later. Under certain conditions,the presence of micro-elementsmust be taken into account. 2. Crops

In this chapter we are mainly interested in classifying water for agricultural purposes. Therefore,the crop is the firstand most importantfactor to be considered.The evaluation ofa water must be based on the tolerance ofa specificcrop or crops in the rotationto the total saltcontent or specificion concentration.The tolerance of a crop to salinityis that concentrationofthe soil solution thatwill give a certain reduction inyield as compared 192

QUALITY OF IRRIGATION WATER to non-salineconditions. In the USA the 50% yield decrement is taken as the tolerance limit for field and forage crops.In Holland and in Algeria the 25%and 20%decrement is taken.The American 50%decrement is taken for the salinity measured at the bottom of the root zone,whereas in Holland the salinity of the top layer is taken as the criterion. Table 7.17 compares the tolerance limits of some plants from three regions. Despite the differences in conditions the similarity ofrecommendation is striking and indicatesthatthe results of one region can be transferred with few restrictions to another one. Table 7.17. The tolerance of some crops to soil salinity,for three regions

USA (ECmmhos/cm)

Crop

Region North Africa (ECmmhos/cm)

Holland

(41)

~~

Barley

Sugar beet Cotton Wheat Oats

Corn Beans

18 16 16 14 12 7.5 3.0

17

12 14 12 7 4

13 14 10 13

4

Some crops are specifically sensitive to chloride and sodium ion concentration. The most important of these crops are the deciduous trees, citrus and avocado. 3. Soils

The behaviour of a soilin contactwith salinewater depends on the initialphysicalproperties and salt content.

The clay content of the soil affects the ion adsorption capacity which in turn influences the hydrophysical properties. Furthermore,the presence of an impermeable layer or of a groundwater table affects the salt distribution in the profile. The initialchemical compositionofthe soilinfluencesthe exchangeprocesses during thewater-soil contact. The application ofsalinewater to a salt-freesoilwill salinise the soil,but the use ofthis same quality of water may reduce the salinity of a saline soil if drainage is adequate. As infiltration and percolation of water may differ greatly for different soils,different degrees of salinisation may be expected with the same quantity and quality of irrigation water. 4. CIimate

Evapotranspiration and rainfall are the two main climatic elements to be considered when evaluating suitability of water for irrigation.The water depth to be applied to a crop during a season depends on the evapotranspiration which therefore affects the irrigation regime and consequently the seasonal dynamics of salts in the soil profile. The amount and distribution of rainfall is the second factor of the climate to be paid attention. A given amount of rainfall distributed uniformly over the growing season will dilute the soil solution,but will not bring about a leaching of the profile as the same amount of rain falling during a shorter time would do. 5. Management of irrigation and drainage

The irrigationmethod influencesthe saltaccumulationin the soil and in the plant.The applicationofamounts of water less than the consumptive use will result in accumulation of salts in the main root zone.Increasing the application will leach the salts out of the root zone,and an equilibrium can be reached between the salt contents of the water and of the soil. Lack of proper drainage in an area with a high water table will result in capillary rise of groundwater, increasing the soil salinity. Relatively saline water applied in furrows on a permeable soil will have no harmful effect on plant growth, while the same quality of water applied by sprinkling might cause reduced yields.

193

IRRIGATION, D R A I N A G E A N D SALINITY

F.

EVALUATION OF IRRIGATION WATER

Various types of water are used for irrigation,therefore it was necessary to set up a particular system of criteriaofwater quality since those in use for geochemical,industrialand sanitarypurposeswere not suitable. The hazards to be considered when evaluating suitability ofwater for irrigationpurposes are thefollowing: salinity,sodium,carbonate alkalisation,chloride and boron. It is not possible to present a classification ofirrigationwater which may be utilised at all sites and underall conditions.Therefore,we will here make a comparativeanalysis of some of the existing classifications and indicate how they are being used.

1. Salinity hazard

In the USSR the evaluation is made as follows: salt content

evaluation

(SI0

water of the best quality water causing salinity and alkalinity hazard water could be used for irrigation only with leaching and perfect drainage

0.2 to 0.5 1 to 2 3 to 7

The standardsset up by the US Salinity Laboratory are shown in Table 7.18. TabIe 7.18. US Salinity Laboratory's grouping of irrigation water

Classification of water

Electricalconductivity in pnhos/cm at 25°C (EC)

Salt concentration in g/l(approximate)

O < ECG 250

< 0.2

250 10000 Group I

where a=soluble salt content of the soil at the beginning of the observations (g/lOO g) b=soluble salt content of the soil at the last observation (g/lOO g) c =salt content in the irrigation water (g/l or kg/m3) d=salt regime constant of the soil (g/100g) v = amount of the irrigation water applied (m3/ha) M=thickness of the soil layer considered (m) tfs= volume weight of the soil This equation can be used to calculate the maximum permissible salt concentration in the irrigation water, by choosing the other factors to result in a stable salt balance; or,if the salt concentration in the irrigation water is given,to forecast the change in soil salt content due to irrigation. For the calculation of the maximum permissible salt concentration with which soil salt content would not be changed by irrigation (a=b),the above equation is used in the following form: c=- d.M.tfs

.105

V

The change in soil salt content to be expected,if a known amount ofirrigationwater of a given salt concentration is applied,can be calculated by using the same equation in the following form: b=a+

(

cv

d + ~ 10-5) . M.t,,

From the data presented here,it can be seen that the total salt content of the irrigation water is of general descriptivevalue only, and the limits established in one classification system are not applicable without restriction to every place and condition. 2. Sodium hazard Due to its effect on the soil and plant,sodium is considered to be one of the major factors governing water quality. Several methods were proposed for expressing the sodium hazard. Previously water quality was defined on the basis of its sodium percentage alone.The soluble-sodiumpercentage (SSP)may be calculated by the formula: soluble-sodiumconcentration (me/l) x 100 SSP = total soluble cation concentration (me/l)

SCOFIELD (1935) and MAGISTAD and CI-IRISTIANSEN (1944) considered water with an SSP of60 as harmful. GREENE (1948) raised this lower limit to 80%for water having a total salt content less than 10 me/l. The sodium hazard,determined by the SSP ofirrigation water,must be reflected in theexchangeable sodium 196

QUALITY OF IRRIGATION W A T E R

percentage (ESP)of the soil. However, in research conducted in western Texas,no correlation was found and LYLERY, 1598). between the SSP and the EST (LONGENECKER Another index for predicting the sodium hazard was developed in India by PURI(1949). Called the ‘salt index’i’c is based on the relation between sodium,calcium and calcium carbonate existing in the water. The empirical formula is : ‘saltindex’= (total N a -24.5) -(total Ca -Ca in CaCO,) x 485

Thisindex is negative for irrigation water of high quality (-24.5 to O) and positive for harmful water. Comparing the sodium hazard as defined by the ‘saltindex’with other classifications no similarity could be found (DARRA et al., 1964). It has been established that the adsorption of sodium depends on the concentration of its soluble salts established thatthe critical and particularly on the bivalent/monovalent cations ratio.For instanceLOBANOVA calcium/sodiumratio could be changed depending on the totalconcentration ofsolution:solutionsofmixtures ofcalcium and sodium chloridescause alkalinity (exchangeableN a > 10%)under the following concentration (C): C=0*58 g/1 only if Ca= 5% and Na=95 % of sum of cations C=6.5 g/1only if Ca=33 % and Na=67% of sum of cations C = 170 g/1only if Ca = 67% and N a = 33 % of sum of cations WILCOX (1958) submitted a diagram,illustratingthe relation between the quality ofirrigation water and the Na+: (Ca+++Mg++) ratio (Fig. 7.7). The sodium classification of irrigation water after WILCOX is as follows: Low-sodiumwater (3,)can be used for irrigation on almostall soils,with little danger ofthe developmentof a sodium problem.However,sodium-sensitivecrops,such as stone fruit,trees and avocados,may accumulate injurious amounts of sodium in the leaves. Medium-sodium water (SJmay present a moderate sodium problem in fine-textured(clay) soils unless thereis gypsum in the soil.Thiswater can be used on coarse-textured(sandy) or organic soils that takewaters well. High-sodiumwater (S,)may produce some sodium problems in most soils and will require special management: good drainage,high leaching and additions of organic matter. Ifthere is plenty of gypsum in the soil, a serious problem may not develop for some time;if gypsum is not present,it, or some similar material,may have to be added Very high-sodium water (SJis generally unsatisfactory for irrigation except on low- or medium-salinity levels where the use of gypsum or some other amendment makes its use possible. ANTIPOV-KARATAEV gave the followingequation for an evaluation of water with respectto sodium hazard :

x,= [Ca+Mgl

[Na1 .Ylois the index of the ‘criticalratio’[Ca+Mg1 in irrigation water, when the ESP in the irrigated soil is 10. [Na1 In this equation X,,=K.C,where C is the total concentration of soluble salts (in g/1) and I< is equal to 0.23.Ifthe ratio is [Ca+Mg1 lessthan 0.23C,then amelioration of quality of water is required (dilution, or

1“

addition,of CaSOJ.

A similar approach can be found in the publication of the US Salinity Laboratory:the sodium hazard of the irrigation water can be evaluated through the sodium-adsorptionratio (SAR),defined as ‘theratio for soils extracts and irrigation waters used to express’.The relative activity of sodium ions in exchange relations

with soil, SAR =

Na+ Ca+++Mg’+ 7 - 2

where the ionic concentrations are expressed in milliequivalents per litre. As indicated below an empirical relation has been drawn up between S A R and ESP. 197

IRRIGATION, D R A I N A G E A N D SALINITY Sodium (mes./¡>

Calcium plus magnesium (m.eq./l)

Fig. 7.7. Sodium diagram (after WILCOX) The classification of water according to the SAR is also related to the water’s electrical conductivity (and therefore its salt concentration). Four groups are indicated:low,medium,high and very high electrical conductivity. For EC= 100 micromhos/cm,the dividing points are at the SAR values of 10,18,and 26, and with EC=750 micromhos/cm,the dividing points are at the SAR values, 6,10 and 18. This relation representing the relative activity of the sodium ion in the cation exchange reaction with the soil is derived from the classical Gapon equation:

=K.

Na, Mg,

/(Ca 4

(Na+) +

+) +(Mgff) 2

In this equation Na,., Ca, and Mg, stand for the exchangeable soil cations (meper 100 g soil). (Na+), (Ca+ +) and (Mg++) are the concentrationsof these cations in the soil solution. Kis a constant,determined by the characteristics ofthe soil,with a value generally between 0.01 and 0.015. An application of the Gapon equation for severaltypes ofwater has been given by SZABOLCSand DARAB. The results are presented in Figs. 7.8 and 7.9. The validity ofthe sodium hazard prediction may be established by the followingempirical relation between SAR and ESP: ESP = 100 (-0.0126+0*01475 SAR) 1 +(-0*0126+0*01475 SAR) Total salt concentration in the irrigation water H.(H.me/l) 300

-

200

-

100

-

O

20

40

60

80

100

Alkalinity quotient of thc irrigation water (Na%)

Fig. 7.8. Graphical form of the Gapon equation-(After

198

SZABOLCSand DARAB)

QUALITY O F IRRIGATION W A T E R Total salt concentration in the irrigation water (mg/1)

20 30 40 50 60 70 80 90 100 Alkalinity quotient of the irrigation water (Na%) ----HCOT .-.-.HCO< SOT-

-HCO,

- Cl-

-

SOT

-

Fig. 7.9. Application of the Gapon equationto some differenttypesofwaters-(After

SZABOLCSand DARAB)

In general,good correlation was found between ESP calculated from S A R and ESP experimentally determined. For example,the following coefficients of correlation (r) were found:for the western Texas soil0.931 (LONGENECKER and LYERLY, 1958), for the Kansas soil,0.81 (JACOBS et al., 1961), and for three different soil types in Israel: grumusol 0.856,grumusolic dark brown 0.906,residual dark brown 0.925 (Israel Salinity Survey, 1964). A good correlation was also found in the Mirrool Irrigation Area, Australia, (GROENEWEGEN, 1961). However,DURAND (1958) has observed in Algeria wide differences between calculated and determined ESP in 40%of the cases,this difference varied between 100 and 200%. LANGELIER has defined a saturation index as the actual p H of a water (pH,) minus the p H (obtained by calculation) which the water will have when it is in equilibrium with CaCO, (pH,). Using a modified Langelier index in combination with the SAR,BOWER(1961,1963) proposed for highcarbonate water and without residual sodium bicarbonate the tentative empirical equation:

ESP=2 SAR+2 SAR (8.4-pHc) In this tentative equation the term ‘(8-4-pHc)’ is analogous to LANGELIER’S saturation index except that 8.4,i.e. the approximate p H reading of a non-sodicsoil in equilibrium with CaCO,, is substituted for the actual p H value (pH,) of the water. The application of this equation to a series of well waters from West Pakistan,has shown a good correlation between calculated and determined ESP.

3. Bicarbonate hazard

The bicarbonate anion is important in irrigation water as regards precipitation of calcium and,to a lesser degree,also ofmagnesium in the soil.This brings about a change in the SSP in the irrigationwater,and therefore an increase of the sodium hazard. In this connection special attention has been paid to the group of alkaline (soda holding) low concentratedirrigationwatersby USSR scientistsin 1946.EATON(1950) introduced the term ‘residualsodium carbonate’ (RSC): RSC=(CO,--

+HCO;)-(Ca++

+Mg++) (in me/l)

WILCOX (1955) concluded that if RSC > 2-5the water is not suitable for irrigation, if 1.25< RSC equals 4.27at 20°C, 7.11 at 30°C and 9.52 at 40°C. Thus, the order of magnitude of the second term is fairly well determined by prevailing temperaturesand its value is linearly dependent upon the windspeed. In practice, one can conclude that in humid and temperate,or humid and warm, climates,the radiation balance determines the evaporation potential at low and moderate windspeeds,but in arid and particularly in arid and warm climates,temperature and windspeed assume an importantrole and the evaporation potential may exceed the radiation balance.This general situation is illustrated in Fig.8.3,computed for noon-time conditions and clear skies.

Fig. 8.3. Potential evaporation computed for a standard surface (roughness parameter 1 cm) for three typical environments as a function of windspeed at 2 m above the surface. Arrow indicates net radiationequivalent at noon,assumed temperature, and vapour pressure in degrees centigrade and millibars, respectively

The form of the wind functionf(u) in (9) can be obtained from empirical data, or from turbulent transfer theory. The latter method is probably better because in that way the roughness of the evaporating surface can be accounted for. The resulting expression is: 1*22 ' mm per m b and unit time f(')= [ln(z/z>lz in which u is the windspeed in metres per unit time,zthe height at which u is measured, and z,the roughness parameter,both in cm. Accordingly,the potential evapotranspirationfrom rough surfaceswill be greater than that from smooth ones. Such a difference would be marked in an arid and hot environment,whereas it may not be noticeable

224

W A T E R , P L A N T G R O W T H A N D C R O P IRRIGATION

in a humid and cool climate. It is also implied that the potential (and actual) evapotranspiration from crop surfaces can be greater than from open water-all other things being equal. It should also be clear now that the water loss from a standardised open water surface such as an evaporation pan will not have any fixed or predictable relation to the losses from another evaporation surface since both H andf(u) will be different in the two situations. A study of evaporation pan records tells us very little about the evaporation from natural surfaces. Combination equations have the great merit of requiring only standard weather data and can therefore be used as means to predict potentid evaporation since statistical information on radiation, windspeed, vapour pressure and temperature is available,though not as widely as one could wish. In practice, it will be the hardest to obtain a good estimate of H and a realistic appraisal off(u). Summarising,we can concludethat the role of the principal weather parameters in the evapotranspiration process is well understood. However, the practical application of this knowledge has not been perfected. The so-called‘advection’effect is implied by any meteorological approach in that the properties of the air mass reflect the general environment on a large scale. The precise nature of so-called‘edge’effects at the periphery of irrigated fields is not well known.

3. Soil factors

When potential evapotranspiration is calculated or measured, it is implied that water is freely available for evaporation at the surface.If such is not the case,the actual evapotranspiration will be less. Physically,this reduction can be explained either by the existence of a diffusive barrier to the vapour flow at the boundary of the evaporating moist body, or as a lowering of the relative vapour pressure, because of a reduction of the water potential. An example of the first instance is the effect of a crop residue mulch on evaporation from a moist soil surface and,of the second,evaporation from highly saline lakes, such as the Dead Sea or the Great Salt Lake. It seems that,in practical agriculture, only the former process has a significant role. W e will now consider the effect of soil water content upon soil evaporation. Evaporation from bare,wet soil proceeds at essentially the potential rate as long as the relative vapour pressure is at least 0-95at the surface,equivalent to a water potential of -25 bars,that is to say,the soil w ill be quite dry at the surface when evaporation starts to decrease. If there exists a water table close enough to the surface to permit water to flow upward at the potential evaporation rate, the surface will never dry out. The required depth depends upon the moisture characteristic and conductivity ofthe soil as well as the potential evaporationrate, as shown by GARDNER (1958). Generally,this will be quite a shallow depth. At greater water table depths,the equilibrium evaporation rate will be less, until at depths of around two metres the presence of a water table is of no account and the soil dries progressively slower with time. In well-managedirrigation projects a situation will probably never exist where upward flow from a water table determines the evaporation from bare soil.Such a condition could rapidly result in salinisation ofthe surface. The common event is the formation ofa dry layer at the soil surface in which moisture transfer is essentially a vapour diffusion process. The diffusive resistance together with the ambient condition determines the evaporation rate.As the dry layer increasesinthickness,the evaporationreduction becomes greater.Although the following analysis is oversimplified,one can obtain a useful grasp of the situation by considering the thickness and porosity of the dry surface layer. For example, if the soil consists of a coarse sand and if a 3.0 cm thick dry layer had formed, its diffusion impedance would at 25°C be equal to 47 seclcm (VAN BAVEL,1952). At a windspeed of 2 m/sec and bare soilwith a roughnessparameter of 1 cm,the corresponding atmospheric ‘resistance’to vapour diffusion would be 0.85 sec/cm. Obviously the evaporation rate will then be much below the potential one,when the impedanceto vapour flow in the soil layer is 50 times greater than that in the air layer close to the surface. The analysis given here is too simple because it ignores liquid flow through the upper soil layer. Also, it considers the surface mulch as a staticlayer,whereas,in fact,this layer accumulates moisture at night when the evaporative demand is small and yields this water again in the daytime. A completely satisfactory description taking all pertinent factors into account has not yet been devised. With the combination approach, one can show that the actual evaporation (EA), potential evaporation (Eo)and ‘surfaceresistance’(Rs)are related as follows:

IRRIGATION, D R A I N A G E A N D SALINITY Equation 11 is very instructive since it demonstrates that the ratio of actual evaporation (EA)to potential evaporation (EO)depends not just upon the surface property Rs,but also upon temperature through E and on the windspeed throughf(zi). For a given surfaceresistance,the ratio E,/Eowill be greater with higher temperature and lower wind velocity, The relative effect of mulching practices, then,will be greater at low temperatures and high windspeed. O n the other hand,we may be more interested in the absolute effects, which would be given by:

AE=Eo (l-E,JEo) (12) showing that the effectiveness of mulches,and so on,depends first of all on the potential evaporation itself. The relation (1 1) is illustrated in Fig.8.4 for a hypothetical case where the surface resistance is 6 sec/cm, corresponding to a mulch of dry material about 0-5cm thick over a moist soil. It is also assumed that the surface roughness is 1 c m and that windspeeds are measured at 200 cm. The role of ambient temperature and windspeed upon evaporation reduction is obvious. Clearly, we should not expect much benefit from mulches at low windspeed,which would apply in practice to mulches used under a plant canopy, Km

Der hour

Wind speed m per sec

Fig. 8.4.Effect ofwindspeed measured at 2m above the surfaceon the ratio ofactualto potential evaporation for a standard surface (roughness parameter 1 cm) having a vapour diffusion resistance 6 sec/cm. Computed at two temperatures,this relation does not depend upon vapour pressure Whereas,evaporation from bare soil is subject to some analysis and prediction, the situation is less simple when evaporation and transpiration both take place simultaneously as under most partial plant covers.Yet this condition is the rule and,therefore,practically important.

It has been a common practice to make measurements of evaporation of bare soil and to subtract the values from evapotranspirationdata to arrive at the relativemagnitude ofthe two components.This evidence is without much value since the exposure of the bare soil in either case was not the same.Equally faulty can be the practice ofcovering the soil on some plots with an impermeable material and comparing soil moisture losses with values obtained on normal plots, by designating the former as transpiration and the latter as evapotranspiration.Available data on the radiation balance of the soil under a crop canopy gives a clue as to the magnitude of soil evaporation,but no definite value (TANNER,1960). Thus,we have no way ofgeneralising about the relativeparts that evaporation and transpirationcan play. When either rain and/or irrigation are frequent, evaporation is bound to be significant, all the more with row crops and partial covers. When surface wetting is infrequent and a complete canopy exists, surface evaporation may amount to only a small fraction of the total. According to BUDYKO’s calculations (1948), in a grass cover of 10 c m height the transpiration is 30% of the total water loss. If the grass cover is 20’cm high, the relative loss by transpiration increases up to 50%. It is importantto note that,according to the experimental data by the same author,the ratio of,the soil surface evaporation rate from the open field to that of thc field under a cover depends on height of plants only,and does not depend on meteorological conditions. ‘ Relative water losses by evaporation from the soil surface would obviously be the largest with a wide-row 226

WATER, PLANT G R O W T H AND CROP IRRIGATION crop. STEPANOVA(1963) carried out experiments in Leningrad using lysimeters;she found out that cabbage transpired from 35 to 48% of total consumptive use during the vegetative period (see Table 8.9). Other data indicate that the majority of crops lose by transpiration from 50 to 75% of the evapo-. transpiration. In the irrigated conditions of Middle Asia (arid climate), i of the water is lost by evaporation and 3 by transpiration. Table 8.9. Soil moisture use effectiveness (by STEPANOVA,1963) ~

Evapotranspiration(mm)

Years

1956

1959

with irrigation

without irrigation

249

166

-

196

Transpiration (mm)

Transpirationof total evapotranspiration,%

with irrigatíon

without irrigation

with irrigation

without irrigation

88 93

64 -

35

48

38 -

I

Evapotranspiration (EA)from alfalfa-17-23 April 1963 (irrigated on 4April) Height of crop (cm)

10-12 25-28 4b-47

EA (averagein mm per day), ,-

.

462 6.80 7.92

When conditions for potential evaporation are met, it makes no difference what the source ofthe evaporated water is-it must be supplied by the soil reservoirone way or other.Therefore,questions of the relative efficiency of sub-irrigation,surface irrigation or sprinkler irrigation have no relevance in this context.When the degree of plant cover or the nature of the foliage definitely limits evapotranspiration,any form of subirrigation that would not wet the surfacecould be much more efficient than any other system.Also,the lowest acceptable frequency of irrigation would be the most efficient schedule. 4. Plant factors Plant factors influencing evapotranspiration can be considered in three categories : (a) The effect of plant morphology and geometry upon the aerodynamic roughness, as it appears in equation 10. (b) The effect of degree of plant cover or leaf area index, accounted for by Rs in equation 11 (c) The effect of stomatal aperture and the circumstances that cause it to vary. This effect can be represented also by the value of Rsin equation 11 (a) In regard to the aerodynamic roughness,the greater this parameter,the more efficient the removal of water vapour and-in the case of extraction of heat from the air-the more efficient the transfer of sensible heat from the surroundings to the transpiring leaves.In the Dalton and combination-typemodels,the effect of roughness is accounted for approximately by the relation (10)in which zois the roughness parameter. Its value varies from 0,001cm for smooth surfaces [ice) to 20 cm for a tall uneven crop. Little is known about orchards or forests.Owing to the In function,the effect of change in zois not very profound. The significance of roughness for the potential evaporation depends upon the other weather factors involved, as can be seen from Fig. 8.3 in which roughness and windspeed effects are, in a sense,interchangeable. Thus,the role of roughness may be minor under tropical or temperate conditions,where the radiationbalance predominates. In contrast,in an arid,warm environment,increased roughness will increase potential evapotranspirationmarkedly.Under such conditionsthepoteqtialevapotranspjrfitionfrom different

227.*

IRRIGATION, DRAINAGE A N D SALINITY cropswill be decidedlydifferent,perhaps in contradiction to widely held beliefs.The following data obtained in Arizona give the daily evapotranspiration from well-watered alfalfa,obtained on the same days in adjacent plots, the only difference being in the height and roughness of the stand. (b) Effects of plant cover are of interest only when they depress evapotranspiration below the potential rate.The exact point at which this would occur would vary primarily with the transpiration resistance per unit leafarea and to a lesser degree with the climate.Since the relation illustrated in Fig. 8.2also applies quantitatively here, the fractional depression of evaporation due to insufficient cover or leaf area would be greater in a cool climate than in a hot climate. The absolute effects, on the other hand, will generally be greater in a hot environment. Reliable data on this problem are difficult to find,PENMAN(1963) has indicated that in a cool, moist climate, the potential, or at least the maximum evapotranspiration,is reached when about half the ground show area is shaded by the crop. Data obtained with alfalfa in Arizona (unpublished data from VANBAVEL) that potential evaporation is reached about fiveto six days after the crop is mowed close to the ground,provided soil moisture is adequate for regrowth. At such time,a complete cover exists with a height of about 15 cm. A certain way of determining whether the crop limits evapotranspiration is by observing the effect that surface irrigation has upon the daily or-if observable-hourly rate of water loss. Measurements made on sudan grass in Arizona showed, for example, that surface irrigations applied to the emerging crop would markedly increasewater loss on the following day until the crop had attained a full stand of about 1 m height at about 50 days after planting. (1963) have arrived By summarising and critically correlating a large number of data, JENSEN and HAISE at a set of figures showing how the evapotranspiration of a large number of irrigated crops rises as the crop develops. The data are normalised against incoming short-waveradiation (solar radiation) and expressed in terms of fractions of the total growth period. Though essentially empirical and not necessarily accurate, these data are most useful to guide our notions on the effect of degree of cover. (c) Finally,we are to consider the effect ofthe plant cover at maximum development.Less-than-potential evapotranspirationmay occur because of the restricting effect of the stomatalmechanism. The cause may be in the total number and the morphology of the stomata,or in their aperture,even when they are numerous and close to the leaf surface. Most agricultural plants have thin leaves with stomata on both sides numbering around 10000-2000per cm2.Generally considered,these stomata are not recessed or covered with hair or other features. Itisfrequentlyassumed thatthe evapotranspirationfrom a well-wateredcompletecover equals thepotential rate. This is the same as stating that the resistance offered by the stomata is negligible in the overall vapour transport process. A large number of measurements made with alfalfa in Arizona by the author confirms such a view. A review by PENMAN(1963) also supportsthe idea of a less than measurable resistance to vapour flow from well-wateredvegetation under a wide variety of circumstances. However, an analysis by Monteith (1963) of available data suggests that, in some cases, stomatal resistance may have been an imporant co-determining influence in the transpiration losses. Experimental and theoretical interest in stomatal leaf diffusion resistance is rather recent and data are lacking on many common crop plants. Most findings agree on a typical value in full light of 0.5 seclcm-l for broadleafed crop plants. Ifthe total area of the leaves that intercept most ofthe radiation (counting both sides) is taken as four times the corresponding land area,we arrive at an effectiveresistance of a still layer of air 0.3 mm thick.Under typical circumstances the resistance of the turbulent boundary layer is equivalent to a still air layer about 2 mm thick. From equation 1I it is evident that under such conditions the effect of stomatal restriction is negligible from a practical standpoint.Yet,one should maintain an open mind in this regard, considering that there are a large number of crop plants about which little is known. The previous statements applied to leaves with wide open stomata. Stomatal closure is-in natural environments-brought about by low light intensity or by dehydration of the leaf.The first cause does have little bearing on the problem at hand since luminous and total radiation are strongly correlated.Thus,from a practical point of view,it matters little if stomata are closed at night since potential evaporation is usually also quite small.A n exception to this rule may occur in windy and arid environmentswhen night temperatures are high. Stomatal closure as a result of moisture deficienciesin soil and plant is certainly possible, although not fiecessarily as common as often assumed on the basis of visual wilting symptoms.Recent laboratory studies

228

W A T E R , P L A N T G R O W T H A N D C R O P IRRIGATION

by the present author and associates have shown that at the onset ofsevere wilting the stomata of both cotton and sunflower were still fully open and did not close until a more advanced stage ofdehydration was reached. Thusthe question arises whether,under acceptable agronomic practices,an appreciable transpiration regulation by the plant ever exists. Some agricultural hydrologists have suggested that the ratio of actual to potential transpirationequals the ratio of available to maximally available soilmoisture in the plant rootzone.There appears littlefirmevidence to support this view that may have originated from confusion between soil moisture depletion and evapotranspiration,or from observation on partial covers,where the effect of drying upon surface evaporation was taken to be an effect upon transpiration. Inducing transpirationcontrolby withholding water from crops seldom seems to be economicallyjustifiable as attested by many irrigationexperiments in which the maximum evapotranspirationis,as a rule,associated with the maximum water use efficiency,defined here as the amount of harvestable material divided by the accumulated evapotranspiration.The latter should not be confused with the accumulated amount of water applied to the crop. Proposalshave been made to controlplant transpirationby biochemically active substancesoperating upon the stomata guard cells. Little of practical significance can be said on this subject other than that stomatal physiology is complicated and may not yield quickly to attemptsto control it.Total stomatal closure can be obtained by a number of compounds,but is not yet, and may prove to be never, a practical agronomic objective. A discussion of the effect ofthe nature of the plant cover upon evapotranspiration is not complete without considering the water use efficiency,already mentioned in a broader sense.This subjecthas been adequately reviewed by VIETS(1962). From this review we may see that any factor that increasesyield,including water availability itself,will either increase evapotranspiration not at all,or will affect it to a smaller degree than yield itself.In other words, the water use efficiency is always increased by effective fertilisation,disease and pest control,use of adapted varieties and planting rates. Often-primarily in humid or temperate climatesthe effect on the absolute value of evapotranspiration is minor. O n the other hand,water use efficiency may-for a given crop and management practice-be markedly affected by the climate.This was demonstratedin a general survey made by DEWIT (1958). It may be recalled that,physically,evapotranspiration is almost solelyaresultof exposing a moist surface to a dry environment. In contrast,dry matter accumulationby plants is a much more involved process,althoughcertain connections with evapotranspiration exist, notably through the radiation climate, the ambient temperature,and the stomatal mechanism. Thus,it is possible to find greatly varying water use efficiencies in the production of such widely grown crops as alfalfa,wheat,corn,forage grasses,and cotton.The wisdom of producing staple crops or feed crops in areas where large amounts of sensible heat are the cause for high rates of evapotranspiration should be closely questioned since the water use efficiencies may be comparatively low.

D. WATER REQUIREMENTS

OF CROPS A N D THEIR DETERMINATION

1. Terminology (a) Water sources Although precipitation represents the direct origin of most sources ofwater, agriculturists generally consider water added to the soil in two major categories:precipitation water which includes all the water naturally added to the soil in the form of rain or any other form of precipitation falling directly on the cropped area, and irrigation water which includes any water artificially added to the soil. Irrigation water in turn can be divided into two categories:groundwaterwhich includesartesian water,pumped water and shallow sub-surface flows; and surface water which includes river runs,such as flood plains and the like and regulated flows of water such as rivers and canals. (b) Water requirements of crops The water requirement of crops is the quantity of water, regardless of its source, required by a crop or 229

IRRIGATION, DRAINAGE A N D SALINITY diversified pattern of crops in a given period of time for their normal growth under field conditions. It includes evapotranspiration and other economically unavoidable losses. Since the major part of the water required by crops has to be obtained from the soil,the amount that a soil can retain in the root zone and the frequency of re-wettingare very significant. (Root zone is defined as the volume of soil or fractured rock occupied or occupiable by roots of the plants from which plants can extract water.) The retained volume of water represents the balance between gains and losses of the water added to or lost from the soil under consideration. Optimum water reyuireinents are the amounts of water required during the growing season to produce maximum yields of different crops,where the amounts include soil moisture supplied by precipitation as well as water delivered by irrigation. Wuter requirement efJiciencyis defined as the ratio ofthe amount ofwater beneficially used during an irrigation interval to the requirement during that interval.A low water requirement efficiency could result from an improperly chosen interval or an inadequate amount of applied water. (c) Water supply to the soil (1) Naturally added water (precipitation) This representsthe volume ofwater actually falling on the area.It can be represented as water depth units or depth area units (volume) per unit area.This volume ofwater will either be lost as surfacerun-offor enter the soil.The fractionthat percolatesinto the soilis either (a) retained by the soil or (b) lostthroughthe soil profile, Part of this last fraction in turn will either be used in leaching the salts out of the root zone,in which case it will not be considered as a loss or moved away from the root zone if in excess of the leaching required 'for the particular soil. The 7iet effectiveprecipitation is the sum of the water retained in the soil plus the volume of water used for leaching. (2) Art$cially added water (irrigation) Irrigation water is the quantity of water artificially applied in the process of irrigation. It does not include precipitation,but is partitioned in the same categories as enumerated above under precipitation. (d)

Water losses

At each irrigation and/or precipitation a certain volume of water is added to the crop land. The farmer's major problem is to store this water in the root zone of the soil. The intelligent irrigator tries to store the maximum percentage of the water as soil moisture in the root zone. Surface run-offand deep percolation representthe most common agents oflosses.Irregular land surfaces,compact impervioussoilsor shallow soils underlaid by gravel ofhigh permeability,small or too large irrigation streams,non-attendanceof water during irrigation,long irrigation runs,excessive single applications-all these factors contribute to large losses of irrigation water and low irrigation efficiency.Improperpreparation of land,and steep slopes also contribute to inefficiency.These losses may be categorised as avoidable or unavoidable. (1) Avoidable losses (a) Conveyance or transmission include losses: losses of water by (1) direct evaporation or deep seepage in transit from the source of supply to the point of service whether in natural or in artificial channels and (2) incidentaltranspiration by vegetation growing in the water or along the banks ofnaturalchannels or water courses. Water conveyance eficiency-It can be evaluated from :

E,= 100Wf

wr where- E,= water conveyance efficiency Wj=water delivered to the farm W,=water diverted from the river or reservoir This term can be applied at field,farm or water project levels. (b) Operational losses or delivery losses: losses due to lack of efficiency in management and breaks in the conduits (c) Farm losses: losses of water on the farm due to uneven distribution, poor handling, evaporation and deep percolation into the subsoil due to over-irrigation 230

W A T E R , P L A N T G R O W T H A N D C R O P IRRIGATION Water application eficiency. Since,in most cases, more irrigation water is applied than the soil can hold, the term water application efficiency (E,)was introduced to measure and focus attention upon the efficiency with which water applied or delivered to the farm (W,)was being stored in the root zone (Ws) of the soil where it can be used by plants. W S E,= 100-

w,

where W,= water delivered to farm

It may be calculated for a project,a farm or a field.In normal practices with surfaceirrigation efficiencies of application are about 60%,whereas in well-designed sprinkler irrigation systems 75%is achieved. A more recent definition of this term, by HALL, is that applicationefficiency is the ratio of the net volume of useful water infiltratinginto the soil in a field to the gross volume of water delivered to the field. Although the two definitions are synonymous,the latter allows for an intentionally leaching requirement or for any other intentional useful application of water in excess of root-zonestorage. There are somefacts to remember about application efficiency defined in this manner. It is entirely possible to irrigatewith an application efficiency of 100%and still failto grow a decent crop (presumablythe purpose of irrigation). This application efficiency tells nothing about how uniformly water has been applied, or whether enough water has been applied to hold the crop until the next irrigation.It may be an easily defined term, but it is quite misleading to the farmers and politicians as well. Water distribution eficiency: The uniformity of the distribution of water on the surface as well as in the root zone is most important. Uneven surface distribution of water has many undesirable characteristics. In a field which is not irrigated uniformly due to uneven levelling,poor irrigation practice or any other reason, ‘dry’areas will show unless excess irrigation water is added.This excess water will lower the irrigation efficiency. At the same time,these ‘dry’areas will show the signs of salt accumulation,whenevet a tendency for such accumulation exists. The water distribution efficiency is calculated as:

where Y=average numerical deviation in depth of water stored from average depth stored during the irrigation d= average depth stored during the irrigation The following terms concerning the water distribution are suggested by HALL(1960): Uniformityfunction-is defined to be the depth of water applied either seasonally or by each irrigation as a functionofposition over thefield.From it may be derived certain statisticalparameters(such as the coefficient of uniformity (used extensively in sprinkler irrigation). It is useful in predicting the seasonal application efficiency. While a somewhat clumsy bit of information,it cannot be eliminated completely in favour of a single number such as coefficient of uniformity, since it is the effect of ndn-uniformitywhich must be determined. Coeficient of uniformity-has been defined by CHRISTIANSON as one minus the ratio of the sum of the magnitudes of the deviations to the mean value of the uniformity function. It is a good single numerical measure of the degree of uniformity,simple to compute and useful in general appraisal. Water storage eficiency-The purpose of this index is to indicate how adequately the irrigation has met the plant needs ofthe crops.It supplementsthe application eficiency which is only a partial index;for example,if only a fraction ofthe water needed is applied,E,is 100%.Consequently the storage eficiency (E,)is used to calculate how effectively the soil water deficit has been removed by the irrigation.When only a fraction of the needed water is being applied,the application efficiency is automatically 100%.Under such conditions, this will be a very poor irrigation practice since only a fraction of the water needed by the crop is added, although the application efficiency is 100%.The water storage efficiency conceptwill assist in the evaluation of this problem. It directs attention to how completely the needed water has been stored in the root zone during the irrigation.

231

IRRIGATION, D R A I N A G E A N D SALINITY

where: E,=water storage efficiency W,=water stored in the root zone during irrigation W,= water needed in the root zone prior to the irrigation (2) Unavoidable losses

These include field evaporation,discussed in Sections C and Dydeep percolation which should allow for the quantity to leach salts from the soil,and seepage. (e) Water uses Beneficially used water or useful water is all the water used to aid the growth of the crop and is identical

with HALL’S definition of useful water. Seasonal beneficially used water or seasonal useful water is defined as the total amount of water used by the plant system during an irrigation season. Water use eficiency is defined as the ratio of the water beneficially used to the water delivered to the project, farm or field. W

E,,=100-

U

wd

where E,,=water use efficiency W,=water beneficially used W d=water delivered Evapotranspiration (E,):Total water loss from plant and ground surface. Potential evapotranspiration (THORNTHWAITE) :The amount ofwater used by an extensive area of short green

crop, completely covering the ground, actively growing and never short of water. Consumptive use, consumptive water use: This is the sum of the volumes of water used by vegetation over a given area in producing plant tissue, in transpiration,plus that evaporated from the adjacent soil or from moisture intercepted on leaves.Sincethevolume ofwater used in producing plant tissue is negligiblecompared with the volumes used in transpiration and evaporation, the consumptive use can be taken to be approximately equal to the evapotranspiration. Valley consumptive use: Consumptiveuse when referred to a valley includes all transpiration and evaporation from land on which there is growth ofany kind,whether agricultural crops or native vegetation plus evaporation from bare land and water surfaces. Consumptive use eficiency: This is defined as the ratio of the normal consumptive use of water to the net amount depleted from the root zone of the soil.

E,,,= 100-W

CI

wd

where: E,,,= consumptive use efficiency W, =normal consumptive use of water W d= net amount of water depleted from root zone soil

This index evaluates the loss of water by deep percolation and by excessive surface evaporation following an irrigation. As suggested by HANSEN a wide furrow spacing and considerable exposed ground surface may result in excessivesurfaceevaporation and continual significantdownward movement ofmoisture beyond the root zone. He states that when irrigating potatoes under moist conditions in ridges of permeable soil with widely spaced rows the consumptive use efficiency may be of the order of 50%. By intelligent management, this value can be increased, Transpiration eficiency or transpiration ratio is the ratio of weight of water consumed by crops during the growing seasonto the weight ofdry matter harvested.Itis also known as relativetranspirationor transpiration coefficient. Irrigation requirement: (This is the same as the term water duty or duty of water used in the old literature and the terms irrigation need or grant in aid.) The quantity ofwater,exclusiveofprecipitation,i.e.quantity of irrigation water that is required for normal crop production. It includes surface evaporation and other unavoidable losses under the given conditions.It is usually expressed in water-depthunits or depth-areaunits per unit area and may be stated as monthly, seasonal or for a crop period or annual. 232

W A T E R , P L A N T G R O W T H A N D CROP IRRIGATION Irrigation water requirements, evapotranspiration minus effective rainfall, net irrigation requirement, crop irrigation requirement,farm delivery requirement,delta at farm is the irrigation requirement at the head of

an irrigation farm and is equal to consumptive use plus percolation minus effective precipitation, or equal to the net duty of water when the latter is expressed in similar units. Irrigation requirement has sometimes been used as synonymous to net irrigation requirement. Diversion requirement, gross irrigation requirement or delta at head of main canal is irrigation requirements at the source ofirrigation supplies.It is equal to net irrigation requirement plus water losses and operational wastes in transit,and is the same as gross duty of water when the latter is expressed in similar units. Irrigation eficiency is the ratio or percentage of the irrigation water consumed by crops of an irrigated farm, field or project to the water diverted from the source of supply. When measured at the farm headgate it is called farm irrigation efficiency or farm delivery efficiency;when measured at the field or plot it is designated as fieldirrigation efficiency and when measured at the source of supply it is called water conveyance and delivery efficiency or overall efficiency. In generalterms,irrigation efficiency has been used for farm irrigationefficiency or field irrigation efficiency.

(f) Frequency of irrigation The frequency of irrigation depends upon several factorswhich have already been discussed,in particular to the moisture content in relation with the stage ofgrowth. It is usually agreed that production is maximum if less than 50% of the available water is removed during the vegetative,flowering and wet fruit stages. With some crops,removing not more than 25% of the available water will produce maximum yields. At least 75% of the available moisture can generally be removed during the dry fruit stage without detrimental results (ISRAELSEN and HANSEN, 1962). (g) Useful calculations The depth (LI)ofwater needed to bring the soil moisture content back to field capacity is calculated as follows: pw

d= .A,. D 100 where P,, = PfcPi Pfc = moisture content (in terms of weight percentage) at field capacity Pi=moisture content (in terms of weight percentage) at the time of irrigation A,= apparent specific gravity of soil D=depth of soil under consideration Having determined the amount ofwater to be added to the soil,the irrigator needs to know the timerequired to apply a given stream of water. The relationship between: g :size of stream t :time of application a :area to be irrigated d :depth of water to be applied is as follows: gt =ad (19) Examples: The moisture content of a certain soil is 27% at field capacity and 19 % at the time of irrigation,then P,=8; the apparent specific gravity of the soil is 1.3 (i)

calculation in metric units: depth of soil to be wetted: 0.90 m area to be irrigated: 4 ha size of stream: 110 1.s-l or 0.11 m3/secW1 8 depth required: 0-9. 1.3 . -=0.0936 m=93.6 mm 100 amount per ha: 0.936.10000=936 m3 936.4 Time required: 0.11.3600= 9.45 hours 233

IRRIGATION, D R A I N A G E A N D SALINITY

(ii)

calculation in British units: depth of soil to be wetted: 3 ft=36 in area to be irrigated: 10 acres size of stream:4 ft3/sec 8 depth required: - 1.3 .3=0.312 ft 100 * amount per acre: 0.312 acre-ft=0.312.41560 ft3 0.312-41560 Time required : = 9.44hours 4.3600

In these calculations given as a siinplifieclexample, the eficiency has not been taken into account. (For further details please refer to Chapter 10.) 2. Determination of crop water requirements

The determination ofthe water requirements ofcrops is one ofthe basic needs for the planning of any irrigation project. Water requirement, as previously defined, is the quantity of water regardless of its source, required by a crop or diversified pattern of crops in a given period of time,for its normal growth under field conditions. It may be expressed as equal to seasonal consumptive use plus such percolation as may be unavoidable;the consumptive use is effectively that lost by evaporation. Evaporation from soils: Evaporation from a saturated or a poorly drained soil or when the water table is closeto the surface is almost equal to evaporationfrom a free-watersurface.The rate ofevaporationdecreases with increasing depth ofwater table until the capillary action becomes so ineffectivethat no water reaches the surface.The results of an experiment carried out in a tank are given by ISRAELSENand HANSEN (1962): the evaporation from a fine sandy loam was 88% of that from a free-watersurface when the water table depth was 10 c m and only 7.2% when this depth was 1.25 m. Where floodingirrigation is practised more water will b st by direct evaporation than in other systems of ter distribution. With light showers as well as with sprinkling irrigation,a considerable amount ofwater is ined on the surface of the leaves. This subsequently eváporates and is lost to the atmosphere without passing through the plant;during this time,however,no water is lost throughthe plant. The effect ofthe type of crop and the stage of growth of a certain crop upon loss by evaporation is attributed mostly to the extent to which the crop covers the soil surface.The effect of the season,however,is basically due to changes in the amount of solar energy reaching the soil surface. The effect ofcultivation or mulching upon direct evaporation ofwater from the soilis still being investigated but without much agreement.There is great variation in the large number offactorsinvolved;initial moisture content,depth of water table,texture and structure of soil and soil water conductivity make it very difficult to arrive at generalised conclusions. Transpiration: Transpiration involves a continuous movement of water from the soil into the roots through the stem and out throughtheleaves to the atmosphere.A relatively smallportion is retained by the plant and the rest is lost as water vapour to the atmosphere. If the transpiration exceeds the rate of absorption of water from the soil,wilting occurs and plant growth is impeded. As discussed previously the transpiration process requires energy for the transformation ofliquid water to vapour,as well as low atmospherichumidity and air movement to remove thevapour from the leaf surfaces. If the energy used for vaporisation,exceeds the net incoming energy then leaf cooling occurs. Evapotranspiration: Evapotranspiration involves two processes,evaporation from the soil surface and transpiration.Both processes are governed by the same conditions as have already been discussed. The relative amounts ofsoilevaporation and transpirationdepend usually on the amount ofground cover.For most crops covering the soil surface only a very small amount of water is lost from the ground surface.Under field conditions incoming solar radiation suppliesthe energy for the evapotranspirationprocess.Wind is important in removing water vapour from the crop area and the prevailing temperature and humidity conditions result from the interaction of these two processes;Temperature and humidity,however,still have a direct effect on evapotranspiration themselves.Usually a very close relationship exists between net incoming solar radiation and evapotranspiration.However,under some conditionswhere alarge amount ofenergy is being transferred horizontally (advective energy) from surrounding field areas the relationsh less close. This supply of ‘oasis’effect.Oasis effects advective energy for evapotranspiration is commonly known as the ‘clothesli 234

WATER, PLANT G R O W T H AND C R O P IRRIGATION may occur on any scale.R o w crops and isolated plants are more susceptibleto higher water loss due to these oasis effects whereby air of lower humidity and higher energy content passes the plant more frequently. The interception of solar energy by row crops is also a function of their orientation with respect to the sun. Depending on the surrounding crop areas,particularly those upwind of the crop in question, air masses passing across the field will vary in humidity and temperature. Thus if the surrounding fields are bare more water will be lost than if the surrounding Íields were cropped and irrigated. Results obtained at Davis,California, show that rye grass loses 2.6 mm per day in early spring while 5-0mm per day are lost in autumn, even though incoming net radiation is the same at both these times of the year. In spring the surrounding fields are heavily cropped and irrigated whereas in autnmn these same fields are dry and fallow.Thus almost 50% of the evapotranspiration occurs as a result of advective energy. Water loss differs with different crops (Table 8.10). This is partly due to crop geometry (leaf angle,height, density) and partly to the different leaf characteristics (see previous section). However, it is due mainly to length of and incoming energy during the growing season. Table 8.10 Growing Crop

Barley Chilli Cotton Groundnuts Jowar (sorghum viilgare) Linseed Maize Mustard Oats Peas Potatoes Ragi Rice

Sugar Cane Tobacco Wheat

season number of days 88 202 202 124 114 88 100 88 88 88 88 127 98 365 132 88

Total water requirement*

Daily water requirement

ma/ha

ma/ha

3600 9850 1O 700 6600 6500 3200 4550 2700 3700 3000 6800 7600 10600 24O00 10000 3750

inches 14.1 38.8 42.2 26.1 25.7 12.7 17.8 10-6 144 12.0 26.7 29.8 41*7 95.0 39.2 14.8

41 48 53 53 58 35 46 30 41 35 76 58 109 66 76 43

inches 0.16 0.19 0.21 0.21 0.23 0.14 0.18 0.12 0.16 0.14 0.30 0.23 0.43 0.26 0.30 0.17

*Includes water lost in transpiration and by evaporation and seepage. The results are applicable to areas round about H yderabad

(a) Direct measurements of consumptive use There are several methods for the direct determination of consumptive use for different crops and many factors to be considered in the selection of method. One of the major factors is the source of water used by the plani: precipitation, surface irrigation or groundwater. The principal methods in use are: tank and lysimeter experiments,soil moisture measurement in field plots and in the field,and water balance measurement. (1) Tank and lysimeter experiments Lysimeter in the commonly used sense of the term involves the growing of crops in large containers (lysimeters) and measuring their water loss and gains.Conditionsin these containershave to be as close as possible to the natural conditions.From the irrigation point of view, lysimeters are set up to enable the operator to measure the water balances: water added, water retained by the soil, and water lost through all sources, evaporation,transpiration and deep percolation.These measurements involve weighings which may be made with scales or by floating the lysimetersin water on a suitable heavy liquid (ZnCl,) in which case the change in liquid displacement is computed against water loss from the tank. The technique yields a measurement of total water loss and is quite useful as a measurement of field water loss provided suitable precautions are taken. The tanks must be permanently buried in the ground and surrounded by a large area of crop of the same height if the readings made are to bear relation to losses from the crop in the field.The water table R

235

IRRIGATION, D R A I N A G E A N D SALINITY is maintained at a specific depth in the tank by connecting it to a supply reservoir provided with a float mechanism which has an arrangement for receivingexcess water that tends to build up in the tank.Water is applied in measured amounts to the soil surface,as irrigation is applied to the surrounding field which is cropped.The overflow and deep percolation are measured. The water either from the reservoir orprecipitated as rainfall,excluding the outflow, constitutes the water used by the crop. Soiltanks equipped with Mariotte water supply tanks have proved successful in consumptive-usemeasurements from water tables at various depths.Double-typesoil tanks,with an annular space between the inner and outer shells,are considered best. Lysimeters have the advantage of being able to measure directly the amount of water supplied and lost by a crop;however,a number ofproblems are involved with their usage.Major problems are concernedwith rooting depth,drainage and tank temperatures. Roots are confined to the lysimeterdimensions and soils are usually disturbed when lysimeters are installed which may create growing conditions not common to the undisturbed soil in the field. (2) Field experimentalplots Measurements of water changes from soil moisture contents of field plots are sometimes more dependable than measurements with tanks or lysimeters. The seasonal water requirements are computed by adding measured quantities ofirrigation water,the effective rainfall received during the season and the soil moisture stored in soil prior to planting of crop. This is given by the formula:

where

U=seasonal water requirements (in mm or inches) I= total irrigation water applied (in mm or inches) R = season effectiye rainfall (in mm or inches) Mbi= moisture percentage before the season in the i-thlayer Mei =moisture percentage at the end of the season A i=apparent specific gravity in the i-thlayer D i=soil depth of the 1-thlayer (in mm or inches)

The method requires that water be measured into a field or plot where positive control is made of its distribution. Accurate measurements of the seasonal total water applied can be obtained,but no information is obtainable on intermediate soil moisture conditions,short-termuse, profile use,deep percolation or peak use. Determination of effective rainfall also presents a problem. The values of seasonal water use may be too high because of deep percolation losses which undoubtedly occur under field conditions.

(3) Soil moisture studies Consumptive use of water for various crops has been determined by intensive soil moisture studies. These studies involve measurement of soil moisture at various depths at a number of times throughout the growth period. The greater the number of measurements the more information can be obtained from such studies. Natural random variation of soil water content often gives inaccurate results and makes the technique impractical. Crop water usage in the field. Consumptive use is calculated from the change in soil water content in successive samples from:

where

U=water use from the root zone in mm for a sampling period within one irrigation cycle n =number of soil layers sampled in the root zone

MI, =moisture percentage on oven-drybasis at the first sampling in the i-thlayer M Z=moisture i percentage on oven-drybasis at the second sampling in the i-thlayer Sbi =bulk specific gravity of i-thlayer of soil D i= depth of soil in mm of the i-thlayer 236

W A T E R , P L A N T G R O W T H A N D C R O P IRRIGATION

Seasonal consumptive (U,,is ) calculated by summation of consumptive use values for each interval.A correction for accelerated water loss in the interval(s) after irrigation(s) and before soil moisture sampling is made. This method is usually suitable for areas where soil is fairly uniform and the depth to groundwater is such that it willnot influence soil moisture fluctuationwithin therootzonewhere the precipitation is low and where good control of irrigation water is possible. Usually a great number of measurements must be taken to obtain the desired accuracy. Soil sampling siteswith respectto plants can be changed to avoid effects on roots. Equipment cost is low. Measurements may include deep percolation losses.Use the following formula:

where U,=water use inj-thinterval as calculated from the previous formula E, = evaporation from U S open pan evaporimeter for the period following the k-thirrigation K=ratio of consumptive use during the first days following irrigation to evaporation from standard pan (E,).K usually varies from approximately 0-5to 0.6 (4) Water balance method The water balance method depends on the hydrologic equation:

Precipitation =

P=E,+O+D+d w Surface Subsurface Change in soil Evapotranspiration + run-off drainage 4- water content

(23)

If all factors except Etare measured, then this may be computed. There are, however, many difficulties associated with the measurement of effective precipitation, surface run-offand sub-surface drainage. This method as the integration method is best suited to large areas (watersheds) over long periods. (b) Calculation of consumptive use from climatic data Calculation of evapotranspiration from a cropped surface may be made either (1) by correlation with water loss from evaporation devices or (2) by estimation based on various climatic parameters. (1)

Correlation witlz evaporation devices

It is assumed that the conditions that affect crop water loss (E,) will also affect evaporation from a free water surface (Ew)in a similar manner. It is then necessary to apply a coefficient (obtained by informed or uninformed guesswork or by long tedious observations) to estimate E,from E,=kE,. Most evaporating devices,because of their shape,bulk, colour or exposure, do not respond in the same way as a crop surface. The coefficient k is usually difficult,as the value varies with the type of pan, its maintenance,its exposure, the season,the crop and growth stage of the crop. The US Weather Bureau Class A pan (USWB Class A), the Bureau of Plant Industry (BPI) and the Australian Standard Pan are commonly used. The USWB Class A pan of 4ft diameter,10 inches deep with water, 8 inches deep and rim 16 inches above the ground is commonly used in the USA. The BPI pan is 6 ft in diameter, 2 ft deep,sunk 20 inches into the ground with the same water level as the ground level. The Australian pan is 3 ft in diameter, 3 ft deep, a 6 inches water jacket around the pan with water and tank being set flush with the ground. Typical monthly values for evaporation from these pans relative to E, for perennial rye grass at Davis,California,range between 0-7to 0.8(USWB), 0.9 to 1.02(BPI) and 0.95 to 1.08 (Aust.). Under extreme weather conditions the pan ratios are very variable. At Davis, California,the values of E,(rye-grass)/E, (USWB) often drop as low as 0.4to 0.5 on days when strong dry winds prevail.Various modifications are made to evaporation pans in an effort to improve their effectiveness.Some observations in low humidity areas indicate that peak use may be as much as 10 to 20%higher than the evaporation from a Weather Bureau pan for a shortperiod of time.Various modifications are made to evaporation pans in an effort to improve their effectiveness.These include colouring the water,screening the surface,as well as many other techniques,all of which help little.The exposure of the pan also has a large effect. Piche evaporimeters,used extensively in many countries,have the same disadvantages. The Piche unit is essentially an inverted graduated test tube filled with water, with a blotter over the open end and installed in

237

IRRIGATION, DRAINAGE A N D SALINITY a conventional shelter with other weather instruments. Because of the small size of Piche units, rates of evaporation are in excess of the rates of water use by crops. Piche values are also larger than those obtained by the USVdB Class A evaporation pan. Measurements of evaporation with Piche evaporimeters are usually poorly correlated with evapotranspiration and evaporation from other pans. The units tend to overestimate wind effects and to grossly underestimate radiationeffects.Typicalvalues for atmometerrelative to consumptive use values of perennial rye-grassat Davis,California,are: white :EJE, =0.003-0*004 black: EJEb=0.002-0.003

A more reliable estimate is found by dividing Etby the difference in total evaporation from black and white atmometers,this value being more closely correlated to Consumptive use and net radiation: LIVINGSTONE and BBLLANI atmometers,which are porous ceramic bulbs filled with water and,attached to a water supply,have also been used successfully as indices to estimate consumptive use. Black and white bulbs are used. E,/E(,-,)=0.012 Limitations of evayoratiuiz methods

Principal limitations of evaporation pans or other evaporative devices are the physical differences of evaporation surface.compared with a crop surface. Moreover evaporation pans are expensive and requireconsiderable wzter. The evaporating surface of Piche evaporometers and atmGmeters is subject to contamination by dust, oil and other foreign material.Blotters for Piche units are inexpensive and can be readily changed. However, since atmometers are relatively expensive porous ceramic bulbs, they are not subject to ready replacement when dirty.Fingerprintswhich leave oilon the surfaceareespecially harmful,Atmometersmust be recalibrated when cleaned.All of these evaporative devices are independent,of course,of the physiology of the plant. (2) Estimation of water loss based on climatic parameters

Itis often inconvenientto use evaporimeters and sinceclimaticparameters are more easily measured,estimates ofevapotranspirationmay be made from these data.Broadly these techniques fallinto two classes:completely empirical attempts to correlate water use with one or more climatic factors, or the application of a more theoretical approach. The latter technique involves vapour flow, heat balance or combination techniques. In all techniques an estimate is made of ‘potential evapotranspiration’ (see Section C).Most empirical with a correlation factorbeing used as before. Since estimates of k are varied and formulae also estimate E,, not soundly based, many methods are correct only to the extent that k is estimated correctly. Empirical equations are simply formulae where a few pertinent factors are related to the actual water use; many have been used in an attempt to estimate crop water loss; however,only some of the more commonly used will be discussed. One of the earliest equations used to estimate evaporation from a water surface was (see Section C,formula (7)). proposed by DALTON ROHWERevaluated constants for the equation to give:

E=0.40 (e0-ea) (1f0.17~~) m m per day

(24)

where the vapour pressures are in mm of mercury and u2 is the windspeed at a height of 2 metres in miles per hour. A correction factor is necessary for estimates of Eoand Et. BLANEYand CRIDDLE developed a simplified formula using temperature and daytime hours as the only weather parameters. By multiplying the mean monthly temperature,t, by the monthly percentage of daytime hours,p, there is obtained a consumptive use factor,F,that is:

U=KF=K.Z(t .p)/IOO where the following quantities must be determined for the same period:

(25)

U=consumptive use of crop;inches for a given time period F=sum of the consumptive use factors for the period (sum of the products of mean temperature and percentage of annual daytime hours) (t .p)/100 K = empirical coefficient (annual,irrigation season,or growing season) t = mean temperature Fahrenheit p = percentage of daytime hours of the year, occurring during the period 238

W A T E R , PLANT G R O W T H A N D C R O P IRRIGATION Usually monthly calculations are made: The conscmptive use of water by a particular crop in some areas being known,an estimate of the use by the same crop in some other area may be made by application of the formula U=KF. Thornthwaite method utilises mean monthly temperature and a constant based on the The equation developed by THORNTHWAITE ‘heatindex’(I)for the location.Iis a complex factorwhich in normal usage is determined from nomographs. The equation has the form: e=cta (26) where e =the monthly evaporation in cm t =the mean monthly temperature in degrees C a and c =coefficients which vary locally To evaluate a,the following equation was developed: ~ = 6 * 7 5 lO-’I5-7*71 . . P+1*792. I+0*49239 where I=annual heat index and is equal to the sum of the monthly heat indices i= (t/5)lV5l.The coefficient c varies inversely with I From these relations the equation for potential evapotranspiration is given : e=1.6 (lOt/Z>.

(27)

(28)

The complexity of the mathematical development necessitates tables and nomographs (e.g. HANTUSH, 1959) which may be found in irrigation manuals for convenient use. Most empirical formulae are useful for the particular type of area in which they were developed. BLANEY and GRIDDLE'S formula is used widely in the western United States. THORNTHWAITE~ methods include no crop coefficients,while the crop coefficient utilised by BLANEYand CRIDDLE is questionable. Most empirical equations are effective only under ‘normal’circumstancesand are not useful for extreme climatic conditions. The above mentioned formulae are most effective in humid areas. Under conditions where large amounts of advective energy are present, estimates of water usage are more difficult. In UAR,a formula,partially based on PICHEdata, has been developed by DIFRAWY: m=d,. t+d,. E

where m =consumptive use d,= coefficient of evaporation from soil surface d,= coefficient of plant transpiration t = mean daily temperature E=daily evaporation measured by PICHE

As evaporation by plant was found to be only 32to 40%of the consumptive use,the author has simplified the formula as follows: m=c. dl .t where c = factor dependency on site, increasing with temperature.

A more basic and theoretical approach to the estimation of water losses involves often expensive and difficult measurement ofvarious parameters.The more commonly used equationsare those Of THORNTHWAITP: and HOLZMAN, PASQUILL and DEACON and SWINBANK based on vapour diffusion equations and PENMAN’S equation based on heat balance and vapour diffusion principles. One of the formulae based upon the theory diffusion was developed by THORNTHWAITE and HOLZMAN: E= ek2(41-42)(U,-U,>/(ln Z,/ZJ2 where E=the rate of evaporation ql, q2= the specific humidities U,,U,=the mean wind velocities at heights 21,ZZ Z,,2,= heights k= Karman’s constant (10.4) e=the density of air

(30)

239

IRRIGATION, D R A I N A G E A N D SALINITY

The equation inthisform requiresextremeprecision in measurement ofthevarious parameters. Thesemeasurements are diRcult and expensive. PASQUILL suggested a more convenient form for the equation: E = B U,(e,-e,)

where

where

B=is constant for any one site and crop of fixed roughness:

M = molecular weight for water R =international gas constant T = absolute temperature

Correlation of actual evapotranspiration with calculated evapotranspiration of rye-grassgrown on lysimeters at Davis,California,showed that the THORNTHWAITE-HOLZMAN equation and modified PASQUILL form to be highly inadequateexcept under wind conditions of 3-4m/sec or more. Both equations gave similar estimates. DEACON and SWINBANKproposed that the difficultiesin these equations were associated with air stability and correctestimation ofturbulent transfer.These authorsproposed an equationutilisingthe ‘dragcoefficient’ (friction force of wind passing over a crop surface which gives a measure of the degree of turbulence):

E=eCnUn2 (ql-qz)/(Uz-u,)

where

(33)

C = drag coefficient at height n e = air density U,=wind velocity at n

U,,U,and Q, q, =wind velocities and humidities at two selected heights C,,should be estimated from wind profilevalues (data at times when air density is constant with height). Estimates of water loss by this method gave better results in the rye-grassexperiments at Davis,California, than the former equations. Again,however,the estimates were best under windy conditions. Techniquesand cost of measuring the various parameters limit widescale usage of the method. PENMAN’S combination heat balance and vapour transfer equation has been widely used throughout the world,althoughit also has equipment limitations (see Section C and PENMAN, 1963). Only afewclimatological stations record the needed data. PENMANreasons that incoming net radiation (R is partitioned mainly the ratio between these being called the Bowen between evaporation (LE)and sensible heat transfer (H), ratio (ß). (34)

where TsTaand ese,=respectively the surface and air temperatures and vapour pressures y= the psychrometer constant and equals 0.49mm mercuryldegree C L = the latent heat of vaporisation (586 ca1.g-l at 20°C) Using aerodynamic considerations he suggested a system of estimating p from air conditions alone,This results in the following equations for E,+,: E, = AR nlLY + E a (35) A/Y+l where A = slope of temperature-vapour pressure curve Ea=0*35(ea-ed) (0~5+0.01U2)in“/day ea- ed=air saturation deficit in mm H g U,=windspeed in miles per day at a height of two metres To estimate crop water use, PENMANuses the form E O = K E wbut assumes a coefficient which varies seasonally.For short grass k is: 0.6from November to February 0.7 March,April, September,October 0-8from May to August

240

W A T E R , P L A N T G R O W T H A N D CROP IRRIGATION The method is reliable in England. It has also been shown to be quite reliable in drier areas although slightly higher coefficientsseem called for.Although it has been suggested thatthe formula fails to respond adequately under dry,windy,high advection conditions,recentwork at Davis,California,suggests the method does pick up the advection effects very well,with some tendency to overestimate the effects. Utilisation of both empirical and basic equations for estimating water use from climatic data, although convenient, has many difficulties.Apart from problems of measurement and cost, these methods have only limited use. The empirical methods have little accuracy away from the areas and crops on which they were developed,failing when used under a wide range of conditions. With all methods estimation of correct crop coefficients is difficult and widely variable.

E. ESTABLISHING AN IRRIGATION PROGRAMME

T o establish an irrigation programme one must have adequate information on water supply,climate,plant, soil and economic factors. An adequate and dependable water supply will make possible and facilitate irrigation applicationin accordancewith the biological needs ofplants.Then,the problem is to determine the suitability ofthe crop to theenvironmentand themarketability oftheproduce.Soilsmay be altered by grading to change slope,operations to modify profile characteristics,addition of amendments and leaching to correct p H and salinity,organic and inorganic manuring to improve fertility,and pesticides for control of soil-borne disease,nematodes,etc. However,factors such as climate can be modified little,if at all, and thus determine the cropping and the irrigation programme. Irrigation programmes can facilitate crop production in several ways, viz., providing moisture control; efficient use of fertilisers; better adjustment of cultural practices such as sowing and harvesting schedules; permit double cropping and inter-cropping; an introduction of high value crops-potatoes, hybrid maize, sugar cane,long staple cotton,fruits,vegetables, etc. The extent to which these objectives can be achieved depends on the total water supply.

1. Total water requirements

In order to determine whether sufficientwater is available for irrigation,estimates must be made of the total farm water requirements.Aspects which need to be considered are evapotranspiration losses and application losses including run-offand deep percolation.In addition,allowancesmay be made for conveyance losses and necessary leaching.It is not adequateto know only the total water requirements,since these change with the advance of season, being low in the early stages, rising to a peak at the time of maximum growth, and decliningthereafter.Thus,each crop will have periods ofmaximum rate ofwater use depending on the stageof growth and weather conditions. The irrigation interval has to be shortened during the peak demands to prevent damage to crops. These peak rates must be taken into consideration in any irrigation programme and water supply must be assured to meet the demands at such peak periods. The procedure for calculating evapotranspiration has been discussed in a previous section. Application which is discussed in Chapter 10. losses determine irrigation application efficiency (Ea) Irrigation application efficiency ranges from nearly 100%to 30%,and sometimes lower. Low efficiencies arise from irregular land surface,irrigation systems poorly suited to prevailing conditions and inefficient operational procedures including too high a rate of water application, excessively long irrigation runs on highly permeable soils,and inadequate control of water distribution. In any irrigation system water is lost during application.Even under sprinkler irrigation these losses can be appreciable particularly under windy conditions. Over and above the estimates of total water requirements as described above,certain alIowance has to be made for leaching the salts from the root zone.In a surface irrigation method, deep percolation losses may provide the required leaching. However,with a sprinkler irrigation system,additional water will be needed for leaching.Leaching of salts may occur naturally through heavy rains. 241

IRRIGATION, DRAINAGE AND SALINITY 2. Soil, plant, climatic, management, economics and water supply factors determining saitable irrigation schedules

Irrigation scheduling is a means of supplyingwater in accordancewith the crop needs.Factors,such as water retention characteristics of soil and rooting depth of crop,which determine the supply of water available to crops,and any factors,such as climate and the extent of plant cover on soil surface,which affect water-use rate,must be considered in determining irrigation schedules.Accordingly before an irrigation programme is planned,the local situation should be analysed in terms of soil,plant, climatic and management factors. Among the soil factors involved are soil structure,texture and depth,mechanical impedence,infiltration rate,internal drainage rate,aeration,moisture retention characteristics,hydraulic conductivity,groundwater table conditions, soil salinity,toxic substances,plant disease and nematodes,temperature and soil fertility. Climatic factors for consideration are temperature, solar radiation,wind, humidity, day length, length of growing season and diurnal fluctuations.Plant filctors include crop varieties,rooting characteristics,drought resistance behaviour, growth stages critically affected by water stress, organs or plant constituents to be harvested, effect of water stress on quality of harvested produce and the length of growing season. The principal management factors include dates of planting,resultant plant population, irrigation scheduling in relation to critical growth periods, fertiliser application,crop protection measures, and dates of harvesting. Irrigation affords opportunitiesfor double or even triplecropping.Selectionand planting ofcropscan be so arranged that the peak rates of water use by different crops do not clash. Studies at Arizona in the southwestern United States reveal that a stream of 71 l/s would be sufficientfor 72 ha (180 acres) of cotton-i.e. l/s/ha;103 ha (257 acres) of alfalfa, i.e. 0.70 l/s/ha;or 136 ha (340 acres) of a rotation consisting of 48ha (120 acres) of alfalfa, 48 ha (120 acres) of cotton, 32 ha (80 acres) of small grain and 8 ha (20acres) of sorghum,i.e. for the whole 0.52 I/s/ha.Such programmes are dependent on climate-particularly the total and seasonaldistribution ofrain,characteristics ofcrop-particularly root depth and density,and water retention characteristics.Intercropping raises irrigation requirements,but the increase production obtained often justifies the greater water use. Operational policies including pricing of water and the scheduling of water deliveries to farms can have a major effect on crop patterns and on water-use efficiency. It is the almost universal experience that water will be misused and wasted where it is abundant and free or relatively cheap. This may ultimately cause drainage problems and increased salinity,loss ofplant nutrientsthroughleaching,and a subsequentreduction in crop yields.Where cost of water is high,efficiency in its use is higher. The optiinal level of irrigation and corresponding net profits are inversely related to the cost per unit of water, as shown from experimental data by PRASHAR and SINGH(1963) in Table 8.11. Table 8.11. Optimal irrigation levels and net profitsfor wheat crop in relation to price of water under differentsources of water suppIy

Sources of water supply Persian wheel Tube-well(working on oil) Tube-well(working on electricity) Canal

Cost per acre inch of water

Optimum level of irrigation

in Rs*

cm

inches

1s 8

19 23

7.6 9.3

28.8 71.4

28 34

11.0 13-6

118-4 163.0

4 0.50

Net profit with the optimum irrigation in R,

*One acre inch= 1024 cm3

Net returns per acre for individual crops become critical in the choice of cropping patterns. The relative ranking of different crops and therefore the optimum choices and resource allocations,will change considerablywith variablewatercosts,or quantities ofwateravailable.The quantities ofwater need to be so regulated that its purchase at a particular price sufficientlyensuresthat each successive applicationis consistentwith the profit maximisation goal. However, when the objective is to attain maximum production, an increase in and HEDGES, 1963), using linear programming water use may be justified.Studies in California,USA (MOORE techniques also revealed that the optimum water quantity for different farm sizcs (from 32 to 512 ha, i.e. 242

W A T E R , P L A N T G R O W T H A N D C R O P IRRIGATION 80 to 1280 acres) varied as the cost for water ranged from zero to $24.30 for 1OGO m3(from zero to 930.00 per acre-foot). 'Stepped' demand schedule for different farm sizes are given in Fig. 8.5.These steps, each representing a combination of a quantity of water and a particular price, are not necessarily parallel among Cost of irrigation water per acre foot (in V.S.#) 32

r

Fig. 8.5. Optimum water quantity in relation to water cost for different farm sizes (80, 160, 320, 640 and 1280 acres,i.e. 32, 64, 128, 256 and 512 ha)

the five farm sizes. The study showed that the amount of water used at a certain price depends upon that specific combination of crops and the acreage of each,which give the maximum total net farm returnsunder the particular water-cost conditions. Thus, water prices are extremely influential in determining cropping patterns, optimum water use,and efficiency in water use.In areas where water is plentiful and cheap, in comparison to other production costs and crop values,there may be little incentive for a farmer to conserve water by improvingthe efficiency ofhis irrigation operations.When low water price is fixed for some reasons, special regulatory measures will have to be imposed to ensure efficient water use: otherwise the inefficient use of water will created drainage problems and a consequent reduction in crop yield. Thewater supplyingorganisation can also affectirrigationefficiencyby its choice ofwater delivery schedules and ofthe size of stream made available to individualfarmers.In some irrigation projects,the water delivery schedule may make it impossible for the farmer to schedule his irrigations at the precise time water is needed by the crop. Therotation system,for example,allows littlechoice.For the farmerwho elects to pass his turn may indeed need water before the time ofthe next delivery,generally a week or two later.Ifhe accepts water on schedule, consideration must be given to the crop that should be irrigated.In other projects, the farmer may receive a small stream of water under a continuous delivery system.Here, he must apply water on some part of his farm at all times when evaporative demand is high in order to cover the irrigated acreage in timefor the next irrigation.Continuous flow systems are often inefficient,but they may be quite efficient under one or more of the following situations:(1) the flow rate can be properly adjusted to evapotranspirationrate and water can 243

IRRIGATION, DRAINAGE A N D SALINITY be shut off wherever it is no longer needed;or (2) flow rate is too small for efficient surface irrigation and sprinklers are available. Maximum flexibility and the greatest opportunity to irrigate on a more scientific basis occurs where irrigation water is available on demand. Water is usually ordered a day or two in advance where project distribution systems have capacities to satisfy crop needs during periods of peak demand. In some irrigation projects,water may be obtained at any time,without advance notice (fully automatic on demand system). Where groundwater is plentiful,the farmer need only turn on a pump to irrigate.In spite of the advantages, many farmers misuse the demand delivery system by irrigation which is more frequent than needed by the crop. Irrigation schedules and applications must consider other farming operations. For example, a farmer might elect to irrigate alfalfa sooner than necessary because he anticipates harvesting a crop of hay the following week. 3. Determination of irrigation schedules

As discussed above,crop irrigation requirements vary with soils,type ofplants,stage ofgrowth,and weather conditions.Itis impossibleto recommenda universally applicableirrigation schedule.The optimum irrigation programme,from the point of view of a farmer,is the one which is most profitable. Crops differ in their toleranceto the depletion of soil water. A crop such as paddy rice responds favourably to frequent irrigations and even to continuous submergence.Some crops such as berseem (a kind of clover), potatoes,and most winter vegetables, require moist conditions and suffer if more than 40-50% of available water in soil is depleted before irrigation is applied even though the evaporative conditions are not severe. Other crops, such as small grains (especially during maturation stage), alfalfa,fruit trees and a number of cropswhich develop deep and well-branchedroot systems,may show littlereduction inyield until nearly all of the available water has been depleted in the soil depth from which extractionhas been most rapid.It must be emphasised,however, that irrigation programmes for specific crops should vary according to the prevailing conditions. Criteria for scheduling irrigation thus vary from one situation to another. Where water is scarce or expensive,irrigation should be scheduled to maximise crop production per unit of applied water. Where good land is more scarce than water,irrigation should be scheduled to maximise crop production per unit of planted area. However, in certain situations irrigation schedules may be modified to minimise irrigation costs to facilitate farm operations,viz. to overcomeproblems ofpoor germination and slow penetration of irrigation water,to control atmospherictemperatures or the groundwater level,to accomplish leaching of salts,and to accommodate schedule of water delivery to farms. (Reference is made to Chapter 10.) The following approaches may be used to schedule irrigation: (a) Measurements or observations of water deficit in plants Requires knowledge of relations between measured or observed deficit and crop yields (b) Calculated schedules Based on (1) estimated soil water depletion allowable without loss of crop yield and (2) an estimate of water use rates (c) Evapotranspiration rates Computed from evaporation or meteorological data.Also requires information on allowable depletionof available soil water (d) Soil water measurements Also requires information on allowable depletion of available water or allowable suctions I

(a) Irrigation sclzedules based on measurements or observations of plant water deficits The most direct approach to the scheduling of irrigations would be the measurements or observations of plant water deficits which can be tolerated without adversely airecting crop yields and/or quality. Plants respond to water stress in a number of ways and although responses vary in magnitude and type between species,they may be useful in some cases as criteria for irrigation need. Measurements of plant water deficits,which have been proposed and used to some extent,include relative water content or turgidity,stomatal opening,transpiration rate,osmotic concentration of cell sap,and total water potentials. (For a detailed review,see HAGAN and LABORDE, 1966.) Unfortunately,the values obtained by all these measurements depend on (1) the plant part selected and its age,(2) time of day,and (3) exposure 244

W A T E R . P L A N T G R O W T H A N D CROP IRRIGATION

,.MI

of the plant part selected. These measurements may also be affected by climatic conditions, soil fertility, disease and other factors influencing plant growth. Many of these measurements are time-consumingand require highly refined equipment.Equipment needed for making the more fundamentalmeasurements is not as yet suitable for field use. Even when measurements are made under carefully standardised conditions, considerable research is needed to establish critical values as the basis for scheduling irrigations. The present difficulties in making plant water measurements suggest that more attention should be given to finding practical ways to use easily detected visual symptoms of water stress to guide irrigations. The most obvious and frequently observed symptomsof water stress are wilting of part or all of the plant. Increasing water stress in many plants causes a darkening of colour and loss of sheen.Water stress in some crops leads to the appearance of carotenoid (yellow and orange colours) and anthocyanin (purplecolours) pigments; shortening of internodes (as in sugar cane and cotton); retardation of shoot elongation (as in grapes); change in leaf angle and apparent colour (as in bean); leaf rolling (as in maize, clover and some pasture grasses); and leaf abscission (as in almond). Unfortunately, by the time water stress symptoms can be observed in most crops the plant has already been under stress for some time and yields may be affected. However, in cotton, grapes, beans and perhaps other crops, visible symptoms are adequate indices for scheduling irrigation. More detailed studies may possibly lead to recognition of useful visual indicators for other crops. Visual symptoms of water stress can be effectively used to schedule irrigation where plants growing on certain areas of a field dry out more quickly because of sandy soil, shallow depth,restricted infiltration, smaller irrigation applications or other reasons.As the dryness of these areas relative to the rest of the field is learned through experience,then the appearance of visual stress in these areas can be useful in scheduling irrigation for the whole field. Another possibility deserving further attention is the use of selected plants or specially treated plants grown with the main crop to indicate irrigation needs. Ifindicator plants can be selected or treated so that they visually show water stress at a desirable interval before the crop is affected,they could be a useful guide to irrigation scheduling.This approach is potentially useful,especially so in the rapidly developing countries, but requires considerable experimentation. With the exceptions indicated above, delaying irrigation until appearance of water stress symptoms will generally lead to reduced yields. (b) Calculated irrigation schedules (irrigation guides) Approximate irrigation schedules,sometimes called Irrigation Guides,can be calculated from the following relation: Supply of soil water useable without affecting crop yield Irrigation interval (days) = Rate of water use per day The amount ofwater which can be stored within the root zone,and which can be used by the crop without being adversely affected,will be termed the total available water and can be calculated as follows: Total available water =

(FC- WP) 100

D,

where FC is the average field capacity, WP is the wilting point (or permanent wilting percentage), Sbthe bulk specific gravity, and D,the effective depth of rooting. In order to avoid any possible crop damage due to drought,the generally accepted irrigation practices recommend irrigating the crop before the soil reaches the wilting moisture content (i.e.before complete depletion of the total availablewater). The depth of water which can be depleted before the crop is adversely affected is termed the usable available water, or the allowable depletion of soil water. The allowable depletion is the equivalent depth of water which can be depleted from the root zone before the occurrence of unacceptable adverse effects on the crop. It can be calculated as follows:

2(E*) n

Allowable Depletion=

.sb,.di.fi

(37)

i= 1

where n is the number of soil layers or increments sampled within the effective rooting depth and FC,,WPi, 245

-

IRRIGATION, D R A I N A G E A N D SALINITY sbly dlandj;are the field capacity,wilting point,bulk specificgravity,depth of soil layer and the ratio of allowable depletion to the total available water all in the i-thlayers. The fractionfreflects the root activity and completeness of water extraction from each depth (j61).Accordingly, it will diminish with soil depth, but it will increaseas roots grow during the crop season.Since information for each soil depth within the root zone is often lacking,the following simplified expression may be used:

(--

Allowable Depletion= Fcl~owp) , Sb.D, F n

where Fis the ratio of allowable depletion to the total available water in the entire root zone (F

Transpiration per unit dry weight of leaves (in glh per g> Suction power of leaves (in atm.) Soil moisture (in %by weight) Soil moisture (in %of water capacity)

Tdble 9.19. Diflerence in the growth,development and crop formation of Gossypium hirsutum L. on saline soil according to water supply conditions

ExperiHeight of Number mental plant of buds conditions (in cm)

Shed

Leaves Remain- Total dried up ing

No.of

bolls

Weight of Yield (in one plant metric cent(g) ncrslha) ~~

~

Irrigated plot Dried-up

74.6

23.0

6.0

2

30.2

38.3

11.2

58.6

20-50

plot

44.0

13.4

10.7

10

18.6

39.3

3.0

26.4

6-49

its yield drops sharply.The soilmoisture before watering should not descend below 70%ofthe field capacity. Thus to improve the water supply ofplants growing on saline soil,measures have to be taken to increasethe water availability of the soil, reduce the salt concentration in the soil solution and provide plants with a satisfactory water supply during the whole of the vegetative period. It is advisable, therefore,to maintain a high level of moisture in both the soil and the root zone by applying small quantities of water at frequent intervals,and watering even when the soil is already moist. Note that plants with less salt resistance need watering more frequently; while highly salt resistant plants should be watered less often, but with larger quantities of water. At the same time,we consider that the amount of water applied on different parts of the same field should vary according to the degree of salinity of each particular plot. Another point is that the salt tolerance of individualplants varies at different phases of their development, and in accordance with the intensity of the growing processes. With annuals, the effect of salts is most marked during sprouting and blossoming;with perennials,during fruit bearing. It is during these periods that special care must be taken in regard to irrigation with saline water. In addition,as pointed out earlier, chloride salinity causes a sharp drop in the transpiration rate of plants, whereas with sulphate salinity,the

284

PLANTS-WATERLOGGING

A N D SALINITY

transpiration rate increases appreciably.In view ofthis,the water consumed by plants through transpiration during the vegetative period will vary also according to the chemical composition of the water used for irrigation. It should be stressed that sprinkling irrigation cannot be used with successin all conditions or for all types ofplants.Tests made by various scientistsshow that sprinkling with water containing 0.012%chloride causes the appearance of scorch on the leaves of citrusplants.There are further data showing that sprinkling done during the day, and more particularly during the hot part of the day, increases the absorption of salts by plants. (c) Mineral nutrition In general measures are needed to raise the fertility of saline soils used for farming. Organic and mineral fertilisers may be used for this purpose. The disturbance of the mineral nutrition and the deficit of main elements,as described in section C.3(e) of this chapter,might suggest that the nutrients in short supply simply be applied.But in fact,as KOVDA (1947) has pointed out,this does not always have the desired effect. Owing to the high concentration of the soil solution in saline soils,some nutritive substances may precipitate.The use of easily soluble fertilisers, such as ammonium and potassium salts of nitric and sulphuric acid and also,in particular,potassium salt,will increasethe concentration of salts in the soil and make them more toxic for plants. The only way of ensuring the effectiveness of fertiliser for increasing the growth of plants is by taking measures,at the same time,to reduce the quantity of easily soluble salts in the soil. In order to improve the mineral nutrition ofplants growing on saline soilsit is recommended,therefore,to use more organic fertilisers and to spray liquid nutrients or use mineral fertilisers in addition.

3. Increasing the salt resistance of plants

Next to special farming and amelioration techniques,methods for increasing the yield of agricultural crops on saline soils include biological measures to improve the salt tolerance ofplants. W e may now speak about the adaptation ofplants,not to salinityin general,but to specific types of salts in the soil.The ways in which plants adapt themselves to a saline substratum depend firstly on the specifictype of salt content and secondly on the biological properties of the plant. T o put it more concretely,the different types of salt content in the soil lead to the emergence,within the same strain and species,of ecologicaltypes of plants possessing certain distinctive physiological,anatomical and morphological features. It is possible to determine the resistance to sulphate salinity, chloride salinity, carbonate salinity and alkaline salinity of individual agricultural plants. Salt tolerant forms of plants are of value not only because of their higher yields but also because they can be used in the development of saline and reclaimed lands. For obtaining salt tolerant strains,artificial selection,intravarietal crossing and hybridisation can be used ; also prolonged cultivation on saline soil and seed-processingprior to sowing. Numerous tests have shown the advisability of using local seeds and plant material in saline regions. It is recommended that only seeds of plants growing on saline soil and possessing the best capacity for adaptation and the highest productivity should be used. Such plants-including, in particular, Gossypium-are frequently to be found on solonchak patches with distinct traces of salt efflorescence on the surface. It is possible, by reproduction of the seeds ofsuch plants on saline soils and repeated selection of the best adapted and most highly productive specimens,to produce strains with considerably improved salt tolerance and productivity. According to the experiments made by MATUKHIN (1963), the salt tolernce of Lycopersicon esculentum can be noticeably increased by growing it for a number ofyears on artificially salinised soil.The salt tolerance of Hordeum, Medicago and Trifaliumwas likewise found to be increased after many years’cultivation on saline soils. Experiments (STROGONOV, 1962) show that intravarietal crossing within Gossypium strains characterised by different degrees of adaptability and productivity produces a marked increase in the weight of the boll. Gossypium grown from seeds produced by intravarietal crossing develops greater salt tolerance in saline conditions and produces a higher yield than the control plants (Table 9.20). 285

IRPLICATION,DRAINAGE A N D SALINITY Table 9.20. Yield of Gossypiuin grown from the seeds of intravarietal crossing Number of bolls per bush

Degree of

salinity of plot

Strain

Weak Weak Strong Strong

Control* Experimental? Control Experimental

Weight of boll in g

18.IX

ll.x

Total

of one boll

of one bush

4.6 6.3 3.7 4.8

6.7 7.7 8.6 12.3

11.3 14.0 12.3 17.1

5.69 6.10 6.10 5.68

64.3 85.4 75.0 97.1

* Without intravarietal crossing

t With intravarietalcrossing

With intravarietalcrossing,little segregation will of course occur in the offspring.This method can therefore be used as a means of raising the general salt tolerance and productivity of a variety. In view of the distinct differences of salt tolerance between different strains and species,a lot ofinteresting work has been done on intervarietal crossing,hybridisation and selection.For instance,the crossing of high quality strainswith salt tolerant strainsproduced some fairly salt resistant strains of Oryzu.It proved possible by selection to breed a strain of Lupinus which gave a very large quantity of seeds and vegetative matter (267% that of the control plant) with a salt content in the soil of 2.5-3.25 % CaCO,, This type of Lupinus can be grown successfully on carbonate soils. Distant hybridisation (between subgenera) produced, for the first time, poplar-touranghybrids with a much higher salt tolerance than the Populus. Budding of Citrus paradisi Macf., using Citrus aurutium and Citrus reticulata as seedling stock, showed that trees grown from stock of Citrus auratium have less resistance to saline drain waters than when grown from stock of Citrus reticulata. BERNSTEIN (Publication 942), selecting over five generations from a cross of two presumably sensitive lines of cotton,and from a cross between two presumably tolerantlines,found a large difference in vigour between the two resultant lines, but no difference in salt tolerance. More promising, however, were some marked improvementsin salttolerancethat were achieved inadvertentlyby plant breeders,with different selections of bermuda grass. Increase ofsalt tolerance offruittrees can be achieved by selectingthe right type ofrootstock. much more may be accomplished ifbreeding and selectionareundertakenspecifically According to BERNSTEIN to increase salt tolerance. Biological measures for raising salt tolerance are based on the principle that the adaptability of plants to the external conditions of their environment is greatest in the initial stage of development.O n this principle, GENKEL and his colleaguesdevised a series of methods for improving the salt resistance of plants. In working out methods for processing seeds with salt,the duration of seed processing and the concentrations of salts used were varied according to the biological properties ofthe plants and the conditions ofcultivation (degree and type of salinity). The proposed pre-sowingmethods for increase of salt tolerance were tested out and proved capable of producing a considerable increase in the yield of agricultural crops grown on saline soils. data,salt toleranceacquired by plants by processing the seeds once with salt solution According to GENKEL’S in working out this method for raising salt prior to sowing is handed down to the plant’s progeny. GENKEL, tolerance by treating seeds with salt before sowing,drew on suggestions previously made by other authors. A point of interest is that the specific adaptation reactions of plants vary according to the salts contained in the substratum.This is illustrated by the results obtained by GENKEL (1954)in his experiments on methods for raising salt tolerance,with different types of salinity, through treating the seeds before sowing. These experiments show that, for Na,SO, soil salinity, pre-sowing processing of seeds of Panicum miliucum is more effective when done with MgS04 than when using NaCl whereas for NaCl soil salinity better results are obtained by processing with NaCI than with MgS04. In addition to these direct methods, work is also being done on indirect methods of increasing the salt tolerance of plants such as damping seeds before sowing and vernalisation of seeds in solutions of nutritive substances and micro-elements. Another indirect method of raising salt tolerance is to treat plants with various kinds of growth-inhibitors (Amo 1618,Phofon,CCC),which makes them more resistant to the toxic effects of salts.However,it will not be possible to continue to use these substances in agriculture until tests have been made on the value of 286

PLANTS-WATERLOGGING

A N D SALINITY

plants thus processed for food and fodderpurposes. It was also observed that substances ofpetroleum origin raised the salt tolerance of secale. (1958) however. Negative findings for the seed treatment have been reported by HAFIZ

4. Diagnosis of salt tolerance Diagnosis of salttoleranceis ofvital importancein seed production and selection.Before selecting seeds from salt resistant plants,tests should be made of the salt resistance of those plants which produce the largest yield when grown on saline soils. One method of testing salt resistance is to grow seeds in a salt solution. This method has,however, the disadvantage of demonstrating salt tolerance at the initialphases of development only,whereas it is essential to ascertain the degree of salt tolerance throughout the entire ontogenetical cycle,since it varies at different phases of development. STROGONOV(1962) proposed a method for testing the salt tolerance of plants by ascertaining the rate of destruction of chlorophyll on leaves immersed with peduncles in solutions of salts.This method is based on recognition of the existence,in plants with high salt tolerance,of a stable chlorophyll-protein complex.The method is as follows:plant leaves,cut off under water,are immersed in a water extract taken from saline soil, in saline groundwater or in a salt solution with concentration of 24%. In order to protect the leaves against wilting, the experiment is carried out in diffused light.The degree of salt tolerance is indicated by the speed with which saltpatches appear as a result of the destruction ofthe chlorophyllunder the influence of the salts. In salt resistant plants,the process of chlorophyll destruction usually begins later and occurs more slowly than in plants which are not salt resistant. In addition,a microscopic method for determining salt resistance,applicable to plants of various kinds, was devised.Microscopic sections ofthe leafepidermis are submerged for severalhours in a molar solution of sodium chloride after which the number of plasmolised cells in the field of vision of the microscope are counted,and the mean number of cells calculated.As a rule,Íive fields of vision are taken (30in all for each section). The number of hours for which it must be kept in the saline solution is determined by means of a preliminary experiment. It should be noted that all methods are applicable to plants grown in saline conditions. Itis clearfrom a brief analysisofthe changes occurring in some ofthe vital activitiesofplants in an external medium with a high salt content that the solution of many of the problems involved in the prevention of soil salinity are closely linked with the physiology of the salt resistance of plants.

REFERENCES AKHMETSAFIN U.M.(1947), New data about vegetation connections with underground water, News of Kasachsk. SSR N 1-2 (in Russian). ARNOLDA.(1955), Die Bedeutung der Chlorionen für

die Pflanze,insbesonderenphysiologische Wirksumlceit,

Eine monographische Studie mit Ausblicken auf das Halophytenproblenl,Bot. Studien.Publ.von W.Troll und H.von Guttenberg,Jena. BATALIN A.(1884), Bull. du Congres International de Botanique et d'Horticulture, St. Petersburg. BEYDEMANNI.N.(1947), Seasonalcourse oftranspiration in certain plants in the semi-aridclimate of North Mugan, Reports of the Azerbaijan S.S.R.Academy of Sciences, HI,7 (in Russian). BEYDEMANNI. N.(194.9), The part played by the plant cover in the salt and water conditions of soil,Soil Science, 7 (jn Russian). BEYDBMANNI. N.(1954), Development of vegetation and soil in the lowlands of eastern Transcaucasia: Problems of improving the €orage base in steppe, semi-arid and arid zones of the USSR,Academy of Sciences of the USSR,Moscow-Leningrad (in Russian). BEYDEMANNI. N.(1956), On inethods of studying the water regime of plants, Botanical Journal, XLI, 3 (in Russian). BEYDEMANFI I.N. (1960), The rhythm of seasonal development in the level of transpiration of plants with

287

IRRIGATION, DRAINAGE A N D SALINITY different types of water regimes in soils under various climatic conditions, Botanical Journal,45,8 (in Russian). BEYDEMANNI. N.(1962), Plants transpiration in the lowland of Kura-Arax under conditions of different humidity and salinity of the soils,Collection:Ecological,geobotanical and agricultural-meliorative investigations in the lowland of Kuru-Arax in Transcaucasia,M.-L.(in Russian). BEYDEMANN I.N.(1964),Significanceof coediphicatorin indicatoryproperty ofplant communities.Works of the Moscow Society of naturulists,VIII (in Russian). BEYDEMANNI. N.(1964), Plants and plant communities’ influence on dynamics of underground water, Works of the Moscow society of naturalists,VIII (in Russian). BEYDEMANN I.N.(1965),Ecological and biological groundsfor evolution of plant communities inplant cover in the lowland Kura-Araxin Transcaucasia,L.(in Russian). BEYDEMANN I. N.and PREOBRAZHENSKY A. S. (1957), Mutual conditioning in the development of soils and vegetation. Transactions of the Botanical Institute of the Academy of Sciences of the U.S.S.R.,series 3, Geobotanika,II (in Russian). BEYDEMANN I. N.and FILENKO R. A. (1959), Hydrological prospecting in geobotanical research. Field geo-botany,1,Academy of Sciences of the USSR, Moscow-Leningrad (in Russian). BERGC.VAN DEN (1950), The influenceofsaltin the soil on the yield of agricultural crops,IVlnt. Congress of the InternationalSociety of Soil Science,Transact. 1. BERNSTEIN L.Salt tolerance ofplants and the potential use of saline waters for irrigation,DesalinizationRes. Confer.,Nat. Academy of Science,Nat. Res. Council,Publ.,942. BERNSTEIN L.(1962), Salt-affectedsoilsand plants,The problems of the arid zones.AridZone Research,XVIU. BERNSTEIN L.(1964), Effects of salinity on mineral composition and growth of plants, Plant analysis and fertilizer problems,1V. BERNSTEIN L. (1965), Personal communication. BERNSTEIN L., BROWNJ. W . and HAYWARD H. E. (1956), The influenceof root-stockon growth and salt accumulation in stone-fruittrees and almonds, Proc. Amer. Soc.Hort. Sci.,68. BERNSTEIN L.and HAYWARD H.E.(1958), Physiology of salt tolerance,Ann. Review Plant Physiol.,9. BERNSTEIN L., MACKENZIE A.F.and KRANTZ B. A.(1955), The interaction of salinity and planting practice on the germination of irrigated row crops,Proc. Soil Sci. Soc.Amer., 19,2. BI~ATTACHARYA R.(1958), Growth of azotobacter in rice soil,Indian J. Agric. Sci. 28. BRAUNBLANQUETJ., ROUSSINE N.and NEGRE R. (1952), Les groupements vegetaux de la France mediterranéenne,C.N.R.S. BUTIJNJ. (1961), Bodembehandeling in de fruitteelt (Soil management in fruit growing). Versl. Landhlc. Onderz.,66,7. CANNON W.A. (1925), Physicalfeatures of roots,with especialreference to the relation of roots to aeration oJ the soil,Carnegie Institute,Washington 368. CHANG H.T.and LOOMIS W.E. (1949,Effect ofcarbondioxideon the absorptionofwater nutrientsby plants, Plant Physiol.,20. DEMIDOVA L. C.,SHAVRYGINAA. V.,KUSINA Z.M.,FADEEVA O.T.and LEVIN V.L.(1955), The experience of the use of geobotanical method in hydrogeological research on Black soils. Collection: Work of the geobotanical method in geological investigationGasgeolotechizdat.M.(in Russian). DHAWAN C.L.C.S. (1958), Pre-irrigationsoil survey of some districts of the Punjab,IndianJ. Agr. Sci.,27,4. EHLIG C. F.(1960),Effects of salinity on form varieties oftable grapes grown in sand culture,Proc.Am. Soc. Hort. Sci.,76. EHLIG C.F.and BERNSTEIN L.(1958), Salt tolerance of strawberries,Proc. Am. Soc.Hort. Sci.,72. FERRIS J. G.(1949), Groundwater,In WISLER C.O.and BRATERE.F.,Hydrology,Wiley,New York. FORSHL.F. (1957), Evaporation and transpiration in the Amu-Daryadelta. Laboratory of Lake Sciences, Academy of Sciences of the USSR, IV (in Russian). GENKEL P.A.(1954), TimiryazevaReadings 12, Publ. USSR Academy of Sciences (in Russian). GRASMANIS V.O.(1958), Manganese excess and bark necrosisin apples. Manganese excess and bark necrosis in pears,J. Austr. Inst.Agric. Sci.,24. GRECHIN I.P. (1961), Role of aeration in changing the properties of dernopodzolicsoil,Dokl.s.-Ich. Akad. Timiryazeva,63 (in Russian). GRILLOT G.(I 954), The biological and agriculturalproblems presented by plants tolerant ofsaline or brackish water and the employment of such water for irrigation,Arid Zone Programme,UNESCO,Paris. 288

PLANTS-WATERLOGGING

A N D SALINITY

HAFIZ A. (1958), Report on UNESCO-Iran symposium on soil salinity problems in arid zones with special reference to Pakistan. Agric. Pakistan,9,4.

HAYWARD H.E. and BERNSTEIN L.(1958), Plant-growthrelationships on salt-affectedsoils,Botan.Review,24. HAYWARD H.E.,LONGE. M.and UHVITS R. (1946), Effect of chloride and sulphate salts on the growth and development of the Elberta peach on Shalil and Love11 rootstocks, U.S.Dept. Agric.,Techn. Bull.,922. HIGASE S. (1959), Physiological studies on flower formation in tobacco, 2.The effect of the duration of waterlogging on the developmental responses of tobacco,Proc. Crop Sci.Soc. Japan,28. HOORN J. W.VAN (1958), Results of a groundwater level experimental field with arable crops on clay soil, Neth. J. Agric. Sci.,6, HOPKINS H.T.,SPECHTANDA. W.and HENDRICKS S. B. (1950), Growth and nutrient accumulation as controlled by oxygen supply to plant roots,Plant Physiol.,25. KELLER B.A.(1910),Minutes of the XII congress of Xussìan naturalscience researchersand doctors in Moscow (in Russian).

KELLERB. A.(1929), Evaporation ofplants,Experimental Station of V. I. Voronezsh (in Russian). KELLER B. A.(1940), Papers of the Laboratory Evolutionary of Ecology of Plants,Moscow Botanical Garden, USSR Academy of Sciences,1 (in Russian). KON’KOV B. S. (1948), Agrotechnical methods for combating soil salinity, State Publishing House of the Uzbek. SSR,Tashkent (în Russian).

KOVDA, V. A. (1947) Originsand regime of saline soils,11,Publ. USSR Academy of Sciences (in Russian). KOVDA V. A. Reports of the TurkmenBrunch of the USSR Academy of Sciences,3 (in Russian). KOVDA V. A.(1950), The study and aims of soil research in the deserts of Central Asia. From the book USSR deserts and their reclamation, published by the Academy of Sciences of the USSR, MoscowLeningrad (in Russian).

KOVDA, V.A.and EGOROVV. V. (1953), Some regular featuresin soilformation in littoraldeltas,SoilScience, 4 (in Russian). KOVDA V. A.,EGOROVV. V.,MURATOVA V. S. and STROGONOVB. P.(1960), Botanical Journal,45,8 (in Russian).

MATUKHIN G. R. (1963), Physiology of the adaptation of cultivated plants to soil salinity, Publ. Rostov University (in Russian).

MEINZER O.E. (1927), Plants as indicators of groundwater, US.Geol. Survey Water-SupplyPaper,557. MOLCHANOV A.A.(1952), Thehydrological effect ofpine forests on sandy soils,Academy of Sciences,USSR, Moscow (in Russian).

MOLINIER and TALLON G.(1947), Note sur les possibilités d’amériolation des prairies en Camargue, Bull. Techiz. des I.S.A., 23. NOVIKOFF G. (1961), Contribution à l’étudedes relations entre le sol et la végétation halophyte de Tunisie, INRAT. Tunis.,34, OGANESIAN A. P. (1954), Salt resistance of some field crops,Pochvovedenie 10 (in Russian). PROTSENKO D.P. (1956), Comparative characteristics of the salt resistance of jiuit trees, Publ. Shevchenko State University, Kiev (in Russian).

RATNER E.I.(1950), Mineral nutrition of plants and the absorption capacity of soils.,Publ.USSR Academy of Sciences (in Russian).

RAUNKIAER (1934), The lif. form of plants, Oxford. RICHTER A. A. (1927), Journal of Experimental Chemistry of the Southeast,3, 2 (in Russian). RIJOV S. N.(1948), Irrigation of cottonplants in the Ferghana Valley,Publ. Uzbek. Academy of Sciences, Tashkent (in Russian).

ROBINSON T.W.(1958), Phreatophytes,Geological Survey Water Supply Pupeu, 1423. SERGEEVL.I. (1953), The endurance of plants,Publ. Sov. Nauka (Soviet Science) (in Russian). SHAKHOV A. A. (1956), Salt tolerance of plants, Publ. USSR Academy of Science, Moscow (in Russian). SKAZKINF.D.and FEDOROVA Yu.N.(1961), Effect of nitrogen and of excess moistening of soil on some physiological processes and yield of barley in relation to its phasal development,Dokl.Akad. Nauk, 139. STOCKERO.(1928), Das Halophytenproblem,Ergebn. d. Biologie,3,Berlin. STROGONOVB. P. (1949), Physiology of the salt resistance of the cotton plant. Publ. USSR Academy of Sciences (in Russian). STROGONOVB.P.(1962), Physiologicalbases ofthe salt toleranceofplants (withdzj5erenttypesof soilsalinity). Publ. USSR Academy of Sciences (in Russian). 289

IRRIGATION, D R A I N A G E A N D SALINITY

TULAIICOV N.M. (1922), Solontsy, tlzeir

improvement and utilisation, 2nd edition, revised and enlarged,

Gosizdat (in Russian).

us SALINITY LABORATORY (1 954), Diagnosis and improvement of saline and alkali soils, Agriculture Handboolc 60 USDA. VOSTOKOVA E.A.(1953), Vegetation as an indicator of geological and hydrological conditions in deserts and semi-deserts connected with cultivation. University of Moscow, M.(in Russian). VBSTOICOVA E. A. (1956), Geobotanical observations in hydrogeological investigations in TimirshkoAktubinskom Priuralje, Soviet geological works 56, Moscow (in Russian). WADLEIGI-I C. H.and AYERSA.D.(1945), Growth and biochemical composition of bean plants as conditioned by soil moisture tension and salt concentration,Plant Physiol., 20 (in Russian). WARMING E.(1901), The ecological geography ofplunts, M.(in Russian). WHITE W.N.(1932), A method ofestimating groundwater suppliesbased on discharge by plants and evaporation from soil, US Gelog. Survey Water-Supply Paper 695A. YANICOVITCH L.(1949), Resistance aux chlorures des plantes cultivées,Anrz. Serv. Bot. et Agron. Tunisie 22. ZNAMENSICY A. A. (1938), Plant cover and fluctuationsof the groundwater,Soil Science,7(in Russian).

290

10.Irrigation Systems and Management* A. WATER SUPPLY, STORAGE A N D CONVEYANCE

1. Introduction

The objectives in a permanent irrigated agriculture are to secure optimum crop yields at least cost to maintain favourable water and salt balances and otherwise preserve soil resources for future production. This will require that the irrigation and drainage systems be properly designed,operated, and maintained. The collection,storage and,distribution of water for irrigation involves investment in the construction of dams,reservoirs, canals,pipelines, and various other hydraulic structures. Water must be conveyed to the farm and utilised there in an efficient manner in order to achieve optimum use of the capital invested. This distribution network must be maintained in good repair and the flow of water closely controlled. The most economical method of irrigation must be chosen on the basis of current and long-termyield potential and efficiency of water use; careful management will then achieve the maximum potential returns. History shows the most common reasons,apart from fertility,forfailure to maintain high yields in irrigated agriculture over an extended time period to be (a) waterlogging, (b) salinisation and (c) alkalinisation of the soil being irrigated chiefly as a result ofseepage from canals and over-irrigation.Insidiousdangers sometimes appear only after a number of years:they should be considered during the initialplanning stage,yet they are often disregarded. For the same reason,irrigation management must be extremely vigilant to detect such dangers in the initial stages,when the remedy is least expensive. 2. Water supply sources and water storage

(a) Water supply sources The success of every irrigation project rests largely on the adequacy and dependability of its water supply. Irrigation projects may obtain water from (1) artesian or pumped groundwater,(2) lift from lakes or rivers, (3) gravity flow diversion from rivers or lakes (without storage), (4)diversion from rivers with storage and (5) springs and other subterranean sources. Water storage in some form is frequently required in irrigation projects. The purpose of storage is to conserve a sufficient quantity of water in surface (or underground) reservoirs for use during the irrigation period when the amount available in natural water courses or water tables is inadequate.

(b) Water storage The discussion of this section is limited to those aspects of surface water supplies and water storage which affect water quality, a detailed discussion of other aspects of water quality is given in Chapter 7. I (c) Dissolved salts The salinity of water stored in reservoirs is influenced by (1) the salinity in the watershed, (2) the salinity of the reservoir bed and (3) evaporation. The salinity of the water flowing into a reservoir may vary to an appreciable degree depending on the rainfall distribution and on the geological nature of the Watershed.The variation is greatest in semi-aridand arid regions. The salinity of the inflows is at its highest concentrationwhen the first run-offfrom rainfall begins after a long dry period during which salts have accumulated on the surface from evaporation of saline soilwater.Salt inflows may also come from springswhich have a variable discharge from one part of the year to another or from one year to the next and which appear either on the watershed or below the water-line in the reservoir itself. The operation of storage facilities can influence the salinity of the water supplied to irrigation projects. *This chapter was edited by R. M. HAGAN with the assistance of J. N. LANDERS from the manuscripts submitted by F.CLINTON and T.H.QUACKENBUSH, A.DARLOT, and S. R. OFFENGENDEN as authors with contributionsby D.S. FERGUSOW

291

IRRIGATION, D R A I N A G E A N D SALINITY For example,the average salinity of flood water, after the initial flush,is low in comparison with the small continuousflow entering a reservoir in the dry season.Therefore,the average salinity of the stored water can be lowered by making provision to retain most of the flood flow. In extreme cases,it may be advisable to over-designa reservoir’s capacity in order to minimise the volume of excess flood water spilled. The salt concentration in a reservoir at any given time can be calculated from the following formula:

where

C,= the salt concentration in a reservoir at the end of a given time period Ci=the initial concentration at the beginning of the time period V,=the initial volume corresponding to Cf V,=volume of inflows T=the net salt addition to the reservoir, T= (salt inflow-salt outflow) E=the depth of water evaporated per unit area over the time period S= the average surface area Yo=volume of outflow S,= volume of seepage losses Ts=net change in total salt due to salt diffusing in from reservoir bottom or lost in seepage The evaporation rate can be calculated from an empirical formula selected to fit the existing conditions

(ICIDpublication pending, S. LELIAVSKY, 1955-60). In the USSR,the undermentioned formulae are used: (1) Evaporation from the reservoir surface can be defined by the formula derived by POLYAKOV:

w= 18.6 (i +0.2w)d2/3 where U=evaporation layer per month,in mm W=mean monthly wind velocity,in m/s d=mean monthly moisture deficit,in mm (2) The formula derived by DAVYDOV is used to define evaporation from water surfaces of large reservoirs: U=0.48De(1 +0*125W)

(3)

where D e= difference between the maximum vapour pressure at the water temperature and its actualpressure in the air under the same temperature. (3) Evaporation can be defined by the formuladerived by KRITSKY, MENKEL and ROSSINSKY, thewater surface being equal to several square killometres: U=9(10- 12002/1+0*15WSoo)

where

(4)

I,=pressure of saturated vapour,in m m , calculated by the mean monthly water temperature 1200 =mean monthly air moisture, in mm Wgo,=mean monthly wind velocity at the height of 9 m

The volume-surface function should be known from design data,but estimation of seepage losses and their salt contentwill be difficult;they can be neglected if small in comparison with the other factors.Volumes of inflow and outflow are standard measurements, additional measurements of the salt content of these waters are desirableby continuously recording electrical conductivity,or at least by frequentsamplingand analysisof the flows. All other things being equal, the higher the surface/volume ratio, the greater will be the increase in concentration due to evaporation. Evaporation also increases with the windspeed,turbulence, and vapour pressure deficit of the air mass (ICIDpublication pending). In certain extreme cases,a reservoir may become ‘self-consuming’.This occurs when large areas of shallow depth (useful for flooding-dampingand farm purposes) cause a substantially elevated surface/volumeratio, and a similarly increased salinity concentration rate. In such a case the desirability of restricting the design capacity to improve this ratio should be examined. 292

IRRIGATION SYSTEMS A N D M A N A G E M E N T

A great deal of research is still being conducted all over the world in an endeavour to devise a practical method to reduce surface evaporation (ICIDpublication pending and CIGR 1958). Attempts have been made to cover the entire surface with a mono-molecular film of an anti-evaporant product, such as hexadecanol or octadecanol; the film, non-toxic,reduces evaporationwhile allowing free passage of rain,oxygen and sunlight.The reduction in evaporation achieved by this method ranges between 15 and 70%for small reservoirs and remains below 40%-under low wind conditions-for large reservoirs. The cost per cubic metre of water saved is variable. Further long-termexperiments will be necessary before these methods are sufficiently perfected to become common practice. The salinity of the reservoir bottom is especially important where a high water table (e.g.in polders) or an impervious substratum prevent seepage losses; the absence of leaching allows upward movement of salts into the reservoir water by diffusion. For example, in lake Ijssel in the Netherlands, a polder area, it is estimated that salt diffusion from the reservoirbottom (i.e.the former sea bed) will provide lo7kg of C1ions annually over a total area of 1200 km2.That is approximately 834 kg per hectare per year. These different phenomena should be analysed in detail on the basis of a systematic sampling programme in order to obtain accurate information on the salinity ofwater in reservoirs at each period ofthe year and to draw the appropriate conclusions as to water utilisation procedures. (d) Esfects on the water and salt balances of nearby lands Where seepage from a reservoir becomes sufficient to cause a rising trend in the neighbouring groundwater level,depending on the seepage flow paths, a drainage network must be established at the toe of the dam or over the entire area likely to be affected.Ifthe seepage water is saline or the groundwater rises through saline subsoil horizons,this drainage network must be adequate to permit leaching of the affected land. The alternative is to seal the dam,the foundations and the reservoir with a bentonite clay layer,bitumen emulsions,chemical sealants or,for small reservoirs,sheets of impervious plastic or asphalt.

3. Water conveyance

In arid and semi-aridareas water is often conveyed long distancesto farmsfrom the point at which the natural resources are diverted. Water conveyance may present a number of operational problems; seepage losses from canals have particular significance in salinity and waterlogging control. (a) Estimation of seepage losses

A sound estimation of seepagelosses is highly necessary for further decision,e.g. lining of canals,density of drainage network,area to be irrigated with a given flow.These losses are usually expressed in litres (or cubic feet) per square metre (or per square foot) of wetted area in 24 hours. For preliminary estimates it may be assumed that,in typical earth canals under usual conditions,about one-thirdof the total water diverted will be lost by seepage,operationalwaste and evaporation.The prediction of seepage must be based onjudgement within the limitsof existing data and natural factors.Many formulaehave been proposed in the world;some of them are no longer in use. Seepage losses can be estimated from flow records at gauging stations in the network. DARLOT suggests the following expression:

S,=

where

SP

100

R . V.86.4’ S,=percentage loss of water (perkm) s,=seepage losses (in m3per m2per 24 hours) R = the mean hydraulic radius of the canal (in m) V= the velocity of the water (in m/s)

In the USSR the conveyance efficiency of a canal or ditch is considered equal to : Qnet E,=---.100

(6)

QgrOSS

293

IRRIGATION, DRAINAGE A N D SALINITY E,= system conveyance efficiency (%) where Qnet=outflowat the tail of the canal (Qnet=Qgross--S,) Qgross=inflow at the head of the canal S, = conveyance losses in the canal The conveyance efficiency of an irrigation system is measured by the ratio water delivered to the farm: water diverted from the source. It may also be obtained from the product of the individual components of the distribution network thus:

where

E, = conveyance efficiency of main canal (%) E,I= conveyance efficiency of laterals (%) E,,= conveyance efficiency of farm ditches or pipelines, etc. (%) Wf=water delivered to farm W,=water diverted from source

When measurements are not available the following equation for estimating seepage losses from earth canals or ditches has been proposed by A.N.KOSTIAKOV:

where:

s=water losses per k m of canal length (in

L=length of the canal (in km) Q =water flow (in m3/s)

%)

s is calculated by the formula: s=

A Qm

(9)

A and m = empirical constants depending on soil permeability (Table 10.1). Table 10.1. Values of the empiricalconstants A and m in equation 9--(After KOSTYAKOV)

Permeability Constant

A m

low

medium

high

0.70 0.30

1-90 0.40

3.40 0.50

It can be seen from equation 9that conveyance efficiency increasesin a given canal as the volume of flow increases,and decreases rapidly with higher soil permeability values. OFP~NGENDEN defines the application efficiency as:

and he has set up the following formula with respect to interfarm canals: l-e, 1 1-e-u"

--_

where

e = application efficiency corresponding to the design system discharge e a = selection application efficiency Qa u =-where Qa = the discharge when e= e a

Q

m = symbol from Table 10.1. 294

IRRIGATION SYSTEMS A N D M A N A G E M E N T Under water rotation,the application efficiency can be obtained by the formula: 1 -e, I P - b l-e am e r = selection application egciency under water rotation where e = application efficiency (as above) K=number of turns among canals (number of cycles of water rotation) b =relationship between the length of interfarm canals functioning simultaneously under water rotation and the total length of these canals

a

=e the ratio between the discharge under water rotation and the design discharge Q

Usually there is 2-cyclewater rotation and by experience b =0*60. For this particularcase (canalsin soils ofmean permeability) the relationshipbetween applicationefficiencies is as follows: l-e, dä =0*85l-e a

Ifwe know thevalue ofthe application efficiency with the discharge decreased (Q, )the applicationefficiency under water rotation will be as follows:

1l-e -er=K" or: with e,=0.85ea+0.15

(14)

K=2,b=0.6, m=05

(15) Ifthe canal runs in the soiIs oflow permeability (m=O-3) maintenance of maximum values of the application efficiency can be expected with the 2-cyclewater rotation. If the canal is constructed in soils of mean permeability the maximum value of the application efficiency under 2-cyclewater rotation can be expected only with a >0.55 while for the soils of high permeability it can be kept with a > 0.70.Other results will be obtained if the comparison is between e, and e,. In this case the use ofthe 2-cyclewater rotation compensates the decrease ofthe application efficiency with the reduction ofthe discharge.This is illustrated in Table 10.2: Table 10.2. e, =y (e,, m),-(After

OFFENGENDEN) ...

0.90 0.80 0.70 0.60 0.50 0.40

m=0.3

m = 0.4

m=0.5

0.93 0-85 0.78 0.70 0.63 0.56

0.92 0.85 0-76 0.68 0.61 0.53

0.91 0.83 0.75 0.66 0-58 0.50

The efficiencyof the 2-cyclewater rotation is given in Table 10.3.

Table 10.3. (Eexpressed as a percentage of ea-(After

OFFENGENDEN)

ea

m = 0-3

m=@4

m = 0.5

0.90 0.80 0.70 0.60 0.50 0.40

3 6

2 5 9 14 21 32

2 4 6 10 15 22

11 17 26 39

295

IRRIGATION, DRAINAGE A N D SALINITY

The duration of each cycle of water rotation is defined by the following formulae: Q'ii e2

The Ist cycle-t,=

Q"A

Q'ne2+

The 2nd cycle-t2 = where

Qf'n

e,

ti

*

.ti

Q'nez +Q"A

ti =period ofwater rotationin days,minus the timerequired to regulatethe discharge and the

run of water

QIn,Qnn=sum of the net discharges of the canal group functioning in each water rotation cycle e,, e2=application efficiencies of the canal groups functioning in each water rotation cycle When water is diverted into a dry canal,the seepage loss is high at first,but in most soils decreases quickly to a much lower value which is maintained as long as the canal carriesthe same flow. and MIRKIN found that on periodically functioning canalsconstructed in the soils In 1939 OFFENGENDEN of mean and higher permeabilities with the occurrence of groundwater the percentage of seepage losses decreased hyperbolically. The values of seepage losses settle in t = 50-60 hours after the water diversionto the canal.The authors propose to use a coefficient d when calculating seepage losses on periodically functioning canals.The value of the coefficient with t > 10 is: 21-55 d=0.54 + -

(18)

t

Other formulae are also in common use in the USSR (N.V.PAVLOSKY; S. A. GUIZSHKAN). In the formula developed by S.A.AVERIANOV many hydrophysicalproperties ofthe ground are takeninto account:retention capacity,porosity, permeability,etc. Moreover,it has been established that the seepage losses are reduced when there is a groundwater at limited depth and a coefficient of correction is to be applied (Table 10.4). Table 10.4. Coeficient of correction on seepage losses-(After

Canal discharge (in m3//s) 0.3 1 .o 3 *O 10.0 20.0 30.0 50.0

3

GUIRSHKAN)

Depth of groundwater (in m) 5 7.5 10

3

15

0.82 0.63

0.50 0.41 0.36 0.35 0.32

0.79 0.63 0.50

0.45 0-42 0.37

0.82 0.65 0.57 0-54 0.49

0.79 0.71 0.66 0.60

0.91 0-82 0.77 0.69

0.94 0.84

The Moritz formula suggestscomputationsof total seepageloss in cubic feet per second per mile of canals as follows:

s=o,2c&

(19)

where S=loss in cubic feet per second per mile of canal Q =discharge of canal in cubic feet per second V=mean velocity of flow in feet per second C = cubic feet of water lost in 24 hours through each square foot of wetted area of canal prism Observations on eight differentprojects gave the following average figures for the value of C in earth canals. These factors are suitable for rough preliminary estimates,but measurements have shown that seepagelosses vary widely within each ofthe general soiltypes.For design purposes,it is usually necessary to make estimates of seepage losses in questionable areas on the basis of field tests.

296

IRRIGATION SYSTEMS A N D M A N A G E M E N T Value of C

Type of material

Cemented gravel and hardpan with sandy loam Clay and clayey loam Sandy loam Volcanic ash Volcanic ash with sand Sand and volcanic ash or clay Sandy soil with rock Sandy and gravelly soil

0.34 0.41 0.66 0.68 0.98 1.20 1 e68 2.20

The following empirical formula is in use in India: K = 5 . Q 1/16 where K=loss in cubic feet per second per million square feet of wetted area Q =discharge of canal in cubic feet per second

In earth canals,the lossses through seepage may range from 75 to 1500 litres per 24hours per squaremetre depending on the nature of the soils. Unlined elevated earth canals tend to have high seepage losses. In a lined canal system in good state of repair, seepage losses should not exceed 5% of the water conveyed. (This is dependent on the length ofthe system.) Similarly for a pipe system,the losses should not exceed 1 although small cracks in pressure lines may discharge comparatively large volumes of water. (b) Seepage control using lined canals and pipelines Water which seeps from canals and laterals often collects in low-lyinglands, thereby rendering them unproductive.Thereclamaion ofwaterlogged land,where this is practicable,by construction ofdrainge systems, even ifpracticable, is costly.Good engineering practice demands that all cost factors-the value of the land, the value of the water, and the cost of engineering features-be properly and carefully evaluated in the economic construction of an irrigation system and that this evaluation be projected carefully into the future. An important part of this evaluation is the consideration of canal linings to conserve water and reduce seepage and waterlogging of valuable land. Canals may be lined for the purpose of conservation,reducing damage to lowlandsfrom seepage,reducing operation and maintenance costs,or increasing structural safety. Usually more than one benefit accrues.The quantitativedetermination of seepage in an existing channel and the location of probable seepageareas in a proposed canal or lateral are of major importance to the planner and designer in selecting the reaches of channel to be lined. The cost incurred to alleviate the conditions caused by seepage losses when they are large enough to cause drainage, alkalinity, and salinity problems may not be immediately apparent,but it may eventually be enormous. The permissible expense for canal lining (orinstallation ofpipelines) in terms ofthe total savings achieved can be computed as follows: (20) cl=[Sp. b . C,+S,+S,

.-I

where C,=permissible lining cost per m length of section,in money terms Sp= annual seepage losses before lining (m3/yr) b = expected reduction in seepage loss ( %) C, = cost of water per m3 So= annual saving in operation costs after lining ,Y,,,= annual saving in maintenance costs after lining D =years of lining life L=length of lined canal or pipeline (metres)

It should be pointed out that this and other formulaeproposed for determining the feasibility and practicability of lining an unlined canal usually include such items as construction costs, the value of the water, drainage problems, protection from failure,increased capacity,factors which consider the life of the lining and its maintenance, as well as others (Portland Cement Association, 1957). Further,for proposed linings for new canals,the formulae should properly include factors to reflect such items as reduced storage and 297

IRRIGATION, DRAINAGE A N D SALINITY diversion requirements, smaller and fewer canal structures,and smaller canal sections which would result from lining.All of these factors are dificult,if not inipossible,to evaluate with accuracy. the advisability of the measures on seepage control can be determined by the According to OFFENGENDEN, formula he set LIP: 10.55 S .T . A a C N= (21) 100 pl

where N=permissible expenses for lining construction,in money terms S=losses from canal before lining,in m3 T=period of work A=number of years of lining life u = expected percentage of losses reduction C=cost of one cubic metre of water,in money terms I= canal length,in kin p =wetted perimeter of the canal (p=4*442/Q)in metres Linings for control of seepage losses in canals are of the following types:hard-surfaceand exposed membrane linings (portland cement concrete, shotcrete,soil-cement,asphaltic concrete, masonry-type linings, exposed asphaltic membranes,and exposed films of plastic and synthetic rubber); buried membrane linings (hot-appliedand prefabricated buried asphaltic membranes,plastic and syntheticrubber films,and bentonite membranes) ; earth linings (thick compacted-earth linings, thin compacted earth linings, loosely placed earth blankets, and soils with admixtures). Soil sealants, stabilisers,and other means may be used for seepage control. The several types oflinings all have advantagesand disadvantages and no single type can be recommended for all conditions encountered.All linings require some maintenance.The planner and designer must take these facts into consideration; and they,along with the operation and maintenance organisation,have a responsibility in the final choice of the type of lining to be used. Increasing use is being made of pipe systems which further decrease losses,permit a desired head to be available in different sections of the system (allowing direct operation of sprinkler systems), facilitate automation for water distribution,and increase ezse ofwater measurement.Pipe systems have the further advantages of avoiding loss of land area for water distribution and of virtually eliminating obstacles to cultivation and transport operations.Pipes may be made of steel, cast iron,concrete,asbestos cement,or plastic. The choice of materials is governed by technical considerationssuch as mechanical strength and corrosion resistance and by economic factors. In pipe distribution systems,buried closed conduits are used for conveyance of irrigation water to the delivery point on the farmer’sland.They may also be used to distribute water within the farm area.The pipelines may be installed both up and down slope if the pipe can be kept below the hydraulic gradient.There is no necessity to contour as in the case of a canal.Design criteria for individual systems open (or limited pressure) 2nd closed (or full pressure) have been developed. Either may be adagted for use at pump or gravity service areas. (c) Water coiztrol structures Standardisationofcanal structures is limited because ofthe wide range ofclimate,terrain,geology,and wateï delivery requirements.Therefore, control structures in current use in distribution systems throughout the world are of many forms.Publications dealing with this topic are given as ‘references’. Regulating structures(control structures) on an irrigation distribution system are those needed to regulate the flow and level of water. These structures include ‘turnouts’or ‘intakes’which are needed to control the rate of flow into laterals and farm ditches,check structuresused for controlling the water surface elevations, ‘division structures’ or dividcrs for dividing flow into two or more laterals, and wasteways for removing unwanted water from the system. The diversion structurein the river must include regulating devices providing required depths,water level, stream velocity, etc. In the USSR the formula of S. T.ALTUNIN is used for ‘watercatching dikes’,i.e.when no dam is erected:

6%

298

L d 8B

(22)

IRRIGATION SYSTEMS A N D M A N A G E M E N T

where: L=length of the diversion dike B=width of the stable channel for the design discharge

The bank strip is protected by restricting the stream with longitudinal and transversal levees. Spurs are erected in the downstream direction at some angle of the stream;the space (L) between spurs is defined by and I.A.BUSUNOV: the formula of S. T.ALTUNIN L=6L, sin a

where L,=working length of the spur a=angle between the axis of the spur and the stream (La) To prevent washing out of the foundation,the actual length of the spur is: La=1-5L,,

(22b)

L,=8 Ho

(22c)

Moreover it is recommended that: H,=mean depth of the river near the spurs Constant water levels in water conduits are desirable so that discharges at canal and lateraljunctions and farm delivery points do not need to be adjusted continually.(Ina major portion of the controland measuring devices used at these points discharge is related to water level.) Also near constant water levels in the conduits tend to stabilise seepage losses and the consequent hazard of waterlogging of land. Control structuresare needed in irrigation conduits where main suppliesmust be divided between branch canals,where small portions of canal flows must be released to distribution conduits,where water surfaces must be raised to facilitate deliveries,and where canal flows must be discharged to natural channels or other suitable outlets. Features planned for differentpurposes are often incorporated in the same structure.For instance,control devices may incorporate system protection features(e.g. ‘spills’)or they may also serve as measuring devices (e.g.weir and check structure combined). Checks are built in open conduits where water surfaces must be raised to permit adequate deliveries of water through turnouts or to permit diversion of canal flows through upstream wasteways. Dividers are used to ‘divide’a flow, which may be variable, into constant proportions. A two-way flow divider consists of a hinged gate which describes a horizontal arc across the stream and can be locked in any desired position. A series of weirs or orifices of different sizes may be used for the saine purpose. Automatic flow controlis also possible using both upstream and downstream regulatingsystems. Upstream systems maintain the water level constant upstream of a constant-levelgate or a very long crested (‘duckbill’ or ‘diagonal’)weir by allowing water entering the controlled section in excess of discharges to pass on to the next section.There is thus no control over the total discharges withdrawn within the controlled section,and the flow from the headworks and throughout the network must be carefully adjusted to balance the deliveries required.This meam in effect that some form of delivery rotation must be practised to simplify management. Downstream regulating systems depend on gates which automatically discharge sufficient water to meet discharges withdrawn downstream irrespective of their ihctuations,and spilling at the end of the system is virtually eliminated.Downstream control at the headworks (or diversion point) makes releases into the network automatic and greatly simplifies management,being well-suitedto ‘ondemand’delivery (see following section). By the same token,the possibjlity for controlling total releases to the network is reduced. This system is limited to use in areas where natural ground slopes are minimal. Use of regulatory reservoirs in critical locations reduces spill and operational waste. It also permits shoit-timedeliveries to adjacent lands. In additicln to automatic control of the distribution network,farm turnouts can be individually equipped with flow control devices or ‘modules’. Fully automated control systems have been and are being investigated in the USSR and in the USA. Telemetering equipment feeds in information on flow rates and water levels to a control centre from which remote controlled telemechanical devices can br: operated to adjust control structures in the distribution network. Tn all cases, canals should be fitted with safety structuresat intervals so as to make it possible to spill off W

299

IRRIGATION, D R A I N A G E A N D SALINITY

the surplus discharges in the event of valves of gates not operating properly or being wrongly manœuvred. The open or limited pressure pipe irrigation system uses low head pipe with open baffle or gate-controlled standslocated at irregular intervals along the pipeline. Equal spacing of stands may cause seriousline surges at partial capacity flow.The baffle serves the same purpose as a check and its top elevation should be set so that static water level will provide sufficient head to all farm deliveries between the stand and the next upstream baffle when the reach is operating at zero flow beyond the baffle.The baffle may operate as a weir either submerged or free fall;however,the latter condition may result in considerable air entrainment and thus required air vents from the pipeline. To limitpressure on delivery valves,to permit use oflow-headpipe, and to make the stands more accessible and less costly,the baffle height is usually limited to less than 6 metres (20 feet). Farm turnoutsfrom canals consist ofgates set in the canal bank (similar to those used in pipelines) or some form of sliding gate. T o control the water level in farm ditches portable check dams are used, permanent structures with an adjustable gate or flashboards serve the same purpose, but may impede mechanical maintenance operations.From the farm ditch,large siphons may be used as outlets to borders,small siphons and 'spiles' give good control over deliveries to individualfurrows.Gated pipe also serves this purpose well. (d) Sediments Much of the total annual flow of arid region rivers often occurs during flood periods when the rivers also transport maximum sediment loads,Silt particles with the diameter of 0.005-0*001mm and smaller may be valuable as a soil conditioner on sandy soils. Many Middle East countries have depended on silt deposition to rejuvenatefertility of the land. On some soils silt deposition may be detrimental (see Chapters 7 and 12). The first instance requires sediments to be carried to the irrigated land and in the second case they must be prevented from entering the storageand distribution network.Sometimes sedimentdepositionmay be utilised in an attempt to reduce seepage from unlined canals. (e)

Silting of canals and reservoirs

This is a major problem in many irrigation projects. The useful life of irrigation reservoirs may be shortened by accumulation of sediment.Once sediment has accumulated,storage capacity is rapidly diminished and the value is lessened.Since good dam sites are very limited,it is not only a serious financial loss when a site is no longer of value,but it is a grave,often irreplaceable,loss of natural resource.In irrigation project planning, allowance should be made for the effect of sediment accumulation upon the usefulness of the reservoir, and all practical steps should be undertaken to minimise the rate of sedimentation.Devices and methods used to control silting of reservoirs are:settling basins, by-pass canals, off-channelreservoir locations,vegetated streams,venting density currents,flood sluicing, dredging, draining,and flushing.Most of the methods are useful only under special site conditions.Watershed protection and special reservoir design which will permit the utilisation of one or more ofthe foregoing means of prevention are the two most useful approaches to controlling sediment. According to experience gained in the USSR,the use of floating stream-regulatingsystems has proved to be effective as this method reduces the amount of bed load entering a system. The operating of stream-regulatingsystems is based on the creation of cross circulation. This method was originated by POTAPOV. The basis is as follows:ifthe stream-regulatingstructure is set up at the canal inlet at a given angle (16"-20") to the direction of the stream,the stream flow delaminates and the upper cleared layers are diverted to the canal,while the lower layers charged with bed load are diverted to the central line of the river channel. The stream-regulatingstructure is composed oftwo rows of baffles,the latter providing structural stability. In appearance the baffles are metal pontoons with hinged joints arranged in sections. The length ofthe stream-regulatingstructure,the size of clearance between the rows of baffles,the dimensions and spacing between the baffles along the axis of the system should conform to design requirements. The stream-regulatingstructure operates with the bames submerged to a depth h =0*2-0.5 H,where H is the water depth in the river. Settling basins on canals have proved to be a successful method for sediment control when the bed load and coarse suspended Sediments cannot be trapped at the head of the system.They are designed to remove, by settling,deleterious coarse sediments and to pass useful fine suspended particles through canals of the distributor system. These basins can have one or many chambers. Washing out these basins by utilising the energy of the

300

IRRIGATION SYSTEMS A N D M A N A G E M E N T stream is the most effectivemeans of cleaning them.The procedure can be performed if thereis a difference in water levels in the basin and the river at the point where the diversion canal joins the river and if the transporting capacity of the section to be washed out is 1*20-1.35times larger than the turbidity of water in the river. The volume of sediments (VS)which can be hydraulically removed is defined by the formula: ri-ar

VS=86*4Q-

P

t

where Q = discharge capacity of the settling basin during washing out (the openings of the head regulator are closed,the openings of the escape structure are open) (in m3/s) r=stream turbidity of the river (in kg/m3) rt=transporting capacity of the stream in the settling basin (in kg/m3) P=density of the sediments to be washed out (in T/m3) t = duration of washing out (in day) a=factor between 1-2and 1.35

Ifconditionsare not favourable for the hydraulic method of cleaning the settling basins the procedure can be performed by mechanical means :suction dredges,tower scrapers and excavators,etc. Combined methods of cleaningthe settlingbasins can be applied :mechanisms should be used mainly during the irrigation period while hydraulic cleaning is acceptable prior to or after this period. Maintaining non-settlingvelocities in the distribution system will carry the silt through to the fields; however, such velocities can cause excessive erosion when silt-freewater is being conveyed. The transporting capacity of a stream is influenced by the hydraulic roughness of the sediments and the hydraulic characteristics of the stream,i.e., velocity,hydraulic gradient and hydraulic radius (LELIAVSKY, Vol. II). According to the USSR standards recommended the formula derived by E. A.ZAMARIN is recommended for calculating the value of the transporting capacity:

where

r,=amount of sedimentstransported by the stream (in kg/m3) V = mean flow velocity (in m/s) R = hydraulic radius (in m) i= stream surface slope W=mean fall velocity of sediments (in mm/s) Wo=function of W:Wo = W if W > 2 mm/s W0=2if WG2 mm/s

Otherformulae,established by V.V.POSLAVSKY, S.A.GUIRSHKAN, A. G.KHATCHATRIAN, are also in common use in the USSR. The farther from the head regulatorthe smallerirrigation canalsbecome,this means that theirtransporting capacities also decrease. Non-silting conditions should be observed, otherwise the settling of suspended sediments occurs.

(f) Weeds From the time irrigation was first practised, weeds have created one of the major maintenance problems on both canal banks and in the channels. Effective herbicides and equipment for combating weed pests have been developed and are constantly being improved (ICID,1954). Weeds growing on the banks of the conveyances catch floating debris which builds berms and sediment bars resulting in a reduction in the carrying capacity of the channels. Submersed and emergent waterweeds growing in the channels create another group of problems. They reduce the capacity of the canals,making it difficult to supply sufficientwater to the crops. The reduction of capacity also decreases velocity of flow,resulting in deposition of sediment and costly cleaning operations. A reduction in capacity necessitates raising the water level of the canal in order to convey the required water supply.This causes increased seepagebecause ofthe greater wetted perimeter,aggravated by the factthat the water above thenormal flowlineis in contactwith soilwhich is usually more perviousthan the normal flowline 301

IRRIGATION, DRAINAGE A N D SALINITY or primed portion of the wetted perimeter. The higher water level adds pressure to the sides and bottoin of the channel which also increases seepage.

4. Water allocation and delivery (a) Regulatory statutes The procedures employed for delivering water to consumers are extremely varied. In some cases,especially as regards older projects,the owners ofthe water rights are not necessarily the users,and those in whom these rights are vested sell them to the users. Sometimes water rights are linked to the holding itself regardless of the size of the areas actually irrigated. There is hardly any need to stress the anachronistic and irrational character of such practices which merely perpetuate age-oldcustoms. It is obvious that the only method that will result in rational use of the available water resources is that which lays down the allocations according to the real needs of the areas actually irrigated.There must be an incentive for efficient use of water and an understanding on the part of the water users of how much water is actually required for the specific crop. In countries like the USSR where water utilisation and delivery,as well as farm organisation,are completely governed by planning of State agencies,the ‘planofwater distribution’ fixes the amounts of water delivered, the irrigation periods and the order or irrigations. (b) Water prickzg In some countries water is made available free of charge.This practice encourages wasteful use of water and makes it difficult to exercise control unless water is allocated by volume; it offers no incentive for efficient ilse ofwater. In countrieslike the USSR,water pricing is not considered,as both land and water belong to the State.In such cases,efficient water use depends on a precise preparation and implementationofthe scheme of water management. The charges made for water supplied from the irrigation system vary considerably throughout the world. There are,however, general charging patterns into which the majtjority of systems fall. The first system is a charge based on the rate of flow. This requires flow rate measurements and adequate records. The second, a volume basis, necessitates a volumetric measuring device or a rate-of-flowdevice combined with a time record ofdeliveries.The third method is a chargebased on the acreage ofcrops matured. The rates vary in accordancewith the crops grown.Such items as the amount of water necessary to produce a certain crop,the season when most water is necessary for the crop (winter or summer), the value ofthe crop and comparative cost of irrigation,from wells or other means,all enter the formula from which the charge is derived. Measurements of flow as a basis for charges are not necessary under this system, but adequate control must be exercised to ensure equitable distribution. The fourth method is a charge based on each irrigation for a given area.In climates that do not demand continuous irrigation for the successful growth of crops or in locations in which the same crop is grown in large areas year after year,a fixed charge is possible. This system is practised extensively in rice producing areas.The fifth method is to charge some proportion of the value of the actual yields of a crop or crops grown by each cnltivator. This involves either assessnients which may be coniplex,or centralised marketing of the crop or crops concerned.But it does mean that the effects of variations in the yields obtained are shared between the irrigation authority and the individual cultivator. Efficient use of water is the primaiy means ofpreventing or alleviating problems of salinity and drainage. In most countries water pricing is an eflective way to control over-use. (c) Water delivery programmes Systems of water delivery can be classified into four groups, (1) continuous delivery, (2)delivery by rotation (3) delivery on demand, and (4)delivery on controlled demand. The way in which water is made available to farmers can have a profound effect on irrigation management and may encouragewasteful practices which lead to drainage and salinity problems. (1) Continuousflow delivery systems Water is supplied continuously to the farm at a predetermined rate.A n open canal distribution system using continuousflow deliveries is relatively simple to operate,but may lead to excessive water applicatioiis where surface irrigation is used. However,if water delivery to large farms under the same management is involved, 302

I R R I G A T I O N S Y S T E M S AND MANAGEMENT a continuous system of delivery can be quite eEcient. It can approximate to a demand system because the water from the continuous supply is redistributed at the farm to each parcel in accordance with crop and soil conditions.Incorporation ofregulatory storage on the farm can increase overall efficiency.Seasonalvariations in water requirements can be accommodated simply by varying the size of the continuous stream and the number of units (basins,borders, furrows,etc.) irrigated at one time.Large corporation farms in the USA are able to make efficient use of continuously delivered water supplies. Continuous flow to large State farms is practised in the USSR and stresses the advantages to be achieved by delivering irrigation water to large blocks of land in the same crop. When land is held in large blocks, with good planning, advantages can be secured by water delivered on a properly scheduled rotation (in accordance with other cultivation works) to blocks of a single crop,served by a minimum length of farm ditches. Table 10.5 summarises experience in the USSR indicating the greater water conveyance eEciency possible by water deliveries to concentrated land and crop areas versus that commonly experienced where water is delivered to scattered areas.The Soviet specialists point out that canals on permeable soil and unlined, must be large and used at full discharge in order to increase water efficiency. Table 10.5. Improvements in conveyance efjicienciesin earth canals resulting from concentrating single crops in large bIOCkS-(OFFENGENDBN)

Conveyance efficiencies (in percentage) Best cases

Scattered land areas Concentrated land areas Increasein conveyance efficiency achieved by land and crop area concentration

83-86 83-86

Worst cases

Averages

57-61 74-76

66-72 80-81

2430

12-21

The figures given are also applicable to rotation delivery systems and point to the benefits of rationalised cropping plans and the limitations of fragmented land holdings. Small continuous streams to smaller land holdings may restrict the irrigation method which can be used and also result in unnecessarily high conveyance losses and decrease,as mentioned above,overall efficiency. Continuous delivery also means that labour for irrigation must be available for 24hours per day. Ifdeliveries are not regulated according to seasonal variations in consumptive use,water will be wasted;such regulation may be difficult. Continuous delivery requires a minimum of controlby the water distributing authority,but it progressively reduces the flexibility possible in irrigation management as the size of the farm holding decreases. (2) Rotation delivery systems Individual farmers are grouped into areas served by a section of canal or a single lateral,and the water distributing authority assumes responsibility for allocating the continuousflow from this canal section,etc., to each farmer in turn on a fixed delivery schedule. Water rotation schedules should not be established merely on the basis of engineering convenience in operating the distribution system.Rather, the schedule should provide for water delivery at intervals which conform as closely as possible to actual irrigation requirements ofthe growing crops.Thus,the schedule must consider evapotranspirationpatterns,soil conditions,the type ofcrop and stage ofgrowth,cultural,and other management factors. Rotation delivery systems are easy to plan and to manage using either pipelines or canals. A fixed delivery schedule requires coordination in the planning of cropping patterns, and even the establishment of irrigated parcels of standard size,for best result. Obligatory use ofwater supplied and failure to adjust rotation schedulesto crop irrigation requirementsare a major cause of serious wastage of water and resultant waterlogging and salinity in many areas and may result in insuEcient irrigation during crucial growth periods. Provision offacilities for emergency spilling of unwanted water is to be preferred.Engineers responsiblefor operatingwater rotation schedulesshould confer closely with agricultural authorities to establish the optimum schedule. In Pakistan the schedule is revised weekly according to irrigation requirements.Where irrigationwater is to be delivered to an area composed of 303

IRRIGATION, DRAINAGE A N D SALINITY

small farms growing a variety of crops at different stages of growth,it becomes nearly impossible to develop an efficient water delivery schedule on a rotation basis. Provided that irrigation is carried out on a 24-hour basis, comparable economy in installed capacity is possible in both continuousflow and rotation systems.Reductions in the working day must be accompanied by proportionate increases in the delivery capacity installed. Rotation of deliveries adapts the system better for use with smaller irrigated parcels. However,at the same time there is a great reduction in the flexibilityof water management which is allowed the individual farmer. When designing a rotation delivery system,the following criteria should be observed: 1. the maximum capacity of canals should be adequate to accomodate the flows required at times of peak demand; 2.the working length of canals should be minimised where possible;3.deliveries in each phase of the rotation should be equalised. (3) Free demand systems With a free demand system (which includes farm wells) the water distribution authority maintains a constantly available supply. Farmers take intermittent delivery at will,depending on the needs of their crops, although they may not take an instantaneous flow rate greater than that for which they subscribe and which correspondsto some extent to the installed capacity. This system gives maximum flexibility,enabling water applicationsto be closely adjusted to crop requirements.Fundamental to eficient management in afree demand delivery system is that thefarmer is closely aware of crop irrigation requirements and has some means of measuring the water delivered. It goes without saying that farmers must be capable of a high level of irrigation

management to achieve good results. There will almost certainly be a tendency to over-irrigateif water is not sold by volume. With surfaceirrigation,the free demand system leads to uncontrolled peak demands during daylight hours and excessive operational losses during night time.Because of this large delivery capacities are desirable and the system must have a rapid ‘responsetime’.Thus the use of pipelines is necessary,in most cases resulting in an increase in the cost of distribution. Free demand systems are well adapted where sprinkler irrigation is used and also where varied cropping patterns and parcel sizes are found.

(4) Controlled demand (mod$ìed rotation) systems Often water deliveries are made according to some kind of controlled demand system which is a compromise between the free demand and rotation systems.The water distribution authority regulates deliveries to meet demands as equitably as possible. Priority for delivery is on a rotationbasis,but actualdeliveries may deviate widely from this depending upon demands. It should be mandatory that once delivery has started it must be continuous until the total delivery required has been taken. Also,the water distribution authority should be responsible for operation of the turnouts. In a controlled demand system some economy in installed capacity is possible over a free demand system by using open channel networks. Better communications between the farmer and the water distributing authority are required in order to coordinate deliveries. This system gives a reasonable degree of flexibility in the delivery schedule permitting a high level of irrigation management. In terms of water control,distribution costs are somewhat higher than either of the ‘parent’systems. As the distance of the irrigated area from the headworks increases water control activities become increasingly complicated and expensive for any of the delivery systems considered. 5. Water measurement (a) Water supply units Analyses of the adequacy of water supplies and the design ofirrigation systemsis complicated by the numerous units used to measure water in various regions of the world. Conversion factors for most of the commonly used units are shown in Table 10.6. (b) Water measuring devices Water control activities require accurate flow measurements to ensure the maintenance of proper delivery schedules,to determine the amount of water delivered to the land,to estimate conveyance losses and their 304

I

Lo

I

m

2

l-4

W

IRRIGATION, D R A I N A G E A N D SALINITY effects on the local groundwater situation,and to detect the origin of these losses for remedial action to be taken. Measuring devices giving instantaneous discharge rates are based on water level or velocity measurements at known cross-sections.Some measure of elapsed time may also be incorporatcd so that the total volume of flow can be determined for a given time period. Discharge measuring devices should be installed at the headworks system so that the total volume of water actually used for each period of the year may be determined. Measurements should be made at the starting point of the different secondary systems in order to define the consumption features of the different sectors and to single out any anomalies. Measurements should also be made at each farm turnout. Canal and lateraldischarges are commonlymeasured by standard weirs,submerged orifices,parshallflumes, and commercial meters in the western United States. Flow in natural channels is usually measured at gauging stations which are established,rated and operated using current meter equipment or water stage recorders. Current meters are use to measure discharges of streamstoo largeto be measured by weirs or other standard measuring devices and for periodic measurements to establish a water surface elevation-discharge relationship at a given location,or to check the accuracy of other measuring devices. The more commonly used devices for flow measurement in closed conduits are Venturi tubes,orifices and liow-nozzles,calibrated gate valves, pitot tubes and various types of commercial meters. It is very desirable to install devices which measure the total volume delivered at all farm turnouts to facilitate sale by volume with all the advantages that this formula implies.Further details on the selection, installation and use of measuring devices may be found in the literature,Table 10.7 contains a summary of the characteristics of various flow measuring devices.

6. Comment

In considering the qGestions discussed under paragraphs 4 and 5 above,it is necessary to take into account the degree of sophistication and experience of the cultivators concerned,What would be quite suitable in a highly developed country might be quite inappropriate in a less developed one, especially where irrigated agriculture is not yet a well-establishedpractice.

B. CRITERIA FOR

THE SELECTION OF IRRIGATION METHODS

1. General considerations

Selection of suitable irrigation methods is vital to the planning of an irrigation project. As a fist step, consideration of the operational characteristics of irrigation methods in relation to soil properties, the feasibility of levelling,drainage and salinity conditions,water supply and delivery, the size of agricultural holdings,crop requii-ements,etc.,limitsthe choice ofmethods to those which are practicablewithin theabove physical limitations. The method selected must be capable of meeting crop requirements with a minimum potential ofinvoking drainage or salinityproblems,hence the degree or controlover uniformity and quantities of water applied may be a decisive factor in selection. Provided that an irrigation completely refills thc soilysstorage capacity in the root zone, or nearly so, application efficiency (Ea) is a useful yardstick for comparing irrigation methods and practices. It accounts for deep percolation,run-offand evaporation losses and is defined by:

E = Water stored in the soil root zone Water delivered to the farm

It should be remembered that the viability of irrigated agriculture, where methods with low application efficiencies are common,is directly related to the existing drainage potential and the rate of salinisationand 306

I R R I G A T I O N S Y S T E M S AND M A N A G E M E N T Table 10.7

Discharge measuring devices

Accuracy in %

cost

Comments

Expensive Inexpensive

Requires no difference in level Requires substantial differencesin

A. Discharge measurements on canals

Limnigraph* and calibrated gauge sections Sharp-edgedor broadcrested ked weirs Variable-liftbottom level discharge meter Parshall flumes Graduated gauge tank discharge meter with orifice or sharp-edgedweir Venturi canal meters

,.

.

level

3 to 5 5

Inexpensive Fairly expensive Inexpensive

Relatively little difference in level Slight difference in level Device that can be adapted to intakes

Fairly expensive

Slight difference in level

E. Discharge measurements on pipe systems Diaphragms and pipes with

measurements by differential pressure-gauges Diaphragms and pipes fitted with a volume integrator Venturi meters with recorder and mechanical discharge indicator Propeller meters Venturi meters with recorder and electrical discharge indicator Proportional meters associated with a negative pressure device Electronic discharge meter Float discharge meter

Q max. 2-3 -

Inexpensive

2-5-5 Q max.

Inexpensive

2-3.5

Expensive

Q

Q Q -max. Q

Expensive

2

Q 3-5 -max.

Expensive

8-10

Inexpensive

7 2-3

Expensive Inexpensive

Q

Easily clogged by weeds or moss

* Wateï stage recorder

alkalisation of the soil. Installation of additional drainage works, when these problems become acute, is an expensive remedy;it could at leastbe postponed,and possibly averted,ifthe applicationand distribution methods initially chosen had allowed better control over water losses,provided the human element of understanding and efficiency had not changed when compared with the initial assumptions. Consideration must be given to economic factors during the selection of irrigation methocls. The selection of a metbod of irrigation which economises on initial investment must be justified not only in terms of the water and labour which might be saved by methods with higher installation costs, but also by its ability to maintain favourable balances in salinity and drainage over a long period. More expensive systems must fulfil the latter requirement and pay for themselves by higher productivity or lower operating costs,The choice of the type of equipment and the irrigation methods to use cannot be made on the basis of simple criteria;it can only emerge from an economic analysis which takes account of the increase in gross and net income following the introduction of irrigation and the pattern of development undergone by areas irrigated to varying degrees ofintensity.These factorsdepend not only on thephysical potential ofthe land and climate and onthe market for the produce,but also on the type ofeconomy and the training and aptitudesofthe water users. 307

IRRIGATION, D R A I N A G E A N D SALINITY

2. Operational characteristics of the various irrigation methods (a) Surface methods Basically all surface methods either pond the water on the soil or allow the water to flow continuously over the soil surfacefor the duration of the irrigation. Ponding methods consist of basins and contour basins and surface flow methods comprise border strips,corrugations and many kinds of furrows. The basin method of water application is based on the rapid application of irrigation water to a level or nearly level area completely enclosed by dikes which retain the water at a relatively uniform depth over the area to be irrigated until the net application has had opportunity to be taken into the soil. Contour basin irrigation (sometimes called rice levee or ‘paddy’irrigation) involves the application of water to nearly level areas, of limited and predetermined size, at a rate sufficiently in excess of the intake rate ofthe soilto rapidly cover the area.Water is retained in place by small dikes or leveesconstructed on the contour and surrounding the individual areas or strips. Except for rice irrigation, water remains on the areas until the desired depth has been infiltrated.The excess is then drained off into another area. Rafter irrigation (check row method) is a system related to basin irrigation.The water is distributed along horizontal large-sectionfurrows between the crop lines. Usually, every two adjacent furrows are made to communicate at the end opposite to the distribution ditch (double rafters). The water diverted from the ditch soon fills up the two furrows which measure 8 to 10 metres long in all. It then percolates at a speed which depends on the permeability ofthe ground.The distribution ditches are set out accordingto thelinesof maximum slope at 4to 5 metre spacings and the horizontal rafters are dug perpendicularly to these trenches. This method can be used forirrigatingsteeply slopingland without engaging in complicatedlevellingworks. Border irrigation is a controlled surface flooding method of irrigation water application.The field to be irrigated is divided into strips between parallel dikes or border ridges and each strip is irrigated separately. The border strips should have little or no cross slope,but should have some grade in the direction of irrigation,thus distinguishing them from level borders or basins which have little or no grade. Corrugationirrigation is a partial surfaceflooding method ofirrigation.Theirrigationwater does not cover the entire surface of the field.The water flowing in the corrugations soaks into the soil and spreads laterally to irrigate the areas between the corrugations. Furrows are small channels having a continuous, nearly uniform slope in the direction of irrigation. They are used to irrigate cultivated crops planted in rows. One or more furrows are used between crop rows except for bedded crops,which often have furrows only along each pair ofrows.Furrows are made in different sizes and shapes depending upon the crop grown,the equipment used,and the spacing between crop rows. Contour furrows are used on gently sloping land. Depending on the circumstances, application efficiencies for surface methods can range from about 40-80 %. (b) Sub-irrigation Sub-irrigationis a method of applying water beneath the ground surface. It is usually done by creating an artificialwater table and maintaining it at somepredetermined depth,usually 12to 30inches below the ground surfaceforfarm crops.Moisture reachesthe plant roots through capillary movement. Application efficiencies vary generally from 30-50 %and in some areas go up to 70-80% under favourable conditions. (c) Sprinkler irrigation In sprinkler irrigation the water is sprayed into the air and allowed to fall on the land surface in a uniform pattern at a rate less than the intake rate of the soil. Three types of sprinkler systems are used to irrigate farm crops:rotating sprinkler heads, fixed jets, perforated pipes. Rotating sprinkler-head systems are the type mounted on to the tractor more widely used type.In the USSR two-cantileversprinklers of DDA-100-M are used. Sprinkler systems may be designed as (1) permanent installations,with buried main and lateral lines, (2) semi-permanent,with fixed main lines and portable laterals, (3) fully portable systems and (4) mobife systems. Each rotating sprinkler head applies water to a given area. This area is governed by the nozzle size and the water pressure. Perforated pipes deliver water through very small holes, drilled at close intervals along a segment ofthe circumference ofa pipe. The trajectories ofthejets provide fairly uniform distribution ofwater over a strip of land along both sides of the pipe. Sprinkling application efficiencies should always be above 308

IRRIGATION SYSTEMS A N D M A N A G E M E N T 75%. The frequency and intensity of the wind will definitely affect these efficiencies. (Technical details on irrigation by sprinkling may be found easily in literature;accordingly this matter is not treated here.)

3. Factors affecting the selection of irrigation methods

(a) Nature of soils (see Chapter 3 for further discussion) Perhaps the most important soil property relating to irrigation efficiency is the surface intake rate.On most soils,intake rates decrease with time after the start of irrigation and finally reach an approximately constant level called the basic intake rate which may be appreciably less than the initial rate. Except in rice culture, where a highly impermeable soilis required,surfacemethods in general are not well suited to either extreme of the basic intake rate.A soil which has a high initial intake rate (often due to cracking), decreasing to a very low basic rate greatly facilitates mutual uniform irrigation with surface flow methods since it allows long runs.High intake rates result in uneven water distribution in surfaceflow methods unless exceptionally short lengths of run are used. The same is true with surface ponding methods unless rapid flooding of the entire area is possible either by use of small parcels or very large unit streams (flow rate per unit area), Soils with extremely low intake rates are highly susceptible to problems of soil aeration,which do not, however, affect rice. In saline soils the deep percolation losses almost inevitable with surface irrigation can usefully facilitate leaching which is periodically required providing these losses are not excessive. A n application rate of 5 mm per hour is suitable for most clay soils,rates of 8-10 mm per hour are normal on loams. Sandy and volcanic soils and many latosols can take appreciably higher rates. Sub-irrigationrequires permeable soils provided that the soil permeability in the rooting zone is homogeneous,and salinityis not a potential hazard.A shallowimperviouslayer below the rooting depth is desirable for good control over the water table and water losses by eliminating downward water movement. Imperviouslayers in the subsoilrequire close control of surface water applications because continued deep percolation would create a waterlogging situation detrimental to plant growth and favourableto upward salt movement in saline soils. Shallow impermeable layers may preclude economic drainage and can also limit crops to shallow-rootedspecies. (b) Relief and land preparation requirements Before levelling land it should first be ascertained whether the topsoil is of adequate depth or at least that the subsoil is potentially fertile. Other soil conditions such as intake rate, drainage and erodibility must also be suited to levelled land irrigation practices. Only moderate slopes and shallow undulations can be profitably levelled,in fact 1000 cubic metres of earth moved per hectare corresponds to only a 10 c m layer of soil.The best slopes for grading are of the order of 0.2% and the maximum is about 4%. Information on levelling techniques and equipment is given in specialised literature(e.g.I.C.MA=,1957; USDA National Engineering Handbook, 1959).

O n flat or nearly flat ground,all the different irrigation methods may be used;however, when the slopes are less than 0.1 %,an artificial slope is often made if surface flow methods are contemplated. Furrow irrigation does not require as much land preparation as border strip or basin irrigation and can be practised on slopes ranging from 0.5 to 15 %; however,erosion becomes a problem on slopesin excess of2%. Sprinkler irrigation may require little or no land levelling prior to the seeding operation. (c) Size of the agricultural holding Irrigation methods are restricted by the size of an agricultural holding. Excessive run-offgenerally.occurs with surface flow methods if the length of run is short and the application rate is not adjusted to the intake rate of the soil.Nearly level furrows can be modified so that water flows in opposite directions in alternate furrows (called ‘rafters’or ‘check-back‘furrows). Sub-irrigationto meet the requirements of mixed cropping at different stages of growth and randomly distributed parcels is not feasible.Basins,rafters, or sprinklers are more readily adapted to smallholdings. Fragmented holdings and cropping areas always increase labour requirements.

(d) Salinity conditions The presence of appreciable salinity in the soil,the irrigation water or the groundwater,demands the choice

309

IRRIGATION D R A I N A G E A N D SALINITY ofan irrigationmethod which can effectivelycontroltheseconditions.In this light,DARLOT assessesthe various irrigation methods as follows: Sutface irrigation: well-suitedto all cases where leachingis necessary since,in fact,one ofits characteristics is that it gives rise to losses through deep percolation when improperly practised. Hence it will prove satisfactory in cases of surface salinity or where saline water is used. All furrow irrigation methods must be practised with considerable care in saline and alkali soils because water moves upwardsfrom the furrow towards the surface ofthe beds. The resulting pattern ofsalt accumulation shown in Fig. 10.1 requires special management as discussed in Section D. Sub-irrigation cannot be used with saline soils or water uiiless the soil is leached periodically by natural rainfall,or surface applications ofwater.The upward movement ofthe water tends to concentrate the salt on or near the surface irrespective of whether it comes from the water table,the soi1 or the irrigation water. ;Sprinkler irrigation: this method can prove suitablein most cases since it can provide controlled uniform applications for both light irrigations on shallow soils and the heavy wsterings required for leaching. However,wind intensity has a definite effect ofl the feasibility of sprinkler irrigation. It should be noted that ifsaline water falls on leaves,considerabledamage can be caused when the concentration exceeds a certain value. For citrus fruits,for example,burning of the foliage has been observed with water containing 800 to 900 ppm of total salts, 69 to 140 m g of N a and 133 of C1 per litre. This damage seems to increase in proportion to the intensity of evapotranspiration. With its high application efficiency sprinkling minimises salt added to the soil from the irrigation water.

0 o.01-o.02 m o . 0 2 - 0 . 1 ..

O.^-^.^

0.2-0.5 0.5- 2 Percent salt in so.il

Fig. 10.1. Percentage of salt in soil.Salt content of soil under furrow irrigation.Arrows show the direction ofthe flow ofwater and salt during and sometimes afterirrigation (Bull.876,Texas A and M,1962 Rev.) (e) Groundwater and drainage It is necessary to maintain a favourable balance between drzinage and water added to the soilin excess ofthe consumptive use requirements. (Refer to Chapters8 and 11.) The importance of adequate drainage to accommodate leaching water in saline and alkali soils cannot be overemphasised. Occurrence of both salinity and drainage limitationssimultaneously demands the use of the most efficient irrigation practices if the land is to remain in production (R.M.HAGAN, 1957).

(f) Water supply The choice of distribution network and irrigation methods must be coordinated.The water requirements at the head of the network (total annual volume, Va,and peak discharge, Q,.) must be compared with the resources available. These requirementsare the sum of conveyance losses,consumptive use of crops,leaching requirementsand application losses. Consider an area of 1000 hectares with a combined annual consumptive use and leaching requirement of

310

IRRIGATION SYSTEMS A N D M A N A G E M E N T 550 mm and a daily peak use of 5 mm. With surface irrigation (E, = 50%)and an earth distribution network in a good state of repair (Ec= 80 %>:

V a=550 10 1000= 13'75 .106 m3/yr *

*

0.5 .0.8

With sprinkler irrigation (E,=80%)and a pipe network (E,=99 %> then:

V,= 550 Io 1000=6.95.106 m3 0.8 .0.99 *

*

Qr

*

lo 'OGo

=0.8 .0.99. 86400=0.730 m3/s=630l/s

This exaffiple shows the considerable influence which conveyance and application losses in relation to the water supply available can have on the selection of a given type of distribution network and irrigation method. For all surfaceirrigation methods there is a minimum 'unitstream' (the ratio size ofirrigated unit-border, basin,furrow,etc.-to raie of water delivery) which is necessary to achieve uniform distribution for a given depth of application. High infiltration rates require comparatively large unit streams and if the supply is limited smaller units must be irrigated.Since furrow streams are quite small the number of furrows irrigated simultaneously can be adjusted to the supply,thus giving flexibility.Large streams are usually required for border strips,basins, and contour basins on account of their areas. Details of water supply requirements for various methods are given in Section C. Sprinkling and sub-irrigation are possible with a sniall continuous supply which allows considerable economy in the installed capacity of distribution systems and pumping plants. T w o litres per second are sufficient to sub-irrigatea plot of 1 hectare,consuming 17 mm per day (losses included). A discharge of 3 litres per second can feed a line 100 metres long equipped with sprinklers 18 metres apart and delivering 6mm per hour,which is sufficientto give 1500m3per hectareper month over an area of 3 hectares,irrigating 16 hours in 24. (g) Quality of water Detailed discussion of dissolved salts and suspended materials in irrigation water and their effects on soils and on irrigation and drainage practices are given in Chapter 7. When irrigation is used specificallyto provide deposition of silt by applying appreciable volumes of heavily charged water,basins or contour basins are required. The deposition of earth particles from sprinkler application on the portions of the plants above ground may prove harmful to some crops,particularly to flowers and fruit. Corrosion of aluminium spïinkler pipes caused by saline water increasesreplacement costs and rapid wear occurs in all appliances when it is not possible to remove abrasive particles found in the water. Both factors weigh somewhat against the choice of sprinkler systems.

(h) Nature of crops and their irrigation requivements Soine crops require frequent irrigations because they are shallow sooted,others are deep rooted and may be irrigated at less frequent intervals. In still other crops,especizlly those producing seed and fruit yields, the soil moisture deficit at certain timesis critical. The way in which certain crops are grown precludes the use of some irrigation methods,e.g.wheat and alfalfa are not grown in furrows and potatoes are not grown on the flat.The method of irrigation must fit in with these physical characteristics of agricultural crops.Table 10.8 summarises the irrigation methods suitable for various crops. In areas of limited water supply, careful irrigation scheduling and the consideration of peak moisture requirementsare very important.High application efficiencies will be required to prevent crop damage and ensure maximum production. Consideration must be given to the depth of the root zone,moisture-holding capacity of the soil,allowable moisture stress during germination,and seedling stages and salinity conditions that may occur during germination. Relatively light frequent irrigations at short intervals are generally advisable to counteract some of these critical conditions.

311

IRRIGATION, DRAINAGE A N D SALINITY Table 10.8. Irrigation methods suited to various crops-(DARLOT)

Method of irrigation

Sub-Irri- Sprinkler Furrows gation irrigation

Crops

All crops except rice

All crops except rice

Row crops

Corruga-

Basins

Contour basins

Border strips

Row Pastures crops Orchards (especially Cereals vegetables) Rice Green fodder

Fodder

tions Cereals

Pastures Bush crops

Green fodder

Orchards

crops

Rafters

Cereals

crops

Pastures Cereals

crops

Orchards Rice

In light of the above information,vegetable growers in the USA often use sprinklers for the germinating and seedling stages and revert to furrow irrigation after the plants are established. This practice is also used in the establishment of alfalfa and mixed pastures in some localities.At times corrugations are used for the first irrigations of cereals planted in borders; and as the depth of application required increases,the borders are irrigated in the normal way.

(i) Power availability Where gravity pressure is not available,the availability ofpower for pumping,its reliability and cost may be major determining factors in considering sprinkler irrigation (O.W . ISRAELSEN and V. E.HANSEN, 1952).

(j) Crop rotations and cultaim1 operations Surface irrigation entails the construction of a relatively complex farm ditch system to convey water from the turnout to theindividualfieldsand to remove surface run-off.These ditchesmay restrictthe use ofmechanised farming;however,ifthey can be made shallow,they may not present too much of a problem. Farm ditches consumepart ofthe cropping area,but part ofthis drawback has been eliminated in somecases by distributing the water through portable gated pipe or rubber and plastic pipe with small outlets.This method does result in a higher labour requirement to shift the pipe system round to the various fields. Soil tillage operations are required with all methods of irrigation. Surface irrigated areas will require more effort to maintain a more nearly level surface than in sprinkler irrigated areas or probably sub-irrigated areas.Crop rotation from a border-typecrop to a crop grown in furrows does not involveextensive additional preparation. The reverse is also true,provided the slope is compatible for both methods of irrigation. Crop rotations and changes in the cropping pattern may require less effort under sub-irrigationand sprinkler irrigated areas. While sub-irrigationcan be regulated to provide any desired moisture regime,the ground surface must be maintained relatively level and it does not allow differentiation between the moisture regimes of adjacent crops within the same parcel.

(k) Crop yields and quality oj'produce Generally,due to the human element involved,surface methods apply water less evenly over the area being irrigated when compared with sprinkler or sub-irrigationmethods.This may result in premature deficits of water in some areas,deep percolation losses in other areas,with a resultant loss of fertility due to leaching. Drainage and salinity problems seem to be more prevalent with surface methods of irrigation,which are primarily the result of the inefficiency of the human element.Sub-irrigation(in the absence of salinity) and sprinklingusually give appreciably better results because ofthe narrowertolerance limitsofthe operation and the higher efficiency of the human element,especially because of cost in connection with a pumped water supply in the case of sprinkling. Differences in irrigation methods become more apparent when light irrigations are necessary, because the unevenness of distribution by surface methods is generally accentuated. Under saline conditions in row crops, sprinkling avoids adverse growth conditions due to salt accumulation in the ridges (Fig. 10.1)by applying water from above-this may be especially important in the seedling stage. 312

IRRIGATION SYSTEMS A N D M A N A G E M E N T

When high inputs of fertilisersare used,the results may be greatly influenced by irrigation practices.Whatever method is chosen, it should allow maximum control of uniformity and total application of the water. With respect to crop quality, no definite conclusions are possible about the relative merits of different irrigation methods-management has a much greater effect. Damage has reputedly been found in certain types of flowers subjected to sprinkler irrigation;and surface methods, if poorly managed, may reduce the quality of crops which are sensitive to waterlogging. All irrigation methods may affect disease incidence and control and thus may have an indirect effect on quality. This aspect is discussed later under the heading of management practices (Section D).

(1) Climatic conditions Most climatic influences on the selection of irrigation methods are indirect. Irrigation requirements vary considerably with climate;for instance,sprinklingor contour methods are suitable for irrigation as a supplement to rainfall when heavy expenditure on land levelling is not warranted. Surface salt accumulation as a result of high evaporation rates necessitates a method giving good salinity control.The possibility of erosion resulting from surface run-offafter heavy rainfallfavoursthe use of surface methods which help to check runoff(basins,contourbasins,rafters) and incidentally direct a major portion ofthe rainfallintothesoilmoisture reservoir. Borders and furrows tend to increase erosion under these circumstances,and their lengths of run must be reduced which increases operating costs. The uniformity ofdistribution with sprinkler methods is directly affected by wind. Conventionalsprinklers do not give good results at windspeeds above 13 to 16 km/h (8-10 mph) and are therefore not desirable where such winds are prevalent. Sophisticated (but expensive) giant sprinklers have now been developed in Hawaii which compensate for wind effects with variable trajectories and rotation speeds. Where light frost occurs on fruit and vegetables,overhead sprinkler irrigation may provide useful frost protection and thus be a factor in selecting the irrigation method (G.O.WOODWARD, 1959). (m) Crop insects and diseases Observations that have been made in a large number of countries show that there is little difference in the probability of spreading plant disease or parasites when comparing the various methods of irrigation. Comparative tests performed on melons and tomatoes in particular have shown that, with comparable fungicidaltreatment,therewas no significant difference in the health ofthe crops being irrigated by surfaceor sprinkler methods.This was also found to be true with respectto cryptogamic diseases on fruittrees. Sprinkler irrigation may have a tendency to reduce the frequency ofinsect attack on trees probably due to the washing ofthe leaves.There has been a move away from the use ofgiant sprinklersin banana plantations,because the frequent applications of both fungicides and irrigation water were found to be incompatible (R.M.HAGAN, 1957). (n) Cost of water The cost of water could be a major determining factor in the choice of an irrigation method. Comparisons should be made on the basis of the total amount of water required by respective methods with the cost of obtaining higher efficiency. Basically, high water prices dictate irrigation methods with a high application efficiency. There is ample evidence that low water charges go hand in hand with excessive water applications and resultant low efficiencies which create or accentuate drainage and salinity problems.High or prudent water prices require or encourage more efficient designs and actual water management practices in all methods of irrigation and result in a successful agricultural enterprise. Costs for pumped water vary considerably depending primarily on power costs and total pumping heads, because the overall efficiency of pumping plants throughout the world is fairly consistent. (o) Depreciation,operating and maintenance costs

Economic comparison of irrigation methods should be made by taking into account the annual k e d and variable costs for irrigation and differencesin yield per unit land area. Depreciation costs comprise the annual revenue required to repay the total capital cost of the investment (initial outlay plus interest charges) over its useful life. Operating costs are divided between labour,power, and water costs. Maintenance is the overall cost of keeping the farm distribution system, the land surface 313

IRRIGATION, DRAINAGE A N D SALINITY and irrigation equipment in working order. On a world scale,these costs vary considerably,and each case must be examined on the basis of local costs. (1) Depreciation costs For surface irrigation and sub-irrigation,the items to be accounted for include the initial land preparation, construction of water distribution and surface run-offfacilities,and farm equipment. In the case of sprinkler irrigation, the limited land preparation, the cost of sprinklers, pump units, and portable pipe, or the cost of procurement and installation of permanent underground pipe must be considered.

(2) Operating costs The amount of labour required for surface irrigation depends on the type and condition of the distribution system,the size ofthe parcels, and on the particular method employed.Man-hoursper hectare per irrigation may amount to 10for rafter irrigation and fall to as little as 1 for 200-metre-longborder strips. Soilpreparation operations such as making temporary ditches,levees,or border ridges and annualland smoothinginvolve considerable labour (and machinery) costs. Operating costs are minimal in sub-irrigation.One man can control 100 hectares or about 250 acres, and land preparation consists simply of an annual smoothing operation. High application efficiency is one factor that should minimise water costs under a sprinkler irrigation system.It is estimated that on the average,30 man-minutesto 2 man-hoursare required to move 200 metres ofpipe,i.e. 1.5 to 6 men per hour per hectare per irrigation cycle. Improved design of sprinkler irrigation equipment could reduce the objection to conventional labour requirements for moving portable irrigation pipelines. Permanent sprinkler installations achieve this at the expense of a high capitalinvestment,while large portable rain gun units and motorised wheel-movesprinkler lines also minimise labour costs. (3) &faintení" costs Maintenance costs in surfaceirrigation and sub-irrigationincrease in direct proportion to the length,quality, and complexity of the system used and the necessity for occasional minor surface corrections. Maintenance expenditure on sprinkler irrigation comprises the periodical repair and replacement of the pumping plants,the pipes, and the sprinklers themselves (WOODWARD, 1959).

@)

Socio-economic considerations

Socio-economicconsiderations may be an important criterion in the choice of an irrigation method as a result of the limitations in both quality and quantity of the basic resources, labour, investment capital, land and water, and their inter-relationshipwith the desired level ,ofagricultural production. Objectives in agricultural production differ widely between a basically subsistence economy and an export economy (see Section D). Limited water suppliesare used to best advantage by irrigation methods giving a high water use eaciency. In general,this requires investment in land improvement (drainage and levelling) or in equipment (sprinkler systems,gated pipe, etc.), either of which is used to increase the degree of control over water applications. Even ifcapital is in short supply,the need to conserve water demands a certain minimum investment in water control. Sprinkling should give maximum control of water applications under most circumstances but involves high initial investment in equipment, particularly if it is required to minimise labour requirements. This equipment must often be imported. Even where foreign exchange is limited, importation of appliances, sprinklers,or gated pipe, for example, may be justified when enough water is saved to allow a substantial increasc in the irrigated area. Whichever inethGd is considered most feasible,a fairly heavy initial expenditure on land preparation for equipment will be necessary to ensure maximum water utilisation;Government loans and farmers' cooperatives can help overcome this barrier. Financial and technical assistance from the State in carrying out the initial land improvement works,distribution facilities,and the procurement of multiple-usesprinkler equipment has been utilised to rapidly expand irrigated areas,using surface and sprinklermethods,without involving the individual farmer in heavy capital outlay. A lack of easily reclaimed land for irrigated farming makes it imperative that irrigations methods be

314

IRRIGATION SYSTEMS A N D M A N A G E M E N T

chosen which will favour adequate control of potential salinity, alkalinity, and waterlogging hazards. If substantial amounts of land and capital are available and the water resources are abundant,it might appear that the best returns would be derived from irrigation methods involving low levels of investment per acre. Infact,a higher levelofinvestment inpermanentinstallationscould resultinloweramortisation,maintenance, and operating charges. In all cases, the total charges stemming from the scheme must be compared with the increase in gross income brought about by irrigation. In regions where long experience has taught farmers how to use certain irrigation methods efficiently, it is usually advisable to retain similar methods, at least for an interim period. C.

ELEMENTS OF IRRIGATION DESIGN

1. Introduction

The design of an irrigation system is complex and not readily subject to quantitative analysis: at least 10 principal criteria are important; all are in turn governed by the overall economics of the entire farming operation.They are as follows:store the required water in the rootzone ofthe soil;obtain reasonably uniform application of water;minimise soil erosion;minimise run-offof irrigation water from the field;provide for beneficial use of run-offwater;minimise labour requirement for irrigation;minimise land use for ditches and other controls to distribute the water; fit irrigation system to field boundaries; adapt system to soil and topographic changes;and facilitateuse ofmachinery for land preparation,cultivating,furrowing,harvesting, etc. 2. Information required for design

The surveys and resource inventories normally needed for sound planning include: (a) Water supply inventory Location of supply points,seasonalvariations and maximum capacity,water costs,and quality. Continuous long-termrecords of precipitation,stream flow,and grbundwater storage should form the basis for intelligent and complete utilisation of all water resources. (b) Topographic surveys Such surveys or aerial photographs having topographic data on them are required. The scale and details required vary with the type of irrigation system planned. More detailed data are required for surface-type irrigation as compared with sprinkler irrigation. M a p scales desired usually range upward from 1 :50. (c) Soil surveys Soil types, water holding capacities, intake rates from field tests (USDA Handbook 82, 1956), subsoil permeabilities;depth of soil profile and compacted or indurated layers;water table depths. (See Chapter 3.) ~

~

- -~

(d) Sources of power I Location of supply points,characteristics and reliability of supply,pricing and standby charges. (e) Cropping patterns and crop marketing potentials Crop enterprises-consumptive use patterns,types ofcrops and areas of each,field boundaries,rotationsand marketing potentials.

(f) Labour supply Size of labour force,seasonal availability,and skill. A detailed account of the requirements of such surveys will be found in Chapter 14. 3. Surface systems

(a) Furrow systems After land preparation is completed,the system can be designed to fit the soil and topographicfeatures ofthe X

315

IRRIGATION, D R A I N A G E A N D SALINITY field. Furrow spacing,furrow grade, cross slope, furrow streams, and length of run should be carefully considered.

(1) Fzirrow spacing Furrows can be spaced to accommodate the desired plant population density and the standard machines used for planting and cultivation.Crops like corn and potatoes have furrows between all rows.Bedded crops, such as lettuce,carrots,and onions,may have pairs of rows between furrows.Wide-spaced crops,like fruit trees and berries, generally require more than one furrow between crop rows. Furrows are spaced close enough that water will spread from their sides into the ridge and root zone of the crop before too much moves down below the root zone. Field tests are the best way of determining optimum furrow spacing which varies with the pattern of water movement in different soils. (2) Furrow grade For planning purposes,DARLOT suggests the following maximum slopes if erosion is to be avoided: 2% on sandy soils and 3 %on clayey or pebbly soils.With careful management,good soils and small streams,slopes of up to 15% are possible. (3) Cross slope O n smooth,uniformly sloping fields,crops are sometimes planted across the slop to reduce furrow grade. Furrows are also used on graded benches constructed across the slope.The slope of the land at right angles to the direction of irrigation (the cross slopes) should not exceed 2%. Where furrows are less than 15 cm (6 inches) deep,the cross slope should be less than 2%, and it may be necessary to use shorter runs to prevent accumulation of water and over-toppingof the furrows.

(4) Furrow streams

At the beginning of an irrigation, the largest stream of water that will not cause erosion is used in each furrow.The maximum non-erosivefurrow stream may be estimated from Q = U-

S where S=furrow slope,in % a =0.6 +Q, in l/s a= 10+Q, in g.p.m. with Q in I/s. This stream,which is allowed to flow until the water reaches the ends of the furrows,is called the ‘initial’stream. Its purpose is to wet the entire length of each furrow as quickly as possible. Reducing the difference inthe time that water infiltrates at the upper and lower ends of the furrow gives more uniform distribution of water and improves the efficiency of irrigation. The size ofthe initial furrowstream is determined,for the most part,by the furrow grade and cross-section, V-shapedfurrows being more easily eroded than furrows of U-shapedcross-section.After the initial stream has reached the lower end of a furrow,the stream is reduced or ‘cutback’ to one that will just keep the furrow wet throughout its length with a minimum of waste at the end. This cutback stream flows until the required amount of water has been applied. To compute the proper size of ‘cutback‘streams,the average intake rate of the soil must be known in litres per minute per 100 m or in gallons per minute per 100 feet of furrow.Actual rates should be determined for each field. The usual range of intake rates for different textured soils is shown opposite (as supplied by QUACKENBUSH). The proper size of the cutback stream is the intake rate per unit length of furrow times the length of run. For example, 800-foot-longfurrows on a silt loam with an intakerate of2 g.p.m.per 100ft would take a cutback stream of 16 g.p.m.(240 in long,25 l/mn/100m, 60 l/mn or 0.06 m3/mn). To determine how long to allow this stream to run,the amount of water to be applied in inches is divided by the intake rate in inches per hour. This intake rate is affected by the spacing of the furrows,and the furrow intake rate must be converted to total intake for the irrigated area. In the example,if the planned application were 100mm (0.1 m)and the spacing ofthe furrows 0-90m,the cutback stream would need to run 0*1.0*90. 240-360 - mn=6 0.06

316

IRRIGATION SYSTEMS A N D M A N A G E M E N T Average intake rate Texture

l/mnper 100 m of furrow

US gallons per minute per 100 feet of furrow

Fine textures:

Dense clays Silty clays to clays Clay loams to silt loams Medium textures: Silt to loams Moderately course textures: Fine sandy loams to sandy loams Course textures: Loamy fine sands to loamy sands

o to 12 6to 25 12 to 25

oto 1 0.5 to 2 1 to 2

12 to 37

1 to 3

18 to 25

1.5 to 10

60 to 180

5 to 1.5

Ifat least three-quartersofthe water to be applied is absorbed during the first quarter of the computed irrigation time,the use ofcutback streams is questionable.As this ratio increases,the practice has even less value. In such cases a stream in excess of the average intake rate of the soil but less than the maximum allowable non-erosivestream is satisfactory. After this stream wets the lower end of the furrow,the irrigator cuts off the water and turns it into other furrows. Such a procedure results in a saving of labour with little or no sacrifice of efficiency. (5) Length of run The optimum length of run is usually the longest furrow that can be safely and eficiently irrigated.Ifthe run is too long, water soaks in too deep at the head of the furrow by the time the stream reaches the lower end.This means that the upper end of the furrow is over-irrigatedand the lower end may be under-irrigated. If the run is too short,extra cross-ditchesare required, increasing labour requirements and percentage of land lost to cultivation. The length of run best for a particular fieldis affected by the erosion and drainage hazards,the kinds of soil,the slope of furrow,and the size of stream that the furrow will carry. Ifthe furrowsare too long on soils having poor internal drainage, run-offfrom rainfall may collect at the lower ends and damage the crop. The danger of erosion from rainfall may also limit the safe length ofrun.The size ofthe furrow also affects the length of run by limiting the size of furrow stream that can be used. The length of furrow which can be efficientlyirrigated depends on the intake rate;it may be as short as 45 m (150 ft) on soils with intake rates in excess of 50 mm/h(2 inches per hour) or as much as 300 m (1000ft) on soils with low intakerates and free drainage.A good guide is to make runs of a length that the maximum non-erosive stream reaches the end of the furrow and needs to be cut back in about one-quarter application time.Field trials can be used to determine the optimum length of run using this criterion,using a rate-of-advancecurve as in Fig. 10.2.Also,the layout should be arranged so that the same furrows do not cross two soils with widely different intake rates.

Distance down furrows-hundredfeet

Fig. 10.2.Rate-of-advancecurve for determining maximum length of furrows (from SCS-USDA Ag. Hundbook 82) 317

IRRIGATION, DRAINA'GE A N D SALINITY Tables 10.9 and 10.10 give relationships between the basic variables in furrow irrigation determined by experience i n the USSR and USA which can be used as guides in design. Table 10.9, F¿irrow irrigation data for the cotton growing Zone of central ASiU-(OFPENGENDEN)

First irrigation on loose soil with little water stored in the soil

Next irrigations on compacted soils, pre-irrigationmoisture equalling 65-75%of field capacity

Length of run

Furrow stream

Furrow stream

(m)

(b)

Length of run (m)

Soil permeability time in hours for 0-3I/s to advance 50 m

Furrow slope ( %)

2-7 2.5 1.25 1.0

< 0.05 0.1-0.05 0.1-0.7 0.8-1.0

60 60-1O0 100-150 100-60

0.25 0.7 0.8-0.9 0.5-0*1

120 120-200 200-300 200-120

0.4 0.25-0.04 0.4-0.8 0.5-0.1

0.6 0.75 1.5 1*7

< 0.05 0.05-0.1 0.1-0.7 0-8-1.0

120 75-125 150-175 120- 75

0.4 0.5

240 150-250 300-350 240-150

0-5 0.2-0.3 0.5-0.9 0.6-0.12

0.3 0.5 1.0 1.1

< 0.05 0.05-0.1 0.1-0.7 0-8-1.0

85-100 150-200 160-200 140- ao

0*1-0.15 0.4

300 170-200 320-400 280-160

0.2 0.25-0.4

0.5-0'6 0.3-0.1

0.5

0.1

(l/s)

04-0.8

0.7-0.12

Table 10.10. Furrow irrigation relationshipsfor various North American soils, slopes, and depths 0f appliCatiOn-(Q~ACKENBUSH)

Soil texture

Furrow slope

Coarse Maximum allowable non-erosive furrow stream (l/mn> (US gallons Per mn)

(70)

Medium

Fine

Depth of irrigation application, mm (inches) 50 (2)

100 (4)

150 200 (6) (8)

50 (2)

100 (4)

150 (6)

200 (8)

100 150 (4) (6)

50 (2)

200 (8)

Maximum allowable length of run, metre (feet) ~

~

150

40

150 220 270 300 250 350 440 500 320 450 530 650 (500) (720) (875) (1000)(320) (1150) (1450)(1650)(1050) (1500)(1750)(2140)

75

20

105 145 180 205 170 240 295 340 220 305 380 445 (345) (480) (600) (680) (560) (800) (975) (1120) (730) (1020)(1250)(1460)

0.75

49

13

80 115 145 165 135 190 135 270 175 250 300 350 (270) (380) (480) (550) (450) (630) (775) (900) (580) (820) (1000)(1150)

1.00

38

10

72 100 120 140 112 165 200 230 150 225 255 300 (235) '(330) (400) (470) (380) (540) (650) (760) (500) (750) (850) (990)

1-50

265

7

57 80 100 115 105 135 160 190 120 175 210 240 (190) (265) (330) (375) (310) (430) (530) (620) (400) (570) (700) (800)

2.00

19

5

80 95 280 115 140 160 105 145 180 205 50 70 (160) (225) (275) (320) (260) (370) (450) (530) (345) (480) (600) (675)

3.00

11

3

36 55 65 75 65 90 110 130 80 115 140 165 (125) (180) (220) (250) (210) (295) (360) (420) (270) (365) (470) (550)

5.00

7.5

2

30 40 50 58 50 70 80 95 65 38 105 125 (95) (135) (165) (190) (160) (225) (270) (320) (210) (290) (350) (410)

0.25 0.50

318

'

IRRIGATION SYSTEMS A N D M A N A G E M E N T (b) Corrugutions (smallfurrows)

The spacing and depth of corrugationsvary with the kind of soil and the slope of the land.Table 10.11 shows the recommended spacing and depth for soils and slopes commonly irrigated with corrugations. Table 10.11. Recommended length of run und spacing of corrugutions-(~UAcIu3NBLJsH) A. Deep-rooted crops on deep soils

Slope (percent)

2

Heavy-texturedclay soils

Medium-texturedloam soils

Light-texturedsandy soils

Length

Spacing

Length

Spacing

Length

Spacing

metre (feet)

cm

cm (inches)

60 (24)

metre (feet) 70 (225)

cm

175 (575)

metre (feet) 130 (425)

(inches)

60 (24)

(inches) 45 (18)

4

The spacing, or distance between corrugations,depends on the rate water moves sideways into the area between them.Corrugations should be spaced so that the area between them is irrigated by the time the water has moved down through the root zone.Medium textured soils usually have the best lateralwater movement in relation to downward movement. Corrugations on these soils, therefore, can be spaced farther apart than on either light or heavy textured soils.On all soils,as slopesincrease,spacebetween corrugationsshould be decreased. Corrugations are usually about 8 to 12cm deep. Since water moves down through lighttextured sandy soils rapidly, the corrugations must be shallow so that lateral movement will start very near the field surface. Deeper ones may be used on the heavier soils.It is important to make them deep enough to keep the water from breaking out. Rough, poorly levelled fields or fields irrigated slightly across the slope require deep corrugations. Corrugations must be run in the direction of steepest slope because they are easily blocked; appreciable cross-slopescould result in serious erosion from water breaking out of blocked corrugations. After the 319

IRRIGATION, D R A I N A G E A N D SALINITY establishment ofpasture or similarvegetation,corrugationsmay be used without significant erosion on slopes of 10% or so with careful management. (c) Border strips Precise land levelling is essential to successful irrigation by the border method. The graded slopes should be uniform throughout the length of the border strips,except the first 9 to 15 m (30 to 50 ft) should be level to ensure even water distribution over the entire width of the border. Some variation can be tolerated to avoid excessivelevelling costs.In such cases,the steepest slope in any border strip should be no greater than one and a half times the flattest slope, or the difference between slopes should be no greater than 0.5%, whichever is less. Where the crop to be border irrigated is to be rotated with row crops irrigated by furrows,the slopes in the direction of irrigation should conform to the requirements of the border method. Ifnecessary the cross-slope can be graded to suit the furrows which would then be perpendicular to the border direction. The border stripsshould otherwise be level crosswise;in no event should the cross-slopeexceed 3 cm (0.10 ft) in the width of one strip. Greater cross-slopescause the water to concentrate along one side of the strip giving uneven irrigation. Size of border strips-the dimensions of border strips depend upon the size of the irrigation stream that is available and can be safely turned into the strip.Where possible, the selected border-stripwidth should be a multiple of the width of the least flexible farm implement to be used in the field. The length of a border strip is limited by the size of the irrigation stream that will flow without causing a flow of 0.9l/s per metre of border-stripwidth (0.10 c.f.s.per erosion. For example,with a slope of 0.5 %, foot) may be used safely.With a slope of 2%, the safe size of the stream is about 0.3 l/s per metre of width (0.035 c.f.s. per foot). Thus, the larger streams that can be used safely on the flatter slopes permit longer border strips to be used. On fine-texturedsoils with slopes ofless than 0.2%, the length of stripsis limited by surface drainage requirements. Border ridges-border ridges need to be enough to control the flow. They must be higher for the flatter slopes than for the steeper ones.The base of the ridge should be broad so that farming operations can be carried on over it. The sides of the ridges should be no steeper than two to one, horizontal to vertical. Thus, on a steep slope where a ridge of minimum settled height of 10c m (4inches) is needed,the base should be at least 0.60m wide (24 inches). On flatter lands where a 20 c m (8 inch) ridge is needed, the minimum base width should be about 1.20ni (48 inches). Irrigation stredms-the size of the irrigation stream depends on the texture and intake rate of the soil, the size of the border strip and the depth of application required.Coarse textured soils with high intake rates require large streams to spread the water over the entire strip rapidly and avoid excessive losses due to deep percolation.Fine textured soils with low intake rates require smaller ones to avoid excessivelosses to run-off from the lower ends of the strips.For the same reason,larger streams are required for large strips than for small ones,and for shallow applications than for deep ones.The water should flow in a uniform sheet over the entire width between the borders. It is important that intake rates of the soil be determined by appropriate methods (e.g. USDA Handbook 82, 1956). When feasible,field trial runs are very useful for evaluating stream size requirementsand optimum border widths. In Fig. 10.3 are two examples of curves plotted to show the rates of advance and recession of the irrigation stream.When these are approximately parallel,the intake opportunity time is about the same in all parts of the border and uniform application results. Shortening the length of run,increasing the stream size or reducing the border width all have the effect of reducing the deviation from the average intake opportunity time, As guides for planning border-stripirrigation Tables 10.12,10.13 and 10.14are included to indicate differences in experience. (d) Basins

The basin method is best adapted to soils having moderate to slow intake rates and moderate to high available water holding capacities. Although efficient systems for water application can be designed for sandier soils with lesser available water holding capacities,the size of the basins may be so small that they become objectionable as related to cultural practices,location of delivery laterals and labour requirements.Alternatively, large basins on sandy soils need prohibitively large irrigation streams,e.g.150 l/s for basins of 1000 m2. Smooth,gentle,uniform land slopes are best adapted and result in the best field layouts for basins.Ifthe 320

IRRIGATION SYSTEMS A N D M A N A G E M E N T Time in minutes

I

O

I

I

60

I

I

I

I

1

1

120 180 240 Length of strip check

,fyt 300 m

Fig. 10.3.Advance and Recession Curves,Circ.408,Calif.Ag. Exp. Sta.,'The Border Method ofIrrigation', J. C. MARR, p. 23. Above: Graph shows nearly equal water intake opportunity for the whole length of a strip check Below: Graph shows nearly equal water intake opportunity for first 180 m of a 300 m strip check (600ft, 1000 ft) Table 10.12. Design for border sfr@s--@moT)

Ground

Discharge per metre of plot width (l/s)

Length of run

% Sandy

0.244 0.4-0.6 0.6-1

10-15 8-10 5-8

60-90 60-90 60-90

Silty-sandy

0.2-0.4 0.4-0.6 0.6-1

5-7 4-6 2-4

90-180 90-180 90

Silty-clayey

0.2-0.4 04-0.6 06-1

2.4-3.5 2-2.5 1-2

180-300 90-180 90

Clays

0.2-0.3

2-3.5

Slope

(m)

360 or more

cross-slope(land slope normal to length of the basin) is more than 3 c m between border ridges, the basin will usually require levelling or 'benching'. An overall fall of 6 cm in the length of the basin is often desirable as a levelling construction tolerance to avoid reverse grades. In areas where wind velocities exceed 25 to 30 km/h during the period when irrigation water is being applied, operational difficulties may arise if the wind direction is opposite to the direction of water flow in the basin. Since wind erosion is also usually a problem in such areas,the basin length should be normal to the prevailing wind direction ifpractical.Ifthe topography ofthe field is such that the basin length is parallel to the prevailing wind direction,the water should be applied at the up-windend of the basin. The adaptation of basin irrigation to gently sloping topography is discussed in the following paragraph on 'contour basins'. (e)

Contour basins (contour levees)

A contour levee irrigation system should adhere closely to the natural drainage pattern to provide for the removal of excess rainfall.The border ridges or levees follow the contour with one end of each ridge ending at the natural drainageway,if one exists.W-or V-shapeddrainage ditches,which also conveyirrigationwater, 321

IRRIGATION, DRAINAGE A N D SALINITY Table IO.13. Design data for border strips-(Quacm"usH>)

Soil texture

Slope of land

Desired depth of application

Size of irrigation stream

Suggested borderstrip size Width

Length ~

% Coarse

0.25 1-00 2.00

Medium

0.25 1.00 2.00

Fine

0.25 1-00 2.00

mm

50 1O0 150 50 100 150 50 1O0 150 50 1O0 150 50 100 150 50 1O0 150 50 100 150 50 1O0 150 50 100 150

(inches)

m

(fect)

m

(feet)

us

(c.f.s.)

(2) (4) (6) (2) (4) (6) (2) (4)

15 15 15 12 12 12 9 9 9 15 15 15 12 12 12 9 9 9 15

(50) (50)

150 240 400 90 150 270 50 90 180 240 400 400 150 300 400 90 180 300 400 400 400 400 400 400 200 400 400

(500) (800) (1320) (300)

225 200 170 78 70 70 35 28 28 200 170 100 70 70 70 28 28 28 113 70 42 70 35 21 28 28 19

(8.0) (7.0) (6.0) (2.75) (2.50) (2.50) (1.25) (1 -00) (1.00) (7.0) (6.0) , (3.5) , (2.5) (2.5) (2.5) (1 *O) (1 .O) (1.0) (4.0) (2.5) (1.5) (2.5) (1.25) (0.75) (1.0) . (1.0) (0.67)

(6)

(2) (4) (6) (2) (4) (6)

(2) (4) (6) (2) (4)

(6) (2) (4) (6) (2) - (4) (6)

15 15 12 12 12 9 9

9

(50) (40) (40) (40) (30) (30) (3 0) (50)

(50) (50) (40) (40) (40) (30) (30) (30) (50) (50)

(50) (40) (40) (40) (30) (30) (30)

(500) (900)

(200) (300) (600) (800) (1320) (1320) (500)

(1000) (1320) (300) (600)

(1 000) (1320) (1 320) (1320) (1320) (1320) (1320) (660) (1320) (1320)

1

i

Table 10.14. Border strip irrigation (mainly for grain crops and grasses). Lengths of str@s (in IIl)-(OFFENGENDEN)

Soil

Slope

0*001-0~005

0*005-0.01

0.01-0.02

GO-80 80-100

80-100 100-120

Sandy loam

with good permeability Clayey,low permeable

40-GO 70-80

are usually located in the natural depressions.Successfuluse ofthe contour leveemethod requires that minor surface irregularities,such as mounds and low areas or pockets,be eliminated. This is usually accomplished by land smoothing with land planes or similar equipment. In some areas, rough grading may be re,quired prior to the smoothing operations.Preciseland levelling is not usually advantageousdue to the costs involved. The maximum land slopefor this method is approximately 2.5%.Land smoothingpermits better alignment of the levees,increasesuniformity ofapplicationand efficiency ofirrigation,and providesmore effectivedrainage. The desirable vertical interval between levees is 0.20 ft. This interval can be increased to 0.40 ft where topography is such that a smaller vertical interval would result in a horizontal interval of less than 40 ft. To provide uniform penetration of water and rapid removal of the excess,strips between levees should not exceed 1600 ft in length;this limitation does not apply if rice is grown. The minimum size of the irrigation stream used must be large enough to permit rapid flooding of the individual leveed areas or stripswithin no more than a quarter ofthe time required for the soil profile to absorb the net amount of water that is to be

322

IRRIGATION SYSTEMS A N D M A N A G E M E N T applied. To estimate the required stream size (Q c.f.s.) or the size of basin (A acres) the following relation may be used: Q = n .A (26) where IZ = 10-20 for sandy soils,2-10 for loams and 0-5-2for clays. The settled height ofthe ridgesmust equal the sum ofthe vertical intervalbetween them,the depth ofwater applied, and the freeboard needed. The freeboard should be at least 8 c m (0.25 ft). An additional 10 c m (0.30 ft) must be provided for settling.Where the levees are temporary, as for rice or row crops, their side slopes should be no steeper than 2 to 1, horizontal to vertical.In permanent pastures the levees should have side slopes no steeper than 4to 1 to permit the use ofwheeled machinery and to resist trampling by livestock.

(f) Wildjlooding Field ditches are run along the contours.The interval between ditches and the spacing of diversion points depends on the land slope,nature ofthe soil and the crop and on the water supply.There are no rigid design criteria for this method and at best it does not give very even application,

4. Sprinkling

Designing a sprinkler system to fit soils,crops, and labour conditions of an individual farm and to deliver water in the amounts and rates required by the farm irrigation plan is an engineeringjob. The parts of the system must be fitted together to operate efficiently and economically.(The steps to be followed in the design of a portable sprinkler system may be found in the specialised literature.)

5. Sub-irrigation

Sub-irrigationsystemsshould be carefully designed and the surfacelevelled so that the depth ofthe controlled water table below the soil surface will be uniform over the entire area. On soils of very high permeability water distribution by open ditches is sufficient,but on soils of lower permeabilitiesthese must be augmented by mole drains or tile lines between the ditches.T w o types of distribution can be used: (a) the drainage works, can be used for irrigation purposes,with the flow direction merely reversed,(b) a supply network’can feed the drains at the opposite end to that terminating in the drainage channels.By controlling the input and adjusting the water level at the drain end,the soil moisture supply can be controlled better,leading to better control of the water level,especially during periods of rainfall. If the ground slopes too much, the drainage networks should be fed from canal reaches which should be staggered by at least 10 cm. Thedistribution system must be capable ofadjustingthewater table to a depthwhich maintains a favourable water content in the root zone of the crop. It must also balance the water withdrawn from the capillary fringe by plant roots and evaporationwith the upward capillary rise. Hence,the water level must be adjusted to the rooting depth ofthe crop,the rate ofevapotranspiration and the nature ofthe soil.Invery sandy soils, the capillary fringe is not very high and hence the moisture content varies appreciably with every slight variation in the level of the water table.In soils containing a higher percentage offines,the capillary fringe is distinctly higher and the soil moisture content varies less when the level of the water table changes. Owing to these factors,in alternating wet and dry periods,the water level must be varied very much more in low permeability soils if it is to provide adequate water in the dry period and not saturate the soil in the wet period. The following example illustratesthis point (Fig. 10.4). For soil B,the height of capillary rise is 50 cm. The water table level must be maintained at -35 c m in the dry period to ensure that the roots are moistened properly and it is sufficient to lower it in wet periods to -40 c m to avoid saturating the profile. For soil A, which contains more fines, the height of capillary rise is 120 m. The water table must be maintained at -60 cm in dry periods since the supply would be inadequate at a greater depth. In wet periods,it should be lowered to -90 cm to avoid saturation since at -60 cm the soil might approach saturation. In addition,all other things being equal,thewater supplied from capillary rise becomes smaller as the soil is more impervious, and hence richer in fines. The water table must be kept close to the optimum level or the declines in yield become too pronounced.

323

IRRIGATION, D R A I N A G E A N D SALINITY P cm

100

50

O O

50

30

H%

uAir P = the distance from the water-table to the Water in retention ground surface (in cm> erise water from capillary .H= the moisture content Water-table

in the soil (in%)

Fig. 10.4. Capillary rises in two different soils Thus, in sandy soils containing less than 4% of particles with a dianieter of less than 16pm, the deviation must not be more than 10 cm. This means that (a) the ground must be absohtely horizontal and (b) the tile drains or moles must be close enough together for the pressure head differentials(Ah) which cause the water to flow throughout the soil to remain under 10 cm. Their spacing should be determined accordingly.The formula for calculating this spacing is as follows:

where

E= the spacing between 2 drains C is a constant q = the rate of water supply required from capillary rise

K = the hydraulic conductivity A h = the fall in pressure head compared with the level of the centre-linesof the drains The required water supply should take into consideration not only the losses due to evapotranspiration but also those due to deep percolation.In the Netherlands it is estimated that evapotranspirationamountsto 4-5 mm per day and that the losses to deep percolation range between 5 and 20 mm per day. The spacing may vary from 25 metres for soils containing 2%of particles less than 16 pm, to 8 metres for soils containing 8%of particles of less than 16p m and a finer sand fraction.The greater the fines content of the soil,therefore,the higher the cost of the system. In California,ditches 1.20-1-50m (4-5 ft) deep are placed up to 170 m (500 ft) apart'on 'muck' soils which have exceptionally high saturated permeability.

D.

WATER MANAGEMENT

Water management is one ofthe most vital facets ofirrigation.Constant vigilance must be exercised to obtain

324

IRRIGATION SYSTEMS A N D M A N A G E M E N T optimum conditions for water-soil-plant relationships. Water management activities cover the supply of water from the source to the field through the distribution system and the removal of excess water from the farm.

1. Quantity and quality of water (a) Quantity An annual water budget must be drawn up for the entire irrigated area served. The estimated water supply must be matched to total water requirements as adequately as possible. The annual water allocation from a reservoir should be based on the catchment yield at the 50 %probability level. Seasonal limitations in the supply from rivers and wells should be recognised.Forward planning is necessary to ensure water deliveries according to seasonal fluctuations in demand and the varying requirements of different crops and irrigation methods. A plan of water distribution through the supply network calls for information on (a) farm irrigation programmes and application efficiencies, (b) irrigated areas under different crops, (c) leaching programmes, (d) domestic and industrial water requirements, and (e) the design capacity and conveyance efficiency for each arm of the network. The plan must be flexible to accomodate unexpected peak demands,changes in cropping patterns, rainfall and emergency spilling of water delivered in excess of requirements.A map of the entire system is required for coordinating deliveries. Water supply shortages may necessitate reduced applications to some or all crops. In the design section consideration has been given to matching the capacity of the farm irrigation system to the available water supply.When this has been done,it is crucial that the designed supply rates for surface irrigation methods be adhered to and that water is applied in accordance with the provisions of the design. Deviations from specified management practices should be made with extreme caution. With sprinkler systems deviationsfrom the designed application rate can lead to reductions in uniformity, surface capping,standing water on the surface and even run-offerosion. Sub-irrigationrequires the surveillance of a trained technician who regulates inflows and outflows in the network to maintain a satisfactory balance of soil moisture. A regular programme of soil moisture measurement (refer to Chapter 8 for technical details) and water level observationsis necessary for the proper control of water application by this method. In all methods of irrigation,careful regulation of the water supply is essential to the uniform application of water to the soil.Efficient application means that less water is required to satisfy the soil moisture deficits and the possibilities for effective salinity and waterlogging control are enhanced.

(b) Factors affecting the quantity of wufer applied per irrigation Irrigation systems,schedules and quantities to apply at each irrigation are established throughoutthe world from the recommendations of scientific and research institutions and previous experience.Determinations of the proper depth of water to apply per irrigation is discussed in detail in Chapter 8 from the standpoint of crop requirements,soil properties and climatic aspects. (c) Avoidance of ivuterloggìng Deep percolation losses irrespective of their origin should be kept ta a minimum.A rising water table is not to occur. However deep the groundwater may be initially, a rising water table at any depth is an incipient danger to crop production and if the practices giving rise to it are prolonged, waterlogging of the crop root zone will result. Every irrigated area should have a network of groundwater wells and staff gauges in drainage channels to obtain basic information on the status of the groundwater (see Chapter 14,Section C). A basic principle in groundwater control is to maintain a favourable balance between flows into and out of the groundwater reservoir. A rising water table may be controlled by efficient application of water and adequate drainage facilities. Effective drainage is an essential adjunct to periodic leaching. OFFENGENDEN points out that the most acceptable is the reduction of the inflowby means of lowering the irrigation rate under the improved irrigation technique and by decreasing seepage losses. As a preventive measure the proper operation of irrigation systems is very important.He classifies the preventive measures as follows: 325

IRRIGATION, DRAINAGE A N D SALINITY

1. planned water use and distribution 2. irrigation development over vast areas (not by small separate fields) 3. closure of canals in winter 4.reduction of the density of irrigation canals 5. prevention of breaches in canal embankments 6. application of seepage control measures 7. prevention of back water occurrences on the irrigation network;lift irrigation for highlands 8. application of proper agrotechiiique and irrigation practices without wastes 9.tree plantings along irrigation canals Weather conditions have a definite effect on sub-irrigatedareas.They must be considered in water management. (d) Quality of irrigation water Recommendations for the application of irrigation waters of different qualities will be found in Chapter 7. '(e) Control of salinity and alkalinity The concentration of salt in the soil solution increases as water is removed by evapotranspiration.The main *resultofthis concentrationis to reduce the amount ofavailablewater in the soil.Hence to prevent the salinity reaching an injurious level irrigations should be more frequent when the water or soils are saline. In the USSR it is considered that during the first period of the development of saline soils,drainage and leaching are used: . 1. to provide reliable desaliiiisation of the entire root zone of the soil, the total salt content being not more than 0.2-0-3%, with chlorine ion content not exceeding 0.01%; 2. to provide demineralisation of groundwater up to the value not higher than 1-3 g/1 and to create the fresh groundwater stratum 8-10 m deep by replacing saline drainage water by fresh (irrigation or seepage) water. When the work of drainage for reclamation purposes is over there comes another period of importance, that is the operation period. Drainage at this stage is meant to maintain the conditions for groundwater percolation and optimaldepth ofthegroundwater table,aswell as to feed the rootzonewith freshened groundwater through capillaries. Removal of salts through leaching may also be required. Usually the application of 10 to 15 % water in excess ofconsumptiveuse of the crop w ill be adequatefor leachingpurposes. Further informationregarding the calculation of leaching requirements is given in Chapter 13. Leaching programmes depend on a large number offactors. Some plant species are more salt tolerant than others.The same plants may be more sensitiveto salts at certain stages in their growth.Plants at the germination and seedling stages are much less salt tolerant than mature plants. Irrigation prior to planting may be sufficient to allow the seedlings to pass the susceptible stage before the salt concentration builds up again within the root zone.Alternatively the modifications in the shape ofbeds between furrows shown in Fig. 10.5. allow the seedlingsto devlop in a part of the bed where relatively little salt accumulates.The best control of salinity in crops grown on the flat is obtained by distributing the water as evenly as possible. 1n"thecase of rice growing,the increase in the salinity of the water through evapotranspiration can only be combated by draining the paddy fields more often. '

2. Crop production (a) Irrigation-soilfertility relations The coordinated use of effective irrigation,fertilisers,soil amendments,deep tillage and measures to improve soil structure will result in optimum crop production. Drainage must always be developed in connection with irrigation.Iflatent soil deficienciesand toxic elements levealed by irrigation are disregarded,water inputswill never achieve the expected result. The soil moisture level and its interrelationship with aeration governs the availability to the plant of nutrientspresent in the soil via complex effects on root activity and growth,nutrient and other salt solubilities and the activity of nitrogen producing bacteria in the soil and in the roots of legumes.Results of local field 326

IRRIGATION SYSTEMS A N D M A N A G E M E N T SALINITY AT PLANTING TIME Single row bed

A,

Moderate

High

=~-

-I_

Water

Too salty for seed to germinate Seed germinate

Double row bed

g&&

--

.. ,.:,..t

Salt accumulation

Sloping bed

fi --

Fig. 10.5 Effect of salinity and bed type on salt accumulation in a seeded area. Germination is delayed or prevented where salt accumulationis excessive (Source Texas Agric. Exp. Sta. Bulletin 876)

experiments should be used wherever possible to establish optimum moisture regimes for different crops and fertility levels. In arid regions soils are most often deficient in nitrogen while the reverse is usually true of other mineral elements which may accumulate to the point of causing toxicity. Nitrate fertilisers applied to the soil are easily leached, thus fertiliser application to the soil should in general be followed by a comparatively light irrigation. Higher levels of fertiliser input require better irrigation management not only to avoid leaching, but to eliminate water as a limiting factor in production. Soluble fertilisers and soil amendments may be applied in the irrigation water. Sprinkler systems are easily adapted forthis purpose,but precautions should be takentoprevent acceleratedcorrosionoftheequipmentand ensure that the fertiliser is not detrimentalto the crop foliage or fruit.Band placement offertilisers in furrow bottoms is used in row crops to increase its effectiveness in conjunction with furrow irrigation. (b) Crops and irrigation management

The water delivery system used and the type of crops produced regulate the frequency of irrigation in a specific area. Priority of irrigation, if necessary, should be given to crops reaching critical stages of growth such as cereals in flowering or seed formation,to control boll set in cotton,and to control salinity in the root zone.Every attempt should be made to schedule irrigations to occur when the available moisture in the root zone is at or near the allowable depletion to ensure maximum production. A variation in cropping pattern may staggerthe peak demand on the irrigation system and resultin less detrimentaleffects on the crops within the area. (c) Cultivation operations Irrigations must be arranged to dovetail into cultivation operations. For instance time is required after irrigation for a soil to drain sufficiently to carry harvesting machinery or to support the hooves of grazing animals without ‘puddling’of the surface. Crop protection inter-rowcultivations and fertiliser application should be considered in the irrigation programme. Care must be taken during tillage to keep the furrows deep enough to carry the irrigation stream.Annual pre-seasontillage and mole drainage every few years on heavy soils are also important operations associated with good irrigation management. (d) Plant disease relutions Certain irrigation regimes may favour the incidence of plant diseases and parasites, first because of their direct effectson the plant environmentand,secondly,throughindirecteffects associated withplant growthand vigour. Salinity and waterlogging effects fall in the latter category and tend to reduce plant growth,generally rendering crops more susceptible to attacks from diseases and parasites. The effects of irrigation on plant diseases are specific to each case. 327

IRRIGATION, D R A I N A G E A N D SALINITY 3. Economic considerations

Two groups of constraints influencethe economic aspects of irrigation management: (I) the physical limitations of land and water availability,and (2) social and economic factors,labour supply,wage rates,interest rates and capital availability.Also, two cases have to be considered separately-(a) subsistence economics (as in the case of some under-developed areas) and (b) competitive economics (or market economy). (a) Subsistence economics In a subsistence economy investment capital is scarce and the goal of irrigated agricultural production is to maximise the total gross product. Ify is the yield per unit area and S is the irrigable area,the total gross product can be defined as:

Y=yS (28) When water resources are not limited,water allocations should correspond to those required for maximum yields per unit area.In Fig. 10.6this corresponds to the point A on the yield function and a water allocation of VA. If the water supplies are limited to a total value R,the area that can actually be irrigated is inversely proportional to the water allocation V (R= SV). In this case,if the optimum water allocation per unit area is V,, the gross product for the irrigable area is: R Y=y, .s=y, .(29) VB

with

2=tan b (Fig. 10.6), i.e. the slope ofthe line OB.

VLi B represents the point on the yield functionwhere the average production per unit ofwater applied begins to decline.

Vrl VA Volume of water applied, V

Fig. 10.6.A typical relation for yield as a function of water applied per unit area (b) Competitive economics The objective in this case is to maximise the net income from irrigated crops.Net income is expressed by:

N=B-C where B=the gross income of the area from irrigation and C = the total cost of irrigated crop prodmtion. If q is the unit sale price of the yield per unit area,y, then: B=y.q.S and C= S (C,-I-AC,, -I- C’+pV) 328

(30)

IRRIGATION SYSTEMS A N D M A N A G E M E N T where C,= the normal operating charges before irrigation AC, = additional operating charges arising from irrigation (added expenditure on soil preparation, harvesting,fertiliser,labour,etc.) C‘= depreciation and maintenance charges for the farm irrigation system p = direct cost of water per unit volume All above are expressed per unit area to correspond with the units of S Re-definingN in terms of (31) and (32):

N= S [y4-(C,+AC,+C‘+ p V)]

(33) In this expression C,,AC,, C‘ and q may be assumed constant for given operating conditions. N o w p =f (Y, S), i.e. water costs vary with the amounts used and the area served by the distribution network. Also,since y =f(V) the basic variables affecting net income are S and V.Two typical cases will be analysed in terms of these variables. 1. If water resources are unlimited,the total net income ofthe area irrigated is related to the net income per unit area thus: y1=

N -= S Y4- (K+PV)

(34)

where K = (C,+AC, + C’)

dn dY =P-. This means that the slope of the curve y =f(V) For IZ to be maximum -must be equal to zero,and dV dV 4 in Fig. 10.7must have a value of tan a=p/q. Corresponding to this is a point M at which the net income is at its maximum,which determines the optimum volume vM to be used per unit area and yM the corresponding yield.

I

/

VM

VA

Volume of water applied, V

Fig. 10.7.Location of the optimum level of production For a given curve,it can be seen that y;M will approach V,,as tan a decreases,i.e.as the price of the water falls and the crop’s sale price rises,production will tend towards the maximum yield at point A. As long as the cost of water remains low, the volume of water applied tends to approach VA,the allocation at the maximum yield point (in Fig. 10.7,M would be close to A). It may happen,even with abundant water resources,that the cost of supplying water increases markedly outside a certain perimeter in the irrigable land area because of the scarcity of economical dam sites or a substantially elevated unit cost ofnetwork construction.Ifit is required to increase the total net income from irrigation by expanding water supplies it is necessary to select the most economical way of achieving an increase in production from two alternatives: (a) for a limited increase in production of the existing area to allow economic optimisation,(b) for other reasons it may not be desirable to develop one agricultural area at the expense of another and ‘scaled’government assistance might be given to assist development of slightly less favoured regions of production. 2. In order to maximise the net income ofan irrigated area when the water resources are limited to a value R, the area irrigated must be restricted to the value: 329

IRRIGATION, *DRAINAGI?, A N D SALINITY SI= R -when N =

V

V

dN For N to reach its maximum, - must be equal to zero,giving: dV

(y being a function of V). The curve F( V) may now be constructed from the relation:

PM

mP= m M - PM= m M - rip .1ZP

(37)

The representativepoint for the optimum volume V,,,to apply per unit area is located at the intersection of K the curve F(V) with the ordinate - as shown in Fig. 10.8.The optimum volume will decrease as the ratio

5

4 grows smaller,i.e. as the total production CO& fall and the sale price of the yield rises.

4

,

vu Volume of water applied, V

Fig. 10.8. Graphical method of determining V,

4. H u m a n health considerations

It is well known that the mosquito, which is the carrier of malaria,breeds in stagnant water.Precautions should be taken to eliminate these breeding areas through effective water management. In rice cultivation standing water is unavoidable,but draining for short periods at critical stages in the development of the larvae can help to control the mosquito population. Other human parasites may be carried in the water of rice paddies. In many countries,bilharia is a real menace,unless constant precautions are taken to control the snails, which are hosts to the parasite during part of its life-cycle,in both canals and storage ponds. When the water is polluted (as when sewage water is used for irrigation) or contains parasites, every precaution should be taken to prevent it from coming into contact with any parts ofthe crop that are to be eaten, especially in their raw state.Such water should not be used for sprinkler irrigation of vegetable crops,and in the case of roots or tubers, analyses should be carried out to check that the produce harvested has not remained unfit for consumption,in spite of the purifying action of the soil.Ifthis is not done,certain crops 1957). must be formally prohibited (R.M.HAGAN, 330

IRRIGATION SYSTEMS A N D M A N A G E M E N T

E. MAINTENANCE

OF FARM IRRIGATION SYSTEMS

This section will be restricted to a brief outline,as a more detailed account of this subject may be found in Chapter 14. Great importance must be attached to the problem of maintaining equipment and installations in a good state of repair. The most perfected appliances and schemes will only continue to operate satisfactorily in conjunction with an efficient programme of maintenance. Maintenance begins immediately after facilities or equipment are placed in use and continues throughout their useful life. The amount of maintenance will depend on the permanence of the materials used in structures, climatic conditions such as amount and intensity of precipitation and temperature variations, stability of soil materials, selection of maintenance materials,preventive maintenance practices and proper use and operation of facilities. Maintenance operations in distribution networks include canal cleaning, lining and earthwork repairs, replacement or repair of structures,weed control operations, and provision of cathodic protection in saline waters to prevent electrolytic corrosion of metal structures. Surface irrigation methods require periodic re-levelling of the land surface and repairs to permanent levees and turnout structures.Sub-irrigationusing mole drains requires the moles to be re-drawnevery two to five years. Sprinkler equipment must be checked annually for repairs or replacements which become necessary and pumping plants should be regularly checked for efficiency and reconditioned as required. Operating instructions should be provided for any major mechanical facility and should be available at all times to operating personnel.Operating instructionsmust be followedexplicitly to prevent damage and failure. Any irrigation or drainage system that was worth constructing is certainly worth maintaining.

REFERENCES ALTUNINS. T.(1956), River bed regulation,Selkhozgiz.,Moscow (in Russian). BABBITT, DOLAND, and CLEASBY (1962), Water Supply Engineering (Sixth edition), McGraw-Hill,New York. BAUZILV. (1952), Traite'd'Irrigation, Eyrolles,Paris (in French). BUREAU OF RECLAMATION (USDI)(1953), Water measurement manual (First edition), Government Printing Office, Washington,D.C. BUREAU OP RECLAMATION (USDI)(1963), Linings for irrigation canals (First edition), Government Printing Office, Washington,D.C. BUREAU OF RECLAMATION (USDI)(1963), Earth manual (First edition,revised), Government Printing Office, Washing,D.C. BUREAU OF RECLAMATION (USDI)(1961), Design standards No. 3, Canals and related structure, Denver, Colorado. COMMISSION INTERNATIONALDU GÉNIE RURAL (CIGR)(1958), Proceeding of the F$th Congress. CONFERENCE OF UNITEDNATIONSON THE APPLICATIONOF SCIENCE AND TECHNOLOGY(1963), Irrigation and the

utilization of water, Geneva.

DURAND J. H.(1958), Irrigated soils, Alger (in French). DARLOT A.(1 955), Land preparcltion for irrigation, Centre de recherches et d'experimentation du génie rural, Rabat (in French). FAO (1958), The Utilization of Saline Soils.

GULHATI N.D.(1955), Irrigation in the world, ICJD,New Delhi,India. HAGAN R. M. (1957), Can man develop a permanent irrigation agriculture?, US National Committee, ICID,Denver,Colorado, San Francisco Conference. HOUK I. E.(1951;1956), Irrigation engineering,Vol. 1(1951) and Vol. II (1956), Wiley, New York. INTERNATIONALCOMMISSIONON IRRIGATIONAND DRAINAGE, Report on the Ist congress (1951), 2nd congress

(1954), 3rd congress (1957), 4th congress (1960), 5th congress (1963), and 6th congress (1966). ICID,

New Delhi,India. INTERNATIONALCOMMISSIONON IRRIGATIONAND DRAINAGE,World-wide survey of experiments and results on

the prevention of evaporution losses from reservoirs, ICID,New Y

Delhi,India (Publication pending). 331

IRRIGATION, DRAINAGE A N D SALINITY

IRSAELSENO.W.and HANSEN V.E.(1962),Irrigationprinciples aiadpractices,Wiley,New York. IZD.MSKH. SSSR. (1960), Approximnte rilles of operation of irrigation systems, Moscow (in Russian).

KALISVAART I. C. (1958), Some principles andpossibilitiesof subsoil irrigation,ISSS Report on the conference on supplementalirrigation (Copenhagen).

KING H. W.and BRATERE. F. (1963), Handbook of hydraulics for the solution of hydrostatic andfluidflow problems (5th edition), McGraw-Hill,New York. KOSTIAKOV A.N. (1962), Fundamentals of reclamation,Selkhozgiz,Moscow (in Russian}. KOVDA V.A.et al. (1956), Drainage effect on improving soilfertility, Izd.AN SSSR,Moscow (in Russian). KRAMER PAULJ. (1959) Plant and soil water relationships,McGraw-Hill,New York. LELIAVSKY S. (1955-60), Irrigation and hydraulic design,Chapman and Hall,London (3 volumes). MAHUB S. I. and GULHATI N.D.(1951), Irrigation outlets, Revised edition,Atma Ram,Kashmere Gate, Delhi,India.

MALATIC J. T.(1968), Sprinkler Irrigation,Soils,Climate,and Land Classification.Proceedings1968 Annual TechnicalConference, Sprinkler Irrigation Association,Washington,D.C.

MALETIC J., SACHSM.S. and KROUS E.S. (1968), Desalting suline irrigation water suppliesfor agriculture-A case study,Lower Colorado Basin, USA;presented at a Symposium on Nuclear Desalination,Madrid, Spain. MARR JAMES C.(1957), Grading landfor surface irrigation,Extension Service, State College of Washington, Pullman,Washington,DC (Extension Bulletin 526). MINISTÉRE DE L'AGRICULTURE (FRANCE) (1963), Retenues collinaires (Small reservoirs in hilly landscapes). Direction générale du génie rural et de l'hydraulique agricole,Service de l'hydraulique (in French). NATIONAL COUNCIL OF APPLIED ECONOMIC RESEARCH,N E W DELHI,INDIA (1959), Criteriafor fixation of water

rates and selection of irrigation projects,Asia PublishingHouse,Bombay.

OFFENGENDEN S. R.and MIRKIN S. L.(1940), Designing ofsomeirrigationnet facilitiesin a cotton collective farm.Hidrotekhnica i Melioracia,2, Moscow (in Russian).

OFFENGENDEN S. R. et al. (1962), Operation of reclamation systems,Selkhozgiz,Moscow. PECKWORTH, HOWARD J. (Editor) (1961), Concretepipe for irrigation and drainage,American Concrete Pipe Association,Chicago,Illinois. POIRÉEM.A. A. and OLLERC.H.(1957), Irrigation,Eyrolles,Paris (in French). SHAROVI.A.(1959), Operation of reclamation systems, Selkhozgiz,Moscow (in Russian).

TSHERKASSOV, Melioration und Landwirtschaftliche wasserversogung (Reclamation and water supply for agriculture) (in German). W-XAS AGRICULTURAL EXPERIMENTSTATION,Salinity control in irrigation agriculture,Bulletin 876. UNESCO (1957), Utilization of saline water. UNESCO (1961), Salinity problems in the arid zones. us DEPARTMENT OF AGRICULTURE(1956), Methods of evaluating irrigation systems,Handbook No.82. us DEPARTMENTOF AGRICULTURE (1956), Soil Conservation Service, National engineering handbook, Section

15 Irrigation;Chapter 12, Land Levelling.

us DEPARTMENTOF AGRICULTURE,Yearbook of Agriculture; Water (1955), Soil,(1957); Land (1958), Government Printing Office,Washington,D.C.

VOLOBUEV V. R. (1959), Leaching rates for reclamation of saline lands,Hidrotekhnica i Melioracia, 12, Moscow (in Russian).

WOODWARD G.O.(1959), Sprinkler irrigation(Secondedition), SprinklerIrrigationAssociation,Washington, D.C.

332

11 Drainage Systems and Management* A. NECESSITY OF DRAINAGE AFTER IRRIGATION 1. General

Probablythe most widely recognised definition ofdrainage in relation to arid,irrigated areasis ‘theremoval of excess water and salts from agricultural lands’. This is a simple definition, but the solution of a specific drainage problem is often complex,perplexing, frustrating and costly. Drainage is one of the most critical aspects on which the maintenance of irrigated agricultural production depends.With good drainage,even the worst irrigation farmer cannot easly ruin his land. With inadequate drainage,even the best farmer usually cannot save it. All properly irrigated Iand in the world requires drainage.This can be natural,artificialor a combinationof both. ‘Properly’infers thatthefull water needs ofthe plant are supplied.In theplanning ofa feasible irrigation project,it is essential that a diagnosis be made ofthe necessity for the provision ofartificialdrainage,because the costs of such work may determine the feasibility of irrigation. A successful drainage system may be simple or complex, depending upon site conditions. Surveys and installations must be held in close tolerances.Judgementand experience determine to a large extent many of the decisions necessary in investigating, planning and designing a successful system. As is true with other professional endeavours, a competent and experienced drainage engineer should plan and direct surveys, investigations,designs and installation of drainage systems. This is basic to ensuring good results towards completion of a comprehensive system which will remove excess water,permit the proper control ofgroundwater and harmful salts,and enable effective and economical operation and maintenance. In general,drainage can be divided into two basic types: (a) Surface drainage Surface drainage is the removal of excess surface water from irrigation and precipitation. It is nearly always necessary;is comparatively simple to plan,design and construct;and is usually rather inexpensive.However, the importanceofproviding adequate surface drainage cannot be over-emphasised.All possible excess water from all sources should be removed from the land surface by this relatively inexpensive method before it can harm the growing crops and percolate to the groundwater table to create or intensify a more expensive subsurface drainage problem. (b) Subsurface drainage Subsurface drainage, as already implied,refers to the control of groundwater heights and salts through the removal of excess subsurface water. The behaviour of the water table in an irrigated area is governed by the schedule and amount of deep percolation;the physical characteristics ofthe substrata;i.e. the depth,stratification,pore space,permeability and continuity; and the topography,including the intensity, location and depth of natural outlet channels into which groundwater will discharge. A knowledge of these various characteristicsis essential to a reasonable diagnosis of the necessity for and the amount of artificial drainage after irrigation.To obtain this knowledge requires thatsufficientinvestigationsbe made to satisfytheparticular purpose or objective. In the planning stage of an irrigation project, the primary purpose of the drainage investigation is to provide a reasonable estimate of the probable requirement and cost of artificial drainage which would ultimately be required under irrigation, Because large areas are being considered for irrigation and time and available funds are limited,the investigations must generally be limited in intensity. However,the investigations must be in sufficientdetail and intensity to provide a general understanding of the overall characteristics and show the uniformity or non-uniformityof the area. * This chapter was edited by C. VAN DENBERGfrom the manuscripts submitted by S. AWRYNOV,V. LEGOSTAEV and N.M.RESHETKINA; J. H.BOUMANS and J. WESSELING; J. G.SUTTON,C.R. MATERHOFER and W.W.DONNAN as authors; with H.N.ENGLAND and V. A. KOVDA. contributions by A. G.ASGHAR,

333

IRRIGATION, D R A I N A G E A N D SALINITY The scientific approach to drainage problems in irrigation projects has to do with the following order of subjects: (a) The permissible groundwater and salinity level of the soil,securing good crop growth (b) The quantities of water added to the groundwater reservoir,the quantity of groundwater draining to existing (mostly natural) outlets or low land,and the quantity of groundwater to be removed from the area by artificial means in order to meet the requirements under (a) (c) The groundwater flow processes (d) The selection,design and installation of a drainage system

It is clear that these subjectswill not be investigatedin every detail for every irrigation project.Comparison ofresultsin similar existing projects and availableinformationfrom formerresearchmay contributeconsiderably to sound judgement.This holds particularly for the more general subjects mentioned under (a) and (d). The subjects under (b) and (c), however, are closely related to the characteristics of the hydrology,soil, topography,water quality and so on,of the considered project and require specific attention. In this chapter the subjects (b), (c) and (d) will be treated in the order given,whereas subject(a) will not be discussed separately, but in connection with the others. Firstly,therefore,the quantity ofgroundwater to be drained willbe considered.For this,thefollowingfactors are of importance: 1. The magnitude of irrigation losses 2. The leaching requirement for salinity control 3. The natural hydrological conditions

2. Irrigation losses Irrigationlosses generally form the main source of water to be drained.The losses to the subsoil (also called : percolation losses) include canal seepage and infiltrationlosses in the field.Percolation water seeps vertically downwards to the groundwater and must be evacuated,just like that part of the rainfall that infiltrates into the soil,by groundwater drainage. Groundwater drains must be deep enough and cut at least in the groundwater. The depth and locations of the drains depend, in contrast to surface drainage, not only on the quantities to be drained but also on the soil conditions, In this chapter mainly the drainage of these subsurface losses will be discussed because surface drainage is considered as part of the irrigation system. The latter is only taken into account as far as it influences the required capacity of collectors into which surface water is discharged. (a) Quantities The magnitude ofthe expected irrigation losses is difficultto predict,being extremely variable and depending on a great number of hardly controllablefactors of which the human factor (skill in handling the irrigation water) is not the least important.Nevertheless an estimation of future or actual losses is necessary for the design of a drainage system. Irrigationlosses differ with the season. For irrigation planning the summer or dry season is normally the criticalperiod. For drainage both the suminer season with relatively low losses, because water is scarce, as well as the winter season in which irrigation efficiency is usually much less,must be considered. Other factors of importance are soil and topography. In very permeable soils percolation losses will be large, surface field losses on the other hand of little importance.In poorly permeable soils percolation losses will be low and surface losses relatively high.A similar relationship exists for the soil slope.With increasing slope the risk of excessive infiltration losses becomes less,but that of surface losses increases. The irrigation method is also an important characteristic to be taken into account in estimating irrigation losses. The last important factor is the type of construction of the irrigation scheme.It will be clear that lined canals reduce seepage losses considerably but also facilitate water distribution which resultsin lkss distribution losses.A n irrigation scheme can be fully controlled by automatically working weirs, keeping the water level either upstream or downstream at a constant level independent of the quantities taken off at the farm

334

D R A I N A G E SYSTEMS AND M A N A G E M E N T inlets. Under these conditions practically all losses of excess water at the end of the canal system can be avoided. In order to obtain an equal distribution of water over the field in gravity irrigation, field percolation losses should theoretically be in the order of at least 10%(CRIDDLE,1950;AMAYON1962). Actual field percolation losses,even when the field is properly levelled and irrigation properly done,will usually be much higher, especially in the rainy season. O n the other hand these losses may be less during the peak period in summer which indicates that the local water supply was insufficient. In not too permeable soils (infiltration rate between 2and 10cm/h)the field percolation lossesfor properly-carried-outgravity irrigationmay be estimated at 20 to 30%of the field supply as an average over the year;30 to 50%during the wet season and 10 to 20% in the dry season.For better permeable soils these losses will be higher and the infiltration rate of 15 cm/h is sometimes accepted as the limitabove which gravity irrigation is no longer possible due to excessive losses. With sprinkling irrigation the water supply can be controlled much better. Still,due to the necessary overlap of the sprinkling circles,percolation losses in the order of 10 to 15%must be accounted for. Canal seepage can be completely neglected for well-constructedlined canals provided with suitable distribution works. For unlined canals in permeable soil these losses may amount to 20 to 30% of the total irrigation supply. Future seepage losses of earth canals may be estimated on the basis of measurements of infiltration rates (ROBINSON, 1957; BOUWER,1962) preferably in similar canals, other conditions being comparable. It should be kept in mind that continuously functioning canals gradually will become less permeable by silting up.This does not hold for canals working in rotationwhich dry out and crack periodically. Table 11.1 gives an example of seepage losses for unlined feeding canals (tertiary canals) in loamy soil,based on actual observations. Table 11.1. Example of seepage losses for canals running continuously and in rotation

Functioning of canal

Continuously

In rotation 66%

loo00 10000 5500 1.5 82 23

10000 6000 3630 3.0

~~

Total length in m Equivalent length permanently in use Average length continuouslyfilled 55% Infiltration (seepage) rate cm/h Seepage losses in ma/h and in l/s

1o9 30

(b) Losses and leaching In the previous paragraph all irrigation water,not directly taken up by the plants,was considered as ‘losses’. This is not correct because field percolation ‘losses’may be very useful and are even necessary for leaching and controlling the salts in the soil.Therefore all field percolation ‘losses’will be considered as fully effective for leaching.The difficulty that percolation losses are unevenly distributed over the field can be substantially overcome by employing an irrigationtechnique which is adjusted in such a way that the differencesin leaching over thefield are offset over a number ofyears by shiftsin theirrigationunits,changesintheirrigation methods and correctionalleachings. In some cases the normal field percolation ‘losses’may be sufficient for leaching, so that no additional irrigationespeciallyforleachingshould be given.Inothercases,however,additionalleachingmay be necessary. In order to solve the question whether this is the case,the salt regime of the soil must be studied.

3. Drainage quantities for salinity control

(a) Salt and moisture balance The general principles of salt and water balances are discussed in Chapter 2.Here they will be used as a quantitative calculation technique which supplies LIS with one of the most important basic informations for drainage design,namely the required leaching quantities and thus the quantities to be drained (natural or 335

IRRIGATION, DRAINAGE A N D SALINITY artificial) for salinity control. The calculation technique which follows was developed by BOUMANS(1963) (VAN DER MOLEN and BOUMANS,1963). and later on extended by VAN DER MOLEN The moisture balance of the root zone of an irrigated field may be written as:

Ir+ N = ET+P+ AV

(1) units

where

Ir=field irrigation supply less surface losses dm/month* N = precipitation less interception and surface run-off dm/month ET=evapotranspiration dm/month P= deep percolation below root zone or capillary water supply from below (Pnegative) dm/month AV=variation of the quantity (Y)of moisture stored in the root zone dm/month

For the groundwater balance below the root zone holds :

P+ S,= D,+ D,+A W =D,+A W where S,= underground water supply D,=natural drainage D = artificial drainage D,=total drainage A W=variation of moisture storage below root zone

units dm/month dm/month dm/month dm/month dm/month

Salts supplied by precipitation or assimilated by the crops can be neglected. The salt balance of the root zone now becomes : Ir.C,,=P.C,+AZ (3) where Ci, = salt concentration irrigation water C,= salt concentration percolation water AZ=variation of quantity dissolved salts (2)in root zone

me/lor g/1 me/lor g/l me/dm2or g/dm2

As already mentioned, P is negative when it represents capillary rise. If, in the period over which the balance is considered,both positive and negative percolation occurs,P is taken as the algebraic sum of both. After a number ofyears ofirrigation the soil salinity will reach an average equilibrium level with an average seasonal variation around it.Also after a number of years the salt concentration of the groundwater below the water table will be equal to the average concentration of the percolation water, C,. (b) Eficiency of leaching For salinity control the salt content of the soil moisture (esm) in the unsaturated root zone is decisive. One might take as an approximation C,, at field capacity equal to C, (Hunclbook60,1954). This,however, is a too favourable assumption since,especially in heavy soils,an importantpart of the percolation losses (leaching water) passes through cracks,fissures and relatively large pores without any leaching effect. If the share of the effective water passage to the total percolation is kt and the ineffective part is 1 -k, the following relationship is valid:

C,=k. CSm+(1-k)C,, (4) where C,,,, is the salt concentration of the soil moisture in the root zone at field capacity and k the leaching coefficient. The leaching coefficient is less than 1. It depends on the soil and, to some extent, also on the depth of the root zone. Some average values for k are given below. * 1 d m = 10 c m = 100 111111. 1 dm/month= 1 litre per month and per d m a 7 This coefficient k is not quite comparable to the leaching coefficient 1 used in Chapter 2. The method used here is somewhat more refined

336

D R A I N A G E SYSTEMS A N D M A N A G E M E N T

(c) The yearly drainage quantities For the average year A W,A V and AZ are zero.The combined salt moisture balance then can be approximated by : 2Ir Cz=(ZP)[k. Cz-t(l-k)Cr;l ZIP = ZIr .CG k .C~+(l-k}'~ and in combination with (1) :

ZP = Cr;Z(ET-N) k(C,, Cd where

-

(7)

Z=year total of indicated quantities Cs = weighed average of Ci over period considered CG =average value of C,,,, over period considered I=P=the total year of the required deep percolation to ma&ain soil salin,,y at the average level of CG. With equation 2 P may be expressed in terms of drainage

If C,,,is the average salt level of the soil which is not to be exceeded,ZP represents the required yearly leaching,and thus also the total required yearly drainage for salinity control. 'and soil salinity Cs~,l in electrical conductivity of the If Ci,is expressed in electrical conductivity (ECI,) saturationextract (EC,),and using the approximation thatformedium textured soilsthesaturationpercentage is roughly twice that of field capacity,equations 6 and 7 may be written as: ZIrECG ZP = 2kEC;+(1-k)ECG

Table 11.2 gives some examples of the calculation of the average yearly required drainage quantities according to equation 9. Table 11.2. Examples of the calculations of the average required yearly drainage quantities in dm ~~

~

Example

1

2

3

Soil type

sand

loam

clay

Leaching coefficient (k) 0.8 Evapotranspiration (ET)dm 9 Precipitation (N)d m 3 Tolerated average salt level (EC,) 4 Conductivity irrigation water (EC,,) 2 Required leaching (ZP) 2.5 Required irrigation (Z,,) 8.5 Natural drainage (+)or seepage supply ()O Total drainage (Dt) dm 2-5 Artificial drainage (Or) dm 2.5

0.6 9 3

4 ,

2

3.3 9.3 +2 3.3 1 *3

0.3 9 3 4 2 6.6 12.6 -3 6.6 9.6

(d) The monthly drainage quantities The average yearly requirementis insufficient as a design criterion.Better information on the seasonal variation of soil salinityand the peak drainage demands can be obtained by studying the irrigation-salt-drainage relationship over monthly periods. The quantity of salt stored in the root zone at the end of any month is:

2,=z, +'42 and

z=

&(2,+Z2) where &=dissolved salts in the root zone at the end of the month 2,=dissolved saltsin the root zone at the beginning of the month of salt per month Z=increase (+)or decrease (-) Z=average of dissolved salts in the root zone during the month

(10)

(1 1) me/dm2or g/dm2 me/dm2or g/dm2 me/dm2or g/dm2 me/dm2or g/dm2

337

IRRIGATION, D R A I N A G E A N D SALINITY For a root zone of thickness T/dm saturated up to field capacity (FCby volume) holds:

Z=Z,-t*AZ=T.FC. CG Combining equations 3, 4 and 12 results into:

dZ=

F- BZI ~

A

in which &’=[Ir-(1 -k)P]Cl, B = k .PIT.FC A= 1 +0*5B With equations 10 and 13 the salt content of the soil can be calculated as a function of the quantity and the saltcontent ofthe irrigationwater and the amount ofdeep percolation,P,which followsfrom equation 1. The required drainage quantities are found from equation 2.If the monthly salt and drainage variations ofthe average year are considered,the starting value 2,has to be chosen such that after 12months the same value is re-obtained.This implies that several trials may be needed before a satisfactory result is reached. The application of the monthly analyses of salt,moisture and water balances will be explained by some examples. (e) Examples Example a. (Table 11.3) The field water requirements for an irrigation-drainagescheme have been determined on the basis of local experience.Furtherare known ETand precipitation on a monthly base.The rootdepth is 80cm,field capacity 0.25,leaching coefficient0-5.It is assumed that the soil is at field capacity at the end of each month (AV=O) and that variation in groundwater storage can be neglected (d W=O). What are the discharge quantitiesfor which the drainage system has to be designed and how to investigate whether this drainage is sufficient to control soil salinity if A. no irrigation is applied in December and January B. two watergifts of 60 mm of leaching water are applied in December and January? The calculations are given in Table 11.3. The table is divided into two parts pertaining to the solutions A and B. It is self-explanatory.In the third column the number of the equation from which a factor is computed is given. In case A the maximum drainage need occurs in March,with a surplus of 0.75 d m or 2.5 “/day. With respect to salinity the calculations show that the monthly averages of the saturation extract vary from 3.20 in April to 5.10 in September. The consequencesof an extra leaching of 6 d m in December and January for the drainage and the salinity of the soil are calculated in part B of the table. The data show that maximum drainage is now needed in December with 9 dm,followed by January and March with 7.5 dm. The result of the leaching on the salinity of the soil appears to be of importance.The maximum EC value dropped from 5.10 to 4.60 in December. The maximum drop occurs in April from 3-2without leaching to 2.3 with leaching. From the standpoint of the drainage it follows from the example that it would be more economic if the December leaching had been spread over the two months November and December,In that case a much lower drainage need in December could be expected. Example b. (Table 11.4) This example refers to a case with winter irrigation and summer fallow.It is assumed that,during the fallow, the root zone (80 cm)dries out by the weed vegetation to 50% of the field capacity and that 80 mm water enters the root zone from below by capillary flow (P=-0.8 cm). The irrigation quantities are based on crop evapotranspiration,rainfalland normal losses.Other conditions are equal to those of the previous example. The moisture and salt balances are calculated in Table 11.4.It shows that percolation has its maximum in December,the required maximum drainage,however,occurs in February as the December percolation could be stored in the profile to compensate the upward moisture movement in the previous fallow.The salt balance shows a rather high salt concentration,especially during the initialgrowing stage.It might thereforebe useful 338

o

to

Tr

ootoo

t'c?fcc 0-00

o00 222 I go

O 0 0 0 o t-cor-d d AOAO o w l

o

O

c?

O

O

z! to

x O

x Y

O O O

2 a

c? O

2 O

B

O

"i

to

O

otoo %r-? r: In rO

A 0 0 0 o

o o

O 0 0 0

O 0 0 0

7-l

O 0 0

otoo ln

\09\c?c? \0

Imr4.b

E

-0

h

3

ci

s cd

.B

6

IRRIGATION, D R A I N A G E A N D SALINITY to use extra water gifts of 50 mm for leaching in October and November. The consequences for the drainage and the favourable effect on soil salinity of this leaching are shown in the second part of the table. Example c. (Table 11.5) Howisthesalt-.drainagerelationship influenced by thetypeofsoil,itsfield capacityand theleachingcoefficient? In order to show the effect of the abovementioned factors,the example b has been computed with various values of FC and k.The result is given in Table 11.5. Table 11.4. Example of the monthly salt-water balances for a winter cropped soil with:

T=8dm FC=25% Oct dm

Ir Cr

41

N ET AV P A W

dm

D*

dm

dm

dm dm dm

. _

1.3 0.20 O +Om20 O O

k=0.5 S=O ECe=0.75xC,,

Nov

Dec

Jan

Feb

0.50

1*o0 1.o 0-60 0.60

1.00 0.9 0.40

4-0.30

O 0.60 +0*10

1.50 0.9 0-30 1.20 O 0.60 O

0.50

0.60

1*2 0.30 0.3 f0.50 O O

0.70 +0.70

O

O

O O

csm

1 12-55 O O O 12.55 6.3

0-60 O 1 12.55 O 0.60 0.60 13.15 6-4

0.65 0.18 1a09 13.15 2-37 -1.72 -1.58 11.57 6.2

ECe

4.7

48

4.6

F B A g

BZI

F-BZI AZ

g g 6

2 2

g

Ir

O

0.80

0.63 1.08 0.15 0.15 1.08 1.08 11.57 10.54 1.74 1.58 -1.11 -0.50 -1.03 -0.46 10.54 10.08 5.5 5.2

4.1

3.9

Two additional leachings of 50 mm in October and November dm 0.50 1.o0 1*o0 1.00 1.50

Mar

APr

1.50

1a50 0.8

0.8 0.50 1.50

Fallow

O (0-9)* 0.40 2.20 -1.00

O

0.40 1-50 O 0.40 O

0.50

0.40

1.o0 0.12 1.O6 10.08 1.21 -0.21 -0.20 9.88 5.0

1-04 0.10 1-05 9.88 0.99 5 5 9.93 5-0

3.7

3*7

4.2

1.50

1.50

O

O 050

-0.80

-0.80 O

0.36

-0.20 0.90 9.93 1-99 2-35 2.62 12.55 5.6

~

DI

dm

O

O

~

0-90

0.60

0.60

0.50

0-40

O

3.2

2.8

2.7

2.7

2.8

3-3

~

ECC

3.8

3.7

* C,for the fallow period should be taken as the average over the preceding cultivation season Table 11.5. Effect of FC and K value on salt-drainagerelationship (Ir,Ci, ET,N equal to firstpart of Table 11.4)

Nov

Dec

Jan

Feb

Mar

APr

Fallow

k= 0.5 O 4.8

O

0.50

41

0.60 3.9

0.50

4.6

3-7

0.40 3.7

O 4.2

O 5.8

O 5.4

0.50

0.60 3.6

0.50 3-4

0.40 3.4

O 4.6

O O 0.50 0.60 0.50 EC. mmhos/cm 9.6 9.8 9.6 9.2 8.9 8.8 as C but two additional irrigationsof 50 mm in October and November 0-60 0.50 0.90 0.60 O D, dm O 6.2 6.2 7.1 6.7 6-3 EC, mmhos/cm 7-1

0.40 8.7

O 9.2

0-40 6-2

O 6.6

Oct

A B

C D

340

T=8dm

FC=25%

DI dm O ECC mmhos/cm4*7 as A but FC=12.5% D, dm O EC, mmhos/cm5.6 as A but k ~ 0 - 2 D, d m O

4.3

DRAINAGE SYSTEMS AND M A N A G E M E N T

Part A is taken from the previous table.It refers to a root zone T= 8 dm,field capacity of0-24and a leaching coefficient k=0-5. Part B is related to similar conditions,but with a field capacity of 0-125only. The average EC; value does not change much, but the seasonal variation increases considerably.If the maximum values cannot be tolerated,additional leachings and additional drainage is needed. Part C concerns a heavy bad structured soil (basin clay,tirs) with a low leaching efficiency (k=0.2).With equal irrigation and drainage quantities as case A the salt level of the zone remains rather high. Increase of the leaching with two gifts of 50 mm in October and in November gives some improvement, but increases the drainage need considerably (partD).

(f) Additional remarks 1. The leaching coefficientk determines the efficiency ofthe leaching operation and thus the required drainage for salinity control.It is an empirical quantity smaller than 1,mainly determined by soil structure,i.e. the pore size distribution and the presence ofcracks.It is thereforecorrelated to soil texture and soilpermeability. As a first approximation the following values may be applied (BOUMANS,1963). Better values for a given type of soil may be obtained by field or laboratory tests on undisturbed soil. Moreover the coefficient varies with the soil moisture content and in saturated soils,with the flow velocity which depends,in particular, on the hydraulic gradient. 2. The calculationtechnique given above for salt and water balances presumes that allsaltsremainin solution. This is only true for such soluble saltsasallchlorides,allsodiumandpotassium saltsand magnesium sulphates, but not for such soluble salts as magnesium and calcium carbonate and calcium sulphate. If these salts constitute an importantpart ofthe totalsaltsin the irrigationwater,it is advisableto carry out the calculations for solublesaltsonly and to add to the finalresult a correctionfor the poorly soluble salts,taking into account their maximum solubility for which the following approximate values can be used:

EC 0.8 EC 4.0 3. In certain regions very saline groundwater may occur in shallow layers and it is known that this situation may lead to heavy salinisation of the surface layers ofthe soil by upward capillary movement in dry periods. In the previous treatment this aspect has not been accounted for as under reasonable drainability of the soil and after installation of a drainage system percolating irrigation water will refresh the groundwater until its upper layer has about the same concentration as the percolating water. A n example of the change in salt content ofthe percolated and drained water isgivenfrom along-termexperimentintheUSSR (Table 11.6). MgCO,+ CaCO, CaSO,

10 me/l 50 me/l

Table 11.6. Drainage water salinity from data of the Soil Improvement Station at Zolotaya Orda

Content g/1

1929 1930 1931 1932 1933 1934 1935 1936 1937 1938 1939 1940 1941

Content g/1 Year

Year

of solid residue

of chlorine

14.70 12.39 10.06 10.34 10.78 10.11 8.72 8.80 8.01 7.69 7.72 7.22 7-21

4.65 3.78 2.77 2.77 2.93 2-57 2.25 2.24 1.92 1-86 1.60 1 -45 1 -75

of solid residue ofchlorine 1942 1943 1944 1945 1946 1947 1948 1949 1950 1951 1952 1953

7-45 7.91 6.80 6.09 6.38 6.07 6.78 6.20 6.07 6.00 6.10 6.30

1.66 1 -70 1.55 1.35 1.38 1 e45 1.32 1.30 1-65 1e 4 0 1-30 1-31

Only in case the soil profile prevents sufficient leaching of the subsoil or if seepage of salty groundwater from elsewhere occurs,the higher salt concentration ofthe groundwater has to be taken into account for the calculation of future drainage requirements. An example thereof has been given in Chapter 2 of this book.

341

IRRIGATION, D R A I N A G E A N D SALINITY

4.The previous paragraphs give a quantitative method to relate irrigation-percolation,drainage and salinity, taking into account the type of soil. It should be realised that the analysis of the salt and moisture relations does not give a direct answer for the required drainage. The drainage requirements must be approached as a compromise between tolerated salt level, available irrigation water and technical and economical factors. Minimum drainage can be related to the minimum percolation losses, being, however, extremely variable; maximum drainage is theoretically limited by the infiltration rate. If additional leaching and drainage is required above the minimum this will depend on the salt level which can be tolerated in relation to the proposed crop rotation,and on the availability of leaching water. Normally it will be necessary to work out different comparative solutions before a decision can be taken. 5. It should be kept in mind that the calculations are approximationsand that the actualprocesses which take place in the soil are more complicated. Field experiments in Iraq,however, have shown that the results of these calculationscorrespond fairly well to the actual situation. It further remains possible to improve the calculations by introducing more variables such as varying depth of the root zone in accordance with the growing cycle of the crop and different values for the leaching coefficient in the winter and summer season.

4. Natural hydrological conditions (a) Introduction The generalhydrologicalcharacteristicsofthe differentformsoflandscapesinrelation to drainage and salinity have been treated in Chapter 6.The drainage designer,however,needs quantitativeinformationon the natural drainage which must be accounted for in his project and on the underground water supply which he has to include in his calculations.H e furtherneedsdata on the transmissibility of shallow and deep soil strata, on infiltrationrates,etc. These data should preferably be supplied by specialistservices having at their disposal qualified experts such as soil scientists, geologists and hydrologists and having the necessary investigation equipment.A discussion oftheir working methods is beyond the scope ofthis handbook.Often,however,the specialistsare not available. In that case the drainage designer,neither disposing of the specialist knowledge nor ofthe equipment,has to obtain the required data in some other way. In this respect it is important to know that simple field surveys and a study of existing data may already supply important and useful information. Some general principles and techniques will be discussed below,while further reference is made to other relevant chapters of this sourcebook.

(b) Landforms, erosion patterns and hydrography General topography and erosion patterns may be studied from realphotographs,topographicmaps and field surveys. Absence of natural drainage courses in regions with a precipitation excess during part of the years indicates free drainage and sufficient infiltration capacity. A rough water balance (THORNTHWAITE) can already give some quantitative information on both. A relatively intensive erosion pattern in sloping lands functioning only temporarily,indicates important surface run-offand low infiltrationrates. In relatively low lying areas surrounded by higher soils,such as basins and valley bottoms, seepage supply is quite common. Ifthe surface layers of the soil consist of rather impermeable material, this will result in artesian conditions.Ifthe soil is permeable,seepagemay cause high water tables or even marshy conditions at the lowest spots, on the slopes and in general where the land surface lies hollow. Analyses of river and drain discharges,if available,especially with regard to their dry season and minimum flow,may lead to interesting conclusions about their effectiveness as groundwater drains. Sometimes the dry season discharge varies sufficiently to establish the relation between drainage and groundwater depth (Fig. 11.1). Such graphs are very useful for the prediction of the development of the drainage conditions as the result of a rise or a fall of the groundwater table, (c)

Vegetation,soil type and salt

A relative by intensive natural vegetation usually indicates either seepage or concentration of surface water, the latter case being distinguished by the erosion pattern.In case of seepage a rough water balance,with an estimation for the evapotranspiration,may give quantitative information.

342

D R A I N A G E SYSTEMS A N D M A N A G E M E N T Discharge ("/sei)

200

160 120 80 Average groundwater depth (cm)

Fig. 11.1. Relation between the average depth ofgroundwater and the discharge ofthe main drainage canals for the Beni Amir project in Morocco If in an area, which is uniform with respect to its slope, vegetation is scarce as compared with the surrounding area it points to very permeable or impermeable or shallow soils. Soil profiles may give useful information on the hydrological conditions. Reduction phenomena as grey, blue and black colours may, as a rule,be related to seepage conditions.Red,brown and yellow colours on the other hand mean sufficient drainage under actual conditions. Saline soilscan be identifiedby their typicalvegetation,by saltefflorescences,by taste and by simpleanalysis of electrical conductivity of an aequous extract. They often indicate seepage supply and shallow groundwater depths. Salinisation may also be the result of evapotranspiration of surface water collected in depressions with insufficient drainage. Salt accumulation originating from surface water being more superficial and rich in sodium chloride can be distinguished from groundwatersalinisation.Efflorescenceofsodium sulphate,accumulationofhygroscopic calcium and M g chlorides (dark patches) and the spongy structure of the so-calledpuffed solonchaks (BURINGH,1960) seem to be related to underground water supply,be it sometimes of very local origin. (d) Groundwater table The position of the groundwater table is ofutmost importanceas a source ofinformationwith respect to the hydrology ofan area. Itmay be studied with respect to its depths below soil surface,its elevation above some reference level and its change with time. Groundwater observations are carried out in existing wells, in specially constructed wells or in bore holes. Observations should preferably be done at regular intervals and cover one or more years. A well chosen survey at the end of the wet season,however, is already of great value. The best procedure is perhaps the regular observation of a limited number ofpermanent wells in combination with one or two general surveys. Observations of water table depths during a longer period can be represented graphically in the so-called groundwater hydrographs (Fig. 11.2). Importantfor the interpretation is the depth where,under existing conditions ofvegetation,capillary water transport to the surface becomes negligible. This depth, further called 'critical depth', depends on soil vegetation and can be taken as equal to the groundwater depth at the end ofthe dry season,ifno seepage or free drainage intervenes (see Chapter 3). It is in the order or 1.2to 1.5 m for the coarse and fine textured soils (sand and clay) and 1.5to 2-0m for the medium textured soils (loamy sand,loam,silt loam). The following cases are distinguished: (a) Ifgroundwater remainspermanently below the critical depth in regionswith at least a seasonal precipitation surplus (deep percolation), free drainage equal to the amount of deep percolation must occur (b) If groundwater comes above the critical depth in regions without a precipitation surplus,subsoil water supply is the cause (c) If,in case of a seasonalprecipitation surplus,the groundwater table is temporarily or permanently above the critical depth, the situation may be further analysed according to the maximum depth occurring at the end of the dry season as follows (Fig. 11.2): 343

IRRIGATION, D R A I N A G E A N D SALINITY maximum depth less than critical depth -- seepage maximum depth more than critical depth = drainage maximum depth equal to critical depth = neutral The groundwater hydrograph also reflects a number of important hydrological features of the region. Areas with seepage generally give only very small fluctuations whereas regions with good drainage give fluctuationswhich follow closely the rainfall pattern (Fig. 11.2). cm groundwater depth 0.

50 100 ’

A 96 Neutral >

4

150

A 75 drainage 100

-f

150 200 250

.

A103 seepage

150

200

Fig. 11.2. Groundwater hydrographs reflectingdrainage and seepage conditions.Critical depth is 150 cm

A quantitative rough analysis ofregional groundwater depth datawith respect to natural drainageor subsoil water supply conditions can be carried out with the aid of the groundwater depth-drainage-seepage curve. Such a curve (see Fig. 11.3) can be empirically approximated if the following are known: -the precipitation excess (12P)in mm per year of the high natural drained soils of the area under study (to construct point C) -the storage coefficient(fvolume) of the soil for the zone where the water table fluctuates(pointC) -the critical groundwater depth (to construct point B) -the evaporation for a free water surface for end-of-dry-seasonconditions (to construct point A) mmjmonthly

I

L O

i 400 cm 200 300 Groundwater depth below surface end dry season

d

1 O0

Fig. 11.3. Natural drainage and seepage conditions in relation to dry season groundwater depth

344

D R A I N A G E SYSTEMS A N D M A N A G E M E N T

Ifthe water tables are not too shallow so that over a longer period capillary rise and uptake ofwater by the crops from the groundwatermay be neglected and feedingby irrigation losses or rainfallare small,an estimate of natural drainage can be obtained.A n example is given in Fig. 11.4.The fall of the water table multiplied by the storage coefficient gives the total discharge. When divided by the amount of days the drainage rate is obtained. Groundwaterdepth (m)

Well 37

91-

110 days

11

13

1963

Fig. 11.4.The fall of the water table during periods with negligible deep percolation enables an estimate of natural drainage The depths ofgroundwater observed for a certainday or the mean value for a certain period may be marked on a map (Fig. 11.5). Ifthe level of the soil surface (or an elevation map) is known,a more accurate map of the water table depth often can be obtained by using groundwater contour maps (Fig. 11.5(b)) as an intermediate stage of the final map indicating the depth (Fig. 11.5(c)). Such a map gives a complete review of the existing situation. In general deep water tables indicate drainage,shallow water tables point towards seepage. The same holds for convex and concave shaped water surfaces which can be obtained easily by drawing the shape of the water height in rows of observations.

Contour m a p with observed depth of water-table in wells

Groundwater contours derived from(a) '

Groundwater depth map composed from (a)and (b)

Fig. 11.5. Example ofthe construction of a groundwater depth map (c), from a contour map for the groundwater (b) and an elevation map (a) Since the flowofgroundwater is perpendicular to the contour lines a groundwater contour map allows the drawing of flow lines, indicating the direction of subsoil water movement. In a homogeneous profile, the groundwater fed by deep percolation, shows a gradually increasing slope in the direction of the drainage course.This explains why in a sloping river valley with underground flow to a deep river cut,the soils in need of artificial drainage after irrigation development may be bordered by two zones sufficiently naturally drained (Fig. 11.6). 345

IRRIGATION, D R A I N A G E A N D SALINITY

Fig. 11.6. Drainage conditions in an irrigation sloping strip intersected by a deep river bed Assuming horizontal water movement the groundwater flow is proportional to the slope ofthe water table along the flowlines.For a situation as given in Fig. 11.7 we have:

Qi=KHi @ai

and in case the thickness of the aquifer may be assu

Qz=KH, @a2

d constant (Fig. 11.8):

Q l=K H tgal

Se=K H

tgaz

-

where h =H2 Hl These basic formulae may be applied to actual situationsin order to determine either K or KH when Q is known or to find Q from known KH. They are also used to predict water tabledevelopmentafter irrigation (increased P-values). They often lead to very satisfactory results for the determination of the amount of groundwater ffow. b

a Percolation

Q,

2 2

Fig. 11.7. Flow in a phreatic aquifer of changing thickness

Fig. 11.8. Flow in aquifer of constant thickness

5. Prediction of necessity and optimal time for installation (a) Classificationof drainage need Many examples of irrigation projects are known where insufficient attention has been given to the drainage

346

D R A I N A G E SYSTEMS A N D M A N A G E M E N T

aspect.Although in most ofthese cases adequateprovisions for surface run-offhave been made, the need for deep drainage was overlooked.Often this is due to the deep groundwater level which existed before irrigation started. After some years,however, the groundwater table rose to a dangerous level, causing waterlogging and salinisation of the soil. If in the original plan no space has been reserved for the rather large drains which in fact are needed, correctionsafterwards generally involve great difficulties. To avoid such difficulties,drainage should be given already full attention in the earliest possible stage of design. Sometimesthe need for future drainage is evident. In other cases a reasonable estimate for it is possible. In a rather large number of cases,however,the forecast of future drainage need is very complicated.In such cases it is advisable to follow the development of the water table depth so that one can modify the planning as early as possible. Basically the estimation of drainage need consists of comparing the supply of groundwater originating from leaching, irrigation losses, seepage and rainfall with the capacity of the natural drainage. Then three classes can be distinguished,namely: 1. Areas in which natural drainage characteristics are highly favourable and experience in similar irrigated areas has shown that little or no artificial drainage will be required 2. Areas in which natural drainage characteristics are generally favourable but, because of various specific deficiencies ofsome characteristics,experiencein similarirrigated areas has shownthe need for some artificial drainage in combination with natural drainage 3. Areas in which natural drainage characteristics are unfavourable and experience in similar irrigated areas has shown that extensive artificial drainage will be required

Areas in category 1 can be characterised as having high topographic position with reference to natural outlets; good agricultural surface sojl textures; thick, highly permeable substrata;and reasonably narrow width ofirrigated land.River terraces and alluvial fans often have these characteristics.Areas in this category seldom need artificial drainage on the irrigated lands themselves;however, interceptor drains may be needed at the base of the escarpment below the irrigated lands to protect lower lying agricultural lands, properties or developments. Areas in category 2can be characterised as having more rolling uplands with good agricultural surface soil textures, reasonably thick and permeable substrata and numerous natural drainage channels. Drainage problems which develop in areas of this category are usually caused by deficiencies in one or more of the specific drainage characteristics. The artificial drainage requirements of this type of area usually consist of outlet and collector drains located in the main topographiclows or natural drainage channels and a random system of interceptor drains. Spaced,relief-typedrains may also be needed on long, contiguous,irrigated slopes. Due to the widely differing conditions of such areas, mathematical determinations of their drainage requirements offer many difficulties. For this reason,the diagnosis of the necessity for subsurface drainage after irrigation must, in general, be based on judgement and experience gained from other irrigated areas having similar characteristics. The determination and proof of similarity often, however, requires that sufficient investigations be made of both areas. Areas in category 3 can be characterised by relatively flat topography;few natural outlets; either slowly permeable or highly permeable substrata;either deep or shallow slowly permeable barriers which restrictthe movement of groundwater;and relatively large, contiguous areas of irrigated lands.Areas of this category usually have little natural drainage and, consequently, most of the drainage must be provided artificially. Deep water tables before irrigation in areas ofthis category, and to some extent in areas in category 2,do not warrant a conclusion ofgood natural drainage and the requirementforonlyminor equipmentforartificial drainage. Many areas with water tables at depths of 30 metres and more before irrigation have developed widespread subsurface drainage problems within a relatively short time because of inadequate natural outlet characteristics.Theoretical,mathematical tools or formulae have been developed and their validity proven, for use in estimating subsurface drainage requirementsfor areas of this category. For all cases it is advisable to incorporatein the planning a possible future drainage need. For larger areas a drainage map can be constructed based on the above classes of regions. For practical purposes a further division ofthe classesaccording to the amount of drainage water,time ofinstallation and drain spacings must be made. z

347

IRRIGATION, DRAINAGE A N D SALINITY (b) Time of imtallation When a future drainage need is established, the time of installation of the drainage system becomes an important factor because it will determine for a large part the efficiency of the planning in connection with the required investments and the rentability of the works. Future drainage need occurs for instancein all areas with deep water tables where the groundwater storage is higher than the natural discharge. If the initial depth of the groundwater is for instance 50 metres and the drainable pore space (effective porosity) of the soil is 0.05(5 %),an amount of 2500 mm of water can be stored in the profile,When the excess percolation amounts to 250 mm per year,it will last 10 years before drainage is required.In fact the period will be longer since natural drainage has been neglected.In addition to this the natural drainage will increasewith a rising water table. Suppose that the natural draining amounts when the water at the beginning to 50 mm per year and gradually increases to a maximum of 150 "/year table is at the surface.Then the mean value is 100 "/year and in this case it will last 2500 :(250-100) = 16-6 years before drainage is needed. Several refinementscan be added to the above calculation technique. If for instance a non-steady-state theory is applied to the above example, one would have found 17.3 years. This is done in Fig. 11.9.The storage coefficient(effective porosity) is taken as 0.05for the whole profile. The left hand part of the figure gives the relation between natural drainage Dn and the depth of the water table. In the right hand part the height H of the water table is computed from:

Hc

P -Dn(t= O)

=-e

-WfP+ 1

Considering all kinds of inaccuraciesin the transmissibility and the drainable pore space,in homogeneities in the soil,etc.,the difference is negligible. In actual cases, therefore,simplifications are often justfied.

I

Present water

Supply of percolation

table

water,P,m/year

Fig. 11.9. Schematicdiagram for the development of the groundwater under continuous constant irrigation

(c) Examples of calculations of drainage need

As the first example an area between two drainage courses is considered represented in Fig. 11.10. In the present situation the drainage system maintains a mean water table at a depth of 15 to 20 metres below surface which is, as may be seen from the figure, an average 25 metres above the water level in the drainage courses.The soil profile consists oflayers of loam and clay,but a good permeable layer exists below the water table. The natural discharge,which may be computed by one of the methods discussed earlier, amounts to 100 "/year. Suppose that the future additional discharge due to irrigation as computed from water losses and Then the future total discharge will be 250f 100=350 "/year. In order to salt balance is 250 "/year. evacuate such a discharge the hydraulic gradient in the area must increase with a factor 3.5. This would

348

D R A I N A G E SYSTEMS A N D M A N A G E M E N T y---\

I-

River

Fig. 11.10. Schematicdiagram ofthe relation between drainage and groundwater depth A =present situation,natural drainage 100 "/year €3 =water table near soil surface,natural drainage 160 "/year C=required head for a natural drainage of 350 "/year imply that the water table mid-waybetween the rivers has to rise to a height of 3.5 .25= 87.5 metres. Hence artificial drainage is in each case necessary,although natural drainage situations look quite favourable. If the water table rises, however, until 2.5 metres below surface,the natural drainage increases to about 40/25.100= 160 "/year. For additional drainage there is left then 350-160 = 190 mm,as an average. For the computation ofthe time after which the water table will reach the soil surface,a mean discharge of &(1CO+ 160)= 130 "/year can be taken into account. With an effective porosity of the soil of 0.1 (10%) this time will be (15000 -0.1):(350- 130)=6*8years. Therefore the construction of the drainage system can wait until some years after irrigation starts. When the future irrigation covers only part of the strip between the rivers, matters become much more complicated. Such a situation is given in Fig. 11.11 in which also a calculation scheme is given. First the required groundwater discharge after irrigation is determined.Next an increase of the hydraulic gradient is assumed proportional to the increase of the discharge. From this the required hydraulic head follows. The latter is used to compute the finalheight of the water table. Irrigation project

+

width zone (m)

BEFORE IRRIGATION

- -I- 30 O00

0-10 O00

free drainage 0.1 discharge to river 4500 41 0 average per zone k4250- -33500 elevation ground w. hydraulic head

AFTER IRRIGATION

f

-

0.3 Tequired total drainage discharge to river 6 5 1 6( 'O aierage per zone 6250. 4500 increase discharge ratio 1.47~ hydraulic head *22* +13 predicted elev. gr. w.t.Q :

3000

0.1 m/year per m length m3/year per m length m 3 /year per m length m m

-

0.1 m/year per m length m3/year per m length m3/.year per m length

-

m

m

Fig. 11.11. Computation of the development of the groundwater for an irrigated strip In the next problem an example of a region in which the irrigated area increases step by step is given. The computations are compiled in Fig. 11.12 in which the calculation scheme is incorporated. From the geohydrological profile an estimation of the storage and the permeability are made. Two different water-bearing strata have been assumed,namely the limestone and the gravel layer. On the basis of this assumption the

349

IRRIGATION, D R A I N A G E A N D SALINITY

Fig. 11.12. Development of groundwater. Computation of the development of the groundwater for an irrigation project with construction period relationbetween depth ofwater table and discharge is drawn in the lefthand part ofthe figure.The right hand part gives the finalcomputation in which an increasingpercolation P is accounted for on the basis ofthe development of the irrigation project. The time for filling the whole aquifer up to 2 metres below surface then becomes 27 years. In order to show what a simplified computation can give, see the following example: -total storagecapacityto a depth of2 metres below surface 20.0~1+10.0~02+5.0~2+13.0~15=5~15metres -average natural drainage +(Oe07 =0.37)+0-22metres per year -percolation supply over t years 2.25+(t 10) .Oe45metres -natural drainage over t years 0.22 t -excess supply (=percolation supply minus natural drainage) over t years 0-23t-2.25 metres -time required for filling (total storage to excess supply) 0.23 t 2-25= 5.15 or t = 32 years -as compared with a more accurate computation the difference is only 5 years or about 20 %

-

-

A classicalexample ofthe problems discussed here is the SaltValley in Arizona.Beforeirrigationstartedthe water table depth was 15 metres. Due to irrigation a rise of 0.6 m per year has been observed and after some 20 years drainage became a necessity (ISRAELSEN et al., 1950). Another example,clearly demonstrating the importance of close inspection of the groundwater develop1957). The project was started in 1938. ment, can be given for the Beni-Amirproject in Morocco (BOTELLI, In 1940 the irrigated area amounted to 3000 ha, in 194.8to 18000 ha and in 1963 to 23000 ha. Since the initial depth of the water table was 15 to 30 metres below surface,rainfall was low (about 350 "/year) and the natural drainage towards the deep river O u m Er Rbia was good,no artificial drainage had been planned.From regular observations ofthe water table a rapid rise was observed,mainly due to seepage from irrigation canals.The situation for part of the region in August 1947 is given in Fig. 11.13(a). At that time drainage works started,Fig, 11.13(b) gives the situation in October 1952.The situation gradually improved with progress of drainage construction as may be seen from Fig. 11.13(c) and Fig,11.13Cd). B.

GROUNDWATER FLOW A N D DRAINAGE PRINCIPLES

1. Drain function and types Surface drains as well as groundwater drains have the dual function of collecting water and transporting water. They are considered as part of the irrigation method and will not be considered further

350

D R A I N A G E SYSTEMS A N D M A N A G E M E N T a

August 1941

b

October 1952

d

November 1960

----

Canal drain

-

O

1

2km

Fig. 11.13. Development of the groundwater in the Beni-Amirirrigation scheme in Morocco here. To collect water the groundwater drains must be laid out properly with regard to depth and spacing in connection with soilconditionsand amountofwater to be removed.The collected water must be transported to the outlet. For this purpose canals and pipes must be of adequate size. Extensive treatments of flow in open channels are given in relevant textbooks (cf. VEN TE CHOW, 1959) and engineering handbooks (see SCHEWIOR PI r > 7cm S>&H Y G $Yo 20>H0.2 H For practical cases it must be noted that it is advisable to measure in bore holes of different depths when layers with different hydraulic conductivity have been observed. and KIRKHAM (1949) is the so-calledpiezometer method. A tube is Another method proposed by LUTI-IIN driven into the soil below the water table with or without a cavity at the end of the tube.The soil is augered out of the tube. Then water is pumped out of the tube and the rate of rise is measured. The hydraulic conductivity is computed from the equation: TR2ln (d-YJl(d-Yd (29) K= A (tz -tl) where R=the inside radius of the pipe A = a function dependent on d,R and r

The A-function has been determined by means of an electric analogue and graphs for this function are The piezometer method is very suitablefor determining given in the original paper by LUTHINand KIRKHAM. the hydraulic conductivity of individual soil layers. For a good result the separate layers to be determined must have a minimum thickness of about twice the length of the cavity.

S

Fig. 11.23. The auger hole method Both the auger hole and the piezometer method are conveniently applicable to a depth of 2 to 2-5metres.

If,especially for the purpose of determining possible deep groundwater flow or for the purpose of installing wells, the hydraulic conductivity must be known for larger depth,then a pumping test must be carried out. This means that the constants must be computed from the observed drawdowns around a well.Both this method and the auger hole method can best be explained by means of some examples. (b) Example of calculation A n auger hole test is carried out with an auger of 8 cm diameter (r=4cm). A hole with a depth of 2 metres has been bored. The original depth of the water table was 74 cm below surface. After pumping,the water tablewas at a depth of 105.2cm.N o impermeable layer at a depth above 3 metres was observed.Observations of the rise have been made every 10 seconds.They are given in the table below. Solution: From the above data we have (see Fig. 11.23) : depth of bore hole depth of original water table depth of bore hole below water table 360

D =200 c m W = 7 4 cm H = 126 c m

D R A I N A G E SYSTEMS A N D M A N A G E M E N T

The observed water levels in the bore hole allow the following computations: t (sec)

Yt

AY*

O 10 20 30 40

10542 104.0 102.8 101.7 100-6

1.2 1.2 1.1 1.1 1.1

so

99-6 Ay=

5.6

-

yo = 105.2-74= 3 1.2 AY =yo -y ,, 5.6 y =ya - y n = 5.6 y = y o -&Ay= 31.2-2.8= 28.4 y/At= 5.6/50=0.11 cm/sec

5.6

Filling in these values in formula 27 we find K=0.67 m/day.Nomographs to facilitatecalculations are to be found in VANBEERS(1958).

4. The design criterion (a) Discharge and depth of water table The agro-hydrologicalcriterion for the design of a (groundwater) drainage scheme have to be formulated as:

1. the quantity of groundwater to be evacuated in determined periods (the drainage coefficient Dr) 2. the minimum groundwater depth which can be tolerated in the same period In literature the drainage requirementis sometimes only specified by a depth to which groundwater must be lowered. Since drainage,which indicatesflow of groundwater,can only be defined by the quantity offlow and the corresponding hydraulic head this is impossible. Moreover a fairly deep groundwater table itself is no guarantee against salinisation. 1. Drainage coeficient

The first question to be answered is whether discharge and groundwater table have to be studied over daily or weekly periods, or whether average values over a month or even longer periods can be used. A definite answer cannot be given. For practical reasons monthly periods are the most appropriate and in most cases quite sufficiently accurate. The normal procedure is to start with monthly averages and to check afterwards whether variations over shorter periods have to be accounted for.It is also possible that periods longer than one month have to be used when sufficient data for a sub-divisionare lacking. The drainage quantity Dr follows from the groundwater balance. The percolation P in this balance is determined by the subsoil losses and the leaching required for salinity control, as extensively discussed in Sections A2 and A3. The supply ofgroundwater by seepageSp formsan extra chargeto thedrainagecoefficient.Naturaldrainage Dn,on the other hand, reduces the need for artificial provisions. Seepage and natural drainage have to be studied for futuredevelopment-i.e. under irrigatedand drained conditions.Themethods ofapproachto these quantities are discussed in the Sections A4 and A5.Often only very approximative values can be given. The possibility of storage ofwater in the soil (A W )is favourable for absorbingthepeaksofpercolationand for evening out the drainage demand.The effect of soil storage,however,can only be appreciated in relation to the second design criterion,the permissible groundwater depth. As a general rule one may proceed to determine the value of Dr as follows: based on the chosen crop rotation the minimum (monthly) crop requirement (consumptive use) is established next the corresponding irrigation quantities are determined taking into account expected losses and rainfall after this the salt balance is studied and if necessary some additional water applications for leachings are projected, taking into account the availability of good quality water and avoiding peaks in the drainage need (see Section A3(e)) 361

IRRIGATION, D R A I N A G E A N D SALINITY

(d) finally the seepage and natural drainage are considered and also,albeit provisionally,the possible importance of water storage

It will be evident that the steps (a) and (b) and to a certain extent (c) are the responsibility of the irrigation and agricultural experts and should at leastbe carried out in close cooperation with them. Examples from three experimental stations in the USSR giving average long-term figures for the drainage coefficient are given in Table 11-7. Table 11.7. Mean drainage coeficient over a period of years in 3 stations in the USSR

Drainage coefficient,l/s/ha*for mean drain depth H (m) Month

January February March April

May June July August September October November December Period mean

Zolotaya Orda soil improvement station H=25 m

Fedchenko experimental station H = 1.5 m

Muganskaya experimental station H=35m

0.194 0.173 0.175 0.189 0.166 0.154 0-127 0.093 0.081 0.077 0-083 0.127 0.137

0.055 0.114 0.124 0.222 0.145 0.083 0.136 0.145 0.067 0.013 0.031 0023 0.097

0.152 0,325 0-193 0-565 0.575 0.365 0.405 0,273 0.125 0.100 0.073 0-053 0.265

*0.12 l/s/ha=l "/day;

1 l/s/ha=864"/day

In UAR the drainage coefficient adopted in cotton districts for secondary drains is about 3 "/day 1-1 "/day for main collectors taking into account the rotations.

and

2. MirzimLm groundwater depth In aproperly drained soil the salinity is completelycontrolled by the quantities of irrigation and percoIation water and by the salt content of the irrigation water as described by the salt balance. It has no direct relation to the depth of the groundwater.The influence of the groundwater on the salinityis an indirect one,through its effect on evapotranspirationand capillary rise during periods without irrigation.In analysing the influence of the groundwater,therefore, a distinction should be made between the irrigation season and the season without irrigation. During the irrigation season a net downward water movement occurs, due to inevitable losses and eventually also to the extra amounts of water given to maintain a low salinity level in the root zone. This downward movement exceeds capillary rise; therefore the soil salinity during this period is not influenced by the depth of the groundwater. If,however,groundwater comes too close to the surface,waterlogging occurs and crop growth is restricted by lack of oxygen in the root zone.During the irrigation season the highest groundwater levelsto be tolerated are therefore determined by the required aeration of the soil and further by the possibilities for soil tillage. For annualirrigated crops on loam,sandy loam and silt loam average depths of 60-80cm can be tolerated. For annual crops on clay loam and clay soils this depth is 80-100 cm. Field observations on loam soils in Iraq and Morocco have shownthat cereals,green gram and beans could withstand average depths of60-80 c m and minimum depths of25 cni (shortly after an irrigation)withoutvisible harm.Perennial crops and especially trees need greater groundwater depths. The situation for the non-irrigatedseason is different,since in this period a shallow water table will lead to excessive capillary risc, followed by excessive evaporation (ET-fallow) and thus to salt concentration near the surface.But water tables only remain shallow i€ the groundwater is €ed by underground supply (seepage). Without seepage supply a shallow water table (e.g. resulting from previous irrigations) will drop to the criticaldepth (Section A3(d)), afterwhich capillarymoistnre transportis too SIOW to be ofimportance.Without subsoil supply,the danger of salinisation during the non-irrigatedperiods is limited with adequatedrainage. 362

D R A I N A G E SYSTEMS A N D M A N A G E M E N T

Ifthe groundwater is fed from outside,be it only temporarily,thedepthatwhich groundwateris maintained during the non-irrigated season becomes of paramount importance. Without drainage the seepage supply will give rise to a continuouscapillary flow towards the rootzone,at a rate approximatelyequalto the seepage supply.Only if the water table is kept deep by drainage,the seepage is intercepted by the drains and upward capillary transport of moisture and salt is reduced. The depth of groundwater to be maintained to reduce capillary rise to negligible quantities depends on the soil characteristics and equals by definition the critical groundwater depth earlier discussed. Usually this depth will be in the order of 1.2 to 1.5 metres for coarse sandy soils and for heavy clay soils (clay loam,clay) and for soils of intermediate texture 1.5 to 2.0 metres. Situationswith seepagesupply are quite common,may have differentoriginsand may locally ortemporarily occur in areas which, as a whole, are subject to a certain free drainage (see also below). (b) The drain depth The minimum depth of the drains is determined by the required groundwater depth. Ifthis is 1 metre for the irrigated periods and 1.5 metres for the non-irrigated season, the drain should anyhow be more than 1.5 metres deep. The choice of the drain depth depends,further,on: -permeability of soil profile: drainsshould preferably be cutintothegood permeableIayerto give a maximum effect -relation of drain depth to drain spacing: this follows from the drainage theory. Deep drains can, as a rule,be widely spaced,but are more expensive -construction:this is influenced by soil conditions.In stable soil side slopes can be constructed steep and the relation of excavation to drain depth will be favourable compared with unstable soils which need very flat slopes. Another aspect is the kind of soil material. The unit price for excavation rock and stony soils will naturally be considerably more than for loose soils (c) Seepage Seepage exerts its influence on each of the two drainage criteria: it increases the drainage coefficient and it determines the minimum groundwater depth required during the non-irrigatedseason. The general aspects of seepage in relation to topography, groundwater table and the geohydrological conditions have been discussed in Chapter 6 and in Section A4 of this chapter. Some less known seepage phenomena of importancefor the drainage design are listed below. Canal seepage: these losses have been examined in Chapter 10;they cause high water tables, increase evaporation and salinisation Leakage from neighbouringirrigated fields: often only part of the area is irrigated,which may lead to salinisation of the non-irrigatedpart. This phenomenon is common in summer when often only a part of the land is irrigated Differences in capillary characteristics of adjacent soil types:capillary transport of moisture is usually most pronounced in soils containing large amounts of silt. In sands it is far less because of restricted capillary conductivity. Due to the more pronounced capillary conductivity in silty soils differences in groundwater level will occur during summer,which in turn give rise to groundwater currents towards the silty soils and to salinisation of the latter. This phenomenon may be observed in flat regions with shallow water table. It explains also why river levee soils of loamy texture are sometimes more saline than neighbouring basin clay soils,notwithstanding the higher elevation of the former 5. Special drainage conditions (a) Drainage for reclamation of saline soils

As has been pointed out in the previous sections, the design of a drainage system must be based on the expected amount of percolation due to irrigation losses and required leaching plus seepage on one hand and the minimum allowable depth of the water table on the other hand. All the quantities mentioned should be based on future conditions of regular irrigation. If a saline soil must be reclaimed before irrigation,the quantitiesof leaching water will generally be much higher than those on which a drainage system for normal irrigation circumstancesis based.As a consequence, AI

363

IRRIGATION, DRAINAGE A N D SALINITY

the planned drainage system will not be sufficientto remove these extra leachings.In fact there are,therefore, two periods €orthe operation ofthe drainage,namely that for the reclamation period and that for the normal exploitation.Since the latter,as a rule,will last as long as the irrigation exists,it is evident that the drainage system is primarily based on the latter period. For the reclamation period, additional drainage must then be applied.This drainage must be carried out in such a way that the requirements set by the extra leachings for reclamation purposes are fulfilled (see Chapters 12 and 13). Generally the way to be followed will consist of installing an additional shallow drainage system. This system may consist of open drains which can be filled up after the reclamation period.Another possibility is the installation of a mole drainage system perpendicular to the tile drain lines.N o general rules can be given for the choice of the additional system, because this will depend completely on local circumstances,i.e. the system ofleaching,the amount ofleaching,soil and degree of salinisation of the soil.Therefore each problem must be considered separately. (b) Drainage oj heavy soils Heavy soils are often considered as impermeable or having a very low permeability. Due to this most of these soils are considered as undrainable. Ithas been pointed out already that the maximum drainage rate will always be equal to the infiltrationrate of the soil. This, however,is only true in so far as it concerns deep groundwater flow.In most of the heavy soils,the upper (tilled) layer often has a much higher permeability.In addition to this better permeable layers are often present at a certain depth. In this case drainage may be possible. The flow of the water then takes place vertically to the better permeable layer and horizontally through this layer. For the mathematical basis of the drainage,i.e. the computation of spacing,not only a careful investigationinto infiltration rates and permeability of the various layers is necessary,but rather complicated formulae as those developed by ERNST (1962) must be used. If a good permeable layer is absent,but the permeability of the soil is not too low,a deep drainage system may give a solution.In this case the larger storage capacity of the soil profile can be used to overcome peak discharges.If the soil has an effective porosity of 3-5%, at least 30-50 mm of water can be stored in the upper metre of the profile. If there is a good permeable topsoil, drainage is often possible due to the fact that a large percentage of the flow takes place through this upper layer.Then,however,in the case of a subsurfacedrainage system,the effect of the system will completely depend on the permeability of the backfilled trench. Care should then be taken that the backfill remains as loose as possible. This may be done by filling either with a large amount of filter material or by using topsoil with a better structure as a backfill. In addition to this, the permeability of the topsoil may be increased by ploughing or application of lime. In cases where the good permeable topsoil is absent or very shallow,the only solution is the use of small furrows,combined with a certain levelling ofthe soil so that surface or subsurface run-offispossible.Apossibility in this respect is to install the furrows above the tile lines. Often a system of narrow spaced mole drains perpendicular to the tile lines can be used as a drainage solution for heavy soils.

C.

SELECTION OF DRAINAGE SYSTEMS

1. Open drains versus covered drains

A choice between open or covered drain systems depends on numerous factors and can only be made after a careful study of local conditions. First of all the choice will depend on the irrigation system applied in the region. For conveyance of larger quantities of water generally only open drains will be suitable. Open drains,however, must allow the land user to get to his land easily and to keep fields of suitable size and shape for efficient machine operation. In addition to this,attention should be given to the transport in the whole region.Open drains often require a large amount of bridges and culverts for roads and the accessibility of the fields.Therefore additionalcosts for these structures must be taken into account. 364

D R A I N A G E SYSTEMS A N D M A N A G E M E N T

In Fig. 11.24a cross-sectionofan open drain is given.The depth ofthe channelmust be adequatetoprovide outlets of subsurface drains where necessary. Therefore a certain freeboard must be applied. The depth of such a ditch once established,the total top width depends on the bottom width and side slopes which are dependent on soil type,flowvelocity,vegetation,etc.O n either or both sides ofthe canal a spacefor deposit of spoil must be keptfree.When stability of the soil does not allow the spoil to be deposited close to the ditch a berm must be kept free.Apart from the spoil banks which may, if not used for filling up depressions in the fields,be levelled and farmed with the remainder of the adjoining field,open ditches occupy a considerable strip ofland.An open drain with a depth of 3 metres and side slopes 1 :2for instance and a bottom width of 1 metre occupies a strip of 13 metres which means 1-3ha per km length ofdrain.This loss of land is a serious objection to the use of open drains in areas of high-priced land. Often the losses are not restricted to the area occupied by the drains themselves but losses in yields along the drains must be taken into account too. For a continuoussatisfactory operation it is necessary to control sedimentation in the channel, growth of vegetation as well as channel and bank erosion. Sedimentation will depend on land use, while bank and channel erosion generally are closely connected with the stability of the side slopes. Especially in irrigated areas exessive growth of weeds may be an important factor in maintenance. In contrastto open drains,covered drains cause no loss of land at all. Maintenance is mainly restricted to cleaning once in a few years. Control of the operation, however, is difficult in most cases and in this respectopen drainspromote better practices because improper operation can be seen immediately. Apart from the factors mentioned above one of the main factors in the choice of a drainage system is the capacity.Itwill be clear that for larger capacities only open drains have to be considered,sinceclosed drains will have a restricted capacity due to the restricted diameter which can be applied in practice. For smaller capacities for which reasonably priced closed drains are available,the choice is partly determined by installaI F T O PWIDTH -,-BERM7f-SPOIL

ic

BANK-,

+ !

BOTTOM WIDTH

Fig. 11.24. Cross-sectionof an open ditch tion and construction costs, the available material and the capacities and functioning of earth moving equipment or manual labour. Neither open drains nor closed drains will be designed to carry peak discharges because this will not be economically feasible.They will be designed in such a way that they can remove either surface or subsurface flow at a rate which will not cause serious damage to the crops. In periods of high run-off rates a certain storage capacity in the open water may be of importance. 2. Deep drains versus shallow drains

The choice of the type of drainage may depend on whether a deep or a shallow drainage system will be applied. It is evident that losses of land in cases of deep open drains will be smallerthan in cases ofshallow ones.Several other aspectswill determine the choice between a deep and a shallow system providing that both fulfil minimum requirements of drainage operation. In general it may be said that a deeper drainage system will have the advantage of a larger spacing depending on the permeability of the subsoil. The influence of drain depth on drain spacing is clearly given by the drainage formulae. In addition to this there will be the advantage of a larger open water and soil storage which is important during periods of excessive flow. Contrary to the advantages,a deep system generally will have higher installation costs,both with respect to the higher excavation costs and the additional costs for larger pipes in the case of covered drains.

365

IRRIGATION, DRAINAGE A N D SALINITY Sometimesmaintenance may be a limiting factor for the depth,especially in those caseswhere the open drains have such a profile that special equipment must be available for maintenance. Under specifictopographical and hydrological conditions a deep drainage may require an outlet provided with pumping while a shallow system can work with a gravity outlet.In addition this depth may be restricted if the drainage water is to be used for irrigation in the same region or in a lower lying area. In cases where a pumped outlet is used,extra costs of pumping due to larger elevation must be taken into account. 3. Mole drainage

Although for mole drainage a maximum life of 10 to 15 years has been reported from some humid areas, moling must at best be considered as a temporary method of drainage. Generally it functions efficiently for the first few years after installation but it gradually deteriorates. For irrigated areas it has to be pulled in at least every two years. The principal advantage of mole drainage is its low cost, but it is not applicable to all types of soils, Important factors are structural stability of the subsoil and soil moisture content at the time of installation. Although several attempts have been made to classify soils according to their suitability for moling, no general method is yet available.Although the clay content of the soil seems to be one of the most important factors,it certainly is no guarantee of a good quality of moling. In any case moles can never be used in sandy soils. Since they will be installed preferably in the heaviest soil layers, they will hardly ever be in the best permeable layers.In addition to this fact the depth is rather limited and often smaller than that required for salinity control.Mole lines need no maintenance except perhaps at the outletsbecause they can be redrawn again. Due to this ever recurring reinstallation, costs may be as high as for any other drainage system. A new development in mole drainage is the so-calledmole lining.Here the mole itselfis protected by means of a thin tube formed from a strip. Plastics especially give a good opportunity to further development.Then the moles will have a much longer life and lined mole drains can be considered as a certain substitute for a normal drainage system. The application of mole drainage,however, must be seen more as an additional drainage, especially where the spacing of the normal system is too large in comparison with the permeability of the soil.In these cases a mole system perpendicular to the normal system can be applied.

4. Pumped well drains In some areas the water table can be lowered and drainage provided by pumping from wells.This method is most effective in areas underlain by free aquifers where conditions are not complicated by upward seepage from deeper lying artesian aquifers.The initial cost of a drainage well field is usually less than that of open or tile drain systems; but the operation and maintenance expenses are higher, and over a period of years gravity drainage systems are in most cases more economical.In some areas,needs for pumped water,aesthetic considerations,complex groundwater conditions,or problems of groundwater quality control make drainage by wells desirable because of the other benefits entailed despite the overall higher costs. Conditions favourable to successful,economical drainage by pumping from wells are: 1. A n adequate thickness,usually 15 metres or more of relatively homogeneous non-artesianaquifer 2. A transmissivity of about 100 m2/dayor larger 3. A satisfactory specific yield of aquifer 4.Distant recharge and boundary conditions which favour the maintenance of a lower water table by pumping from wells 5. Non-corrosive or at least only slightly aggressive soil, water and biological conditions. An aggressive environment could cause uneconomically short well and pump life 6. Availability of low cost power Where the quality of water pumped is satisfactory, added economic benefits can accrue from using the water for irrigation or other uses. When water is reused for irrigation,elaborate calculations have to be made of the water and salt balance. In the latter the initial salt reserves of soils and of groundwater have to be included. This is necessary because the salt balance may be seriously disturbed as the salts leaching from 366

DRAINAGE SYSTEMS AND MANAGEMENT upper layers will in the end increase the saltconcentration ofthe groundwater.From the calculationsit has to be decided what part ofthe groundwater has to be evacuated from the area in order to maintain a favourable salt balance. Considerableattention (RESHETKINA, 1960,1954,1964a)is given to pumped well drainage inthe reclamation schemes of Uzbekistan (USSR).Where saline groundwater occurs without seepage to the area concerned, and with a non-salinetopsoil,pumping is considered favourable because the water table can easily be kept at about 10 metres depth by pumping only the irrigation water losses. The deep water table prevents any danger of secondary salinisation. The pumped saline groundwater has to be removed from the area. Where the soil is saline throughout the profile due to a certain amount of seepage from higher lands and from the main irrigation canal, mostly horizontal drainage is installed,but experiments are underway to compare drainage with pumped well drains. In areas already under irrigation and insufficiently drained by open channels, high amounts of leaching water are required to prevent secondary salinisation, even when irrigation water is of good quality and groundwater is only slightly saline.In such areas pumping wells have been installed,because improvementof existing drainage channels proved difficult,due to unstable sandy layers. Some examples of well fields in Uzbekistan (USSR)showing a diversity of results in relation to different aquifer characteristics and well constructions are given in Table 11.8.These tests showed a good agreement between the adopted calculation scheme and the actual results. The tests were carried out during periods with negligible evapotranspiration or recharge of groundwater. Other examples ofdrainage by wells are to be found in India and West Pakistan,where secondary salinisation is a seriousproblem, but where the groundwater in deeper layers is of good quality.The greater part of the pumped water is reused for irrigation resulting in a gradual lowering of the water table. The lack of adequate outlets and the intensified use of land were here serious drawbacks for the use of a usual drainage system.Part of the groundwater will,however,have to be evacuated by main drainage channels or pipelines irì order not to create future unfavourable salt conditions. Table 11.8.

Test results of vertical drainage in Uzbekistan

The Golodnaya Steppe Technical parameters

State farm ‘Socialism’ Total area,ha Number of wells Thickness of cover aleurite,m Thickness of waterbearing layer,m Depth of wells,m Length of filter,m Type offilter Diameter of well,mm Diameter of strainer,mm Porosity,in % Results of pumping Yield of wells,l/s Specific yield,l/s/lm Permeability coefficient of aleurite,m/24hours

Town of Yulistan

Ferghana Bukhara arca State farm Kirnov region Kagan region Palchta-Aral

3000 28

300 9

10800 72

700 7

1800 16

20-25

20-30

15-25

9-12

8-8

50-100 65-80 25-40

10-20 40-50 10-17

15-50 50-70 20-36

20-30 35-40 13-18

Slitted-pipe filter with a gravel .fill

Perforated filter

700-900 18-20

500 300 25

100-200 10-15

5-8

300

0.07-0-1 Permeability coefficient of water40-45 bearing layer m/24hours Rate of groundwater table 0.02-0.03 lowering m/24hours Action radius of a well Q = 100 l/sec in metres X=700 m Q= 500 I/sec R=1800 m

60-80 0.03-0.07 27-30 O*01-0~02

Q = 50 l/sec R=OOO m Q = 110 l/sec R=llOO m

Slitted-pipe filter with a gravel fill 900-1000 400 1417 60-80 5-8

0.3 27-30 0.05

Q = 50 l/sec R=650 m Q = 100 l/sec R=800m

10-13 35-40 8-10 Perforated frlter

500 400 20

500 400 22

25-50 3-4

25-40 4-5

0.3-0.5 16-5 0*17-0*18

0.5-0.7 40-50

0.22

Q = 20-25 l/sec Q = 25-30 l/sec R = 3 W O O in R = 400-450m Q= 50-60 l/sec Q = 40-50 I/sec R = 700 m R = 550-500 m 367

IRRIGATION, D R A I N A G E A N D SALINITY In general,installing pumped well drains when the groundwater is too saline to be used for irrigation, will seldom be economicallyjustified as,in addition to the wells,a system of open channels or pipelines will have to be installed to drain the total quantity of pumped water outside the area. Where drainage problems are caused or aggravated by significant upward seepage from an underlying aquifer,sometimes the only physically feasible solutionis pumped wells.In some places,open or pipe drains supplementedby pumped wellsto lowerthe piezometric surfaceofthe artesian aquifersand reduce the volume ofupward seepage have been successful. In some areas,flowing wells in the bottom of open drains have been beneficial,but pumping may be required to obtain sufficient widespread and adequate lowering of the artesian head. A comparison ofcosts ofdifferent types ofdrainage systems has been made under the conditionsprevailing in Uzbekistan (Table 11.9). Table 11.9. Cost comparison of drainage systems

Type of drainage Open horizontal drainage

Closed horizontal drainage Vertical drainage

Specific expenditure in roubles/ha(1 rouble? $1.10) construction operation 570-590 1400-1450 200-300

20.0-27.0 7.5-10.0 30-50

In comparing these figures it should be kept in mind that the closed horizontal drainage consisted of large diameter pipes at a depth of about 3.5 m. For difficult cases the best approach is usually experimental or test plots of adequate size within which wells, horizontal drains or both are installed to determine the most effective and economical procedure for . the problem as a whole. The question of what part of the pumped water can be used for irrigation may be decisive for the selection of the most economic system. 5. Other drainage practices

Drainage systems may involve prevention of overflow. In such cases dikes and levees are needed along the drainage channel. Drainage pumping plants are used where necessary to lift drainage water to a higher level.As pointed out before this may depend on the depth of the drainage system. Where outlets enter lakes,streams or tidal waters pumping may be needed during high water. In order to get rid of seepagewater from irrigation canals trees are sometimes planted along the borders of the canals. The lowering of the water table may be successful due to the high transpiration of the trees. This ‘vegetal’or ‘arboricultural’or so-calledbiological drainage has also been used to drain swampy areas. Eucalyptus is a tree well known for this purpose.

D. DESIGN A N D INSTALLATION

OF DRAINAGE SYSTEMS

1. General

It is importantthat a competent and experienced drainage engineer should plan and direct the surveys needed for the design and installation of drainage systems. The principal structural works and practices include open drainage ditches, closed drains,auxiliary structures and,where required,the pumping from wells,pumping to removeorutiliseexcesswater,leveesor dikesto prevent overflow,tidal gates,and other auxiliary structuralmeasures required by the plan and site conditions. 368

D R A I N A G E SYSTEMS A N D M A N A G E M E N T

The quality and permanence of structuralwork is dependent on the life span planned for the system.Part of the drainage systemsmay need major rehabilitation or reconstruction within this life span as well as annual maintenance.All the componentparts must fit together so as to provide effectivedrainage for the land to be developed. Even though construction may be commenced on the basis of a preliminary plan, additional construction surveys are required to set location,grade and slope stakes and supervise construction. Construction plans (for instance as a basis for a contract) have to be made. They should be clear, complete and specific. Construction plans usually include: (a) Drainage area and project map (small scale) showing location,proposed lay-out of the system, and auxiliary structures together with all features affectingdrainage design or maintenance (towns, roads, irrigation facilities,etc,) (b) Profiles and cross-sectionsof open ditches (c) Plans and specifications for all structuressuch as bridges, culverts,drop structures,etc. (d) Plans (and eventually specifications) for clearing,piling and spreading soil,seeding ditchbanks, etc. (e) Plans for tile systems (f) Specifications for type and quality of tile drains (g) Cost estimates Detailed discussion is limited herein to open and closed drainage, mole drains and pumped well for drainage. Many details on design and installation,however, will have to be left out. Reference is made to the following literature: Land Drainage Techniques and Standards. Reclamation Instruction Series 520, US Department of the

Interior,Bureau of Reclamation,Denver, Colorado (on design problems and procedures). Open Ditchesfor Agricultural Drainage. Chapter 6,Section16,Soil Conservation Service, National Engineering Handbook, US Department of Agriculture, Washington,DC 20250 (on design problems and

procedures). Open Channel Hydraulics. VENTECHOW. McGraw-Hill Publishing Company,New York (including open

channel design in USA and USSR). Mole Drainage. Chapter 12,Section 16,National Engineering Handbook, Soil Conservation Service, US Department of Agriculture,Washington, DC 20250. Tile Systems and Appurtenances.Chapter 5,Section 16,National Engineering Handbook,US Department of Agriculture, Soil Conservation Service, Washington, DC 20250 (including specifications).

2. Adequacy of outlet and quality of drain emuent Drainagesystems should dischargeinto an adequate outlet.Thismay requireextension ofthe drainagesystem or channel improvement below the outlet.A study of the frequency and stage of high water often is needed for proposed outlets in lakes, streams, and tidal waters to determine their adequacy as an outlet and to establish the design gradient. The quality of drainage water and salt content of the stream or body of water and character of disposal area need to be studied.Data should be obtained to determinefeasibilityofuse ofdrainagewater forirrigation by direct application to land or by mixing with water used for irrigation,and to determine the effects on quality ofthe stream or water in which drainage water is discharged.This study may need to be inconsiderable detail with respect to quantity and quality of drainage water, and water where the effluent is discharged. Dilution in periods of maximum,minimum and average discharge of drains and streams used for an outlet need to be studied. In large drainage systems which discharge waters having a high salinity content, it is especially important to plan a satisfactory method of disposal. In some areas, where adequate disposal was not provided in original project plans, subsequent drainage disposal has become difficult and costly, and salty water has affected downstream use ofwater. Thishas been the cause ofmuch trouble and the planning and satisfactory installation of methods of drain water disposal is a prime necessity for a successful drainage system. 369

IRRIGATION, DRAINAGE A N D SALINITY 3. Open ditches (a) CZassijication Open drainage ditches are classified by function as follows: I. Mains and laterals transport water from collection drains to point of outlet, disposal or re-use II. Open drains for subsurface drainage act as collector drains (similar to tile drains) to lower or control the groundwaterlevel and salinity,Drainage field ditches are small open ditches necessary for collection of drainage waters within farm or field boundaries. In general both types are used to effect a general lowering of groundwater over nearly level areas where the source is percolation from irrigation or precipitation and where gradients of both the water table and subsurface strata permit little lateral movement of the groundwater III. Interceptor drains are used to cut off groundwater which is moving downslope from whatever source IV. Diversion ditches divert water from its natural course to provide protection for land or property and may provide for use of water collected. Such ditches are usually reinforced with a dike or levee on one or both sides of the channel V. Floodway ditches transport flood water and contain it within adjoining dikes or levees

In the following we are mainly concerned with the types mentioned under I,II and III. (b) Location The layout ofthe irrigation system and the topography are the main factors determining the location ofmains, lateralsand drain ditches. Inflatland areas all channels and ditchesare usually laid out in a grid system, alternating with the irrigation canals. Straight ditches promote hydraulic efficiency and efficient farming. In cases of sufficient large farms,mains and laterals are often spaced 4-2 k m apart so that each farm is provided with an outlet for farm drains.Where farms are small it may not be practical for a public enterprise such as an irrigationor drainage district to provide all the lateraldrainsto reach each farm.Groups offarmers will then need to construct a lateral as an outlet for their farm ditches. Topography or less stable soils may not make it advisable to construct straight ditches. Low areas, old stream beds, etc., should then be used for location of the drains.Where feasible,smooth curves should be used rather than sharp bends. It is often desirable to establish a minimum radius or curvature based on size of ditch and velocity. If serious erosion is probable, another location using a longer channel on a non-erosivegrade or in a more stable soil may be feasible. Occasionally, using a wider and shallower channel to decrease the hydraulic radius and the velocity is a solution. If these alternatives are not feasible,then grade-controlstructures or bank protection may be needed. O n the other hand it is sometimesconsidered as good practice to locate drains along the maximum possible slope of the terrain. Under favourable conditions erosion of the ditch may lead to a deeper equilibrium depth. A discharge of open drains,following the slope,has been reported by the Fedchenko Experimental Station (USSR)to be 2.5 times greater than for drains with smaller slope.The Central Reclamation Station at Zolotaya Orda (USSR)has found a discharge 1.4times greater in drains following the slope. Other factors important for site selection are existing physical and natural features.Farm boundaries especially should be planned so as to avoid unnecessary bridges and crossings. Interceptor ditches are located near the edges ofhilly or sloping land and sometimesalong irrigationcanals to interceptgroundwaterwhich is moving downslope from whatever source.They often need to be deep to be successful interceptors of seepage.On hilly land their function may also be to divert surfaceflow.Excavation from the channel is then often placed to form a levee or dike on the lower side for added protection. (c)

Design of open charznels

In semi-aridareas,the run-offfrom rainfall may control the capacity ofdrainage ditches.In humid areas this is alwaysthecase.Drainage coefIicientsapplied inarun-off formula may furnish a basis forcomputingcapacity. In arid areas, the flow from subsurface drains,the amount of surface waste and the eventual amount of seepage water controls the required capacities ofdrainage ditches.The rate at which run-offmust be removed to provide a specified degree of drainage, called the drainage coefficient,has been discussed in Section B, part 4,of this chapter.Under conditions of good management,average infiltrationrate ofthe soil,irrigation water of reasonable quality and limited or lacking seepage the drainage coefficient in arid areas will seldom 370

D R A I N A G E SYSTEMS A N D M A N A G E M E N T

be higher than 2 mm per day (about $l/s/ha) as an average over weekly or monthly periods. This quantity is close to the leaching requirement (see the examples given in Sections A3(e) and B4(a)). With the drainage coefficientand the degree of drainage as a starting point, other factors to be considered in designing open drains are: 1. The water surface or hydraulic gradient of laterals and mains should be placed at a depth adequate to provide a free outlet for the subsurface drains. When the latter are pipes, the normal practice is to keep the water surface in the outlet drain about 30-40 c m below the inlet of the pipe, taking into account the discharge from subsurfacedrainsand irrigationwaste.This is commonly called ‘normalffOW’. Where the depth of collector or outlet drain is determined by the depth of subsurface drains, the capacity is usually sufficient to carry flood flows without unduly affecting the efficiency of the subsurface drains during a short duration of the flood flow 2. The need for installation of subsurface drains may not occur for several years after the beginning of irrigation. However, surface drains will be needed immediately and their installation should be made concurrently with the installation of the irrigation facilities.Where it is apparent that deep collector and outlet drains willbe needed within a relatively short time afterthe beginning ofirrigation,it is often more economical to construct them initially to their ultimate depth requirement In the design of ditches a formula is used for flow computation.For a complete treatment the reader is referred to textbooks like VEN TECHOW(1959).

MANNING’S formula for computing velocity is as follows: n

or

Q = A-R2/3S1/2

n where v=average flow velocity in mlsec A =cross-sectionalwet area in m2 n = roughness coeficient R = hydraulic radius in metres (cross-sectionalarea divided by wet perimeter) S= hydraulic gradient in metres/metre

The dimensions of the channel are computed by substituting a certain constant flow velocity v or capacity Q,a roughness coefficient n and a hydraulic gradient S. L o w velocities result in large channel cross-sectionsand increased maintenance because silting of the ditch may occur.High velocities on the other hand may result in serious erosion,particularly at bends.The highest permissible velocities are therefore preferable. FORTIER and SCOBEY(1962) worked out permissible velocities for aged straight channels (Table 11.10). The values for water transporting colloidal silts, column 3,applies to most drainage ditches. The selection of a roughness coefficientis one of the most difficult problems in drainage channel design. The coefficient varies particularly with the vegetation, the size and shape of the cross-section,the bed load and suspended sediments and with the hydraulic radius.Some typical values for non-linedchannels,given by VENTECHOW (1959), are compiled in Table 11.11. In designing drainage ditches the possible future weed growth and the expected degree ofmaintenance has to be taken into account. Immediately after construction, ditch banks are smooth and the value of n is lower than after the channel has aged. As bare soil is less resistant to erosion a check should be made to determine the erodibility ofchannelsimmediately after constructionand to determine whether erosion control practices should be installed. The cross-sectionof a channel is determined by the bottom width, the side slopes and the total depth. The bottom width of trapezoidal ditches should be determined by the design capacity or by the construction equipment requirement,whichever is greater.The minimum bottom width for mains and laterals is generally about 1 metre. The depth of open drains should be determined by the cross-sectionrequired to carry the design flow below the design water surface or based on depth required to provide a free outlet for normal flow from subsurface drains.The side slopes are mostly constructed as steep as possible to conserve land,A study at the site may need to be made to determine the natural angle of rest of the soil and to observe old ditches, so that stable slopes can be determined. 371

IRRIGATION, DRAINAGE A N D SALINITY Table 11.10. Permissible canal velocities

Clear water

Originalmaterial excavated for canal

no detritus

1

Fine sand (non-colloidal) Sandy loam (non-colloidal) Silt loam (non-colloidal) Alluvial silts when non-colloidal Ordinary í%m loam Volcanic ash Fine gravel Stiff clay (very colloidal) Graded,loam to cobbles when non-colloidal Alluvial siltswhen colloidal Graded,silt to cobbles, when colloidal Coarse gravel (non-colloidal) Cobbles and shingles Shales and hardpans

Velocity after ageing of canals carrying: Water transporting Water transporting noncolloidal silts colloidal silts, sands, gravels,or rock fragments

2

3

(mid

(m/s)

4 @/s)

0.45

0.75

0.45

0.50

0-75

0.60

0.60

0.90

0.60

0.60 0-75 0.75 0-75

1.05 1.05 1 *O5 1-50

0.60 0.70 0.60 1.10

1.10

1-50

0.90

1.10

1-50

1.50

1.10

1.50

0.90

1*20

1.65

1 e50

1 a20 1.50 1.80

1.80 1.65 1.80

1.95 1.95 1.50

Table 11.11. Some values of the roughness coeficient n (takenfrom VENTECHOW 1959)

Type of channel

Minimum

Mean

Maximum

0.016 0.022 0.023 0.030 0.050

0.018 0.027 0.025 0.035 0.080

0.020 0.033 0.030 0.040 0.120

Excavated channels

(a) earth straight,clean earth straight,with grass (b) earth winding and sluggish earth with aquatic plants (c) dense weeds

The steepest slopes satisfactory for ordinary conditionsare as follows: Soil

Side slopes

Loam Clay and clay loam Peat,muck,sand and loose soils Field lateralditchesfor silt,clay and muck under 1 m deep

2:l 1:l to 1g:1 1 :1

Q:1

Maintenance requirements may also control selection of side slopes. (d) Auxiliary structures and practices Auxiliary structures and practices are necessary to nearly all open ditch systems and need to be considered in all stagesofproject development.Severalhandbooks are available describing design offootings,foundations, 372

D R A I N A G E SYSTEMS A N D M A N A G E M E N T

soil mechanics and structural design in connection with drainage systems.Here only a few remarks will be made on auxiliary practices. Berms and spoil banks should be shaped so as to:

1. Provide roadways 2. Eliminate the need for moving spoil banks in future operation 3. Prevent surface water entry except at controlled inlets 4. Prevent excavated material from washing or rolling back into ditches 5. Lessen sloughing of ditch banks Good design of berms and spoil banks may effect costs ofright-of-way,spoil disposal in construction,and ditch maintenance.A roadway along one or both sides of an open ditch is usually an essential feature to obtain good maintenance by use of equipment. Erosion control is needed for differentcircumstances: 1. Where surface water enters a ditch or where a lateraljoins a main, pipe drops,chutes,drop spillways or other suitable structures need to be installed. Pipe drops are commonly used to drop waste water and surface run-offfrom the land side of spoilbanks into drainage ditches or from shallow into deeper ditches.Concrete drops are more costly than pipe drops and their use should be limited to conditions where pipe drops are not satisfactory 2. At some sites grade-controldrop spillway structures are required to stabilisethe bottom grade ofdrainage ditches. Concrete structures may be needed 3. Where stream bank protection is needed, use is made of vegetation, riprap or brush mats. For large channels the more costly construction ofjetties,piling or tetrahedrons may be applied

Other design problems, often encountered in design of open ditch systems,are connection with bridges, culverts and other obstructions.These structures cause losses of head in the system. Wherever possible, bridges should be used in open channels having low gradients in preference to culverts that may offer serious resistance to the flow of water especially when they may become clogged with debris.However,culverts are economical near the upper ends of open ditches carrying a small flow. (e) Construction Grade, location and slope stakes will be set for every channel immediately before its construction. Drain ditches are usually constructed from the month upward so that water running into the newly constructed ditch does not hinder the work, but flows freely into the outlet. The bank slopes of ditches with non-cohesivesoils or a high groundwater table may not be stable immediately after initial excavation because of sloughing of saturated banks before the water table recedes to lower levels. Under such conditions it may be desirable first to excavate a pilot channel of lesser depth than the designed section for instance to a depth of 40-60 c m below the existing groundwater table. After lowering the groundwater,the excavation is repeated until the required depth is obtained. For the construction of open channels many types of excavators and earth moving machines are available. Bulldozers are sometimesused to remove the top layer ofthe soilwhen constructinglargechannels.Bulldozers and graders may be used to prepare the projected course and produce the required slope. For digging the power-driven dragline, the clamshell, the shovel and the backhoe excavators are whereas the operational radius recommended.The capacity ofthe bucket varies from 0-15m3 to over 50 m3, may go to over 45 m and the excavating depth to over 30 m. Chain-bucketexcavators are,under circumstances,more productive than the single-buckettype excavator. Where labour is abundant and cheap, as many developing countries,manual labour should be seriously considered,at least for the smaller channels. Often manual labour is preferable from the standpoint of the national economy.

4. Tile drainage (a) Types of tile-drainagesystems

In general a subsurface drain system should be planned and located with respect to other drainage practices, the irrigation system and topography.The plan of many systemsand the location of outlets are controlled by

373

IRRIGATION, DRAINAGE A N D SALINITY the irrigation system and location of main ditches.Land grading or levelling for irrigation and surface water drainage systems need to be planned together with the tile system for subsurface drainage. The most common types of tile drainage are the gridiron-tile system,the herringbone system,the double main system,a random system,and the interception system. The gridiron-tile system consists of parallel laterals located perpendicularly to the main tile. It is used on fiat,regularly shaped fields and on uniform soil.This system is often preferred because it provides uniform and complete drainage throughout a field. The herringbone system consists ofparallel lateralsthat enter the main at an angle usually from both sides. This system is adapted to fields where the main or submain lies in a slight depression. It may also be used where the main is located in the direction of greatest slope and better grades for laterals are obtained by angling the laterals upslope. The double-mainsystem is a modification ofthe gridiron or herringbone systems and is applicablewhere a depression which is frequently a natural watercourse divides the field where tile is to be installed.Placing a main on each side of the depression may serve to drain the waterway and provide an outlet for the laterals. Parallel mains may also be used under some conditions to reduce the size of the main. A random system of drain is used where the topography is undulating or where soils vary and fields contain îsolated wet areas. The interception system intercepts seepage moving down a slope. The interceptor usually should be placed at about the upper boundary of the wet area as determined by drainage investigations. Under some site conditionswhere artesianflow occurs,drainage may be assisted by one or more reliefwells sunk into the strata under artesian pressure. Such wells may be constructed of large tile or of other pipe. Water flows upward through the well and discharges into a drain (Fig. 11.25).

H=Effective head

Fig. 11.25. Relief well drain discharging into tile drain

(b) FZow and tile drain size For computation of the flow in tile drains the MANNING formula is the most used:

where

cl= tile diameter in c m IZ

=roughness coefficient (normally 0.015)

Dr=discharge in "/day A = area in m2 S= grade in metresimetre Mostly graphs or tables are used. An example is given in Annexe C. In most countries a minimum tile size is used.In the U S A this is,for example,10cm.It may be less in cases where expensive material like plastic is used.The problems of maintaining an accurate grade and alignment and the possibility of some settlementencouragethe use ofa minimum tile size to obtain a longer useful life of tile systems even though not required by capacity needs. Local experience will determine the use ofminimum tile size for such specific conditions as peat, muck, non-cohesivesoils,or seep areas. The maximum diameter (sometimes used in the USSR for drains of special quality and durability) is

374

D R A I N A G E SYSTEMS A N D MANAGEMENT considered 50-60 cm.In long lines the pipe diamcter may increasefrom the beginning to the end, due to the increasing discharge in the same direction. (c)

Types and qualities of tile drains

The most common types of pipe for close drains are clay and concrete.Good quality concrete and clay pipe has been proved permanent under most rigorous conditions. Perforated bituminised fibre,metal or plastic pipes are also used. Formerly pole drains, stone drains and woodbox drains have been used. The selection ofthe kind oftile to be used depends on local conditions and economy.Concretetiles may be used when good clay for manufacturing clay tiles is not readily available.In countries where labour costs are high, the easy handling of plastic pipes may be attractive, although the material costs are higher than the costs of clay tiles. Metal pipe is chiefly used for outlets for tile drains,shallow soil cover of drains,road-crossings,and other situations where required strength,economy and reliabilityjustify use. The quality requirements ofpipe are determinedby the load on the tile,the resistance to acids or sulphates in the soil or drainage water,the resistance to frost,sufficient space to permit flow of groundwater into the pipe and other site conditions. In order to use pipe of sufficient strength, determination of loads that will be imposed on conduits is important.The load on a pipe in a trench depends on the unit weight ofthe soil,compressibilityand character of back-fillin the trench,and width and depth of the trench. In soils that are subject to corrosive acids or sulphates in the soil or soil water a special quality of dense concretetile is required,ifthis tile is preferred for other reasons.Where such soilconditionsoccur,representative soil or water samples to determine concentrations of acids or sulphates need to be analysed. A hazard to clay drain tile is freezing and thawing aciion. Even where tiles are below frost depth, as in arid zones,outdoor storage during frost may cause damage to clay tiles, where pipes are wet or in contact with the ground.The absorption of water by tiles is a valuable index ofthe resistance to freezing and thawing. The space between tiles depends on the installation method and on the smoothness of the edges. Some methods ofinstallationmay not provide sufficientspacebetween jointsto permit flow ofgroundwater into the pipe,Spacers or lugs may be set on one end of each joint of pipe so that an opening will remain when joints are butted together. O n the other hand,the edges should be smooth enough to limitthe space between pipes in order to prevent coarse soil or filter material to settle in the drain. To ensure adequate quality, detailed specifications are desirable. The reader is referred to the well-known specifications of the American Society for Testing Materials (ASTM) and other specifications, listed in: Tile systems and appurtenances,Chapter 5, Section 16,National Engineering Handbook,USDA Soil Cons. Service, Washington,nc. (d) Grade, alignment and connections It is common practice to install tile lines with a certain grade. Generally the grade increases when smaller pipes are used. With an internal diameter of 30 cm,lines are mostly laid with a grade of at least 0-05-0.1%. Where conditions permit or in sandy soils greater grades are maintained. With pipes having 10 c m diameter grades are usually 0-15% or more. The grade of tile lines should be uniform throughout their extent. Zero grades should be limited to short upper reaches of the lines.A reverse grade should never be permitted. O n slopes under 1 %it is desirable to lay long, straight laterals and reduce curves to a minimum. This is due to economy in construction and maintenance rather than hydraulic losses,as even sharp curves have little erect on hydraulic losses.T-and Y-shapedconnections and 45 degree bends should be used if readily available. Gaps between tiles on the outside of curves should not be excessive. Occurring gaps may be covered by broken pieces of tile. Laterals to the main tile line may be connected at the most convenient point, usually the midpoint of the tile. (e) Filter materials

The purpose of encasing or covering tile lineswith a filter material is to increase the permeability around the pipe and to prevent fine soil particles from being washed into the pipe. The increase of permeability is particularly important in less permeable soils and with small diameter pipe. In some cases the base soilsin which the tile lines are laid are quite permeable and composed ofwell-graded 375

IRRIGATION, D R A I N A G E A N D SALINITY sand and gravel particles. N o filter material is needed in such cases. They same may hold for soils rich in gypsum. Types of filler materials are:gravel,peat,straw,wood,glass wool or glass fibre.Gravel filter is most commonly used. The filter material should be quite permeable. The gravel material should be well graded and the coarse particles should be sufficient in size and distribution to prevent movement of filter material through pipe openings and at the same time of soil particles through the filter. The thickness of gravel filter surrounding the tile drain varies greatly in project design. In the USA about 8 cm is considered as minimum thickness. In the USSR large diameter pipes are laid on a 20-60 c m gravel layer whereas the width of the gravel layer is at least twice the pipe diameter. On the upper side the joint openings are then covered by tarred paper or plastic film in order to prevent any loose soil moving from the trench into the drains. Lately glass fibre sheets have been tried as a filtering agent. Such material is effective in retaining coarse silt size, sand and larger particles of soil. Questions which need to be answered by use are whether a thin filter sheet is as effective as a thick filter layer,and whether fine particles will stop up such insulation after a few years’use. Experience has shown that the permeability of glass fibre may be seriously reduced when groundwater contains reduced iron components,oxidising when in contact with air. (f) Other practices Manholes are used in tile systems to gain access for inspection,cleaning,and collection of sediment. They are used in some systems at junctions where mains and submainsjoin,where three or more lines join at one point and at intervals of, for example, 100-200 m along the mains. In some systemsthey are not considered necessary. Manholes are constructed in a wide variety of sizes, designs and shapes. They should be large enough to permit cleaning of the line with sewer- or pipe-cleaningequipment and the ready removal ofany sediment collected. Protection against erosion and undermining is needed where a tile line outlets into an open ditch. This is usually accomplished by a length ofcontinuousrigid pipe embedded in the ditchbank. O n tile outlets protection may be needed to exclude small animals. Surface water inlets may be needed in a depressional area or where it is not feasible to construct an open drain to remove surface run-off.A substantial screen or grate should be used to admit water which will exclude trash and debris from the tile system. Collection and removal of sediment may be accomplished by setting the bottom of the inlet structure well below the elevation of the tile line. Unless absolutely necessary,however, surface water should not be admitted into closed drains. (g) Laying tile drains Close inspection is required during installation ofdrainsto control excavation to grade and width oftrenches, the laying of drains and the use of filter material. Reasonable variations from planned grade of the bottom of the trench and the layer of gravel filter should the error for individual tiles should not be greater than about 1-3 cm. be allowed.O n land slopes up to 0.1 %, On steeper grades,greater tolerances are allowed. Tile drainsmust be joined with care so that therequiredgap ismaintainedbetweenpipes.Gravelfiltershould be placed as called for in design. Backfilling of the trench should be completed soon after the filter material is placed. Some levelling of fields may be needed after settlement of backfill. When meeting unstable pockets of soil,a board cradle or stable material like gravel in the bottoin of the trench can be used to support the pipe.

5. Mole drainage The unlined,egg-shaped,or round earthen furrows,formed by the bullet-nosedplug ofthe moling plough are usually from 5 to 10 c m in diameter. As they are mostly used as a drainage system, supplemental to the normal system,design criteria are mostly fixed by experience under local conditions. 376

D R A I N A G E SYSTEMS A N D M A N A G E M E N T

Adequacy and durability of mole drains depend on the soil in which they are drawn,on the adequacy of their outlet,on the length,depth and spacing of the mole lines and on conditions under which they are constructed such as moisture content of the soil,kind of equipment and so on. Where tile drains provide an outlet for mole drains,good entrance conditions may be provided by use of porous backfill material around the tile line. Maximum length of lines draining in one direction should not exceed about 200 m on the steeper grades. Best grades for mole drains are between 1 and 2%.As to depth,best results are usually obtained at depths of 50-60 cm.Uniform drainage requires the moles to be drawn as nearly parallel as possible, mostly at intervals of 2-3 metres. Plastic-linedmole drainage is still in the experimental stage.

6. Wells

The design and installation ofdrainage wells and well fieldsis complex and each project is a separateproblem. Hydrologists should obtain active participation of soil scientists,agronomistsand othersto obtain favourable results. As a starting point the required or permissible lowering of the water table and the volume of water to be pumped out in order to maintain the required average groundwater depth must be known. The aquifer characteristics obtained, mostly with the help of pumping tests, a suitable flow equation and the probable yield ofwells are then involved in estimating well spacing,discharge per well,pumping schedules and so on. (a) Design The well type most commonly used is a pipe screened over a certain length, encased in gravel. Water is pumped from the well by special pumps with a vertical shaft or by pumps submerged in the groundwater. The nature of the aquifer and the required yields will determine the depth and diameter of the well, the length and the depth of the screen and the capacity of the pumps. As conditions are too variable to discuss the design adequately in this chapter,reference is made to the available literature. Only a few remarks will be made here. Usually,the estimated installed pump capacity is 1.5 to 2 times larger than that required to remove the estimated volume of groundwater.This is desirable and necessary in order to establish a gradient to the wells and permit pre-seasonlowering of the water table where pumping will be on a seasonal basis. Also rapid lowering of local water levels may be required when an unanticipated rise occurs because of failure of a pump, rainfall or similar factors. With increase in the area to be drained, the volume of required excess capacity usually declines. The design of the screen depends on the texture of the surrounding soil. In principle part of the fine particles are permitted to enter the well where they are removed by pumping.With very small apertures the filter may be easily stopped up by soilmaterial and crystallisationofsalts.Unsatisfactory design offiltersisone ofthe major causes of poor functioning or failure of wells. The location ofwells depends on site conditions,but in a well field a rectangular network grid is preferable. A well should not increase seepage from a nearby irrigation canal so that mostly locations midway between irrigation canals are selected. As much use as possible shodd be made of the existing or planned road system in selecting locations for wells,in order to facilitate operation and maintenance. Where the water table is deep before the start ofirrigation,a few years are usually required before drainage is needed. Observations of groundwater during this period permit selective placing and timing of the pump drainage system. Channels to dispose of water pumped from the wells should be considered in conjunction with other drains and channels normal to irrigation development.Where no increased seepage losses are to be feared, a simple solutionwhere quality ofwater permits is to pump the well water into a nearby irrigation canalfor reuse.This,of course,influences site selection of the wells. Where such a disposal is not feasible,channels or pipelines of sufficientcapacity have to be designed concurrent with well design. Also outer communications (electricity,access roads and so on) should be designed at the same time. Under some aquifer conditions bottom-fedwells are used. Groundwater enters only through the open bottom of the pipe. They work satisfactorily when a sufficient cone of gravel at the bottom ofthe pipe is present after construction.

377

IRRIGATION, DRAINAGE A N D SALINITY (b) Instullution Actual construction of a well field seldom conforms to the preliminary plan. The first few wells are constructed where prcliminary investigations show the water table to be at the shallowest depth, subject to a more rapid rise,or delayed decline as compared with the area as a whole. As each well is completed,it is step-testedto determineits performance and permit proper pump selection.Measurementsare made of drawdown and discharge in the pumped well and ofwater levelsin adjacent wells and observation holes during the test. Analysis of these data will show whether the capacity of the well being tested is different from that originally estimated and whether a change in spacing or in design of planned adjacent wells is required in the interest of economy and efficiency. Such changes are usually necessary since aquifer conditions are seldom sufficiently uniform to give the balanced pump and well performance estimated during the preliminary planning stage. In making such adjustments,however, consideration should always be given to maintaining to the extent practicable standard pumping units and motors.Ifpossible, all motors and pumps should be of the same size and manufacture in order that parts may be readily interchangeable,operation and maintenance are simplified. Thisfactor may result in pumping some wells to less than full capacity and keeping to a closer spacing, or similar adjustments. As production wells are completed,it may be necessary to install additional shallow observation holes in order that more significant measurements and better control can be obtained in areas where well distribution differs significantly from that developed in the plan. Details on well construction may be found in the references AJXRENS (1957), AM. WATER WORKS ASSOC. (1958), and DUMBLETON (1963).

E.

OPERATION A N D MAINTENANCE

1. Plans and observations

Once the drainage is installed,only the pumping well system has to be operated. Open ditch and tile systems operate automatically unless a pumping plant is included in the system. Operation ofthe wells is based on the position ofthe water table as shown by measurementsin observation holes.When the water table rises over the required depth,pumping of the wells should be started.It must be taken into consideration that time is required to develop adequate gradients to move continually the deep percolation to the wells. Sufficient storage capacity should be available in the aquifer to store the deep percolation prior to and during development ofthe gradient. The rapidity with which an adequategradient to the wells will develop is controlled by the aquifer characteristics,spacing and discharge ofthewellsand rateof recharge. The initial shut-offelevation is also a matter ofjudgement. Weekly measurements of water levels during the first two years of operation will permit refinement of the initial control criteria and determination of the water-levelelevations at which pumps should be stopped and started in various cells ofthe drainage well field.Theresult should be an economical and efficientpumping and observation schedule,consistent with adequate control of water levels within the project area. Besides for operation of wells,regular observations of the groundwater levels should be made in every improved or new irrigation project. Where drainage has not yet been installed, the measurements will demonstrate the increase in level after irrigation,permitting comparison with eventual former expectations, whereas the need for installation of a drainage system can be determined at the proper time. Where drainage has been installed,the observations will show whether the average groundwater table is at the required depth,Serious deviations from this depth can then be corrected by additional drainage. Measurements generally need not be done in a very dense network, but one point per 100-200 ha should be considered a minimum. When the groundwater is close to the required level,weekly observations should be made.It is also desirable to follow the daily fluctuations of the water table in between two irrigations,at least at some points. Such measurements permit the calculation of the average groundwater depth during the irrigation season. After this period of intensive control, the frequency of most measurements may be diminished to twelve times a year or less.

378

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In addition to groundwater level measurements regular determinations of the salt content of soils and groundwater should be made,together with measurement of drainage discharge and salt content of the drain water. A programme of the measurements mentioned should be made and carried out regularly, immediately after full or part completion of the project. Together with data of irrigation water quantity and quality the results enable one to follow the water- and salt-balances.From such studies it can be decided: (a) whether the salt-balanceis developing in the required direction (b) whether the soil salinity remains under the required limits (c) whether leaching is satisfactory,or has to be intensified or could be limited The planning ofleaching is another necessity from the start.The leaching schedule has to take into account the availability of sufficient water,the crop rotation,and the salinity conditions of soil and water. Regular and level fields,promoting good irrigation practices, are also required for efficient leaching. Planning for maintenance begins in the early stages of project design. The design of drainage works, particularly of open ditches and tile drains,may be controlled by maintenance methods later to be used. Open ditches rapidly lose their effectiveness unless they are properly maintained. A good maintenance programme is just as necessary as proper design and construction.Drainage systems often become clogged with uncontrolled growth of vegetation and partially fill with sediment soon after installation. Some systems may need renovation of ditches to original grade within one to three years after construction to remove slides,sediment bars, and unusual sediment accumulation. Some practices may be classed as preventive maintenance and should be installed where economically feasible.Sediment and debris basins on streams tributary to open ditch systems,channel diversions,channel protection on tributary streams,pipe drops and other structural protection work come into this category. Knowledge ofpast maintenance efforts,or the lack ofthem,on similar areas helps in developing a maintenance plan. If certain practices are accepted and carried out regularly by farmers or farmers’organisations, these practices should be considered in developing the maintenance programme. 2. Organisation

As for the operation and maintenance of an irrigation project,some people should be made responsible for the execution of the observation and maintenance programme, and for the operation and maintenance of wells. Where an irrigation project is to be operated by a corporate body ofprivate farm enterprises,the responsibility for the drainge system will generally be with the irrigation district.Maintenance ofmains and laterals is carried out by the drainage maintenance section,in a small district often combined with the irrigation maintenance section. Farmers and eventually other citizensbenefiting from good drainage will be charged for maintenance costs according to their benefits. As the maintenance of farm drainage and leaching operations are the responsibility of individual farmers,the latter shouId at least be regularly advised on these subjects. The organisationwill be comparable,when the Government is responsible for the operation oftheirrigation and drainageproject. A section for maintenance ofthe drainage system is a requirement from the start of the project. Where the ownership of land is the collective type and the Government carries out the reclamation and improvement of land,as in the USSR and countrieswith comparable political systems,usually a specialland development section is set up in the project. After the construction ofthe drainage system,this section is not only responsible for the regular inspection of the drainage system and its maintenance, but also for the collection ofdata on soil salinity,groundwaterlevel,groundwater salinity and the instructions for the leaching programme. 3. Maintenance

(a) Open ditch The method of maintenance frequently controls or influences the design of the ditch cross-sectionincluding side slopes,spoil banks and roadways ofditches.Maintenance work usually includes such practices as cutting, BI

379

IRRIGATION, DRAINAGE A N D SALINITY burning or chemical control ofvegetation,removing sedimentregularly as it accumulates,repairing structures, and doing such other work as necessary to retain the original effectiveness of the systems. Maintenance practices discussed below should be considered and applicable measures included in the maintenance plan. The surface irrigation and run-offwater from behind spoil banks needs to be channelled safely into a deep ditch.Structural measures to handle surface water not included in the original design should be installed as needed. Usually the same type of equipment used in construction can be employed economically for maintenance. Difficultiesin obtaining effective maintenance work by handtools emphasise the need for efficient equipment for maintenance work. Draglines and clamshells are used most frequently in maintaining larger drains and tractor-powered blade equipment for sinall field ditches. For best results,the needed annual maintenance work is required in addition to periodic cleanouts of open ditch systems. Usually the use ofvegetation on side slopes and banks of drains in irrigated lands is quite limited.However, where the climate is favourable vegetation can stabilise the banks of ditches. In some locations the natural vegetation,aided by application of fertilisers or soil amendments,provides some degree ofvegetative control and prevents serious bank erosion. Local experience should guide the use of vegetation and methods of establishment. Hand cutting of undesirable vegetation is widely used where labour is available. In some locationscontrolled burning ofvegetation has been used.Winter burning ofbrush and weeds when vegetation is dead or dormant is often effective.The methods include the use of oil-burning equipment. The use of oil burners has declined in somelocationsin recent years in favour ofchemicalmethods of control. This is due to cost factors and fire hazards to adjoining structures,fences and crops. Chemical treatment for ditch maintenance has become a well-establishedpractice in several countries. In selecting methods and time of application for ditch maintenance,a study should be made of the effectiveness and cost of chemicals available,also possible damages chemicalsprays may do need to be considered. Also detailed information on the kind of spray rig needs to be obtained.If possible,trials to establish the most effectivemethods should be arranged. Usually more chemicals need to be applied the first year than later and applications at regular intervals are necessary for success.Trials carried out over two or three seasonsusually give a good basis for estimating future costs. A project study of the use of chemicals is often desirable to establish the most economical methods. Where not properly used chemicals may damage adjoining crops or poison livestock or fish or cause injury to operators.These hazards can be reduced by applying the best adapted chemical for the specific purpose, proper timing of the chemical treatment, training of operators and by using suitable equipment. The chemicals 2,4-D(2,4-dichlorophenoxyethylsulphate) and 2,4,5-T(2,4,5 trichlorophenoxyacetic acid) have been used widely in recent years.They are known as selectivechemicals because they control most broadleaf and woody vegetation but do not permanently injure most grasses. Thus sod-forming grasses however,kill or damage crops such as cotton, remain to stabilise ditchbanks. Minute quantities of 2,4-D, tomatoes,legumes,many flowers,bush fruits and truck crops at certain periods. Many other effective patented chemicals are available for specific purposes (SUTTON,Farmers Bulletin 2047,US Department of Agriculture). Spray equipment may be mounted on a truck or on a tractor and run along the ditchbank. Boats and amphibious vehicles may be used wherever water is sufficiently deep. Airplane and helicopter spraying has been used successfully on larger drainage systems.A knapsack sprayer carried on a man's back is adequate for trials and for some farm or similar use. Using herbicidal agents in aquatic weed control is a relatively new and changing field. Further, the wide variety of soil types, quality of waters, climate,and plant species,materially affects the required dosage for control.Plants that normally grow beneath the water, such as pondweed,waterweed and naiad,may be controlled by aromatic solvents,chlorinated benzenes,or mixtures of petrol and polychloro-benzene.However,most of these chemicals are poisonous to fish. Such chemicals may be mixed with non-ionic emulsifiers and sprayed into flowing water at points with naturalturbulence or introduced under high pressure to create turbulenceand thorough mixing ofthe chemical in flowing water. In ditches that are more than 3 or 4 kilometres long it may be necessary to apply more chemical mixtures every 3 km or so along the canal to replace that absorbed by weeds and silt or lost through dilution of the emulsion.For additional information on the use of aromatic solvents in canals reference is made to Agric. Handbook No. 231 of the Agric. Res.Service,USDA. Most floating and emergent aquatic weeds can be controlled by chemicals. Some aquatic plants such as

380

D R A I N A G E SYSTEMS A N D M A N A G E M E N T

water hyacinth have been controlled with chemicals only to be succeeded by submersed waterweeds just as bad or worse. It is as well to know through trials what will happen when chemicals are used regularly.

(b) Tile clrains Free outlet conditions should be maintained on tile drain systems if maximum benefits are to be obtained. Sediment and debris sometimes gather over the outlet and may entirely fill the drain pipe outlets.Rodents also are frequently found to nest in drain pipes. Outlet ditches serving tile systems must be maintained as needed to provide satisfactory tile outlet conditions. Proper working of the drains may be checked by measuring discharge ofthe pipes and the level ofthe water table in the field.Failures may be located by observation wells above or close to the lines.Low water tables downstream and high water tables upstream of the observation point indicate the location of the failures. Silting of the lines, occurring particularly during the first years after installation, require cleaning of the tiles. This may be done by pushing rods with sectional joints into the line at the outlet. Modern practice is now to use a water jet connected with a high-pressurepump. When the jet is pushed in,the settled sediments are loosened. Quite often holes develop over improperly designed or constructed tile lines.These holes may be caused in construction by leaving too widejoints.Other causesmight be a broken tile orimproperlymade tilejunctions. If repairs are not made immediately,the damage will increase.To make repairs,the tile must be exposed at the hole and repaired by replacing the broken tile, cementing the tile junctions, or covering the wide joints with tar-impregnated paper,flat pieces of broken pipe or other suitable material. If trees near the tile line are not removed at the time of construction,the tile may become clogged by tree roots. Roots may be removed from tile by hand,sewer rods with cutting heads and sectionaljoints,or with rotary cutters using power driven,flexible cable machines, especially manufactured for this purpose. Unless the trees are removed or killed,repairs may prove temporary and bave to be repeated at intervals. One way to prevent such occurrence is to use sealed-jointpipe or closed conduit through and for a distance on either side of the trees. The roots of some trees such as cottonwoods and willows may enter tile as much as 30 metres from the tree. Hardwood trees generally do not cause trouble if 15 metres distant.Many tile drains through apple orchards have functioned effectively for years without difficulty.Safe distances from tile lines ofvarious species of trees should be established and included in local specifications. The life and value of a tile installationmay depend on the repair of auxiliary structuresinstalled to protect the tile system.Ifthey are not maintained,the effectiveness ofthe system will decrease. Regular inspection of the entire system is essential for maximum economy. (c) Drainage wells Operation and maintenance of a drainage well field entails considerably more man-hours,material and equipment than other types of drainage installations.Automatic controls are available which will reduce the man-hourrequirements,but in countrieswith low labour costs they are difficult to justify.Usual practice is to provide automatic controls only as they are needed to protect the installations from damage. Minimum instrumentation usually consists of:

1. Controlswhich will ensure a motor runningfortheminimum timerequired to dissipate therapid heat buildup that occurs during starting 2. Thermostaticcontrols in the motor which stop it when operating temperatures rise above maximum safe levels regardless of cause 3. A signal light or horn which switches on when a motor stops by reason of any of the above conditions 4.An air line and pressure gauge on each well by which both pumping and static levels can be measured In addition to the determined schedule ofmeasurements in observation holes,the pumping wells should be visited weekly. During each visit, the oil reservoir of the pump, drive head and motor should be checked and oil replenished ifnecessary.Ifthe pump is operating,the pumping level should be measured and,if shut down,the staticlevelinthewell.Themeasurements and date should be recorded.Routinewater levelmeasurements in pumped wells are best made with an air line and pressure gauge,which should be integral parts of each pump installation.Pump operation should be checked visually and by sound. High air content of the discharge,or a broomy or surging appearance,usually indicates cascading,excessive drawdown in relation to bowl setting, or other causes of entrained air,which result in poor operating efficiency,cavitation and 381

IRRIGATION, D R A I N A G E A N D SALINITY

aggravated corrosion of the pump. If, in addition to the above phenomena, the pump sounds as though someone were throwing gravel on a tin roof,cavitation is almost certainly occurring,which will lead to pump failure.Worn bearings and other mechanical difficulties can also be detected and identified by sound.Any unusual operation condition or noise should be noted and investigated as soon as possible by the pump service staff.

4. Economics of maintenance

A maintenance programme must be justified economically. Ditches overgrown with brush and small trees may have only one-halfto one-thirdtheir original capacity.Tile drains may become impaired ifditches into which they discharge are not maintained and standing water or sediment covers the tile outlets. Damage to tile drainsand the resulting development of poor drainage conditions and crop damage is one of the most costly results of lack of maintenance.The loss or damage to crops on only a small percent of the irrigated area may exceed the maintenance costs for several years.Adequate maintenance provides insurance for good crop production. Using a dragline or clamshell to remove undesirable vegetation and to clean ditches lightly as often as necessary is one kind of maintenance that deserves widespread consideration. The availability and cost of manual labour is a major factor in selecting the method of maintenance. The economy and feasibility of methods need to be determined locally.These considerations and the feasibility of establishing maintenance methods as discussed before should be included in working out a maintenance plan. Annexes A-C follow

382

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IRRIGATION, DRAINAGE A N D SALINITY

ANNEXE B Annexe B Sheet 1

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Sheet 2 S/h> 100

Nomographs for the computation of the drain spacing in a two-layered soil (BOUMANS1963)

384

D R A I N A G E SYSTEMS A N D M A N A G E M E N T

ANNEXE C

Nomograph for the hydraulic design of drainage (For explanation see p. 359)

REFERENCES AHRENS T.P. (1958), Water well engineering, Water wellJournal,Urbana,Illinois. AMAYON(1962), Synthèse des assdis effectuéspur I'OBceNational des Irrigations du Maroc pour la détermination des cardctéristiques des dispositifs d'arrissage en irrigation collective,Rabat. AMERICAN WATER WORKS ASSOCIATION(1958), Standards for deep wells,AWWAA-100-58,New York. BEERSW.F.J. VAN (1958), The auger hole method, Bull. 1 International Inst.for Recl.and Improvement,

Wageningen.

BOTELLI (1957), Drainage par pompage,premiers resultats des études entreprises dans les Beni Amir, Beni Moressa, Ann. Bull.Comni.Int. d'Irrigation et de Drainage,Maroc. BOUMANSJ. H.(1963), Een algemeen grafische oplossing van het stationaire ontwateringsvraagstuk met toepassingen van isotroop en anisotroop doorlatende gronden,Polytechn.Tìjdschr.,14B,545-51. BOUMANSJ. H.(1963), Some principles governing the drainage and irrigation of saline soils,In: Reclamation of saltafectedsoils in Iraq.Editor:P.J. Dieleman.Publ. 11 InternationalInst.for Recl.andImprovement, Wageningen.

BOUWERH.(1962), Variable head technique for seepage meters, Trans.Amer. Soc. Civ.Eng.,127,3,434-51. BURINGHP. (1960), Soils and soil conditions in Iraq,Ministry of Agriculture,Baghdad. CRIDDLE W.D.(1950), A practical method of determining proper lengths of runs,sizes and furrow streams and spacing of furrows in irrigated Idnd, US Dept. of Agriculture, SCS Res. Division. DONNAN W.W.(1946), Model tests of a tile spacing formula,Soil Sci.Soc. Amer. Proc. 11,131-6. DONNAN W.W.(1954), Drainage of agricultural land using interceptor lines,J. Irrigation and Drainage Division,Proc. Am. Soc.Civ.Eng. DUM~LETON J. E. (1963), Wells and boreholes for water supply,Technical Press, London. DUMM L.D.(1962), Drain-spdcing method used by the Bureau of Reclumdtion,ARS-SCSDrainage Work-

shop,Riverside, California.

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IRRIGATION, DRAINAGE A N D SALINITY

DUMM L. D.(1964), Transient-flowconcept in subsurface drainage: its validity and use, Proc. Am.Soc. Agri. Eng.

DUMM L. D.(1967), Transient-flowtheory and its use in subsurface drainage of irrigated land, Proc. Am. Soc. Civil Eng. DUMM L.D.and WINGER R. J. Jr. (1964), Designing a subsurface drainage system in an irrigated area through use of the transient-flowconcept,Proc. A m . Soc. Agri. Eng.

EDELMAN J. H.(1964), Over cle berekening van grondwaterstromingen,Thesis,Delft. ERNST L.F.(1962), Gronwaterstromingen in de verzadigde zone en hun berekening bij aanwezigheid van horizontale evenwijdige open leidingen, Versl. Landb. Onderz, 67,15,Pudoc,Wageningen. FORTIER and SCOBEY (1962), Permissible canal velocities, Trans. ASCE,89,940. HOOGHOUDT S. B. (1937), Contribution to the knowledge of certain physical properties of the soil, Bodem Kundig Institute,Groningen,the Netherlands:Bul. No. 43413) B,461-676. ISRAELSENO.W., PETERSEN D.F. JR. and REEVE R. C.(1950), Effectiveness of gravity drains and experimental pumping for drainage, Delta Area Utah, Utah Agric. Expt. Sta., Bull 345.

JACOB C.E.(1946), Radial Bow in a leaky artesian aquifer, Trans. A m . Geophys. Union, 27, 198-208.

KIRKHAM D.(1958), Seepage of steady rainfall through soil into drains,Trans. Amer. Geophys. Union, 39, 892-908

LUTHENJ. (ed). (1957), Drainage of agricultural lands, Amer. Soc. of Agronomy, Madison,Wisconsin. LUTHINJ. and KIRKHAM D.(1349), A piezometer method for measuring permeability of soil in situ below water table,Soil Sci., 68,349-58.

MAASLAND M.and HASKEW H.C. (1957), The auger hole method of measuring the hydraulic conductivity of soil and its application to tile drainage problems, Proc. Third International Congress on Irrigation and Drainage, S,69-1 14. MAHDI S. HANTUSH (1956), A n analysis of data from pumping lest in leaky aquifers,Trans.A m . Geophysical Union,37,702-14.

MAIERHOFER C.R.(1958), Drainage inrelationto a permanent irrigationagriculture,Proc.Ant. Soc. CivilEng. MAIERROER C.R.(1967), The importance of drainage in irrigation development, Int. Conjerence on Water .for Peace.

MOLEN W . H.VAN DER and BOUMANSJ. H.(1963), Drainage in relation to salinity, Syllabus2ndPostGraduate Training Centre on Land Drainage, International Inst. for Land Recl. and Improvement,Wageningen. The flow of homogeneous fluids through porous media, J. W . Edwards, Ann Arbor: Michigan. POLUBARINOVA-KOCHINA P.Y.A. (1962), Theory of groundwater movement, translated from Russian by J. M . Roger de Wiest, Princeton University Press. REEVE R. C.(1957), Flow equations and basic relations (Chapter 4). Drainage of Agricultural Lands (ed. J. N.Luthin). Amer. Soc. of Agron.,Madison,Wisconsin. RESHETKINA N.M.(1960), Hydrogeologicalfoundationsforprojectihg verticaldrainage in the Golodnuyu Steppe, Academy of Sciences of the USSR,Tashkent. RESHETKINA N.M.(1964), Fundamental propositions to the vertical drainage projecting on an example of old irrigation zone of the Golodnaya Steppe,Problems of Hydraulic Engineering, J. 27. RESHETKINA N.M . and YABUKOV K.H.(1964a), Effectiveness of vertical drainage in the general complex of reclamative measures on an example of the Golodnaya Steppe, Trans. of tlze Meeting on Economics of Investments into Irrigation, Inst.for People’sEconomy,Tashkent. ROBINSON A.R.(1957), Measurement of canal seepage,Trans. Amer. Soc. Civ.Eng., 122,347-73. ROTHE J. (1929), Die Strangenentfernung bei Dranungen im Mineralboden,Kulturtechniker 32,155-69. SCHEWIOR-PRESS H. (1958), HilfstufeZn zur Bearbeitung von wasserbaulichen und wasserwirtschufticlien Entwurfen und Anlagen, 7,Auflage, Berlin, Hamburg. TALSMA T.and HASKEW H.C. (1959),Investigation ofwater-tableresponse to the drains in comparison with theory,J,of Geopliysical Researcli, 64,11, 1959. THEIS C.V. (1935), The relation between the lowering of the piezometric surfaceand the rate and duration of discharge of a well using groundwater storage,Trans. Amer. Geophys. Union, 16,519-24. us SALINITYLABORATORY (1 954),Diagnosisand improvementof saline and alkali soils,Agriculture Handbook 60,USDA. VENTE CHOW(1959), Open clzanizel hydraulics, McGraw-Hill,New York. WINGER R. J., Jr. (1960), In-placepermeability tests and their use in subsurface drainage, Int. Cornm. on Irrigation and Drainage, Pourth Congress, Madrid, Spain.

MUSKAT M. (1964),

386

12. Some EKects of Irrigation and Drainage on Soils* I NTRO D U CTI ON THEdestructive effects of irrigation on the productivity of many soils have been so drastic that it has long been doubted whether irrigated agriculture can be permanent on arid lands. By far the great majority of these occurrences have been due to the development of waterlogging,nearly always associated with excessive concentration of salts,within the root zone of plants. If allowed to proceed unchecked,such developments lead ultimately to a situation where the land cannot produce any useful vegetation.This happened with the irrigation systems put down by the Hohokam people in the Salt River Valley ofArizona up to about AD 1400.These systemswere changed successively to positions higher and higher up the slopes,until they could go no further.This was due evidently to waterlogging and salinisation of the land below the canals,and when these could be taken no higher the Hohokam culture vanished. Again, as long ago as 1122 BC, the Chinese used some form of drainage in the attempt to control these problems in irrigated lands.In the oases of Central Asia,irrigated since ancient times,those which have poor natural drainage now have an average salt content greater than 0.7%in the surface metre of soil and salt groundwater close to the surface.Other examples,includingthe Tigris-Euphrates area,willbe mentioned later. The appalling truth is that deterioration of this kind is still going on at a huge rate in some parts of the world. On the brighter side, technical irrigationists believe that, in the light of the knowledge accumulated in the last 50 years,irrigated agriculture on arid lands can be permanent,provided there is wise selection of the land and irrigationwater,choice ofappropriate crops and good management.A key factorinmanagement is drainage,which must go hand-in-handwith irrigation. Good management also implies a knowledge of, and an ability to control,many other kinds of change in soils which may take place under irrigation.Those which are most important are discussed later. Several others have had to be omitted;these are mainly those in common with changes in unirrigated soils such as those associated with the use of herbicides, germicides and insecticides. A.

CHANGES IN TOPOGRAPHY

1. Alteration af micro-relief

When preparing soil for irrigation, the surface must be thoroughly levelled in order to promote efficient irrigation and leaching.Levelling is especially important when surface irrigation is used. Grading of land for irrigation,as indicated in Chapter 10,obviously will have very digerent effects on the subsequent behaviour of the soils under irrigation,depending whether it is done on a deep or a shallow soil. Clearly,in order to achieve a uniform slope of acceptable gradient,much deeper earth-movingmay be done on a loessial or colluvial soil several metres deep than may be done on a soil profile showing only a few centimetres of clay loam above intractable clay. Micro-reliefof the 'gilgai'type leads almost inevitably during grading to exposure ofheavy subsoils on the sites of the former mounds, due to shifting of surface soil to fill in the former hollows. Difficult conditions are associated also with shallow soils overlying gypsum,limestone or other stone. Levelling is usually done by special equipment; primitive methods of levelling by hand or with horsedrawn smoothing harrows have gradually given way to powerful tractor graders, scrapers and levelling machines. Major levelling operations in areas with complicated relief involve moving from 1000-1500 m3of earth per hectare. However,big machinery,best for efficient grading,may have a detrimentaleffect on soil structure,especially if operated on moist soils. This and the exposure of raw subsoils may have drastic effects on productivity. Investigations in Colorado showed that both troubles may be ameliorated by organic matter and mineral * This chapter was edited by V.A.KOVDA from the manuscripts submitted by F.PENMAN, N.G.MINASHINA (partsA,B, C and F), M.M.KONONOVA and N.P. MALINKIN (partsD and E) as authors with contributionsby €I. H A M D I and V. A. KOVDA

387

IRRIGATION, DRAINAGE A N D SALINITY

fertiliser.The use of 25 loads per hectare of farm manure together with 142kg of phosphate raised the yield of barley grain plus straw from 717kg/haon land graded to remove surface soil to 2237 kg/haon the treated land.In an area ofMallee soilin north-westVictoria,land subjectedto heavy ‘cut’in grading yielded irrigation cotton at the rate of only one-thirdof that on adjoining ‘fill’ land.In the USSR sierozem soils on loess may require the removal of200-400m3/ha.As in such cases the soil cover on 25-50 %ofthe area may be destroyed and heavy quantities of manure are needed to restore soil fertility,it is recommended to limit the earthmoving to 10% of the surface, 20-30 cm deep. Such considerations might well favour on uneven shallow soils a decision for sprinkler irrigation not involving grading.The following specific methods of levelling are reported from the USSR. (a) Central Asian people made use of wind to level sandy soils by setting up shields in a certain formation, The area levelled was then flooded with muddy water (b) In cases with soil horizons differing strongly in fertility, parallel cuts on the higher parts are made alternating with untouched strips. The mixed layers of the cuts are deposited in the low parts of the land and later cut and uncut strips are levelled. The laborious work has the advantage of preventing too much variation in the new topsoil (Fig. 12.1) (c) Hydraulic excavators or a free water stream flowing down a steep slope may be used to transport soil material and to level ancient irrigated regions where humps and basins have been formed. Supplementary advantages are the decrease of salinity in depressions between irrigation canals and the improvement of the mechanical composition of the soil Salinity,swelling and leaching of irrigated soils may cause variations in micro-relief,requiring frequent levelling. Many centuries of levelling operations in older irrigations have eliminated all traces of the original relief, e.g.consisting ofridges and basins in valleys and deltas.These zones are now entirely flat,except for residual hills. It demonstrates the enormous transformations wrought by men in the process of developing land for irrigation.

Fig. 12.1. Lay-outof strip levelling 1. Soil surface before levelling 2. Soil surface after levelling 3. Soil grooves 4. Micro-depressionfilled in with soil 2. Changes in level (a) Subsidence The most marked changes in the surface of irrigated plots and alongside irrigation canals occur at the beginning ofland reclamation work by irrigation.One ofthe main factorsin the alteration ofthe land surface is the decrease of consistency in the subsoil due to wetting and compaction of the soil,leading to subsidence (Rozov,1956). Compaction occurs in soils which are characterised by high porosity silty composition,an insignificant content of swelling clayey particles and,frequently,containing soluble salts.Surface subsidence usually occurs in areas with deep groundwaters (groundwater table more than 8 or 10 m deep). Thereareseveralkindsofsubsidenceofirrigatedlandin arid regions.Because oflong exposureto desiccating conditions,in which the native vegetation uses virtually all the available water (rainfall minus evaporation directly from the soil surface), the packing effect of water penetrating to the deep subsoils is lacking.For this reason, the phenomenon of collapse after clearing is known in many arid areas of the world, e.g. South Africa,and parts of Australia,especially on the poorer and more porous soils.This type of collapse,which in Australian mallee soils may amount to 30 cm,may occur before irrigation if sumcient rain comes after the clearing ofthe vegetation.It refers to the first real wetting the soil has had for ages and is shown in an extreme form in some areas of the San Joaquin Valley in California.Here, subsidences of as much as 3 metres may occur in one year of ponding, usually associated with the saturation of initially highly porous low-density deposits of mud and pebbles. 388

EFFECTS OF IRRIGATION A N D D R A I N A G E O N SOILS

Another kind of subsidence is associated with continued pumping of underground water, leading to a compaction of the aquifer. Such a subsidence may be a slow process, although hundreds of k m 2are sinking in the San Joaquin Valley at rates of 12 c m to 45 cm a year and some portions have sunk about 6 metres altogether.A portion of Mexico City has sunk about 8 metres within the past 50 years for the same reason. Peat lands under irrigation and drainage frequently subside.The surface of the Sacramento-San Joaquin delta in California,a large peat area,has sunk about 2 metres since 1922. Oxidation of peat and compaction by machinery are usually factors in this kind of collapse. Other factors in subsidence are slow deep-seated earth movements, earth tremors and such things as ‘sink-holes’in irrigated lands and irrigation channels. The extent of subsidence is importantin the case of soils formed on loesses of foothill uplands and ancient upper river terraces. Considerable subsidence occurs also on the soils of dry and semi-desertsteppes,formed on top of proluvial foothill slopes,and dry river alluvial fans and deltas.The irrigation of meadow soils on contemporary delta-alluvialdeposits does not lead to a marked subsidence. The soils most commonly found on top of sagging rocks in irrigated farming zones are: sierozems,cheilutu (black earth), grey-brown and chestnut soils and, less commonly, brown soils (ROZANOV, 1951; KOVDA, 1959). Among these soils,sierozems and cheilutu have the most friable and silty compositions,besides which they show little swelling and high porosity,large soil-faunacanals and cells and frequently contain salts in the subsoillayers.Irrigation causes regroupingand compaction of the soil particles,decrease of soil porosity and leaching of salts and,consequently, subsidence (Table 12.1). Table 12.1. Porosity (in %)of sierozem and loess before and after irrigation-(From Russian data)

Depth (cm) 0-10 10-20 2&30 30-40 40-100 100-150 150-200

Porosity (%) virgin sierozem irrigated sierozem 57 55 55 55

54 52 51 virgin loess

210-300 320-350 380-400 550 630-650 700-730 830-850 930-950 1030-1 150 1230-1250

49 51 51 51 51 50 51

53 48 53

54 46 44 48 46 45 45

loess after wetting for 38 days 50

49 45 46 48 46 45 47 46 46

As a result of irrigation,compaction and subsidence of the soil and the wetted subsoil stratum causes a shrinkage of 4-8 cm for every metre of subsided soil.In the case of the greatest subsidence ever observed in the course of irrigation operations in the USSR,the surface of the soil dropped 2 metres. Particularly acute complications arise when irrigation systems are installed on certain types ofsoil.Cases have been observed offunnels and clefts 50-80 c m wide forming along side canals within a distance of 80 m. Subsidence occurs as a rule a few days after irrigation;but it may also happen someyears later.As a means of preventing subsidence,the soil should be wetted gradually,the surface rolled,the canals lined and the soil silicatised (Rozov,1956). (b) Expansion Soils which have sunk on initial heavy wetting and those not subject to that effect, may expand a little on continued irrigation. Such rises in surface level may be attributed to the swelling of the soil profile as a 389

IRRIGATION, DRAINAGE A N D SALINITY result of wetting. Irrigation of the heavy,dificult permeable soil of the Gezira, Sudan,led to a rise of 11 cm in the soil surface due to a summation of expansion effects at depths down to 1.5 metres. At Werribes, Victoria, 11 years’irrigation of pasture with 90 c m of water each year led to a rise of 5 cm in soil level while a mallee soil in north-westVictoria rose from 5 c m to 1 1 c m in 10 years of irrigation.A vertical movement of 8 c m at the surface has been reported in a soil profile at Adelaide,South Australia,under seasonal wetting and drying. 3. Alteration of relief by deposition of suspended matter

The waters of many arid zone rivers used for irrigation contain large quantities of suspended matter. This applies to a number of large rivers, such as the Nile, Tigris, Euphrates, Amu-Darya,Syr-Darya,Kura, Colorado, Hwang-Ho,and many others. The mean annual turbidity of most of these rivers is between 1 and 5 kg/m3;and the mean monthly turbidity during the flood period from 5-36 kg/m3;and the mean monthly turbidity during the flood period from 5-36 kg/m3or more (Hwan-Ho,50-25Og;Vaksh,9;AmuDarya,6). Turbid river water, on entering the irrigation system, loses some of its sediments owing to the deposition of the large particles (Table 12.2). Table 12.2. Sediments in water (gli)in diferent sectors of the P. Shuvabad canal irrigation network (Vaskh ValZey, Sheinkin,1957)

Sector of network Head of canal Middle and shallow network Head of irrigation furrow End of irrigation furrow

5.12.1952

26.8.1953

5.8.1954

1.7

3.4

4.3

043-0.9

2.2-2.5

2.75-3-5

04-0-8

14-1.5

1.4-3.3

o-1

0.5-0.9

0.8-1-8

Approximately 30 to 50% of the suspended matter of the river water remains in the irrigation network. After the canals have been in operation for many years,the level oftheir beds gradually rises. After cleaning the deposits form ridges along the canals.In some places,the old canals have been abandoned on account of the size of these deposits,and new ones dug alongside. Uneven distribution of deposits in irrigated territories leads to the formation of ‘secondaryirrigation relief‘,characterised by raised ridges along the canals alternating with depressions between canals, the differencein altitude being between 3 and 6 m,and the area of the depressions measuring anything from a few hectares to 500 hectares or more. The surface slope from the canals down into the depressions is 0.2 to 0.3 % (as against an average slope of less than 0.05% usually observed in delta areas); while in valleys the slopes of secondary relief are often considerably higher: 2 to 3 % (ZHIGACHEV). The most marked relief occurs in those river valleys and deltas which have the highest figure for water turbidity in the spring and summer period, at the very time when water requirements for irrigation are maximum. This applies to rivers such as,for instance,the Amu-Darya,Syr-Darya,Tigris,Euphrates,etc. (Fig. 12.2). In the hile delta,the irrigation reliefis ideal-perfectly flat,without ridges along the canals such as are to be found,for instaiice,in the Amu-Darya delta,the Mesopotamian valley and other oases in arid zones in Asia. This is due to the relatively low turbidity of the Nile waters, and the relatively heavy mechanical composition of the suspended matter. Another reason is that,in the Nile delta basin,irrigation was applied, for a long time,promoting a more even distribution of deposits.Another important point,evidently,is the quality of the Nile silt which is highly fertile. The Nile silt in canals was quickly removed and used for fertilisation,whereas in other river basins the deposits accumulated for centuries in ridges along the canals. Soil surfacesirrigated with turbid waters rise at an average rate of 0.5-2 m m a year:Nile delta,1 mm per 1937); Murgab, 1 mm (MINASHINA, year; Amu-Darya delta, 1-1.5 mm;Zeravvshan, 1-5-2inm (ORLOV, 1962).Thus over a period of200-300 years,the arable layer or irrigated soils is renewed.In ancient irrigations

EFFECTS OF IRRIGATION A N D DRAINAGE O N SOILS I’. t,

Turbidity in kg/m3 4.8 r

8

/

i 1.6

-

.,.

‘,

...

/;-. ,At!-

>.Y* \ , --/0.8 1. : 0.05mm ~0.01 mm

(b) Humus (in %) Nitrogen (in %) (c) Nitrate nitrogen (in mg/kg) soil, 20 days after composting (d) Nitrate nitrogen (in kg/ha)on nonfertilised field at budding period of cottonplants (e) Volumetric weight of sub-arablelayer (in g/cm3) (f) Field moisture capacity of top metre layer of soil (in m3.ha) (g) Yield of cotton plant,without fertiliser,using efficient farming methods (in 100 kg/ha)

20

1O0

23.0 35.0 0-75 0.07

5.0

11.98

37.74

traces 1.25

75

2500

3300

10-15

20-25

65.0 1.55

0.12

1.5

The fact that suspended matter contained in water used for irrigationis distributed unevenly throughout the area leadsto a differentiationin the mechanical composition of deposits;whereas silty and sandy particles accumulatein the irrigation system and along canals,clayey particles are deposited in the inter-canaldepressions.The fraction distributionin relationto the distance to irrigationcanals in the Murgab oasis is given in Fig. 12.3.Unlike stratified natural soils on alluvial deposits,having a mixed physical composition,ancient irrigated soils are uniform in composition.As a result of prolonged irrigation,finer particles accumulate in the soils (Table 12.5). Table 12.5. Change in the mechanical composition of soils due to silting by irrigation deposits (in %of dry soil)-(After MINASHINA)

Nature of deposits

Depth of Size of particles (in mm)

soil

samples Irrigation

Alluvial

(in cm)

< 0.001

40-50 70-80 95-105 130-140 175-185 225-250 260-3 10 310-337

25.8 25.3 23.8 23.3 18.2 10.1 8.3 7.1

0.001-0.005 0.005-0.01 0.01-0.05 18.2 19.9 17.3 17.0 12.8 7.3 5.0

4.3

8-2 10.8 9.5 9.2 7.3 4.8 4.4 3.6

32.6 30.8 35.2 35-2 42.9 44.5 49.3 45.2

0.05-0.1

0.1-0.25

9.3 9.3 9-5 10.4 13.0 22.5 24.5 30.8

5.6 3.9 4.7 4.7 5.8 10.8 8.5 9.0

393

IRRIGATION, DRAINAGE A N D SALINITY fractions

150250

500

Bank alongside canal

1350 m Distance from ancient canal

Fig. 12.3. Distribution of irrigation deposits, according to size of fractions, from the canal towards the inter-canaldepressions in an ancient oasis (Murgab) 1. content of fractions measuring 0-05mm at depth 0-25 cm 2. content of fractions measuring 0.05 inm at depth 100 c m 3. content of fractions measuring 0.001 mm at depth 0-25 c m 4. content of fractions measuring 0.001mm at depth 100 c m Another factor which plays an important part in determining the physical composition of irrigated soils is the use of earth manure; this method is very commonly practised in ancient irrigated territories (ORLOV, 1937), where eroded matter from mud walls, discarded material from ancient canals,weathering products of shale and loess and specially prepmed compost from clung and irrigation ditch silt serve as manure. The extent to which minerals inside the soil are weathered as a result of irrigation is more diEcult to determine. ORLOV(1937) and ROZANOV(1951) have taken the view that irrigation promotes argillisation of soil minerals through weathering,but they h w e produced no definite proof that this is so. In the Murgab oasis, argillisation through weathering occurs when steps are taken to spread evenly flue dust and clayey particles in soils and alluvium on irrigation deposits (Fig. 12.4.)In irrigated soils,there is a decrease of the 0.01-0-005mm fraction as a result of weathering,with a parallel increase of the fraction composed of particles smaller than 0.001 mm whereas in clayey and sandy alluvium the ratio of these two fractions remains the same. Increased weathering associated with much greater movement of water in the soil profile,particularly in conditions of irrigation with saline water, may lead to notable changes in soil texture.Soils in the Salt River Valley inArizonachanged from sandy loams to clayey loanis or even clay after 28 years ofirrigation;this was attributed to breakdown of original fresh granitic fragmentsin the soil,hastened perhaps by the use ofsaline irrigation water at various times. Eluviation of clay from the upper soil horizons and its deposition in lower horizons may take place to an accelerated extent under irrigation as compared with natural rainfall conditions,particularly when the soils are alkaline and have a significantproportion of sodium on the exchange complex. Such movement might be expected to follow the use of an irrigation water of low salt content,and woulcl,of course,affect texture in upper and lower horizons. Dispersion of heavy clay soils under irrigation is well known,as on the Riverina plains of southeast Australia,where part of the dispersed colloid on drying forms a smooth skin on the surface,which cracks on further drying. At Werribee in southern Victoria, it was concluded that some 14% of the clay content of the surface loam had been washed downward following irrigation of pastures by applications up to 92c m of water annually during 9years.Under moderate irrigation the displaced clay was mainly from the finer fraction (0.001 min), under heavy irrigation there was some loss of the coarser clay as well.Hassan Hamdi has reported from Egypt that a mechanical illuviation takes place in alluvial soils there. The depth of the downward movement of clay varied from 15 to 25 cm. In view of its dominant role as a determinant of soil texture,migration of clay could be of special importance. That such migration can occur has been stated by Californian authorities:movement of particles from the surface centimetre or so of irrigated soils has been indicated. 394

EFFECTS O F IRRIGATION A N D D R A I N A G E O N SOILS Content of fractions (A)

Fig. 12.4.Distribution of particles of soil and alluvium, by fractions 1. sandy alluvium 2. clayey alluvium 3. heavy loam,agro-irrigationdeposits (Murgab oasis) Investigationshave shown that migration of clay particles in soil columns reduces permeability by decreasing pore size.In such cases the presence of sodic types of colloid and insufficientsalt to floculate the colloid are of special importance. Soil texture in irrigated soils may also be influenced by accelerated breakdown of the colloids as e.g. the accumulation of silica in 'szik'soils of Hungary illustrates.Some silica 'flour'was found in soils irrigated for pasture for 9 years at Werribee,southern Victoria. Work in New Mexico suggested that the colloids of Gila clay under irrigation were changingby preferential loss of silica and that they could be of quite adifferentsort after 400 to 2500 irrigation years. 2. Influences on soil structure

Changes in soil structure under irrigation may be due to a number of causes.Unfortunately they are often adverse. The great protectors of soil structure are organic matter, and calcium in the exchange complex of the colloids. Many conditions ofirrigation (mechanical action of irrigation water,the dissolution of humus, the leaching of salts and so on) tend to dissipate one or both of these soil resources. In addition,there are compactive effects due to incidence of water, heavy machinery and trampling by stock. These reduce pore space,while the number and nature ofposes,together with water stability of aggregates,arethedeterminants of soil structure. The aggregation capacity of long-irrigated soils depends to a considerable extent on their physical composition (Table 12.6). Table 12.6. Connection between the aggregation capacity of soils and their physical composition and humus content-(BALYABo, 1954)

Physical composition of long-irrigatedsoils

Humus

Aggregates > 0.25 mm

%of dry soil

~~

Sierozems:

silty loamy

Swampy-meadow

clayey slightly loamy loamy

clayey

0.66 1.38 1-54 1.67 2.27 3.90

2.4 18.3 20.1 8.7 27.8 33.0

The physical composition is often related to secondary relief, as loamy soils on slopes and clayey soils in depressions are more humified than lighter soils on higher spots along irrigation canals. Different soil types have different aggregation. Virgin sierozem soils have a good aggregation capacity CI

395

IRRIGATION, DRAINAGE AND SALINITY

(ROZANOV, 1951), whereas desert soils have a high degree of dispersion. Other Russian investigations lead to the findings presented in Table 12.7. Table 12.7. Content of aggregates > 0.25 m m in soils of Central Asia

Soils

Sierozem soil zone dark sierozems

Virgin Irrigated

Desert soil zone

light sierozems

meadow

takyr-type

sierozems

desertmeadow

23-34 15-40

15-30 5-25

40-82 14-32

0-1o 2-5

5-1 3 2-5

typical

27-49 27-53

There are practically no water-solubleaggregates over 0.25mm in desert soils.In these soils,clayey matter forms compact films of peptised clay,covering the sand particles and lining the pore walls. Takyr-typesoils are therefore characterised by poor permeability,high viscosity and other properties,unfavourable to plant growth. Clean cultivation between irrigated row crops in arid climates leads to dissipation of soil organic matter and deterioration ofaggregates.Other crops may have a favourableeffectasis showninsierozems oftheUSSR (Table 12.8). Table 12.8. Aggregate content in light sievozems--(AfierKOZLOVA)

Depth in cm

Virgin soil

Cotton plant 4 years after grasses

Lucerne second year

third year

Mixed grasses second year

third year

1.3 > 0.25 6.4 11.7 6.0 9.4 4.1 7.1

1-3 > 0.25 9-9 22.9 8-5 21.4 2.5 10.0

Size of aggregates in mm 0-10 10-20 20-30

1-3 > 0.25 4.5 14.8 2.3 18.9 2.1 26.9

1-3 3.1 3.4 3.3

> 0.25

5.3 6.4 7.3

1.3 > 0.25

2.8 3.4 1.9

8.2 14.1 8.5

1.3 > 0.25 4.9 12.3 2.6 10.3 1.9 8.0

Citrus irrigation for 10 years at Griffith,New South Wales, showed that cultivation reduced aggregate stability and that sod between the trees was considerably better in this respect than bare-surfacetreatments. Although sod reduced the total non-capillary porosity of the soil,larger pores were present than in other treatments,44% of them being in the form of insect burrows,2 mm or more in diameter. Eecause irrigation frequently leads to salinisation and/oralkalisation,the effects of these on soil structure are well known through the irrigation world. While a soil carrying sufficient salt to flocculate the colloids and may have a floury structure,those sodic soils with less than the threshold concentration (after QUIRK SCHOFIELD)of salts in the water applied to them will disperse,puddle and become,from the present viewpoint, structureless.It is sufficient to say here that favourable effects on soil structure may follow from the use ofgypsum or more solublecalcium salts,sulphur and other chemicals by way ofsoil dressings or additions to irrigation water. Additions of organic matter to soil and provision of appropriate drainage are other methods of promoting good soil structure. Many soils of the Riverina plain of south-easternAustralia are sodic soils,poorly structured and of low permeability. Natural content of organic matter is low and the content of soluble salts in virgin soils is fairly high. Irrigation with water of high quality on undisturbed native grass on such soils does not leach chloride and soil structure does not deteriorate even under frequent irrigation. When all vegetation is removed, however, such soils may become very unstable under watering by deffocculation of clay and disruption of aggregates.When the amount of vegetation is increased,e.g.by sowing Rhodes grass (Chlorisgayana Ku&) and supplying nitrogen and phosphorus,soil stability is increased.After 8 years' growth of Rhodes grass,the surface inch of soil contained 30-40 % of water-stableaggregatesgreater than 1 mm diameter compared with 10-20% in virgin soil. In such cases the protective effect of organic matter added to the soil by vegetation more than compensates for the disruption of soil structural aggregatescaused by ffood watering per se or by reduction in the electrolyte content of the soil. 396

EFFECTS OF IRRIGATION A N D DRAINAGE O N SOILS

It has been noted in Israel that the cumulative effect of irrigation even with moderately saline water may lead to deteriorationof soil structure.It is stated in connectionwith the Punjabthat soilirrigated with waters containing considerable amounts of bicarbonate,but having no ‘residualNazco,, may accumulate amounts of exchangeable N a considerably in excess of that predicted from analysis of the water. This means that structure may deterioratemarkedly in such cases,as it does inevitably if ‘residualNa2C0,’ is high (see other chapters). Even high quality irrigation water may compact soils.Instances have been given in this chapter and have been noted on the Riverina plains of south-easternAustralia, in New Jersey and on irrigated chernozems of the Rostov region where water-stableaggregation was decreased to a depth of 40 cm.In irrigation of rice, in Japan, it was observed that topsoil aggregates were destroyed by puddling, but stablised by organic matter afid active oxides. Subsoil aggregates were never dispersed, being cemented by active iron and manganese oxides translocated from upper horizons. In Guyana, flooding for rice changed subsoils from stiff blue-grey clay with ferruginous mottling to bleached yellowish-whitestructureless clay. The compactive effects of heavy machinery, especially on moist soils,have been well investigated.It was found in California that excessive use of tractors on irrigated loam could give compactive effects to a depth of 60 cm.Queensland experience is that notable degradation of soil structure was caused by mechanical working of soil at unfavourable moisture content. Apart from the compactive effects on certain soils of irrigation water applied to the soil surface,as discussed above,many soils are subject to compaction by sprinkler irrigation when droplet sizes are large and the water streams have long paths from sprinkler head to soil, so that terminal velocities of drops may be high.Thisis in line with the shattering and dispersive effect of heavy rain. Italian investigations have shown that structural degradation was greater in clayey sand than in sandy clay after repeated sprinkling at intensities of 74-84 mm per hour, while marked compaction resulted from disintegration of aggregates in a calcareous clay soil.Work in New York State showed that,while rate of water application from a sprinkler had no significant effect on the extent of disruption of aggregates when droplet size was kept constant, when droplet size varied over the ranges 0-5 mm diameter, 5-15 mm and 15-25 mm the percentage of disruption varied from 5 to 24 and 36 respectively. The beneficial effects of drainage on soil structure apparently are due to the ability this gives the soil and subsoil to undergo repeated cycles of wetting and drying instead of remaining wet indefinitely, and to deeper penetration of roots and ameliorants. 3. Change of infiltration rate

Initialinfiltrationrate is related to the condition ofthe surface soil and to its moisture content and vegetation cover, while infiltration rates over periods of hours depend on the overall hydraulic conductivity of the portions of the soil profile wetted. As infiltration proceeds, a minimum infiltration rate for the particular conditions is approached. It is therefore important to ensure that measured infiltration rates really are comparable before deciding that a system of land management has caused a change in this characteristic. Moreover, high variability in measurements on an apparently uniform soil has been found in California; one experienceis that eight hours’irrigation may allow water to penetrate 1.20 m into the soil when orchards are young,but after some years the same land may require a watering for 48 or more hours to give similar penetration.This was attributed to the adverse effects offertiliser (other than organic), cultivation and heavy traffic.Investigations in New Jersey show that irrigation of sweet corn tended to increase run-off,indicating a decrease in infiltration.In the Murray River Valley of Australia,20%reductions in infiltration rates have been found on sandy loam soils after periods varying up to 25 years of irrigated horticulture. Compaction could be a factor in some of these cases in which decreased infiltration rate was associated with higher apparent specific gravity for sub-surfacesoil.So it has been found in the USSR,that whereas virgin sierozems have a volumetric weight of 1-2-1.3g/cm3and a porosity of 56-58%, irrigation stablised the volumetric weight at 1-3-1-7 g/cm3and the porosity at 40-45 %.With takyr soils,however, an increase in porosity and infiltration rate has been found after prolonged irrigation,compared with virgin soils (Table 12.9). The method of tillage,the fertiliserused,and the type and degree of soil salinity all influence infiltration. The water-absorption rate,after ploughing a grass layer,was found 5-6 times higher than that of old arable soilunder cotton plants (BALYABO, 1954). According to figures given by Egorov solonchak soil absorbed 850 m3/hain 10 minutes,whereas non-salinesoil under cotton plants absorbed 270 m3/hain the same time.After 397

IRRIGATIQN, DRAINAGE A N D SALINITY Table 12.9. Changes in volumetric weight,pososity und permeability as a result of irrigation in the soils of Central Asia-(Data by Yu P. L"x, 1948 and ZIMINA, 1957)

Light sierozems

Desert (takyr) soils

Depth

Volu-

Porosity Infiltra-

Depth

(cm>

metric

tion rate

(cm>

%

weight

(mmlmn)

(!z/cm3)

Virgin

0-10 15-25 50-60 70-80

1.17 1.18 1.16 1.26

57 57 58 56

0.29 0.32 0.27 0.21

Irrigated soil, cotton plants

0-10 20-86 50-60 70-80

1-34 1.70 1.50 1 a36

46 33 46 50

0.07 0.03 0.34 0.26

0-10 10-20 20-30 60-70 80-90 Ancient 0-10 irrigated 10-20 soil, 20-30 cotton 60-70 Virgin

VoluPorosity Infiltrametric % tion rate weight (mmlmn) (s/cm3> 1.53 1.57 1.55 1.67 1.66 1-41 1.52 1.58 1.46

43 42 43 38 39

0.05

49

0.25

50 44 48

plants leaching, the infiltration rate of saline soils usually drops owing to the disperson of the soil particles. Subsequently,when the soil is cultivated,its water permeability improves. Infiltration rate may be changed also by the translocation of clay and other materials from surface to subsurface horizons;when conditions of irrigation are such as to encourage the development of large pores and channels in the upper soil,it may be increased notably,Irrigated citrus soils at Grisith, New South Wales, showed final infiltrationrates (during the third hour oftest) of 11.5 cm/hourin treatments under sod culture, compared with 2.7 for cultivated treatments and 2.5 for bare uncultivated treatments. This difference was attributed to faunal burrows under the sod. Infiltration may be controlled by a very thin surfacefilm ofsoilmaterial in dispersed or puddled condition. Heavy rainfall,large droplets from sprinkler irrigation,also flood or furrow watering of certain types ofbare soil, may induce such a condition. Deposits from silty irrigation water may diminish infiltration rate very considerably.This is not always undesirable as is pointed out elsewhere in this chapter. 4. Effects on soil permeability

Changes outlined earlier have obvious implications on permeability also. Any process which will alter the pore size distribution in a soil will alter its permeability to water, both for saturated and unsaturated flow. The process of dispersion or peptisation of soil colloids and the opposing process of flocculation are of paramount importance in this respect. Whatever may be the exchangeablesodium percentage,the soil may be kept as permeable as its constitution will allow by a suitable concentration of salts in the applied irrigation water. For a sodic soil, these salts should in general be calcium salts. Where sodium adsorption in the subsoil is increasing by irrigation,swelling of the soil colloids decreases the permeability. Migration of clay particles,with concurrent blocking of fine pores,has the same influence. Increases in soil permeability have been attributed to drainage. The main effects are due to wetting and drying cycles ofthe soil and the formation,if only during drying,ofcracks.The influencesof soil disturbance and mixing of subsoil materials such as calcium carbonate and gypsum with other portions of the profile in making drainage trenches could also be important. Among instances in which decreased permeability has been sought is that of rice fields on the alluvial fan of the Kurobe River in Japan. Here the highly permeable,shallow,gravelly soil overlying sands and gravel was treated with red clayey soil. This was given an appropriate mechanical treatment then jetted into the irrigation channels upstream of the rice fields.A remarkable decrease of permeability was achieved. This in turn is expected to lead to higher temperatureof the water and increased yields.Compaction procedures have been used for decreasing permeability and infiltration in headland ends of highly permeable soil irrigated by surface methods. 398

EFFECTS OF IRRIGATION A N D D R A I N A G E O N SOILS 5. Formation of impermeable horizons Irrigated soils of the arid zone having compact,hard and impermeable layers in various parts of the soil profile are often found.The presence of these horizons hampers the introduction of irrigated farming;often causes over-wettingof the surface soil during irrigation;impedesthe leaching of saline soils,and prevents the natural development of plants. Impermeablehorizons are of various types. Some are formed of carbonates and gypsum; but the majority are caused by consolidation and hardening of the silicate soil which forms a 1939). surface crust and a compact sub-arablelayer (so-calledplough sole) (RYZHOV, The last type ofimpermeable horizons and crusts are most likely to form in soils offairly recent irrigation. soils of heavy meachanical composition are particularly liable to form crusts. Soils of the humus-carbonateclayey type, formed in dry climate conditions under the influence of waterings, are dispersed owing to peptisation of the soil particles,leaching out of the salts,increase of alkalinity or other causes. Under prolonged irrigation associated with good drainage, the clayey part ofthe soil becomes aggregated, loses its mobility, and mixes evenly with the silty and sandy particles thanks to humus and calcium in the irrigation waters, to the perforation of the soil by earth-worms,and to tilling operations. In long-irrigated soils of old oases of Central Asia therefore no clearly defined impermeable silicate horizons are observed nor is there an observable consolidation of the sub-arablehorizon.Periodical interruptionsof the irrigation, accumulationofhighly dispersed irrigationdeposits containing organic substances and irrigation with alkaline water all delay the formation of crustal soils. It has been observed in Egypt that when groundwater rises, gley-formationmay begin, at first along the roots and then in a solid band. This is associated with the formation of sodium clay, an alkaline medium having been developed by Microspora desulphuricarzs under reducing conditions with the formation of iron sulphides. Another type of claypan in irrigated soils of the Nile Valley formed under conditions of high groundwater has a compacted clay horizon,high in exchangeablesodium and cemented by magnesium silicate and perhaps calcium carbonate. Experimental production of claypans has been shown in California. Alternate treatmeirt with a positively charged soil of ferric hydroxide and a negatively charged clay suspension gave a claypan formation in a column of sand.Related experiments illustrated the protective effect of humus against flocculationof sodium clay and demonstrated the formation of a compact clay layer when clay suspension was permeated through very fine quartz particles and the intensification of this process by the presence of salts,including lime and gypsum in solid form. There is evidence suggesting that a kaolinitic clay can fixiron oxides descending Ïrom surface horizons and provide, after saturation,a suitable surface for the initiation of an iron pan. The maximum amount of iron oxides that can be associated with a kaolinitic clay without the formation of iron concretions is 12 %. Black insoluble deposits high in manganese dioxide with less ferric oxide were found clogging tile drains in California in soils dominated by fine to very fine sand and silt fractions. In long-irrigatedterritories,we often find soils with solid gypsum and carbonate horizons forming in the middle and lower part of the soil profle (Fig. 12.5). Such soils are located in inter-mountaindepressions and foothillplainswhere carbonate and calcium sulphate groundwatersare near to the soil surface.The formation of these horizons is always associated with a fluctuating water table close to the surface.Marling occurs also ander rice fields when they are watered with calcium water. At Merbein, in north-westVictoria, outlets from tile drains from irrigated soils containing much finely divided calcium carbonate were found to be blocked by vesicular deposits consisting largely of calcium and magnesium carbonates.

6. Changes in hydrology of soiIs (a) Fluctuation of lzumidity In arid zones,under natural conditions,the upper horizon of soils may be wetted to fieldcapacityintherainy season and in the dry period the water content decreases to wilting point or below,so that plant growth does not exist or is of ephemeral nature, unless roots can reach groundwater. Thus,under natural conditions, moisture varies between 5-10 % and 100 % of field capacity. Crops may already suffer from water shortage at soil moisture contents of 60-65% of field capacity. Therefore irrigation aims at keeping the moisture content of soils between narrow limits,e.g.between 70and 100 % of field capacity during growth of crops. 399

IRRIGATION, D R A I N A G E A N D SALINITY O

20

40

Soil composition (%) 60 80 100

Depth (cm)

Fig. 12.5.Soil with carbonate-gypsum horizons Terminology of water in the root zone of plants, and soil and water relationships (including the concept of field capacity), have been discussed in Chapter 4,the reaction and requirements of crops to humidity in Chapter 8 and irrigation practices in Chapter 10.

(b) Flow of soil moisture During watering a mainly downward flow is created, not only rewetting soil horizons but also removing salts from the root zone.This infiltrationprocess is comparableto that in natural soils where it is caused by rain,but it is often repeated with irrigation crops. Capillary rise of soil moisture, the reverse of infiltration,will occur in the root zone of the soil during evapotranspiration,but is particularly important in cases of high groundwater tables. Rising groundwater tables due to irrigation will therefore tend to increase capillary water movement to the root zone. Both infiltration and capillary rise of moisture are extensively discussed in Chapter 4. (c) Formation,fluetuution andflow of groundwuter Before irrigation,groundwater is fed by surplus rain water,eventual seepage from surrounding higher areas or (in deltas and valleys) by river water. This results,in dry regions,in limited fluctuations ofthe water table. With irrigation,surplus applied water on the field and seepage water from canals are added to the sources feeding the groundwater. Consequently a rise in water table is the result of irrigation and an increased outflow ofgroundwater in the irrigated region will occur,either by natural outflowor by artificial drainage or by both. Moreover the fluctuation of the water table increases considerably,not only seasonally but also between water applications.For further particulars reference is made to Chapters 2,6 and 11. The increase of the groundwater body may lead to the formation of swampy meadow soils or-by lack of drainage-to swamps in the lowest areas within an irrigation project or in low areas surrounding the irrigated region. Flow in groundwater may be further increased in irrigated soils by the varying salt content of different

400

EFFECTS OF IRRIGATION A N D DRAINAGE O N SOILS

groundwater layers. MINASHINA found in a certain case the most intensemovement of solutions in a stratum at 10-15 m depth. (d) Increased evapotranspiration Because of the much higher average level of moisture in irrigated soil than in comparable unirrigated soil, evapotranspiration is able to approach much more closely the energy accessions given by incident radiation of the sun.This means that relatively less of the sun's radiation will be used for heating the soil or the air and more for plant requirements. Evaporation and transpiration are particularly high in cases of high groundwater levels. So in the Bokhara showed that a cotton crop needed 767 mm of water for region lysimeter experiments made by KABAEV evapotranspiration with a groundwater table 3 m deep,this quantity increased to 1185 mm with groundwater at 1 m.In this case 418 mm more groundwater was used,whereas only 89 mm less irrigationwater had been given. Summarising the changes in hydrology, irrigation promotes the maintenance of even soil moisture, intensifies the moisture exchange in soil horizons, and accelerates the circulation of groundwater. These changes in turn promote the displacement and differentiation of salts and cause the last processes to extend to greater depths than under natural conditions. It should be added that the changes in physical characteristics of the soil,brought about by irrigation and discussed in former sections of this chapter,have a profound influence on the quantitative hydrological relations.

7. Changes in thermal properties of soils Water on one side,solid and gaseous soil constituents on another side,have different thermalcharacteristics (e.g. specific heat, thermal conductivity). (a) Specijk heat According to the unit system,the specific heat ofwater is 1.0while it is about 0.2 for the various constituents of the soil (e.g.quartz sand,0.19;calcium carbonate,0.21) except humus (0-45). Consequently,adding water increases the specific heat,e.g.for kaolin,when dry its specific heat is 0.23,half saturated 0.54 and saturated 0.85.It is obvious that the wetter a soil is, the larger is the time of warming up under incident energy. (b) Thermal conductivity When the pores of the soil are reduced or filled with water,the thermal conductivity is increased, e.g. if the heat conductivity of an air-dryloose loam is taken as 100 (as a reference basis) it rises to 120% when the loam is moist and loose and to 175%when moist and compact. (c) Consequences The change of the water content of a soil modifies the abovementioned characteristics and also the evaporation.Without going further into this problem,we may clearly see that the energy balance of a soil depends on its water content and this will result in changes as regards the temperature not only of the soil (i) but also of the air layers near the soil surface (i). (i) Temperature differences as high as 5°C have been found between drained and undrained lands in springtime. Such differences are very significant for germination of seeds,such as cotton. On the other hand,the cooling effect of irrigation water may be an advantage in particular cases,e.g. as in the culture of irrigated potatoes on light chestnut soils of the arid regions of the Volga. (ii) The air temperature is also modified: irrigation not only compacts the surface cultivated soil but also raises the specific heat and thermal conductivity,so that the ability of the soil to supply heat to the lower atmosphere at night during radiatiod frosts is raised.In the Murray River Valley ofAustralia whole settlements are irrigated before a certain date in the spring,when the youngvshoots and inflorescencesof irrigated this has vines are very vulnerable to frosts.Although this may give a rise in temperature of only 1" or 2"C, been a successful practice since 1922,when a disastrous frost occurred, giving rise to the adoption of this method. Recent investigationshave shown that a combination of light irrigation with compaction by rolling

401

IRRIGATION, DRAINAGE A N D SALINITY may be as effective as the rather heavy irrigations usually given at a time when the vines may not be in need of water.

C. CHANGES

IN CHEMICAL PROPERTIES OF SOILS

I. Dependence on drainage and water balance Since changesin chemical properties during irrigation depend to a large extent on the general drainage,great importance is attached to natural drainage in areasto be irrigated.Drainage is one of the main factors in the classification of areas of irrigated territory worked out in Chapter 6. Experience of prolonged irrigation shows that on well-drainedhigh terraces, foothill strips and the upper parts of alluvial fans with subjacent pebble beds, sadinisation and swamping affect small areas only (Vaksh Valley). In such areas,irrigationhas the effect of desalinising the soil,and intensifying biological soil processes favourableto the development of plants. The irrigation of uiidrained or poorly drained territories-which include the lower terraces and deltas of rivers, flat and non-intersected ancient alluvial plains and depressions-causes a rise in groundwater, secondary sailinityand swamping.In order to offset the undesirable effects ofirrigation,artificialdrainage is needed. When irrigating poorly drained territory, local conditions exert a great infiuence on the processes of desalinisation and salinisation. The highest parts have the best drainage conditions.In many ancient oases, located in the valleys and deltas of large rivers, only 30 to 5Q% of the total area may be irrigated. The remaining part of the land serves for natural drainage,receiving surplus water to be evaporated. In the Valtsh Valley,when the general drainage conditionswere improved,the zone oflocal desalinisation increased to 74%; the area ofthe saline zone decreased to 10%; and the size of the intervening strip between the two zones decreased to 16% of the total area (Table 12.10). Table 12.10. Percentages of desalinisedand saline soil areas in ancient irrigated territory with varying conditionsof natural drainage ~~

Area and drainage

Zone of local desalinisation; soils either non-salineor slightly saline

conditions

Zone of salinisationldesalinisation.Medium

Zone of local accumulation. Solonchaks,

Mounds, banks

meadow-swamp and swampy soils

~

1. Vaksh Valley, 3rd

terrace,territory with satisfactory natural drainage (Sundukov) 2. Vaksh Valley, 3rd terrace,zone where groundwaters lie close to the surface. Poor natural drainage (Kuteminski). 3. Central part of the Murgab delta; undrained territory (Minashina)

74

16

10

no data

36

34

19

12

25

35

33

7

2. lIEect of irrigation on soi1 minerals and exchange complex

As a result of irrigation, the original composition of soil minerals undergoes considerable changes, due to the accumulation of irrigation deposits. The changes occurring are more marked the greater the variety of 402

EFFECTS OF IRRIGATION A N D D R A I N A G E ON SOILS

sources of supply of both soil-formingmaterial and irrigation deposits. Colour and chemical composition of deposits vary widely, e.g. the deposits of the Nile contain 2.5 times more humus, twice as much Fe@, and much more Algo, than those of the Amu-Darya,but only one-twentieththe calcium carbonate.The Nile deposits are dark coloured, strongly weathered and clayey,whereas those of the Amu-Darya are grey and have a much coarser particle composition coneisting of primary rock-formingminerals. Within the same irrigation project,differences in the mineralogical composition of the soils are due mainly to variations in the size of the particles suspended in the irrigation waters (Table 12.11). The alterations in mineralogical composition in older irrigated areas may be considerable. Table 12.12 gives an example,comparing a fraction of fresh suspended material with the same fraction in sierozem soil. The fresh deposits contain more non-weathered minerals,feldspars and rock fragments.The minerals in soil are more quartzified and clayeyfied. Table 12.11. Mineralogical composition percentage of deposits,Amu-Darya delta,Kyzcitken Canal (KOVDA, ZAKHARINA, SHELYAKINA,1959)

Size of fractions in mm

Quartz

0.1-0.05 0.05-0.01 0’01-0.005 < 0.005

39.62 43.62

Feld- Micas Frag- Chlo- Clayey Phy- Amphi- EepidoteOres aggre- tolit boles zoizite spars and ments rite hydro- of gates micas rock

29.89 6.79 11.30 0.14 2.01 - 4.80 1-20 2-53 33.51 15.41 - 0.24 0.84 0.30 2.90 - 0.70 0.71 2-52 58.26 34.41 - 0.98 mainly hydro-micaceous,with admixture of montmorillonite,quartz,etc.

3.72 0.44 1.10

Rutile tourmaline titanite etc. 0.31 0.13 1.19

Table 12.12. Mineral composition (percentage)of fractions of 0.01-0.1 m m takenfrom sierozems and irrigation deposits in the Tashkent region,after destruction of carbonates by HCl (Datafrom V. P. ICOSTYVCHENKO,1957)

Depth (cm)

Weath- Micas ered and ferruginised brown micas

Ores

Epidote Horn- Quartz Feldzoizite blende spars

Weath- Hydro- Fragered micas ments of rock mineral

1.1

Sierozem 30-40 On loess 150-160 Suspended material from the Dxhun

1 *9 2.0

2.6 4-0

0.1 0.2

0.2 0.2

25.7 20.7

20.3 26.9

27.7 15.7

11.5 29.3

8.8

0.9

canal

1.1

1.2

0.1

1.0

0.2

13.6

34.3

23.7

6.5

17.6

-

It has been reported by HAMDY from Egypt that an examination of soil profiles in different localities indicated that in the surface soil the 0-002mm fractionscontain mostly illitic clay minerals,whereas in deeper layers the percentage of expanding lattice minerals of the montmorillonite type is increased. This led to the suggestion that the natural subsequence of illitic clay over thousands of years weakens the attractive forces bonding the ISx layers.K ions are removed,water molecules enter between the layers and the lattice becomes expanding. The alteration has been encouraged by the relatively high content of alkaline earth salts in the groundwater. In Section B1 of this chapter the breakdown of granitic fragments in soils of Salt River Valley, Arizona, as well as the loss of silica from colloids in Gila clay have been mentioned already. Irrigated soils will often undergo a change in the exchange complex.This can be due to enrichment of the soil with hydro-nicas and clay-mineralsand with humus, both occurring in irrigation water, but also to changes in the existing minerals as described above. Slight increases of exchange capacities are reported for 403

IRRIGATION, DRAINAGE A N D SALINITY sierozems and desert soils in the USSR.Chernozem soils in Nebraska increased in cation saturation and p H after 10 to 20 years’irrigation. O n the other hand changes in exchange capacity of irrigated soil may be lacking.Irrigation of four New Mexico soils for 25 to 40 years is an example of this.Partial breakdown of the exchange complex with release ofsilicaresults from irrigation ofsome sodic soils (e.g. ‘szik’soilsin Hungary). This lowers the total exchange capacity. The composition ofadsorbed cations depends on the quality ofirrigation and groundwaters.Irrigationwith water of large rivers will often lead to saturationwith Ca,as these waters are high in calcium. Often irrigated soils contain larger quantities of adsorbed M g than soils not irrigated (Table 12.13). Table 12.13. Composition of adsorbed cations in long-irrigatedsoils (mg1100g)

Nile delta Tashkent oasis Soil of Permation irriga- nently deposits tion irrigated basin soil 0-10 30-40

75-85

0-22

22-40

40-50

50-75

Na K

38.0 13.6 1 -0 0.3

42.1 15.4 O O

27.1 15.6 O O

7.2 2.3 O O

8.3 1e 4 O O

5.9 2.9 O O

6.0 2.0 O 0.6

6.0 2-4 O 0.6

6.4 2.0 0.5 0.4

4.8 2.4 O 0.4

Sum of adsorbed cations

52.9

57.5

42.7

9.5

9.7

8.8

8-6

9.0

9.3

7.6

Adsorbed cation

Ca Mg

Murgab oasis

Irriga-

Depth of samples in cm

Irrigation and groundwaters containing appreciable amounts of carbonate of sodium are specially liable to give trouble with soil sodicitybecause the precipitation ofCaCO, in the soil gives an opportunityfor sodium to build up on the exchange complex.In a recent consideration of Punjab groundwaters it has been pointed out that this process may be cumulative towards virtual saturation of the complex with sodium and an accumulation of free Na,CO, and NaHCO,. This is because soil p H usually increases as exchangeable sodium rises, and this enhances the precipitation of CaCO,. Irrigation waters contain large quantities of water soluble salts, which are distributed throughout the irrigated area in accordance with the actual water and salt balances. As the salts circulate with water in the soil,their composition becomes differentiated,owing to the different degrees of mobility of the various ions,to cation exchange and to the concentration of soil water by evapotranspiration.Less soluble salts,like calcium and magnesium carbonates and calcium sulphate,willprecipitate as a result of increased concentration whereas the easily soluble salts (NaCI,MgCl,, Na,SO,, MgS04) are leached and carried away to places oflocal accumulation round the edges ofthe irrigated area or beyond this area. Examples of accumulation of the less soluble salts are found in the Murgab oasis where the irrigated soils contain 3 to 5% more CaCO, than young alluvial soils with the same physical composition (16-20%, as against 12-15 %)and in the Nile delta where the river deposits contain (on an average) 1.5 %CaCO,, whereas irrigated soils contain 3.3 % (mean of 7 analyses). When poorly drained territories are irrigated,there are often accumulations of gypsum (CaSO, .2H20) in combination,frequently,with glauberite (CaSO, .Na2S04). The extent of accumulation depends on the content of Ca and SO, in the irrigation and groundwaters. In the ancient irrigated soils of Central Asia, gypsum deposits constitute as much as 1-3 %or on occasion up to 10-15 %of the dry soil.Gypsum is found mostly in the middle and lower sections of the soil horizon accumulating in the greatest quantities along the lower parts of irrigation relief slopes. In ancient irrigated areas,soils with a large content of chlorides are fairly common (GRABOVSKAYA, 1961 ; MINASBINA, 1962). Accumulations of easily soluble calcium salts (CaCI,) occur most frequently on ruins of someformer settlements and alongside large ancient canals,together with nitrates (in quantities of 5-10 %), times also phosphorus (up to 2%),potassium and sodium chlorides,

404

EFFECTS OF IRRIGATION A N D DRAINAGE O N SOILS 3. Changes in nutrient elements

Irrigated soils mostly contain larger reserves of nutrient elements per unit of soil than do non-irrigatedsoils in the same area. Although sierozem soils may be relatively rich in arid zones, their nitrogen content is low. After reclamation,organic matter is decomposed rapidly and the need for nitrogenis increasing.Withgood management,the fertility of sierozems increases and when irrigated these soils may contain finally more humus and nitrogen than non-irrigated soils. In desert zones, however, the increase of humus and nitrogen, due to irrigation,is greater. Differences in relief, caused by irrigation, often lead to differences in humus content and mechanical composition,and at the same time to varying accumulation of nitrogen and other nutrients. High temperatures can induce rapid movement of nitrate towards the surface after irrigation especially into locally elevated areas,as has been found in Egypt.Soil water logging depresses the uptake ofnitrogen by young Phalarisplants as was found in western Australia. Irrigationresultsin the transformation of ammonia N into nitrate N and an increase in the mineral N when applied to sharaqi (fallow) soils for maize in Egypt. A finding in the U S A is of large losses of nitrate from irrigated citrus orchards by leaching;in a lysimeter experiment in California,leaching losses up to 570 kg/ha of N occurred in a single winter from treatments heavily fertilised with N. Large-areainvestigationsin the Yakima River Valley of Washington have shown that for 150000 hectares growing a variety of crops approximately 70 kg/haof nitrate is removed from the irrigated area by drainage to the river. Phosphorus. Although the mobility of phosphorus is low, it may accumulate in depressions of irrigated areas. Annual losses of only 2-5kg/ha of soluble phosphate is reported from work in the Yakima Valley, Washington,A lysimeter irrigation experiment in California during 20 years showed an annual loss of 18 kg phosphorus in the crop only. The same was found at Werribee, southern Victoria, where during 9 years phosphorus uptake by herbage was counter-balancedby annual applications of 255 kg/hasuperphosphate. Fertiliser regime, the kind and quantity of crops produced are very important factors in the soil P picture, Potassium. Although many arid soils,and sometimes the irrigation waters used on them,are well supplied with potassium, to the extent that luxury consumption of this nutrient by crops may occur,there is evidence of mobility of this element under irrigation. Loss of K from the root zone may lead finally to K deficiency in crops. Several cases are reported where less exchangeable K was found in irrigated soils than in comparable unirrigated soils (Australia,Palestine,USA,etc.). On the other hand,field irrigation of citrus for 28 years in California showed no decrease in exchangeable K in the soil at any depth in the treatments without K or manure.This was attributed to release of K from non-exchangeableform. Manganese and iron. Soil changes under irrigation,which increase alkalinity, may depress the availability ofthese elements.Manganese deficiency ofcitrus occurs in highly alkaline soils,and soil conditions and plant physiology of such irrigated crops as vines and citrus may be so deranged that chlorosis due to lack of available iron in parts of the leaf tissue may ensue. O n the other hand, manganese toxicity may occur in irrigated fruit trees given soil-acidifyingtreatments. Copper andzinc. The solubilitiesofthese elements respond to changes brought about in the soil by different conditions of irrigation. In 28 years of differential fertilisation in a citrus experiment in California,soluble zinc was low in extremely acid soil and limed soil,and was highest as a p H of around 6.5.Soluble copper was low where lime was added and decreased as organic carbon increased. Boron, lithium and other micro-elements.Movements of these elements may have to be considered in some situations.Boron in soil or applied water is a factor in production under irrigation in parts of the Coachella Valley of California,the Hungry Steppe of Uzbekistan and Murray River Valley of Australia. Soluble boron may have toxicity effects on crops,but it can be reduced by leaching ifits concentrationin the irrigationwater is not high. Lithium toxicity in irrigated citrus has been reported from California. For both boron and lithium,very small concentrationsmay produce toxicity (refer Chapter 7). In other special cases,movements of elements like selenium and fluorine may have to be checked. It is difficult to predict the behaviour of many elements when a soil is to be irrigated: whereas many elements are increasing in the soil,increased crop growth will remove larger quantitieson irrigated soils than

405

I R R I G A T I O N , D R A I N A G E AND SALINITY

on non-irrigated soils. Generalisation of the behaviour of the elements is further hampered by soil and management differences.

4. Effects on pH and redox-potential (a) PH There was no change in soil p H after 30 years’irrigation of a calcareouschestnut soil in Nebraska, but p1-I increased during 10 to 20 years’irrigation of chernozems in Nebraska. Such increase is the usual effect when arid soils are put under irrigation.During 28 years’irrigation of citrus soil in California,p H rose from 6.8 to 8-0for the surface 15 cm, when irrigation water was the only material added. When various fertilisers were added as well,theeffectsvaried from extremeacidification with large amounts ofsulphate ofammonia to extreme alkalisation with sodium nitrate. The increase in p H is associated in the first instance with leaching of soluble salts. This is illustrated in Table 12.14where the effects are shown of leaching bare mallee soilfor three months with a total application of 10 metres of water at Merbein, Victoria. Table 12.14. Changes in p H associated with leaching at Merbein

Depth of

Salt content as C1

PH

Sample

(cm)

Before leaching

After leaching

( %)

After leaching (%)

0.370 O.160 0-123 0.123 0.138

0.013 0.010 0.010 0.015 0.018

7.8 8.5 8.9 9.0 9.0

8-6 8-7 9.3 9.7 9.8

Before leaching

0-30 30-61 61-92 92-122 122-153

In the same region, vine areas irrigated by normal methods for 6 years showed similar changes in pH, typically from 8-5to 9.5in subsoilsfrom 30 to 90cm probably due to the presence of sodium bicarbonate and sodium carbonate. (Often such cases of alkalisation can be provoked by irrigation with slightly alkaline waters.) Apart from the artificial dealkalisation there is a natural process which can markedly decrease the p H and which follows the drainage of some waterlogged soils. The oxidation of sulphideswhich may be present in such cases is equivalent to the use of sulphuric acid. Most older irrigated soils of arid zones have a neutral or slightly alkaline pH. When the soil is moist and paste-like,the p H usually varies between 7-0arid 8.2.The HCO,content of a water extract is 0.02-0.05 %. Less frequently,irrigated soils have p H values of up to 9 and more, and HCO, content in a water extract of 0.07-0-2%; this usually occurs in connectionwith alkaline soils or soda salinity.When the soils are irrigated, and the soil solutions diluted,the degree of alkalinity is often increased as a result of hydrolysis and increased dissociation of alkaline and alkali-earthbases. Since the soil moisture and the soil temperature, which determine the degree of ionic dissociation,vary widely, the same holds for the MCO, indices and the p H ofmilieu.A n increasein soil moisture,as a rule,makes the p H of milieu more alkaline.This phenomenon is well known from laboratory dilutions of soil (Table 12.15). Table 12.15. Dependence of p H on the soillwater ratio-(After MOSTAFA)

406

Depth (in cm)

1 :0*3

1 :0*5

1 :1

1 :2.5

1:5

0-25 25-50 50-75 75-100

6.40 6.90 6.95 7.01

7.82 7.80 7.65 7-62

8.25 8.18 8.10 7.92

8.62 8.75 8-45 8.31

9-04 9.25 9.00 8.72

EFFECTS OF IRRIGATION A N D D R A I N A G E O N SOILS By raising the temperature of soil solutions extracted from soda-saline soils from 5 to 30"C,the p H is increased from 1 to 2.5 units. The increaseofthe p H value and ofalkalinitywhich occurswhen irrigating saline soils,is one ofthe reasons for the deteriorationof the physical properties of these soils,and the incïeased dispersion ofthe soilparticles. Subsequently,however, when soils are desalinised and cultivated, the alkalinity is neutralised and the p H stabilised at 7-7.5, thanks to the accumulation of carbonates and gypsum and to the intensification of biological processes conducive to enriching the soil solutions in bicarbonates of calcium. (b) Redox-potential The redox conditionsin the soil are determined by the index BN-valueofthe potential in mV on the electrode: and the index rH,-value of the negative logarithm of the quantity of gaseous hydrogen in the soil solution. The value of rH,is calculated from CLARKE'S formula:

The redox conditions of the soil are frequently determined by the EH reading.In conditions of a neutral and weakly alkaline reaction of sierozem soils,a decrease of the EH reading to values of approximately 300-250 indicates intensification of the de-oxidisingprocesses. The chief reactions involved are nitratesaminonia,sulphatessulphide, ferric ironSferrous iron, and manganic manganese%manganous manganese. Here the oxidised state is mentioned first in each pair, the reduced state second. Nitrate may be reduced to nitrite or to the gases nitrous oxide or nitrogen a5 well as to ammonia.Phosphate may sometimes be reduced to the gas phosphine. Organic materials in waterlogged soils may be fully reduced,as to methane gas,or partly reduced,as to aldehydes.Reduced aeration ofthe soil associated with increased water content, restricts oxidation processes ; frequently this effect is intensified because of increases in the carbon dioxide content of the remaining air. It has been found in Germany that at 5 c m below the surface in a flooded organic soil,76% of the iron was in the ferrous form,24%as ferric iron,while at 15 cm depth,all the iron was ferrous.As much as 10 g of ferrous iron per kg of soil may be found in rice soils of the USSR, but this is not harmful to the crop because of the high oxidising capacity of rice roots. Indian investigations have shown that manganese toxicity in rice may result from uptake of manganous carbonate after reduction of large amounts of manganic compounds under intense swamp irrigation conditions. On the other hand,the deposit of MnO, with Fe,03in tile drains in California,followed successive dehydration and exposure to active oxygen sources. The production of sulphides in waterlogged soils is well known.When such soils are drained,equilibrium shifts towards production of sulphate,often with considerable decrease in pH. Although the onset of reduction may be quite rapid after a soil is flooded,the attainment of equilibrium may take a long time.Eighty days have been found for some ofthe processes.In normal irrigation,as distinct from prolonged submergence, the tendencies are for cyclic fluctuations of soil mositure to induce cyclic changes in redox-potentialat any rate in those horizons with restricted permeability. The extent of these changes may be judged from the following figures for exchangezble cations (as m g per 100 g) in a marsh soil: oxidising conditions:iron 3,manganese O, aluminium O; reducing conditions: iron 298,manganese 19, aluminium 8. An interesting reduction process appears to take place in soils waterlogged for a portion of the year. Here some ofthe soil phosphorus is converted under reducing conditionsinto a soluble form which is then leached down the soil profile. Comparative data on the redox-potentialof irrigated and virgin soils respectively indicatethat the changes it undergoes vary according to soil conditions and season.In virgin sierozems of Central Asia, the highest Ex$readings are obtained in the dry (summer) season of the year (Fig. 12.6). Irrigation of sierozems causes a slight drop in the EH readings for the spring and summer seasons.For the autumn period, the EH readings for irrigated and the non-irrigated soils are similar. Meadow soils (Kashka-DaryaValley) give much lower EIIreadings than sierozems.At the same time,the Efxindices rise sharply between spring and autumn with the lowering of the groundwater level and the decrease in soil moisture. Cultivation ofmeadow soils causes the Ex$readingsto rise noticeably,but they still remain lower than those obtained for irrigated sierozems. It may therefore be concluded that irrigation 407

IRRIGATION, DRAINAGE A N D SALINITY exercises a beneficial eirect on the redox conditions of the soil,slightly reducing the tension of the oxidising processes occuriiig in automorphous soils and raising in it hydromorphous ones. Research on the variations of redox conditions (KONONOVA, 1932)shows that the E, reading drops sharply within three days after watering. The decrease of the EIIreading is greatest when watering is done by the flooding method (Fig. 12.7); with furrow irrigation,the decrease of Erfis less abrupt. The irrigated meadow soils of the Ferghana inter-mountainvalley give lower EH readings than those of the Kaskha-DaryaValley,where the groundwater level is less stable.After watering,the EH readingsformeadow soils are from 107-170 mV lower than before watering;in sierozems,from 230 to 200 mV lower. In other words, the variations of the EH readings are smaller in meadow soils than in sierozems. In addition, the time taken for the EEIvalue to rise again to the pre-watering level is considerably shorter in meadow soils than in sierozems(4-5 days as against 14-15 days). This is because in meadow soils,the groundwater table rises after irrigation and the moisture content in the arable horizon remains high for a longer period of time than in sierozems. Eh mV 550

r

3

5

0 Summer

Spring

5 Autumn

Fig. 12.6.Seasonal changes in the E,-value of the 0-10 cm soil layer (average of 10 readings) (Data by MURAVEVA, BRATCHEVA, ALEKCHEENKO, 1959) 1. typical sierozem 2. meadow soil 3. irrigated sierozem 4. irrigated meadow soil Eh mV 650 550 500

--

300 250 200

I

l

l

l

l

l

l

1 2 3 4 5 Days after watering

I 14

Fig. 12.7. EIIreading of soils irrigated by flooding I. Typical sierozem (data from M.M.KONONOVA, 1932)

1. at depth of 2 c m 2. at depth of 5 cni 3. at depth of 15 cm

II. Meadow soil of inter-mountainvalley (data from I. P. SERDOBOLSKIand P.I. SI-IAVRYGINA) 4. arable horizon, control plot 1 5. arable horizon, control plot 2, July

408

EFFECTS OF IRRIGATION A N D D R A I N A G E ON SOILS

D. CHANGES

IN BIOLOGY OF SOILS

1. The infiuence of micro-organisms (a) Efsects in diifSent soils Irrigation and soil salinisationhave a great influence on soil micro-organisms.In the USSR much research has been done on various types of sierozem,on takyr soils of the desert area, on solonetzand solonchak of the dry steppes and on desert-steppesoils. In takyr soils,in the south-westof the Central Asian desert zone,the biological activity is very weak.The development of micro-organismsin takyr soils is restricted by moisture deficiency during the period of seasonal high temperatures and by the low content of organic matter, which in the upper soil layers (O to 20 cm) amounts to no more than 0.5 %, and a nitrogen content of 0.04to 0.06 %. Nevertheless, takyr soils contain,even though in insignificant quantities, all the important groups of bacteria (ammonfying bacteria, nitrifving bacteria, cellulose-fermenting bacteria, butyric bacteria, etc). Nitrogen-furing bacteria are rare; takyr soils contain mouldfungi and actinomycetes.All the same,the total number of micro-organisms,even in the upper layers, amounts to no more than IO5 to lo6 per gramme of the soil. Seasonalvariationsare observed in the development ofmicro-organisms;they are most active in the spring, which is the rainy season;in the summer,the drying of the soil reduces the activity of micro-organisms. Saline takyr soils have a particularly scanty microflora. Algae,in particular the blue-greenvarieties,are offundamentalimportancein the formation oftakyr soils; their continuous cover on the surface of takyr soils during wet periods is the main source of organic matter and nitrogen.Algae also play an active part in the destruction of the mineral part of the soil,in particular of alumosilicates. During the autumn and winter months diatomic algae develop in takyr soils. When takyr soils are reclaimed by means of irrigation, the number of micro-organisms increases, their composition becomes more varied, and their activity more intense. The greater abundance of microflora in irrigated takyr soils can be partly explained by their being brought in with the irrigation water, although the quantitative aspect of this phenomenon is not entirely clear.The agricultural reclamation of takyr soils (tilling, sowing and the use of organic fertilisers) brings about an increase in their biological activity; in saline takyrs,the increase is stimulated by leaching (Table 12.16). Table 12.16. Microjlora of takyr-typesoils (in thousandsper gramme of soil) in Turkmenia (PALETSKAYA, KISELYEVA, 1961)

Soil

Depth (in cm)

Total number

Ammonifying

Nitrifying

of micro-

microorganisms

micro-

organisms Virgin takyr-type

0-7

1600

254

soil total salts in the soil 0.3%

7-1 5

900

15-40

Azotobacter

organisms 0-25

0.01

6

0.01

nil

300

26

0.03

0.01

0-10

2250

520

0.06

0.62

10-25

2000

140

0.27

0.60

first year.

25-40

1900

280

0.27

0.03

Irrigated takyr-type soil under

0-10

9500

6170

2.57

61.71

10-25

2600

640

2.67

0.03

25-40

900

1400

0.06

0.06

Irrigated takyr-type soil under cotton in the

first year

lucerne total salts in the soil 0.07%

409

IRRIGATION, D R A I N A G E A N D SALINITY Research on the important sierozem soils of the Central Asian Republics has shown an abundance of micro-organismswhich occur in quantities of approximately lo8 to 1040 per gramme of soil.The physiological groupsinclude ammonifying bacteria,nitrifying bacteria,nitrogen-fixingbacteria,cellulose-fermenting bacteria and other groups.The mould fungus and actinoniycetes content of sierozem soils is also high. In non-irrigatedvirgin sierozem soils,the activity o€ micro-organismsis seasonal, depending on rainfall. Micro-biologicalactivity is at its peak in the spring,when natural wetting and a suEciently high temperature provide the soil conditions for a luxuriant development of ephemeral plant vegetation. Inadequatemoisture during the summer months in non-irrigatedsierozem soils causes a decline in the activity ofmicro-organisms, until it is revived by precipitations in the autumn. By cultivation of virgin sierozems,especially under irrigation,the quantity of micro-organismsincreases and their activity rises; during the summer period (May to August) micro-biologicalprocesses are most intensive. The data shown in Table 12.17should be regarded as an example of the effect of irrigation on soil microflora. Table 12.17. Mumber of micro-organismsper gramme of soil

Soil

Mould fungi (in thousands)

Ammonifying bacteria (in millions)

Nitrifying bacteria (in millions)

Virgin,non-irrigated sierozem soil Irrigated,cultivâted

1.0

10.0

0.1

sierozem soil

3.3

29.0

10.0

Azotobacter,

number of colonies developing on the cnp nil 20-200

These data were obtained from the sierozems of the Vakhsh Valley. For a long time the soil was under irrigated crops,mainly cotton and lucerne. The distinctive property of sierozem soils,particularly under irrigation,is the high intensity of mineralisation of organic matter and, in particular, of organic nitrogen. These processes are accompanied by the formation of CO,and NEI,,and by the oxidation of the latter to NO,.The accumulation of NO,in the sierozem soils of certain parts of Central Asia is so high that it can produce nitrate salinisation. The favourable moisture and temperature conditions in irrigated lands of the arid regions,together with the addition of fertilisers and increased organic matter from greater plait growth, lead to much larger populations of soil micro-organismsthan those which occur in comparable unirrigated soils.Experiments on the irrigation of sugar beets in the USSR showed that the number of micro-organismsincreased from 17 to 20 million up to 97 to 127 million per g of soil as the result of only two waterings. The number of bacteria has increased also after irrigation of light chestnut soil for herbage production in Kazakhstan. When the natural soil of the area is saline,very strilcing changesin the number ofmicro-organismsmay take place very soon after leaching. The figures shown in Table 12.18 were all obtained within one month in the case of initially highly saline soil in the Jordan Valley of Israel. While some of the greater number of micro-organismsshown in Table 12.18 came from the Jordan water, they were due mainly to multiplication in the soil once the salt content was lowered sufficiently. The salinisation (chlorideand sulphate) ofsierozems-a fairlywidespread phenomenon-has a considerable influence on soil microflora. The results of a coaparative study of micro-organismsin saline and normal sierozem soils,as well as the data ofthe experiments with the artificial salinisation of non-salinesoils,suggest the following conclusion. Chloride salinisation exerts a stronger negative influence on soil microflora than sulphate salinisation.At lowconcentrations(up to0.3%ofsoilweight), sulphatesslightlystimulatethe development of actinomycetes,fungi,nitrogen-fixingorganisms and cellulose-fermentingbacteria. It has been found that in strongly saline soils many important micro-organisms,and in particular the nitrifying bacteria, are missing. When the salt content in the soil is 1 %,three to four times less NO, is formed than in non-saline the NO,formation is halved. In strongly saline grassland sierozems,no nitrifying sierozem soils;at 0.7%, bacteria, azotobacter and aerobic cellulose-fermenting bacteria develop. Butyrous, ammonifying and denitrifying micro-organismsdevelop to a limited extent only. Leaching of salt in a light sierozem under lucerne gave an increase of the number of bacteria from one million per gramme of soil up to 1.5 to 3 million.

410

EFFECTS OF IRRIGATION AND DRAINAGE ON SOILS

As well as changes in the numbers of micro-organisms,irrigation may bring considerable changes in the activities of particular groups of organisms. Soil salinisation brings about adaptation to salts of certain micro-organisms,such as actinomyces and pseudomonas. Data indicative of the salt tolerance of Rhizobia are available. These bacteria, found in nodules of legume plants growing on saline soils,are more active nitrogen-fixersthan bacteria from nodules of plants growing on normal soils.The salt tolerance of bacteria from lucerne has also been experimentally HOFFS’S solution (up to 25%) to the culture medium; however, a established by the addition of VAN’T further increase in the salt concentration (up to 50 %)has a detrimental effect on nitrogen assimilation by nodule bacteria. Table 12.18. The number of micro-organismspresent in the soil before arid after reclamation (in upper 20 em)

Condition of the soil

Total salts in the soil

t %>

Chloride in the soil (%>

Number of bacteria and actinomyces per gram of soil

Saline Before reclamation was completed After reclamation After reclamation and manuring

Ammonia nitrogen converted into nitrates after 15 days in incubation

(%I*

10.30

4.84

15200

0.0

0.80 0.28

0.20 0.08

1135400 1890000

4.4 67.5

0.33

0.05

4409O00

81.8

* 150 mg of sulphate of ammonia were applied to 100 g soil

(b) Micro-organisms concerned with the nitr.ogeizcycle In soils of low salinity and nearly neutral, ammonifying organisms in hot climates will normally develop sufficiently under irrigation and cultivation to ensure that no notable increase occurs in organic matter. This matter is converted to ammoniacalcompounds,thereforeit tends to disappear.The ammonifiershowever are sensitive to salts. Investigations on a Californian soil have shown that 0.20% of NaCl or Ma,SO, greatly reduced ammonificationwhich disappeared almost entirely at a concentration of 1.0 %ofeither ofthese salts. Nevertheless,Na,CO, up to 1.0 %stimulated the activity of these micro-organisms.In the Beit Shean Valley of Israel the percentage of organic matter,found after the seventh crop (in rotation), in soilsirrigated with salinewater was higher than in those irrigated with fresh water,in spite of an inferiorplant and root development. This was attributed to inhibition by salts of the micro-organismsparticipating in the breakdown of organic residues. It is clear from Table 12.18 that the activities of these organisms entirely disappear under highly saline conditions.In less saline situations there is a partial suppression ofthese activities,as it was found in the Beit Shean Valley of Israel,where the nitrate content in the upper 20 c m of soil after the ninth crop in rotation, decreased from 35.1 to 18.2 ppm with increasing salinity of the irrigation water. Early US investigations showed that the kind and the concentration of salts are both to be considered.It has been found that 0.10 % ofNaCl or 0.20 %ofNa,SO, stimulatednitrification,but that Na,CO, was highly toxicto nitrifying organisms even at 0.05% concentration. Recent work in Pakistan has shown that nitrification is reduced with either 0.5%NaHCO,, 0.5%NaCl,0.5 % MgCl, or 0.4%Na2S0,. Increasing alkalinity appears to be unfavourable for nitrifying organisms so that they may be inactive when p R exceeds 9.There is evidence that they may be limited by phosphate deficiency in very alkaline conditions. The information here given allows an understanding of the effects of salinity or alkalinity-which may develop under irrigation-on nitrifying organisms; it is obvious,however, that in normal irrigated soils the number and activity of these organisms are high. The free-livingnitrogen fixer Azotobacter seems to be stimulated by irrigation conditions; when found, their number is usually about 1000 per g of soil, this figure rises to 8000 in fertile Nile silt, with N-fixing DI

411

IRRIGATION, DRAINAGE A N D SALINITY Clostridia aswell.Non-symbioticbacteria capable offixing atmosphericnitrogen are sensitive to salts,and the effects come in quite sharply with increasing salt concentrations.Investigationsin the USA have shown that these micro-organisms are virtually unaffected by NaCl concentrations lower than 0.50% or by Na,SO, below 1.20%.At slightly greater concentrations,toxicity was marked. Similarly,with Na,CO, at 0-40%, N-fixationwas little depressed,at 0.50%it was markedly depressed.With high saltcontentazotobactercannot be found. This was the case of the Jordan Valley soils (Table 12.18). After leaching of salts,this organism appeared. Blue-greenalgae seem to be the most effective of all non-symbioticN fixers.In flooded rice soils of India these organisms may fix 80kg/haofnitrogen in six weeks,ifsufficientphosphate and molybdenum arepresent. They can also operate symbiotically. Considerable changesin nitrogen and organic matter content of irrigated soils can be achieved by Rhizobia in association with the roots of leguminous plants. Rhizobium is favoured by non-salineconditions and a nearly neutral reaction,especially if the soil does not contain much nitrate, but will be depressed if the conditions of irrigation are such as to induce salinity or sodicity. The activities of nitrifying organisms are increased under conditions of good soil aeration. The disappearance ofnitrate under reducing conditionshas been mentioned above.The role ofdenitrifying organisms as regards this reduction is well known in dry land and irrigation agriculture and it is favoured by wet,hot conditions as well as the presence of nitrate in appreciable quantity,so that it is specially important under conditions of temporary waterlogging. Other reducing organisms will be considered in the next paragraph.

(c) Organisms concerned with oxidation and reduction processes The production of sulphides in waterlogged soils is due largly to the activity of the bacterial genus Desulphvibrio.Theproducts are typically insolubleiron sulphideswhich accumulatein the soilprofile.Similar accumulations may be found in recent marine sediments.When such soils are drained,the onset of aerobic conditions leadsto oxidation ofthe sulphides,so that the resulting sulphuric acid may depress soilp H to 2.0if the sulphide content is high and CaCO, is lacking.Although this effect may come from chemical processes a microbiological action through such organisms as the ubiquitous Tlziobacillus thio-oxidans provides the usual mechanism. This kind of bacteria is responsible for oxidation of elemental sulphur used as a soil ameliorant on sodic soils. Another organism concerned in the sulphur cycle is Microspora desulphtiricans as mentioned in SectionB5. The bacteria concerned with the iron and manganese cycles,and perhaps with phosphzte transformations, are less known than those associated with nitrogen and sulphur changes,but there is plenty of experimental evidence that bacteria may enter strongly into the ferrous ferric and manganous manganic equilibria.

(d) Other non-pathogenic organisms The bacteria which decompose cellulose are inhibited by increased soil salinity under irrigation.They were absent from the natural saline soils in the Jordan Valley area in Israel but they decomposed cellulose rapidly in the soil after leaching. Actinomycetes occurred in considerable numbers in irrigated light chestnut soils of Kazakhstan and increased when the soil dried out. This is because these organisms,like the fungi, are typically aerobic. Actinomycetesstudiedin a solonetzsoilwith saline groundwater in Hungary were found to vary considerably in their tolerance to sodium carbonate and other salts,the least tolerant species being confined to the Ahorizon and the uppermost part of the B-horizon. Certain fungi which are predatory upon nematodes have to be considered because of important nematode infestation in many irrigated soils. A good organic matter content and aerobic conditions in the surface horizon would give the fungi concerned the best environment. (e) Pathogenic Organisms Irrigation and drainage waters may carry pathogenic organismsfrom infested areas to previously clean areas. The tobacco black shank fungus Phytophthora parasitica var nicotianae and many other soil fungi have been distributed in this way. They may be spread from an originally infested area in the upper valley throughout the basins ofrivers as happened with Phynzutotrichzrm Omnivomm,a destructive root-rotfunguswhich attacks cotton on irrigated alkaline soils.

412

EFFECTS OF IRRIGATION A N D DRAINAGE O N SOILS 2. Effects on soil fauna

The larger burrowing animals are usually displaced quickly to the margins of irrigated lands,or destroyed. Those which are useful for meat or fur get short shrift,because of the greatly increased human population when irrigation begins in arid areas. The ripping of burrows during land preparation and other methods of killing virtually cleared rabbits from the more intensively irrigated settlements in Australia. While irrigation of the Colorado Desert in California drove out such rodents as the round-tailedground squirrel,woodrat,kangaroo rat and pocket mouse,it increased the populations ofharvest mouse, cottonrat, beaver, muskrat and gopher. With the exception of the gopher,the second group are all water or streamside dwellers. It was not on the irrigated land itself, but on channel reserves and such areas that rodents increased. Most rodents cannot stand flooding,in fact many are drowned in flash floods after occasional heavy rain in arid areas. Hedgehogs,desertfoxes,Indian gerbils,prairie dogs, birds and voles are among the fauna likely to disappear under irrigation conditions. In Australia, a crustacean,the yabbie,mainly a small species of Cherax, which can adapt itself to long rainless periods even in the central deserts,can persist under irrigation but its activities then are largely confined to channel banks. Soil fauna play an active part in the processes of soil formation.Many forms of macro-,meso- and microfauna are actively engaged in all such processes as the loosening of the soil,the transfer of organic residues into and out of the soil profile,the redistribution of salt reserves,the disintegration and transformation of fresh and decayed vegetable matter, and the formation of the water-resistantstructure of the soil. It has been established that termites, earthworms and wood-lice play an active role in soil-formation processes in desert soils.The activities of termites are a factor mainly in virgin lands.A study of the distribution of termite habitations-termite nests-in the Golodnaya steppe showed that they occupy up to 10% of the whole area. It has been established that termite tunnels help to increase soil porosity. Termites bring up soil and subsoil particles from great depths spreading them over the surface of the ground,and concentrating them in their nests. In the case of saline soils,the transfer of particles leads to an increase in the salt content (chlorides,sulphates and,to a lesser extent,carbonates) in the upper soillayers. Since they make use ofvegetation residues,termites carry them down into the lower layers;inside the termite nest,the humus content is somewhat higher than in the average soil mass. When a soil is irrigated and cultivated, also the life of termites is disturbed. When adjacent areas are under irrigation,the places where termites are present in virgin soil are sharply delimited by patches of surface secondary solonchaks; and because of the irregular moisture in the soil depressions are formed around the termite nests. Termites have not been found in irrigated pasture soils in an area of south-easternAustralia where they manifest considerable activity in unirrigated land.It is known,however,that they persist in irrigated land in which a good deal of the soil surface is clean-cultivated for part of the year. They attack wooden trellis posts and sometimes old vines in irrigated vineyards in Uzbekistan and Australia,and have been known to attack the roots of young irrigated citrus trees. Even in such cases,termite activity seems to be on a much reduced scale in irrigated land as compared with that in adjoining unirrigated areas. Most ants can survive fairly long periods of immersion under water so that temporary flooding of land as in irrigation does not affect ants to any significant extent. Nevertheless, notable changes of distribution of individual species of ants have been noticed. For example, in inland Australia the meat ant Iridomyrmex detectus Sm was far less common than Iridomyrmex viridiaenus Vichmeyer. After irrigation for some years I. detectus has formed large populations whereas I. viridiaenus is now rarely found. Earthworms are found mainly in cultivated,irrigated soils.While some kinds of earthworms and Enchytraeids are severely damaged by water,many earthworms and wireworms, Elaterid larvae,are able to persist and multiply under irrigation. It was found in Middle Asia that the natural soils of the desert zone do not contain earthworms,whereas in irrigated soils under lucerne they number more than 5 million per hectare. Their activity is such that the volume of their bore-holesis 94m3/haand during one spring they will throw 15 to 20 tons of casts per hectare on to the soil surface. In light-coloureddesert-steppe soils,natural soil may contain about 675 O00 earthworms per hectare while soils irrigated for 10 to 18years may have 1 500 000, their activity being 3 to 7 times more intense. While the annual activity of earthworms in unirrigated soils may be limited to 50 to 60 days in spring,in irrigated soils there are spring and autumn activities totalling up to 190 days. Work inNewZealand has shown that,in astony silt loam,an areanot irrigated for many years had 1250O00 413

IRRIGATION, DRAINAGE A N D SALINITY earthworms per hectare, while an area irrigated for 8 years had about 3875000.O n the niverina plains of south-eastAustralia,where no earthworms are found in unirrigated land except near watercourses,numerous earthworms of the Allolobophora caliginosu complex were found under some eight-year-oldirrigatedpastures on loamy soils.The heavy clay soils of the region,however,do not offer suitable conditions for earthworms even when irrigated throughout the year and covered with a large amount of litter and dung. Eight years after inoculation of such soil with worms, only scattered groups of Micvoscolex dubius, a peregine megascolecid,could be found. Sandy loam soil carrying irrigated pasture,similarly inoculated,had after 8 years a population of about 300worms per m2,weighing about 160 g. These were mostly A. caliginosa. Where dense populations were present, earth had been mixed with the mat and the area between the basis of plants was covered with casts.The mat referred to may be 4 cm thick,composed of partly decomposed plant litter and dung and distinct from the underlying mineral earth.It may contain as much as 1200 kg/ha of nitrogen so that its incorporation with the underlying soil by means of earthworms may be valuable. Attention should also be paid to the part played by wood-licein soil formation in sierozem and takyr soils.The presence of wood-liceis determined to a large extent by the composition of the plant associations; wood-liceare particularly numerous when there is a growth of annual and perennial saltwort and wormwood.Their activity considerably improves the physical properties of the soil,whereas their excretions assist in the development of higher plants. Nematodes are essentially aquatic and are mobile in water so that irrigation and drainage waters may spread nematode infestation. They are also mobile in soil water. The potato-rooteelworms, Heterodera rostochiensis (Wollenweber), for example, moves 2 to 3 cm a day in sand. The larvae of Ditylenchus dipsacì (Kuhn) may move more than 3 c m in 5 hours in sand. While conditions of irrigation are favourable for multiplication of nematodes in general, especially in light soil, some appear to be suppressed.Thus Rotylenchus gvacilidens (Sauer) apparently tends to disappear under irrigation in north-westVictoria. It is an interesting point that the kinds of nematodes associated with irrigation of a particular crop may be quite similar in differentparts of the world. As an example,the genera and species of phytonematodes in irrigated vineyards of north-western Victoria are broadly the same as those associated with irrigated vineyards in California. If an irrigated soil dries out, nematode movement ceases and the organisms desiccate. In frequently irrigated well-drainedsoil the tendencies are for percolating waters to carry eelworms downwards in soil as has been shown for larvae ofpotato-rooteelworm and for the burrowing nematode Radopholus similis which may travel considerable distances down in a short time in sandy soils.Ascaris eggs are transported similarly, while hookworm larvae may be brought to the surface by upward water movement. The optimum teinperature for development or hatch of phytoparasitic nematodes is 20-30°C,a range common in irrigated soils of arid zones. With regard to increasing salinity under irrigation, phyto-parasiticnematodes in citrus and papaya plantings are more tolerant than the plants themselves. Little information is available on the distribution and functions of the protozoa in soils of desert areas. The presence of Amoebae and Flagellatae, and, to a lesser extent, of Infusoria, has been established. In irrigated soils they are found in the vitally active state,capable of reproduction and independentmovement. Irrigated non-salinesoils contain from a few hundred thousand to a million Protozoa per gramme. It has also been established that most Protozoa feed on soil bacteria and therefore their number may be expected to fluctuate according to bacteria numbers. The major factors determining the general nature of the dynamics of Protozoa are moisture,temperature, and the presence of organic material,i.e.the same factors as for soil micro-organisms.It has been found from observations that soil salinisation,and in particular the presence of Na2S04salts,adversely affects both the quality and quantity of Protozoa. It is worthwhile to mention here also the possibility of infestations harmful to man.

3. Roots, organic matter and humus status

(a) Roots

It should be noted that the scantyplant vegetation,characteristic for non-salinevirgin sierozems,has a varied composition;it has well developed root systems,though they are concentrated in the upper layer only no deeper than 10 cm. Consequently,despite the extent of the root formations,which m y amount to as much

414

EFFECTS OF IRRIGATION AND D R A I N A G E O N SOILS as 20 tons per hectare,the fact that they have no proper contact with the soil mass, does not help to keep up the supply of humic material forming in the soil profile. In the arid interior of Australia,Spin$ex-Triodiu spp,or Plectrachne spp may occupy only 10%of the land surface.The root systems of spinifex,however,ramify densely throughout the entire soil to a depth ofat least 3 metres so that the plants have excellent prospects of intercepting whatever rainfall penetrates. Unirrigated native vegetation areas in the Santa Ana River Basin of California had root zones under grass from 180 to 340 cm (6to 11 ft) deep,under brush from 340 to 600c m (1 1 to 20 ft). Nevertheless,under many conditions of irrigation, more extensive and active root systems are formed than in dry-land conditions. Thus after eight years’irrigation of Rhodes grass pastures on the Riverina plain of south-easternAustralia, about 4.5 tons/haof ‘macro-organicmatter’was added to the soil,in comparison with adjoining unirrigated native pasture.Macro-organic matter consists of coherent organic particles of diameter greater than 2 m m . Large amounts of macro-organic matter accumulate under irrigated perennial pastures in south-east Australia;thus three-year-oldpastures on a solonetzous sandy loam accumulated between 9 and 16 tons/ha of oven-drymacro-organicmatter,the concentration decreasing sharply with depth.From one-halfto threequarters by weight of this material found in the O to 47 c m depth was in the top 8 cm. On the other hand, work in France showed that the root system of cocksfoot was unfavourably affected by irrigation, 80% of the roots being in the top 5 c m of soil. Cultivation,with or without irrigation,will reduce the macro-organicmatter in soil. In comparison with undisturbed native grassland, 8 years’cultivation and irrigation of a sandy loam on the Riverina plain in Australia reduced this constituentfrom 0.14%of soil in the surface 15 cm to 0.04%;in a clay the change was from 0.22to 0.08%.Root growth of rice for one season in the same region added 3.75 tons/ha of macroorganic matter to the soil,but the net increase was only 0.75 tons/ha.This means that the accretion from root growth little more than counter-balanced the loss by decomposition before the inundationperiod. The root system of irrigated lucerne may be used to demonstrate the general principle that light,frequent irrigationsindmea shallowroot system comparedwith that which resultsfromlessfrequentheavierwaterings. As an example,it has been found in N e w Mexico that lucerne root penetration increases from 92to 122cm, up to 153 cm,when the water application depths are respectively 5 and 13 cm.Light irrigation,as can be done by sprinkling,frequently causes difficulties due to restricted root penetration. Root growth of many crops is inhibited by high groundwateï level. Investigations on a sandy loam in Taiwan showed that 67% of the root mass of sugar cane occupied the O to 40 c m layer with groundwater at 50 c m depth,while with groundwater at 100 c m there was little concentration of roots in any particular layer. Salt concentrations at various depths resulting from irrigation also discourage root penetration, as was found for irrigated pasture species at a depth of about 45 c m in the Riverina plain of Australia. Deep cultivation and inefficient irrigation reduced the root system of irrigated citrus on a sand in South Australia by 50%with similar reduction in crop production. In the same region, a serious reduction in the volume of soil utilised by citrus roots under furrow irrigation as compared with efficient sprinkler irrigation was found.This was due to poor root development under furrows due to excessive leaching and under the non-furrowedareas due to accumulation of soluble salts there.

(b) Organic matter and humus

The humus content in virgin sierozem varies from 1 % in light sierozem to 1.5 % in ordinary sierozem and 2.5% in dark sierozem. Only in grassland sierozem may the humus content reach, and in certain cases exceed, 3 %. The high rate ofthe processes of renewaland decay ofhumic substancesgives sierozem humusan ephemeral character;in their compositionthe amount of humic acids is low (20% of the total humus content), and the initial forms of humic matter (fulvic acids) predominate,malcing up 4.0% of the total humus content. A low carbon-nitrogen ratio,not greater than 8,is characteristic of sierozem soils.This ratio is significant due to the abundailce ofnitrogen in sierozemhumus,which may be attributed to the large numbers ofmicroorganisms found in these soils,whose plasma forms a part of soil humus. Humus and nitrogen contents in sierozem stand as follows:in the 0-100 cm layer of light sierozem there are, per hectare,67 tons of humus and 6.4tons ofnitrogen;in ordinary sierozem,the correspondingfigures are 83 and 7.5;and in dark sierozem, 128 and 11.8. For purposes of comparison,the humus in the 0-100cm layer of chernozem soils amounts,per hectare,to 700 tons and nitrogen to approximately 40 tons;in chestnut soils the corresponding figures are 230 and 13. Sierozems contain about0.3tonsofP,O, (under organicform) and up to 1ton of organic sulphurper hectare. 415

IRRIGATION, DRAINAGE A N D SALINITY

The high rate of mineralisation accounts for the rapid decrease in the amount of humus and of organic matter when virgin sierozem soils are ploughed and irrigated. Three or four years after cultivation,the total reserve of organic matter is reduced by 40-50%; the process is later stabilised,due to the more complex nature of the remaining organic substances,which are less accessible to micro-organisms. Replenishment of the supply of fresh organic material is absolutely essential in irrigated sierozem lands. Lucerne plays a considerable role in increasing the humus and nitrogen content of sierozem soils. Ifproper agricultural methods are used on irrigated land,the lucerne root stock will amount to 15 tons per hectare by the third year of growth;this contains approximately 200 kg ofnitrogen and 40to 100 kg ofP,O,. During the lifetime of lucerne,the soil accumulates up to !O to 12tons of humus per hectare,with a nitrogen content of up to 1 ton per hectare. In irrigated sierozem areas, annual leguminous plants also constitute an important source of organic material and nitrogen. In good conditions,these crops accumulate 30 to 40 tons of stems and leafage per hectare,which,together with the roots,contain 100-200 lg of nitrogen. The high rate ofmineralisation or organic matter creates a risk of unproductivelosses ofnitrogen.For this reason,rational use of the resources accumulated by leguminous plants is an important problem for agriculture in irrigated zones. A n important factor in the balance of organic matter in sierozem soils is the introduction of such matter with silt from irrigation water. In sierozem soils that have been under irrigation for long periods,a marked increase in both humus and nitrogen content is due to irrigation deposits. In the Tashkent region (mainly with cotton and lucerne) the 0-100 c m layer of non-irrigated sierozem has a humus content of 72 tons per hectare which rises to 92 after 20 years of irrigation,and to 160 after 500 years. The salinisation of sierozem soils adversely affectsplant life and decreases the amount of remaining roots; this influence increases with the salt content (Fig. 12.8). The adverse effect is particularly marked in soils that are insufficiently watered,with a low-lyinggroundwater table. The xerophytic vegetation developing in these conditions (saltwort,wormwood, cereals and so on) leaves little vegetable residue behind it. Amounts of plant roots 60 in tons/ha

20 4 I

O

I

I 0.4

I

I

I

0.8

1

1.2

I

I

1.6

I

I

2.0

1

1

2.4

3

2.8

Solid residue (%)

Fig. 12.8. Changes in the amount of plant roots in relation to soil salinisation in Azerbaijan soils: 1. Grassland and ‘sazy’2. Irrigated agricultural soil ‘Korukh‘ 3. Sierozem grassland 4.Solonetz 5. Solonetz-solonchaktype 6. Solonchak In more moist soils with a groundwater table nearer the surface,grassland vegetation withstands a considerable salinity of the soil and leaves a larger root mass. Saline grassland sierozems are therefore higher in humus than saline sierozem soils with inadequate moisture. When,for instance,total soluble salts increase from 0.1 to 1-5-2% in poorly watered sierozems,the humus content decreases from 3.16 to 0.58%.In the humus content sierozem soils with a high groundwater table,when total soluble salts rise from 0.5 to 5 %, drops from 5.41 % to 1-1%. The processes of humus formation are affected not only by the degree,but also by the type of salinity. Investigationsin the USA have shown increases in nitrogen and carbon in irrigated soils after cropping for sixteen years or more with legumes and that irrigated alfalfa surface soils and subsoils contained 44 and 58 % respectively more nitrogen than adjacent virgin soils. In New Mexico the soil under irrigated alfalfa showed increased nitrogen and carbon after six years. In Nebraska thirty years’ cropping of an irrigated chestnut soil showed 30%decrease in nitrogen content of the surface 30 cm without manure or alfalfa in the rotation.The soil nitrogen content was maintained by either 15 tons/ha of manure annually without alfalfa, or 5 tons of manure annually with alfalfa half-timein the rotation.It was found in Utah that soil nitrogen content was not quite maintained by a manured seven-year rotation including three years of alfalfa, but organic matter was increased.

416

EFFECTS O F IRRIGATION A N D D R A I N A G E O N SOILS

O n the Riverina plains ofAustralia two years’growth ofirrigated pasture sifter gypsum treatment increased the nitrogen content of the top 7.5 cm of soil by 10 to 15mg/100 g on all treatments.On the treatment given 20 tons/ha of gypsum,nitrogen at 7.5to 30 c m depth increased by 20 mg/100 g. Generalising the data, one can conclude that correct long-term irrigation with normal crop rotatior: including alfalfa,and with the application of fertilisers,promotes the increase of the content of the humus in irrigated soils. The less humus the soil contains before irrigation, the more will its content increase after long-termirrigation;and in the case ofvirgin soils rich in humus the irrigation in the first few years leads to a decrease of humus and nitrogen contents.Under long-termirrigated cultivation,these contents are stabilised.

E.

EFFECTS OF LONG-TERM OPERATIONS OF IRRIGATION A N D DRAINAGE O N WATER A N D SALT DYNAMICS OF SOILS UNDER G O O D MANAGEMENT

1. Leaching

The soils on high plateaus in arid zones are characterised by a non-leachingregime. The residual and newly formed easily soluble salts circulate within the limits of the soil horizon,rising to the surface in dry seasons, and sinking down during wet ones. Gypsum and carbonates accumulate on the bottom of the soil layer periodically wetted by precipitation. In sierozems, carbonates concentrate at depths usually between 40100 cm, sometimes more;underneath,a horizon of gypsum and easily soluble salts is formed.In desert takyr soils,carbonates are practically immobile,owing to the extreme dryness;but gypsum and sodium sulphates accumulate at a depth of 15 to 25 cm,whereas sodium chloride circulates throughout the whole of the wetted soil horizon. At the beginning of any irrigationperiod, soluble salts are leached but this leaching is easy and permanent only in well-drained areas with deep groundwater.Therefore desalinisation of soils with a high water table requires artificial drainage. In poorly drained areas,desalinisation in the initial period of irrigation is later followed by secondary salinity.At the same time,there appear points of salt accumulation,increasing with prolonged irrigation.An example of salinisation on high terrace soils is found in the long-irrigatedareas of the third terrace of the Vaksh Valley: in such an area 6 k m wide, along the steep bank of the Vaksh River, where a fine-grainedlayer of loess and irrigation deposits 3-8 m thick prevented drainage into the subjacent pebble bed; and caused formation of bogs (6% of the area) and saline soils (21% of the area). Good management practices imply that either natural or artificial drainage is sufficient to ensure that a leaching ratio appropriate to the conditions can be maintained and that the level and salinity of groundwater are such that no deleterious effects are produced on the range of crops grown. The landscapes where natural drainage will probably be sufficient to remove increased subsoil water after irrigation have been described in Chapter 6. In the numerous cases in which the groundwater rises under continued irrigation and salinity either of soils or understrata or irrigation water is in evidence, artificialprovision of sufficient drainage is usually essential for long-termoperation.An alternative sometimes available is the use of crops tolerant to salinity and high groundwater togetherwith careful irrigation and other management to minimisefurtheraccessionsto groundwater. This is done for irrigated pastures in many parts of the Riverina Plain in Australia, and in parts of California and Chile. With very careful use of water, irrigation continued for more than 1COO years on some of the saline,waterlogged soils of Iraq by a system alternating fallow with cropping. During fallow, deep-rootedshrubs and weeds dried out the subsoil,allowing subsequentirrigations to leach salts from the root zone. Where artificial underdrainage is likely to be necessary at some time,not only should the levels of groundwater be observed at regular intervals but positive steps should be taken to defer as long as possible the time when the groundwater will rise to the critical depths for the crops concerned. This involves provision for surface drainage, including that for road and railway embankments,minimising seepage from channels and drainage ditches,and the most efficient management possible of irrigation water on the farms.Often there are big opportunities,not only of saving water but of minimising accessions to the groundwater.Reference is made to other chapters.

417

IRRIGATION, DRAINAGE A N D SALINITY The choice ofmethod of artificial drainage is an important one.In many irrigated horticultural settlements, including those along the Murray River in Australia, agricultural tile drains or slotted plastic pipe drains have been effective.Pumping from deep-seatedaquifers has been used in many US areas,in the McAlister District of Victoria and in the Murrumbidgee Valley of New SouthWales.Whether suchsystems,open drains or any other are used,it isessential for long-termoperation that theroot zonesremain desalinised to an extent capable of meeting the requirements of the crops grown and that the system is capable of keeping the level of groundwater and its salinity permanently below the critical ligures. A n example where previous drainage methods were ineffcctive is the Salt River Project in Arizona where deep pumping was finally installed. Another similar case was the Modesto District of California where gravity drains used until 1922 failed to control salinity.Since that date,not only has deep pumping controlled salinity but the re-useof the pumped water for irrigation has recouped all drainage costs.Desalinisation can be very rapid with appropriate drainage by deep pumping, as indicated for a northern Victoria area In developing saline soils for permanent irrigation two periods can be distinguished:(1) the reclamation and (2) the normal exploitation change. The most important point is the desalinisation by leaching the soil, so as to eliminate the danger of secondary salinisation in the exploitation period.As a consequence,not only salts from the topsoil,but also those of deeper soil hoïizons have to be removed. Elimination of salts may be difficult in virgin areas,where salts occur throughout the whole region. With up-to-dateirrigation and drainage techniques and correct management, including special leaching and watering, the reclamation period may last 20 to 30 years. In existing irrigations,with distinct patches of secondary saltaccumulation,desalinisation may be achieved within afew years.Such situationsmay originate from earlier days of irrigation,when reclamation operations were carried out on one small plot at a time,so that the development period lasted several centuries. The problem of reclaiming such salt patches remained long unsolved,due to poor drainage. Further particulars on reclamation are given in Chapter 13. 2. Examples of effective work of drainage and desalinisation in the USSR

Drainageconstructionin theVakhsh Valley became extensiveinpostwar years.At present the extension ofthe collector-drainagesystem over the entire area averages 12-13 m/ha.The depth of collectors is 3-0-3.5 m y and that of drains 2.0-2-5m. As a result of the collector-drainagesystem the groundwater table sank by 0.51 m and the mineralisation of groundwaters decreased. With an improvement of the collector-drainage systeli1 and an increase in the supply of water in the oasis,the leaching of salts from the irrigated territory reached 1.2million tons for 1960. The reclamation of saline soils proceeded with a constantly increasing effect (Table 12.19). Table 12.19. Changes in the area of saline soils on old arable lands of Vakhsh Valley belonging to collectivefarms during the period froin 1945 to I961 (inpercentage)

Years

Category of soils

1945

1950

1954

1961

1. Non-saline,weakly and medium-salinesoils 2. Strongly saline soils and solonchaks

49.6

65.6

75.1

85.0

50-4

34.4

24-9

15.0

At the beginning,40 to 50 % of the total quantity of irrigation water was used for leaching. Later 25 to 30% of the total water is estimated to be required for favourable salt conditions. The experience obtained here indicates that a deep open horizontal drainage system in a good statefully guarantees the desalinisation of soils in the entire oases. Before the construction ofthe drainage system in the Kliorezm oasis many soils were subject to salinisation. The construction began in 194.5-47with a drainage-collector system 2825,km long (17 m per ha). The average depth of the drains is 2.2m and of collectors 3.3 m. The total yearly salt discharge beyond the oasis through the drainage system amounts to 17841 tons. 418

EFFECTS O F IRRIGATION A N D D R A I N A G E O N SOILS

The discharge of drainage waters beyond the oasis comes to 15-25% of the total quantity of irrigation water. Progressively also the top layer of the groundwater became fresh (1-3 g/1 water). The removal of mineralised ground and leaching water beyond the limits of the oasis (1957) permitted considerable increase in the area under cotton and raised the average yield for the oasis from 6-65metric centners/hain 1947 to 31 c/hain 1963. An example of a more complicated reclamation project is the Kura-Araxlowland.Here the construction of a deep horizontal drainage overcame the extremely complex forms of primary and secondary salinisation that may arise under conditions of rising saline groundwater under pressure. Data on the salinity of the upper 100 c m of soils are presented in Table 12.20. Table 12.20. Soil salinity in the inassifs of Kuru-Arux lowland during 1937-40

Water-soluble salts in the soil 0-100cm (in

%)

Northern Mugan

Salian steppe

(%of the area)

(%of the area)

34.8 33.6 27.3 4.3

2.3 29.3 46.0 18.8

~~~

less than 0.5 05-1.0 1 *o-2.0 > 2.0

The experiments with an artificial horizontal (closed) drainage on Mugansk stationproved to be successful. However, a further construction of open drainage in a number of regions of the lowland encountered numerous natural and economic obstacles. Only since they were overcome,have significant successes been recorded. By this time,within the Mugano-Salianpart of Iba-Araxlowland alone,3700 km of drains and collectors of an open type have been constructed over an area of 159000 ha. The collector-drainagesystem with the help of 16pumping stations removes yearlyanaverageof about 850000m3of drainage water beyond the limits of Kura-Araxlowland. During the period from 1954-62 up to 50 %ofthe saltreserveshave been removed from the drained area. The coefficient of land use reached 0.58 in 1961 and doubled as compared with the pre-reclamativeperiod. 3. De-alkalisation of soils

During reclamation of saline sojls,there is a short period when their properties deteriorate:mobile, easily soluble nutrient elements are lost as a result of leaching, de-oxidationprocesses occur during leaching periods,hydrolysis of alkali-earthcarbonates is promoted,and permeability decreases.In view of this,in the operations carried out in the reclamation period, have to be included :cultivation,application of very large quantities of organic and mineral fertilisers, special treatment for improvement of the physical properties of the soil (deep and plantage ploughing), ‘reclaimingcrops’,green manure,etc. After prolonged irrigation with good water, even without special measures, soils eventually become dealkalised,but the process may take a very long time. With the modern usage of gypsum or acids, and special methods of tillage,the process of de-alkalisation is greatly accelerated. Ifa soil has suficient reserves of calcium,biological de-alkalisationcan be very effective.The liberation of CO,from roots of certaiíí plants may bring into solution enough calcium to reduce markedly the sodicity in one or two years.The ‘blackalkali’soil at Fresno,California,showed a reductionin sodicity (Table 12.21) as the result of frequent irrigation of Bermuda grass, Cynodun DactyZun during summer. Similar effects follow from the use on sodic soils of a variety of organic materials including farm manure and compost,which decompose with evolution of COz. Irrigation and drainage by themselves can reduce sodicity of some soils. By this means ‘whitealkali’soil in the Imperial Valley of California was reduced in two years from 38 milli-equivalentsof exchangeable sodium per 100 g of soil to 1 milli-equivalent,for the surface 30 cm,with lesser reductions down to 120 cm. N o better results were achieved when animal manure,gypsum or sulphur were used. In fact the autodealkalisation through leaching only-without special amendments-is a common

419

IRRIGATION, DRAINAGE A N D SALINITY Table 12.21. Exchangeable sodium percentage (ESP)in Fresno soil

Depth (cm)

1930

1937

57 97 90 43

1 4

0-30 30-60 60-90 90-120

13

4

phenomenon for the salinealkaline soils ofthe Middle East and North African regionwhere salinisationoften originates from ground water and irrigation water containing soluble Ca salts (sulphates). A n example is shown in Table 12.22. Table 12.22. Decrease in exchangeable sodium and salts with leaching in Iraq-(BOUMANS, HULSBOS 1960)

0-30 cm

Before leaching after 12 days’leaching 15 c m

net percolation after 42 days’ leaching 52 c m net percolation after 69 days’leaching 84 c m net percolation idem + 1 cropping season idem +3 cropping seasons

30-60 c m

ECe

ESP

ECe

ESP

106

34

37

38

6.5

15

13.5

33

3.0

8

2-6

23

2.6 3-8 3.6

7 5.5 4.2

2.2 2-0 3.9

21 20 10

ECe = conductivity saturation extract mmhos/cm ESP= exchangeable sodium percentage

Because calcium carbonate is only slightly soluble at high pH, gypsum or its equivalent is the usual chemical amendment for sodicity. Gypsum applied at the rate of 13.5 tons/ha will replace about 3 milliequivalents of sodium per 100g of soil for a depth of 30 cm.Used at the rate of 20 tons/ha on a saline sodic heavy soil of the Australian Riverina,gypsum decreased p H by 0-6units in the surface 7-5c m and had some effect down to 30 cm.In California,30 tons/ha of gypsum reduced p H of a light sandy loam from 10 to 7.3. While calcium chloride has been used to some extent,in the absence ofcheap supplies of gypsum,the readiest means to reduce sodicity is to use an equivalent of sulphuric acid;suchan equivalentis sulphur,which usually oxidises readily to sulphuric acid as described in Section D1. Other equivalents are sulphates of iron or aluminium.In replacing capacity for sodium,2.5 tons of sulphur or 17.3 tons of alum are equivalent to the 13.5 tons of gypsum mentioned earlier in this paragraph. Other equivalents to acids are certain fertilisers which are often used in considerable quantity under the conditions ofintensive production, possible with well-managedirrigation and drainage.This applies specially to nitrogenous fertilisers because those other than nitrates are usually oxidised in the soil to nitrate so that their use is equivalent to applying nitric acid.Ammonium sulphateapplication therefore is equivalentto use of nitric and sulphuric acids. Used at the rate of 445 kg/ha of N annually ammonium sulphate in 16 years reduced p H ofsandy loam citrus soilin California from 7.6 to 4.0 for the surface 15 cm;in 28 years reductions of similar order occured down to 70 cm.Similar effects were found, for surface soil,after 13 years’heavy drainage with ammonium sulphate on citrus irrigatedon heaviersoil inthe MurrumbidgeeValley ofAustralia. Clearly it is part ofgood management to ensure that de-alkalisationofthe kind brought about by fertilisers such as ammonium sulphate does not go so far that the soil reserves of calcium and magnesium become insuficient for balanced plant nutrition. This happened on sandy soil irrigated for citrus near Mildura, Australia,when continued use of ammonium sulphate led to magnesium deficiency in the trees. 420

EFFECTS OF IRRIGATION A N D DRAINAGE O N SOILS 4. De-mineralisation of groundwaters

Prolonged irrigation and artificial drainage may not only desalinise the soil,but also groundwater to considerable depths.When this happens the danger of secondary salinityis eliminated,In many ancient irrigated regions,with favourable drainage conditions,the soil and subsoilare desalinised down to a depth of 20-30 m with a mineralisation of the groundwater as low as 1-3 g/l. In such conditions, high groundwater tables (about 2 m deep) are favourable. The amount of water required for irrigation is reduced considerably; 50-70% of the water consumption has been reported, by NICOLAEV, to be provided by groundwater. The drainage network has to regulate the groundwater level and to remove some salt.Under this regular moisture regime soils develop meadow characteristics. Conditions for the system of sub-irrigationare,that below the surface soil must be a layer permitting rapid upward movement of water,a horizon of limited permeability at some distance from the root zone to minimise water loss by deep percolation, irrigation water of good quality, uniformly level terrain, and sufficient rainfall to keep salinity down,Clearly,salinity must be watched closely in such systems. This has been demonstrated by the histories of several areas. In the years before 1922,sub-irrigationin the Modesto area of California had induced salinity to such an extent that where abundant crops had formerly been produced,yields decreased and orchards and vines died. De-mineralisationwas achieved by vertical drainage using pumped wells,Some of the sub-irrigationon the peat soils of the Sacramento-San Joaquin delta of California had to be replaced by sprinkler irrigation because of accumulationof salts.In most other cases leaching once in 5 or 10years has to supplementannual leaching by rain.In Madras,sugarcane is watered during the dry season by a sub-irrigationprocedure very economical ofwater. Concentration of salts near the surface is controlled by trash mulch and by flushing out when full water supplies become available.

F. CHANGES

IN FERTILITY OF SOILS UNDER CORRECT MANAGEMENT

1. The main soils in arid zones

In the arid belt of the world there are many different soils ranging from very fertile ones to fully barren. The most widely spread among them are the following: Black cotton soils and regurs of the tropics (monsoon and savannah areas) Sierozems of various sorts Cinnamon and grey-cinnamonsoils Chestnut soils Brown desert soils Takyr desert soils Sand desert soils Salt-affected soils of various chemical composition Alluvial, meadow and swampy soils of valleys and deltas, etc. Sometimes in Africa and Asia pure geological sediments, i.e. sand deposits, stone pebble surfaces,clay crusts,have been irrigated. The first irrigated areas were probably valleys and deltas, then more and more lands of higher elevation were irrigated. At this time,practically all of the best non-saline soils in the world have already been developed. However,the futureprogress of irrigation particularly requires the reclamation and development of vast territories of salt-affectedsoils. Most fertile under irrigation are the black cotton soils,the sierozems,cinnamon and chestnut, lying on sand horizons. A n advantage of sandy soils is that they are very rarely affected by secondary salinisation;however,under irrigation they become silty and muddy. The alluvial, meadow and swampy soils of valleys and deltas of arid zones are very often affected by alkalinity or salinity. Naturally from the very beginning of the development of such land for irrigation, a 421

IRRIGATION, DRAINAGE A N D SALINITY system of preventive measures against an increase of salinity is required,and leaching and drainage installations must be introduced to reduce the initial salinity. If non-saline the alluvial soils are the most fertile among irrigated lands of arid zones. The most dificult objects ofirrigation in arid zones are the salt-affectedsoils of different types.The worse the conditions of natural drainage and aridity of the climate,the more saline these soils are. Any form of irrigation of saline and alkaline soils of deserts and semi-desertscould not start without preceding proper, sometimes very expensive, reclamation through deep and effective drainage and heavy leaching.

2. Characteristic changes of profiles and landscapes Under prolonged irrigation there is a change and a redistribution of humus, carbonates and gypsum (Fig. 12.9)in the profile (ORLOV, 1937;RozANoV, 1951). Under the influence of irrigation,the character of the carbonate profile changes, the lime nodules,very noticeable in the central part of the profile of virgin soils,disappear;in ancient irrigated soils the carbonates are spread evenly throughout the whole of the soil profile: this applies both to the original contents,which are re-distributed,and to new reserves of CaCQ, brought in with the irrigation waters. A certain increase in carbonatesis observed in the lower part ofthe profile. The soilprofile ofirrigated soils ofsierozem and desert zones have a similar structure,differing slightly only as regards humus and carbonates content. Long-irrigated soils acquire a monotonous grey colour and uniform physical structure and composition on top of thick irrigation deposits. Long-irrigated soil is thoroughly worked through by earthworms, and has good micro-aggregation;the topsoil of 30-40 cm contains the largest quantity of humus. Irrigation deposits at a depth of more than 40 cm contain less humus; however,this content is always higher than in the same layers of non-irrigated soils. Long-irrigatedsoils are classified in a special category known as ‘ancientcultivated and irrigated soils’. Soil particles 0.01 inm O 20 4060%

Humus

O 0.5 1001.5

Carbonates

Gypsum

%

80

.‘ 200

80 120

,

Depth (cm)

Fig. 12.9.Soil profile of virgin and long-irrigatedsoils in Central Asia I. virgin soils 2. long-irrigatedsoils

422

EFFECTS O F IRRIGATION A N D D R A I N A G E O N SOILS

Irrigation also determines the agricultural use ofthe secondary features ofirrigationrelief.Strips alongside canals,which are higher than the surroundingterritory,have,in former times,been used for building farmers’ cottages,for gardens and vineyards,taking advantage of the good soil permeability.The slopes,where silty clay and clayey soils occur, are used for growing mainly industrial and food crops;these soils have to be carefully levelled,tilled and manured: they are the most fertile in the irrigated area. W e have in mind these soils when speaking of the beneficial effects of irrigated farming. Lastly,there exist,in depressions between canals, fine clayey soils which are frequently overwetted and sometimes also saline. In addition to excess groundwater moisture, they are periodically inundated and are used less intensively for farming. Owing to the alternation of canals and the depressionsbetween them,a specialtype ofirrigation landscape came into being in ancient irrigations,consisting of dense green strips of plantations and gardens, where settlements are located;wide fields of regular crops on the mild slopes; and lastly,depressions, frequently overgrown with rushes and thistles,and sometimes with lakes. 3. Increased fertility of soils under correct management

Fertility refers to the ability of a soil to produce good crop yields of satisfactory quality under irrigation with the least possible expense. As physical, chemical,and biological conditions have been discussed in preceding sections,here the chief interest will be in crop yields. A very demanding production in terms of soil fertility is that of vegetables, especially early ones. The fertile plains near Murcia, in Southern Spain,have been irrigated since the time of the Moorish occupation, which ended in the thirteenth century. In the hot dry climate of the ‘Garden of Murcia’, skilled and intensive cultivation by hand continues to produce excellent crops of early vegetables under irrigation. In Burma and Bengal, rice has been grown for centuries on the same land,apparently without diminution of yield. It seems that the soil can supply all nutrients required with t’ne exception of nitrogen, and this is provided sufficiently by the nitrogen-fixingblue-green algae (see Section D.)1 In the Gezira area of the Sudan, heavy alkaline soil has been irrigated for cotton since 1911.After long detailed studies on soil improvers and other measures,it was decided that it was only necessary to continue irrigation with the good waters of the Blue Nile.Apparently this soil is slowly improved under irrigation, even though there is no drainage. Thus,until now this arid region is an exception to the common experience that artificial drainage is often essential with irrigation. Ancient irrigation systems in North Africa and Arabia have persisted for more than 2000 years without deterioration ofthe soil,and have continually given harvests ofa variety of crops,in spite ofthe use ofhighly saline groundwater,5 to 7g/l;this has been possible only because the natural drainage through the irrigated sands is perfect. Spectacular increases in productivity are possible in a short period with appropriate management, even when conditions to begin with are difficult. This is well illustrated by some data for ‘blackalkali’irrigated soil at Fresno,California;Table 12.22shows the quick improvement due to the use of sulphur,the cost of which was paid back by increased yields during 1926 to 1928.This soil responded very well also to frequent irrigation of Bermuda grass (Cyizodon Dactylon) during two summers. Table 12.23. Yields obtainedfrom black alkali soil (kglha)

Year

Crop

Untreated

Sulphur

1925 1926 1927 1928 1929 1930 1931 1932 1933 1934

Melilotusalba Alfalfa hay Alfalfa hay Alfalfa hay

Ploughedunder

Ploughedunder

90 700 1900 5800 2300 los00 16300 19000 20O00

12500 20400 22100 16800 6300 16000 20300 20800 20600

Alfalfa hay

Barley hay Alfalfa hay Alfalfa hay Alfalfa hay Alfalfa hay

-

423

IRRIGATION, D R A I N A G E A N D SALINITY

Very rapid increase in productivity can be achieved by suitable drainage. In northern Victoria, an area of heavy clay,which had produced no worthwhile vegetation for many years because of a highly salinewater table close to the surface,was provided in 1960 with vertical drainage by pumping from a depth of 7.5 metres,there being an aquifer below the clay bed. Ponding of water on the surfacefor 10 days together with pumping for a total of only 56 hours gave reduction of sodium chloride content of the surface 15 c m of soil from 1.60%to 0.05%.This allowed the establishment of a crop of millet, which was succeeded by perennial pasture which carried 11 sheep per hectare. In another area of northern Victoria, a diflicult heavy clay soil is irrigated for fresh grape production. While in the hands of neighboursmuch ofthis soil has passed out of horticultural production because oflack of underdrainage,careful management on a particular property has maintained production for 28 years with gross return from 4 hectares averaging AS 6800 each year and going up to AS.llOOO. In light soils irrigated for citrus in South Australia, 100 %increases in fruit production are attributable to good management,including provision of efficient sprinkler irrigation instead of furrow irrigation. By a combination of drainage,deep cultivation and more suitable general management it has been possible to raise yields of fine fibre cotton in the Vakhsh Valley of Tadjikistan by 1.7 times since 1940. In the Murrumbidgee irrigation areas of Australia, successful management in rice production has built up the average yield gradually from 3.5 tons/ha for the 5 years ending 1929 to 7 tons/ha in recent years. The choice of species is of great importance,an excellent example of this subject is the finding that in Chile the about 400000hectares of irrigated pastures,on shallow soils with high water tables,could produce three to five times more than they do now, with better mixtures and better management. Recently introduced mixtures of Ladino clover and grasses such as Dactlis glomerata and Festuca arundinacea thrive on this land and give good yields. In addition to attending to nutrient requirements of plants along lines suggested by Section C3 and the numerous fertiliser trials on irrigated crops throughout the world,provision of adequate drainage is usually the most successful step that irrigation management can take, apart from ensuring adequate water supply (see Section Cl). It is a tenet of good irrigation that it permits leaching of the soil profile only to the extent required to maintain a favourable salt balance. This implies that effective underdrainage is available to the irrigator and that he avoids waste of water,and waste of plant nutrients. Nitrogen is specially liable to loss in drainage waters. Remarkable diminutionsofyield ofexcessively irrigated sugarbeet,even with considerable applications of N fertiliser,were due to this effect.

G. CHANGES

IN FERTILITY OF SOILS UNDER POOR MANAGEMENT

Decrease of fertility of irrigated soils or deterioration may take place because of erosion, alkalisation and disaggregation,secondary salinisation and waterlogging,exhaustion of nutrient elements,and so on.

1. Alkalisation Alkalisation occurs both in the first stages of soil salinisation and in the final stages of de-salinisation, During the salinisation of soils formed on top of slightly mineralised groundwaters,alkalisation occurs as a result of the concentration of alkaline cations during evaporation of the soil solutions and also of the fact that carbonates and sulphates are precipitated. At the same time,there is a relative increase of sodium chlorides and sulphate,also sometimes of bicarbonate. Sodium bicarbonate and soda may appear in soils due to the processes accompanying the de-oxidation and de-sulphurationofsodium sulphate.This phenomenon has been observed alsoincertainancientirrigations (Vaksh,Murgab), though they are in this case limited in scale.De-sulphurationleads both to an increase of alkalinity and the appearance of soda and also to an increase in water-chloridity,with a parallel decrease in the relative content of sodium sulphate. The appearance of soda in soils may be due to the process of de-salinisation.The dilution of the soil solutions which occurs during leaching is accompanied by hydrolysis of the humates and alumosilicates of sodium (KOVDA, 1947); also by the formation of soda. In carbonatic,solonetic soils,soda is formed as a 424

EFFECTS OF IRRIGATION A N D D R A I N A G E O N SOILS

result of exchange reactions between CaCO, and the sodium of the adsorbing soil complex. When soda is formed,calcium and magnesium are transformed into an insolublestate.When the soils are high in calcium carbonate and gypsum,soil desalinisation does not lead to alkalisation. W e shall not deal here with cases of alkalisation of soils due to irrigation with strongly alkaline waters, since this is something which calls for special analysis. 2. Secondary salinisation of irrigated soils

Apart from cases of natural salinisation,influenced and complicated by the effects of prolonged irrigation, there have also been cases of salinisation of soils after irrigation which, formerly, were non-salineor only slightly saline. This is what is known as secondary salinisation. The main role in the development of this salinisation is played not by natural processes, but by faulty irrigation and farmingmethods due to inability to consider natural conditions and to foreseethe geochemical changes which occur in soils under irrigation. Other importantfactors in the salinisation of irrigated soils are human failures-waste of water and land resources, attempts to grab more water for irrigation,destruction of natural vegetation, lack of irrigation systems maintenance after wars and invasions and inefficient farming on small plots. Also mistakes have been made in the irrigation works particularly at the end of the nineteenth and beginning of the twentieth centuries. Irrigation systems, still working, have generally a number of defects:primitive canals not sufficiently equipped, making rational use of water difficult; unlined canals, so that seepage is considerable; flood irrigation methods on fields with unfavourablemicro-relief.Consequently,the efficiency ofirrigation is often low. As has been pointed out repeatedly in this book,the first result of these imperfections (particularly over irrigation and seepage)is a risein groundwater.The average speed in rising may be inthe order of0.5 to 1.5 m a year and this may easily lead to secondary salinisation and swamp formation. In southern Asia groundwater came close to the surface in 60-70 years, with an original level at depths of 50-60 m. The resulting saline and waterlogged soils were completely unforeseen. Cases are known of groundwater rising to the surface from 5-7 m depth and causing disastrous salinisation within 2 or 3 years.Evaporation of capillary water and salinisation on the surface increases sharply when a critical groundwater depth is reached (see Section B6). Rising groundwater will dissolve the salts scatteredin soil strata,thus increasingits mineralisation.Salts in irrigation water are another source. It is therefore essential, as a means to prevent and retard salinisation,to take all possible steps to limit the amountofwater fed into the irrigatedarea.With modern progress in agriculturaland engineering sciences, it is usually possible to prevent secondary salinisation in new projects in non-saline areas, as well as to eliminate the harmful effects of salinity in existing irrigation projects. For new projects this requires careful consideration ofthe natural conditions and particularly the existing and the expected water and salt balances. For existing projects this may require a complete re-organisationof the structure and use of the irrigation systems,as will be described in Chapters 13 and 14. The study of experience acquired in age-old irrigations and of traditional methods of local people in combating salinity may be helpful. Examples of decreased fertility and deterioration of soils by salinisation are abundant throughout the world. The history of the ancient civilisations in what is now Iraq is well known,but what happened to their lands,irrigated from the Tigris and Euphrates rivers has become clear only in recent times. As far back as 2400 BC the first salinity was recorded for East Iraq, probably caused by intensive irrigation and rising water table. The area did not recover. Salinity was recorded for northern Babylonia about 100 BC. The important point is that there was no artificialdrainage, and natural drainage is virtually non-existent, especially below Baghdad where the valleys ofthe Tigris and Euphrates widen to a single vast plain. Because of very low rainfall and very high temperatures, salts from long ages of irrigation have accumulated in the soil,although the salt content of the river waters is low.It has been estimated that in central and southern Iraq the soil to a depth of 5 metres in the area suitable for crop production now contains 1000 million metric tons of salts over an area of 150000km2.

425

IRRIGATION, DRAINAGE A N D SALINITY

The deposition of suspended material from irrigation water has the effect of accentuating sometimesby as much as 3 metres, the ‘saucer’reliefwhich is present in any case because irrigation channels follow the higher ground.The coarser material tends to depositin the channels or close to them, the lower undrained ground receives the clayey particles. In the hollows solonchaks or saline swamps may develop and may pass out of production as is reported from the USSR and Australia. In the western states ofthe USA over one million acres ofland were abandoned for irrigationbetween 1929 and 1939 due to salinity and alkalinity;in addition much land there suffered production loss up 25%for the same reasons.It has been stated that there is hardly an irrigated region in the western USA which has not had a loss of fertility in many parts. In India and West Pakistan,large,formerly fertile tracts have been rendered barren as a result of a rise of the water table following the introduction of canal irrigation. A kind of decreased fertility under irrigation is that in which the range of crops for which the land can be used effectively is narrowed by adverse changes. In northern Victoria, areas of loamy, reasonably fertile soils which formerly could have grown a very wide range of irrigated crops other than rice,were developed for irrigated horticulture about 50 years ago. Elevation of groundwater and secondary salinisation caused abandonment of horticulture and the areas have since been devoted to irrigated pasture,many of which are highly productive. Also the Mildura districtin Australia was developed originally (1887) for horticulture.An early mistake in land use planted citrus in the hollows,vines on the upper lands.The citrus died from salinity,waterlogging, root rot and adverse effects of heavy soil.New plantings on sandy,elevated soils have since been effective. A great part of the settlement suffered from secondarysalinisation after 10 years’irrigation,and it was not until 1934 that a comprehensivetile drainage scheme replaced earlier temporary expedients,giving significant increase in yield of dried vine fruits and allowing the bringing back into production of considerable areas previously abandoned for horticulture. Similar findings on various scsles of magnitude could be listed from many countries. 3. Other unfavourable effects

Repeated waterlogging usually ends in salinisation,but also incidental waterlogging may do serious harm. The cotton cïop of 1909 in Egypt was a miserable one,although it was a year of good water supply and other conditions. The reason was that the soil had become partly waterlogged because of inadequate drainage. In the Murrumbidgee and Goulburn Valleys of Australia, many thousands of irrigated peach trees died after wet autumn and winter months in 1931, 1939 and 1956. Heavy rain after irrigation did not allow sufficient aeration for the sensitive peach roots,except where appropriate drainage had been provided. The deposition ofsuspended material from irrigation waters has been described in SectionA2.Deposition in irrigation channels almost rendered derelict parts of the Egyptian system in the 1880’s. Erosion of moderately deep soils in Idaho overlying basalt mused a loss of 2.5 cm of topsoil. This loss meant a real reduction in yield of the irrigated vegetables.The amount of lost soil could be arrived at during two irrigations with too large a furrow-stream. Aspects of management such as adequate use of fertilisers,efficient weed and pesi control, choice of appropriate species and varieties and so on, which can lead to less than optimum productivity, are too numerous for detailed listing,but may be very important factors to be considered together with amounts, frequency and kind of irrigation. So,in the Murrumbidgee Valley of Australia it was found necessary to balance phosphorus with nitrogen nutrition of irrigated citrus in order to harvest fruit of good quality. Rice illustrates the importance of knowledge that timing of irrigation and drainage may help in control of certain insects, such as root maggot which has reduced yield by as much as 29 %,and of certain diseases, such as straighthead,stem-rotand blast.

4. Losses suEexed by the national economy resulthg from salinisation

The losses suffered by the national economy as a result of the salinisation of irrigated soils are tremendous, as will be clear from the examples mentioned in the former section.Whereas the structure and fertility of soils 426

EFFECTS OF I R R I G A T I O N A N D D R A I N A G E ON S O I L S

are the outcome of lengthy processes,their destruction by secondary salinisation may be a matter of only a few years.The restoration of their fertility is extremely difficulton account of the labour and capital involved and because the soils themselves may be almost irreparably damaged. Estimates show that saline soils ofvarious types in irrigated territory in fact cover something like20000000 hectares-i.e. the equivalent of all the irrigated territories of the USSR and USA put together. The largest area ofirrigated saline soilsis to be found in Asia and North Africa;but there are also vast areas of secondary saline soils,covering some 3.5-460 million hectares,in the irrigated territories of North and South America; and the problem exists as well in tropicalAfrica and south-easternAustralia.When we remember that many of these territories,now partially or wholly abandoned,were once developed,with irrigation systems,roads, farms and villages,we shall realise the scale of the loss to mankind. The problem of salinisation and waterlogging of irrigated soils is all the more urgent because these processes are still continuing at a rapid pace in many arid-zonecountries,both in ancient and contemporary irrigation systems such as, for instance,in Iraq, Pakistan, China, India, Iran, Egypt, Tunisia, north-east Brazil,Chile and so on. Not only must total loss of soils be considered,but also the quantity and the quality of agricultural crops grown on soilsofvarying degrees of salinity.In these areasthere often form,on irrigated fields,salinepatches covering,e.g.,10-20 %of the surface where crops die,meaning a 10-20 %drop in yield.There are in addition patches of medium and weakly saline soils affecting crops and reducing both the amount and the quality of their yield substantially. The importance of this fact tends to be overlooked simply because medium and slightly saline soils are so common. The fact that old tax records from Mesopotamia show that crop yield for cereals amounted up to 2500 litres/ha,whereas now yields of barley are only one-quarterto one-halfof this,may be attributed to the effect of salinity,at least partly. The average yield of cotton in some irrigated areas in the Soviet Union,which attained 2000 kg/hain the period 1935-55, has now risen to between 3580 and 4000 kg/ha.But during the same period, cotton in parts of Uzbekistan,Turkmenistan. Tadjikistan and Azerbaijan where large areas of saline irrigated soils still remained,showed a much smaller increasedespite allthe resources and labour expended thereon by both the State and the collective farms. Humus reserves decrease considerably by salinisationas is shown by the following figuresfor the top 50 cm of a light sierozem soil in the Hungry Steppe (USSR).

-Total humus in tlha Virgin soil Irrigated cotton Irrigated cotton on saline soil Irrigated lucerne on saline soil Solonchak

62.5 56.6 43.3 48.2 37-4

In an area of more than 1000 ha, 97.6% of which consisted of more or less saline soils,humus exceeded 1 % for only 20.3 % of the area, whereas in a comparable area with only 10.8 % saline soils a humus content over 1 % was found in 50 % of the area. Average raw cotton yields were 2710 and 3660 @/ha respectively. The eaciency of fertilisers on saline soils is generally low.At Ferghana Experimental Station the yield of raw cotton increased by only 90-330 kg/ha by application of fertilisers on soils with 0-7-3-3%salt. After cleaning the drainage system and leaching the effect of fertilisers increased to 930-1660 kg/ha.The good results with leaching continued in succeeding years. Apart from quantity,also the quality of crops is affected by soil salinity:cotton fibrebecomes shorter and brittler;sugar beet has lower sugar content;fruit trees bear small fruit of poor flavour; vines produce poor quality wine;grain becomes puny and undersized; and tobacco gives leaves of inferior quality owing to the high ash and nitrogen content. Other losses include fruitless labour expended on saline soil of solonchak patches dotted over the fields; expenditure on seeds,fertiliser, irrigation water and so on. Apart from this, high concentrations of easily soluble salts in soils and groundwater cause serious deterioration of metal constructions, the foundations EI

427

IRRIGATION, DRAINAGE A N D SALINITY and walls of concrete,brick and adobe buildings,and cement or asphalt road surfaces,all of which may be destroyed within 5-7 years. When secondary salinisation is accompanied by swamps,people will sufler seriously from both intestinal diseases and malaria. The loss ofsoilfertility,combined with disease and the destructionof roadsand buildings,may insomecases cause the populations ofafflictedterritoriesto move away to other as yet non-salinelands.When this happens, the land left behind untilled and unwatered will after a few years become completely barren, and turn into solonchak desert totally unfit for cultivation. 5. H u m a n failings as causes of salinisation

After the discussion in foregoing sections and chapters of natural and technical conditions involved in salinisation,something will be said on human causes in this section. In the first place the former and still existing ignorance of people regarding the processes connected with irrigation, drainage and salinisation should be mentioned. In the past it was impossible for individuals or groups of farmers to recognise favourable natural conditions.H o w favourable natural conditions such as elevated level of soils with respect to drainage base, good hydrogeological conditions, good quality of irrigation and groundwater would influence the duration of promising irrigation on the selected spot simply had to be found out by trial and error. More recently,project-engineersin the nineteenth and at thebeginning ofthetwentiethcenturiesoftenbased the development of large projects on the success of existing,but limited,irrigation within the area without understanding the consequences of irrigation extension over the whole area.Such difficult features as ‘areahydrology’were sufficiently understood only recently. In the second place the society as a whole and particularly its density relative to usable land has played a considerablerole.Ina societywitha limited population,partly used to movingingroups,even the displacement from one irrigated area to another would occur, as soon as land diminished in production too much by salinisation.Where reclamation could be performed with little pains and at low costs,as in long,wide river valleys, such shifting of land may be preferable to the attempt of preventing salinisation from the purely economic point of view. Growing populations, increased knowledge of and care for natural resources are now imperative for prevention of such practices. Instead of haphazard settlements there is a need for long-term planning of resource development.Among those resources land and water are of the greatest importance.The need for such planning has now been recognised in many countries. Another feature of society conditions has to do with ownership and division of land.Foreign ownership of land,division of holdings in scattered parcels of land,and the existence of many small-holdingsmay be mentioned here as conditions promoting inefficient use of water and impeding methods of reclamation. Lack of sufficient interest, technical difficulties and lack of capital may all play a role in the failure of improvements for the prevention of salinisation. In the third place the cooperation of farmers has an important influence on the success of an irrigation project, particularly when improvement of the project are desirable. Already normal irrigation practices require such a cooperation,as one person’s actionsmay have an effect on others.A n example ofthis may be found in the tendency for farmers to use abundant water when their land is situated upstream of a river, thus causing lack of surface water, and seepage leading to salinisation danger downstream. The collaboration may be organised in different ways: on a voluntary basis, on a basis of voluntariness butcontrolled by the Government,or founded by the State as iiithe USSR.In the latter case the Stateusesthe need for reclamation or improvement as an opportunity for rationalisation of State aiid collective farms. This includes the enlargement of working units, replacement of former drainage systems, substitution of wild flooding irrigation by furrow irrigation and so on. It is intended to reconstruct here many of the country’sirrigation systems,and care for the prevention of salinisation plays an important role. Many failuresin the past may be excused by lack ofknowledge.But in modern times such an excusecannot be considered valid any more, as science has found explanations for the processes going on with irrigation and means to prevent ill-effectsof water use. Every effort should now be made to disseminate the insight obtained,in order to achieve sound planning, benefiting all people concerned.

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EFFECTS OF IRRIGATION A N D DRAINAGE O N SOILS

REFERENCES ACADEMY OF SCIENCES USSR (1958), Application of drainage in the development of salinised soils (in Russian). ACADEMY OF SCIENCES USSR (1963), I@uence of irrigation on soils of Central Asian oases (In Russian). ACADEMY OF SCIEI'JCES USSR (1948), soils of the Hungry Steppe as an object of irrigation und amelioration,

XXIX (in Russian). AMERICAN SOCIETY OF AGRONOMY (1957), Drainage of agricultural lands, Ed. J. N.Luthin.

ANTIPOV-KARATAEV I.B. (1940), Reclamation of solonetzin conditions of irrigationin theory and practice, Trudy Pochv. Inst. i. V. V. Dokuchaeva,24,7-20, 40-60 (in Russian). AVERYANOV S. F. (1958), Demineralising effects by drainage. International commission on irrigation and

drainage,Ann. Bull., 4.

BALYABON.K.(1954), Increase of fertility of irrigated cotton zone of the USSR.Moscow. Selkhozgiz (in Russian).

BOUMANSJ. H.and HULSBOS W.C. (1960), The allcali aspect of the reclamation of saline soil in Iraq. BOWER C. A. (1961). Prediction of the effects of irrigation waters on soils.Salin. Probl. Arid Zones Proc. Teheran Symp., 225-322. FIRST INTERNATIONAL SOCIETY CONFERENCE ON IRRIGATION AND DRAINAGE (1957), Ed.

R. M.Hagan, san

Francisco.

GRABOVSKAYA O.A. (1961), Processes of soil leaching under amelioration in valleys of South Tadjikistan, V. I. Academy of sciences,Tadjik SSR. HAGAN R. M.and VAAIDA Y.(1960),Principles ofirrigated cropping,Arid zone research,XV,UNESCO,215. KELLEYW.P.(1951), Alkali soils, Reinhold,New York. KONONOVA M. M . (1932), Redox-oxidationpotential, as a method of classifying soils conditions under various irrigation methods, Pochvovedeniye 3 (in Russian).

KOSTYUCHENKO V. P.(1957), Irrigated sierozem soils of Tashkent oasis, Trans. Pochv. Inst. 52,Academy of Sciences USSR (in Russian). KOVDA V.A. (1947), Origin and character of saline soils. Parts 1 and 2 (1946) and (1947). KOVDA V.A. (1959), Sketches on nature and soils of China, Academy of sciences USSR. KOVDA V. A. (1960), Trans. 4th Congress. Int. Commission on irrigation and drainage, V G13,1-13, 28. MINASHINA N.G. (1962), Ancient irrigated soils of the Murgab oasis. J. Pochvovedeniye, 8 (in Russian). ORLOV M . Q.(1937), O n sierozems and oasis-culturalsoils. Trans. SASU,Series VII,6 (in Russian). PALETSKAJA L.N.and JSISELYEVA N. T.(1961), The micro-floraand cultivated Takyr-typesoils which were irrigated in the past,primarily in the area of the Kara-Kum canal,Isvestiya Academii Naul Turkmenskoy SSR (Newsof the Turkmenian SSR Academy of Sciences), Biological Sciences Series, 2 (in Russian). PENMANF.(1940), Soil changes under irrigated pasture, J. Dept. Ag. Victoria,37, 83-100. PRATTP.F. and CHAPMAN H.D.(1961), Gains and losses of mineral elements in an irrigation soil during a 20-yearlysimeter investigation,Hilgardia,30,445-67. PRATTP.F. and CHAPMAN H.D.et al. (1959), Chemical changes in an irrigated soil during 28 years of differentialfertilisation,Hilgardia 28,381-420. RAVIKOVITCH S. (1946). The saline soils of the lower Jordan Valley and their reclamation.Ag. Res. Station, Rehovot, Bull.,39. RIJOV S. N.(1939), Causes of crust formation on irrigated soils of Central Asia and methods of fighting it. Collection Problems of Physics, Chemistry, Soil Amelioration and Cotton Fertilization, Soyuznkhi (in Russian). ROZANOV A. N.(1951) The sierozems of Central Asia, Academy of Sciences USSR (in Russian). Rozov L.M.(1956) Amelioration pedology,Selkhozgiz,Moscow (in Russian). W.C. (1963), Reclamation of soil-affectedsoils in Iraq. International SLUE P.M.VAN DER and HULSBOS institute of soil reclamation and improvement,Publication II. SUCHKOV S. P. (1947),Irrigation erosion phenomena on irrigated soils, Bull. Academy of sciences of Uzbek SSR,9 (in Russian). THORNE D.W.and PETERSON H.B.(1954), Zrrigatedsoils,2nd edition,The Blakiston Company,New York. WEST E. S. (1933), Observations on soil moisture and water tables in an irrigated soil at Griffith,NSW, CSIRO,Australia, Bull. No. 74.

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13. Reclamation of Saline and Alkali Soils* THEpurpose ofthis chapter is to review the methods ofreclamation of salineand alkali soils underirrigation; to indicatehow the appropriate method can be selected in specific cases;to suggest cropping practices during reclamation;and to note precautions necessary for a permanent successful production.

A. REVIEW OF METHODS OF RECLAMATION OF SOILS 1. Physical amelioration Severalmethods have been used to improve saline and alkali soils by mechanicaltreatment: deep ploughing, subsoiling,sanding,and profileinversion.The purpose ofthe firstthree treatmentsis to increasesoil permeability directly by mixing fineand coarse textured layers and to obtain a more uniform soil (deep ploughing), by breaking up impermeable layers (subsoiling), and by incorporatingsand into a fine textured soil (sanding). Profile inversion covers an undesirable soil layer with better material from a lower layer. The term ‘deepploughing’,as used in the United States,refers to a depth ofploughing from about40c m to 150 cm. Deep ploughing is most beneficial on stratified soils having impermeable layers lying between permeable layers.This is the situation inthe trans-Volgaregions ofthe USSR,in much ofthe Salt River Valley inArizona and the Rio Grande Valley in New Mexico and Texas. In places where sodium-affectedsurface or subsoils are underlain by soil containing considerablegypsum,deep ploughing has been done to put the gypsum layer on top of the soil.This serves to break up and bury the sodium-affectedsoil while supplying soluble calcium to bring about reclamation. Subsoiling consists of pulling vertical strips of steel or iron,called ‘hives’or ‘chisels’,through the soilto open channels to improve soil permeability. Customarily,the knives are set about 60 to 90 cm apart; a powerful tractor is usually necessary to pull a subsoiler. The beneficial effects of subsoiling usually persist for only one cropping season unless an indurated ßhorizon or lime layer is broken,in which case the benefits continue for several years. Sanding is an effectivemeans ofmaking a finetextured,but not heavy clayey,surface soil more permeable by mixing sand into it,thus relatively permanent change in surface soil texture is obtained.When properly done, sanding results in improved root penetration and better air and water permeability which facilitates leaching of salts in fieldswhere surface soil permeability limits water penetration. Investigations in the province of Almeria in Spain have shown that shore sand,washed in the sea, and spread over a formerly unproductive,flat salt marsh to give 10 c m depth of sand, allowed production of out-of-seasonvegetables,including salt-sensitivetypes such as beans. Irrigation waters of indifferent quality were used and within two years the soil lostits saline and alkaline nature to a depth of 30 cm.For high priced crops,hauling sand up to 10 k m was economic. O n the heavy clayey solonetz soils of the Hungarian Great Plain,however, the sanding method yielded no permanent and satisfcatory results. In Hungary the sandy type of solonchak and solonchak-solonetz soils could be reclaimed successfully by sanding (Arany). In the irrigated areas ofTurkmenia in the USSR,talcyr soils can be fullyimprovedifdeep ploughing (about 50-70cm) is combined with sanding (500-700 tons/ha) and followed by heavy leaching(about 1000-1 500 mm). Profile inversionis used when the surface soil is good but the upper subsoilhas undesirable characteristics. This condition frequently occurs in solonetz soils having a favourable surface soil undcrlain by a prismatic, slowly permeable,sodium affectedß-horizonwhich is underlain,in turn,by a more permeable horizon sometimes containing gypsum. The objective of profile inversion is to retain the surface soil while inverting the subsoil and substratum.This is accomplished by removingthesurfacesoil,deepploughing the subsoiland sub* This chapter was edited by V. A.KOVDA and C.VAN DEN BERGfrom the manuscripts submitted by I. N.ANTIPOV-KARATAEV (part C),H.DREGNE (parts A, B, D,F and G)and V.R.Vo~onuev(parts A,B,E,F a n d G)as authors and by A.G.ASGHAR, A.S. ARANY,V. A. KOVDA (parts D,E and G)F. PENMANand I. SZABOLCSas co-authorswith contributions by B. OZTAN.

430

R E C L A M A T I O N OF SALINE A N D ALKALI SOILS

stratum,then replacing the surface soil (Fig. 13.1). If the soil is a solonetz with gypsum in the substratum that is placed above the B-horizon,reclamation may be accomplished without applying a soil amendment. Even though gypsum may not be in the inverted,more permeable substratum,soil reclamation after profile inversion is facilitated by the greater depth of permeable soil and greater root penetration. I

II

Fig. 13.1. Schedule for ploughing solonetz in the chestnut soil zone with a PT 2-30 three-tierplough I. Profile of virgin soil II. Profile of cultivated soil 2. Biological amelioration Living and dead organic matter has two principal beneficialeffects on reclamation of saline and alkali soils: (1) improvement of soil permeability; (2) release of carbon dioxide during respiration and decomposition. In addition,the shadingeffectofplants leadsto reduced evaporationfrom the soilsurfaceand slower build-up of surface salt by upward movement of water. In the Murrumbidgee irrigation areas (Australia) lucerne promoted the reclamation of saline soils lacking underground drainage by lowering the water table with its deep root system and thus allowing the salt leaching. Incorporating large amounts ofmanure in the soil likewiseimproves surface soilpermeability by loosening the soil and increases carbon dioxide evolution when the manure decomposes. In both saline and alkali soils,the former effect is beneficial,whereas the latter effectis of greatest benefit on calcareous alkali soils. 3. Chemical amelioration

Chemical amendments are used to neutralise soil reaction,to react on free soda and to replace exchangeable sodium by calcium. The success of the reclamation of alkali soils depends on the healthy arrangement of the hydrological(surface and underground water) question.Amendmentsused in alkali soils reclamation depends on the genetical type and chemical character of the soil and fall into three categories:

1. soluble calcium salts such as calcium chloride and gypsum 2. slowly soluble calcium compounds such as limestone (CaCO,) and waste lime from sugar mills (a mixture

of alkaline calcium compounds) 3. acidifying materials such as sulphuric acid,sulphur,and iron sulphate.Besides the direct effect ofhydrogen, the acidifying amendments serve to reclaim alkali soils by neutralising soda and reacting with lime in calcareous soils to produce gypsum which furnishes the desired soluble calcium. Gypsum is, by far,the most common amendment for alkali soil reclamation. Calcium chloride is highly soluble and would be a satisfactory soil amendment,especially when added to the irrigation water,if it were not so expensive. Limestone (lime)is not an effective amendment for reclamationofalkali soilswhenused alone.Itssolubility in an alkaline medium isvery low,thereforc it is avery slow-actingmaterial. Experiments in Oregon and Hungary have demonstrated this.When combinedwith a large amount ofmanure,lime has some beneficialeffect, presumablybecause ofthe calcium bicarbonate formed as the manure decomposesand releases carbon dioxide to react with lime. Lime should be effective on acid soils such as solodised solonetz (USSR and Hungary). Sulphuris an inert material until it is oxidised to sulphuric acid by soil micro-organisms.Like other microbial transformations,sulphur oxidation requiresoxygen and time.The delay in reaction time-chiefly on very clayey alkali soils-and the strong acidity occurring around sulphur particles, which may be harmful to 431

IRRIGATION, DRAINAGE A N D SALINITY plant roots, are the principal limitations in the use of sulphur. This disadvantage appears very rarely on sandy types of solonchak and solonchak-solonetz allcali soils (Arany). All other sulphur-containing amendments (sulphuric acid, polysulphides, iron sulphate, aluminium sulphate) are effective because of the sulpliuric acid they contain originally or the acid formed upon microbial oxidation (polysulphides) or hydrolysis (iron sulphate and aluminium sulphate). In addition,calcium polysulphide supplies calcium directly. 4. Hydrotechnical amelioration

Leaching and drainage are two basic requirementsfor the successful reclamation of saline and alkali soils.

The only practical way known at present to remove excess soluble saltsfrom the soil is by washing them out; provision must then be made to remove the salty drainage water in order to prevent re-salinisation or realkalisation.When the underlying layers are permeable,artificial drainage is not required. Since such a condition is rare in areas where saline and alkali soils occur, it is usually a mistake to believe that a drainage system will not be required. A drainage system in saline soils must be designed to de-salinisenot merely the top soil layer but also the upper subsoil and water-bearinghorizons. Thusit regulates both the water and salt balances of the soil and subsoil. Various types of drainage are used all over the world: vertical, deep horizontal, shallow horizontal, and so on. (See Chapter 11.) Vertical drainage has given good results in certain soil conditions,i.e. when the deep horizons are highly permeable. Shallow horizontal drainage (about 1 m) has been widely used,but has not always been entirely successful. Leaching with shallow drainage has not resulted in a lasting improvement,since this type of drainage fails to eliminate secondary salinisation. On the other hand, deep horizontal drainage lowers the water table to such a depth that it is no longer harmful,thus reducing the risk of secondary salinisation: leachingwith deep drainage produces lasting desalinisation.Moreover,this method has the great advantageof desalinising the soil and groundwaters to a depth 2-3 times greater than the actual depth of the drain.When leaching is to be done under specially difficult conditions or at a high rate,it is worthwhile to combine deep and shallow drainage. Better results are likely to be obtained by placing the drains in depressions and the irrigation canals on higher ground. On plains, the drains should be located in the centre of the areas between canals. Unless canal seepage is the cause of a high water table, it is most inadvisable to place drains and canals side by side. As regards construction, a drainage system may be open or closed. N o essential difference in efficiency between these two types of drains has been established if the required depth is maintained; open drains are frequently more difficult to operate because the channel walls tend to crumble; whereas closed drains, made, for instance, of tile pipes covered by gravel, work perfectly for long periods, requiring virtually no repairs. 5. Electro-reclamation

Investigations have shown that treatment with electrical current may stimulate reclamation of saline-sodic (and soda) soils,although it cannot replace conventional proceduresinvolving controlled leaching.A remarkable result was attained by the US Bureau of Reclamation on a field trial in Montana where saline,boggy land that had produced no crop for 19 years had been improved to such an extent that a heavy growth of grasses and sweet clover covered the area 50 days later.In this work,after the current had been applied for a short time,the sodium content of the drainage water from the area increased from 1500ppm to 37000ppm. The expenses for electrical current amounted to $6/ha. Experimental study of electro-reclamationof the solonetz soils has been started in the USSR.But it is too early to recommend this method for practical use in agriculture. 6. Synergic effect of combinations of reclamation methods

Reclamation can be speeded up by combining various amelioration methods described above.Deep ploughing or subsoiling to open up slowly permeable layers in the subsoil increases the effectiveness of leaching saline

432

RECLAMATION OF SALINE A N D ALKALI SOILS soils.Similarly,the combination of profile inversion, application of large amounts of manure, and leaching will hasten reclamation of alkali soils. The choice ofreclamationmethods should be based on reclamationprinciples,with wide latitude permitted in the selection of particular treatments. Saline soils require leaching and drainage, so improvement in permeability by some physical treatment is most likely to be beneficial.Alkali soils require neutralisation of free soda and replacement of exchangeable sodium with calcium,followed by removal of the sodiuni salt resulting from the reactions. Any treatment, whether chemical, biological, or physical, which provides soluble calcium will help, and all three together will usually hasten the reclamation process. Since leaching and drainage are necessary on alkali soils after the exchange reactions have begun, improvement in permeability by physical and biological means will augmentthe improvementresulting from the removal ofexchangeable sodium.Saline-alkali soil reclamation is similar to that of alkali soils,in principle.

B. SELECTION OF METHODS

OF RECLAMATION

1. Data needed The success of a reclamation project depends to a large extent on the choice and implementation of the method;it requires,firstly,a detailed,comprehensivepreliminary survey of local conditions,and secondly,a careful definitionof the requirements of each reclamation method. The possibility of detrimental changes occurring in the soilunder irrigation have also to be considered.Hence experimental data have to be obtained by wide research as a basis for planning successful soil improvement work. The first step is to investigate climatic conditions.The next step,equally important,is to collect data on the soil properties. Soil improvement research must deal with a great depth (4to 5 m below the surface); and it is of the utmost importance,when planning soil improvementmeasures,to get comprehensive data on the whole of the soil and subsoil profile, down to a depth of 5 to 10 metres, including the groundwaters (depth,chemical analysis,dynamics). The geomorphological analysis of the area is extremely important in assessing the geographical factors governing changes in soil and subsoil conditions, working out a correct classification of soil and subsoil types and investigating the conditions of water and salt migration. It is, furthermore, essential to know the chemical composition of the water for irrigation. In other words,plans for soilimprovement must be based on climatic,geomorphological,hydrogeological, soil and hydrochemical conditions. To obtain the best results,research on all the above factors should be carried out simultaneously. O n the basis of all these data, decisions will then be made on the following points: (a) the genetic relations between the area under consideration and the neighbouring territories; (b) the type and extent of natural drainage facilities;(c) the places where irrigation is likely to lead to desalinisation,secondary salinisation and/or waterlogging. The most importantfactors are the salinity and alkalinity of the soil. Salinity must be defined on the basis of the quality and the total quantity of salts in the various soil horizons. Salt reserves are usually expressed in terms of the mean percentage of salts found in each particular layer. As regards the distribution it is important to determine at what level the salt content is dangerous (right from the surface,below the root zone,or deeper). The chemical composition is determined on the basis of the ratio of predominating anions and cations. Soil alkalinity is defined by p H data and titration alkalinity from carbonates of sodium.Solonetzquality is assessed from the quantity of sodium in the adsorbed cations. Solonetzmay be of various types: residual or contemporary,weak or strong;and may be located either on the surface or confined to the centre of the soil profile. The purpose ofstudying the soil structure and the chemicalproperties ofthe groundwater is to provide data for definingthe agro-ecologicalcharacteristicsand evaluating drainage,irrigation and leaching data (porosity, water capacity,wilting point,permeability,capillaryproperties,water and saltregime,tendency to subsidence, etc.). Many of these data can be obtained in the laboratory,but a great deal of important research has to be carried out in the field. On completion of research,a soil improvement map is drawn up,showing in detail the lay-outof the soil, 433

IRRIGATION, DRAINAGE! A N D SALINITY

its agro-ecologicalproperties and possibilitiesfor improvement.The map should contain details of the main types of soil,an estimate of improvements possible, and notes on the conditions required for application of reclamation measures. 2. Choice of reclamation methods in relation to origin and type of soil (salinity, alkalinity), water quality and drainage condiîions

The abovementioned surveys make possible the selection of the most appropriate methods. Where salinity is the main problem,reclamation will consist ofleaching.Installation of drainage will then often be required. Areas with good natural drainage (e.g.,fore-mountainplains,plateaux and high water sheds) do not as a rule need artificial drainage since they are usually characterised by deep-lyinggroundwaters.Saline soils in such areas can easily be leached.Even so,the size ofthe area to be leached every year,and the leaching norms to be used,must be calculated on the basis of the storage capacity of the subsoil,to make sure that the water table does not rise to a dangerous level, In foothill plains, alluvial fans of rivers (‘drydeltas’) and so on,characterised by the gradual rising of the groundwaters to the surface,the main problem is to control canal seepage in the higher lying parts. Where the water table is near the surface,drainage must be installed from the beginning and a vertical drain may be the most appropriate especially when the water is of usable quality for irrigation. Leaching may have to be done,but only in certain places. Soil improvement conditions may be much less favourablein low lying areas (marine deltas), where the groundwaters are close to the surface,and are highly mineralised.In this case it is essentialto install a dense network of drains and collectors both for the drainage of the leaching water and also, to some extent, for lowering the water table. Drains, in this instance, are run along the depression between ridges. Leaching is usually necessary over large areas,and it may cause marked accentuation of the solonetzcharacteristics of the soil which is often clayey on plain foothill strips. Areas with drainage classed as ‘inadequate’-lowlands and the lowest terraces of valleys, the deltas of rivers-callfor more drastic methods. In these areas,leaching and drainage are necessary immediately.Here again,though,the measures to be applied will vary from one place to another. In the relatively less saline areas where only one-quarterto one-thirdof the soilis salt affected,treatment of the saline patches takes the form of selective leaching after extensive preliminary operations including the installation of main collectors.When salinity is both more intense and more widespread, the land can be successfully reclaimed on a lasting basis only by installing a dense drainage network,so that the soil can be leached within the stipulated time limit,and the top layer of the groundwaters subsequently desalinised. Lastly,areas without natural drainage (i.e. marine and continental deltas, continental depressions) need the most drastic measures.Drainage and leaching have to be done before irrigation starts,with quite a dense network of drains and large leaching norms. Xt is usually more expensive and more difficult to reclaim soda-alkali soils than saline ones. For rapid achievement,a simultaneouscombination ofmethods is commonly much more effective than the application of several methods in sequence. For example,on an alkali solonetz soil having gypsum within 80 c m depth, combining practices such as profile inversion,cultivation of a sodium tolerant crop like Bermuda grass, and irrigation with a calcium water, will result in fast reclamation. Soils only moderately sodium-affectedmay sometimes be reclaimed by a close-growingcrop like irrigated pasture,ifthe water quality is good and there is some free lime in the soil.Alkali soils are commonly fine textured,and their permeability may be low even after removal of excess exchangeable sodium. For this reason, physical and biological amelioration by practices such as fallow,subsoiling,heavy manuring, and in some cases, artificial drainage should be considered.Leaching water oflow salt and high sodium content can delay reclamation:the applied gypsum may be washed from the top surface soil before exchange reaction has been completed.The soil remains alkali and its permeability impedes further leaching.Applying part of the gypsum in the leaching water can overcome this problem. 3. Laboratory and field experiments

Laboratory and field experiments should be designed to supplement previously collected data and to help in selecting methods.

434

R E C L A M A T I O N OF SALINE A N D ALKALI SOILS

Although laboratory experiments, whether on disturbed or undisturbed soil samples, are only approximations of field experiments, they can serve as useful guides in the selection of field reclamation methods. On saline soils,the main laboratory experiment to carry out is to simulate field leaching;this experiment, as well as the other mentioned below, must be done with the kind o jwater which will be used in the field. Much useful information is obtained by building models, in special trays, to analyse the water movement and the desalinisation process of the soil with various types of drains,leaching systems,and so on. Soil layers which are expected to have a low permeability should be tested to determine the depth of the least permeable layer.Thisindicateswhether deep ploughing or subsoiling is practical and what drainage problems can be expected. A second item of information is the leaching requirement (see Chapter 11) to reduce the salt concentration in the soil (measured by salt in the leachate) to the desired level.Another point to be checked is the change in soil Permeability when most of the salts are removed.A permeability decrease may mean that addition of a calcium salt is necessary to avoid soil dispersion: tests can be made in the laboratory, too. A simple test may provide information on the leaching programme (continuous or intermittent leaching). On alkali soil, as with saline soil, leaching experiments are the most important ones to conduct. Here, however, both amendment application and leaching should be combined. Among the points that can be tested are the effectiveness of profile inversion in supplying soluble calcium, effects of different kinds and amounts of amendments on soil permeability and exchangeable sodium percentage,usefulness of manure applications,application of gypsum,reaction time of sulphur,effectiveness of alternate wetting and drying as compared with continuous leaching,and the possibility of using saline waters at the beginning of reclamation followed by low salt waters after sodium removal. The laboratory is the place to screen proposed reclamation methods, after which the best can be tried in the field. Since laboratory permeability will,in all likelihood,not be the same as field permeability for any one treatment,a range in amounts of leaching water and amendments,as well as in other factors,should be included in the field trials,using the laboratory results as a guide. Field experiments are a necessary step in deciding upon the reclamation methods to be used because the effectofplant roots,plant cover,manure,and mechanical treatments (deep ploughing,subsoiling,fallowing, etc.) cannot be evaluated very well in the laboratory.Then,too,there are almost always some soil variations from place to place in the field that will influence the effectiveness of treatments. The way in which the groundwater table,if present,will fluctuate after leaching is begun is another factor that can be determined only in the field.The influence ofirrigation canalsand drains,if any,on thewater-saltregime must be investigated.Field plots should usually be rather large because lateralmovement ofleaching water can be considerable,especially in stratifiedsoils.Duration ofthe field experimentmust be sufficientlylong.Field experiments on leaching,solonetz improvement,etc., are likewise useful (even when confined to small plots) as well as field tests on the salt-resistanceof agricultural plants under ordinary agro-technicalconditions,measurement of the transpiration rate,and so on. All this research of course requires time (not less than 2-3 years) and is done,as a rule,either during the period when the detailed plans are being prepared or at the early stage of the reclamation work.

4. Consideration of local practical experience Local practices used by farmers to reduce the adverse effect of salt and sodium in established irrigated areas should not be ignored. One reason is that farmers are commonly reluctant to accept new practices but are willing to make small changes in accepted practices. Another is that they may have learned,from trial and error over many years,what crops will or will not thrive in theirparticular area,and their experience can be an invaluable guide. A n example ofthis is the experience ofthe people ofthe CentralAsian Republics in dealingwith saline soils. From ancient times, they have used ‘zaury’-sunken canals (0-7-2-5 m deep)-which served as drains. One of the largest collector drains at Bokhara was built in the time of Tamerlane (fourteenth century). The Chinese people too have used drainage from time immemorialas a means of drying and desalinising the soil;while the irrigation wells in India have also served as a means to land improvement. In Europe, as early as the seventeenth century,we find descriptions with drawings of the technique of drainage. Central Asian farmers,from earliest times, buried the remains of mud huts as a means of improving the soil quality, while the Chinese peasants spread silt over their fields.Critical observation of these practices may well serve to indicate the best methods for present-dayuse.

435

IRRIGATION, DRAINAGE A N D SALINITY

A good way ofintroducing new methods is to include the usual practices in field experiments.This provides farmerswith an opportunity to compare their methods with the new ones and demonstrates a willingness to give consideration to their practices. 5. Preparing the programme and sckedule for soil improvement work

World experience in irrigation and soil improvementhas shown that failureeither to prepare detailed plans or to observe a rational time-tablein their implementation may have very serious consequences.A particular instance of this is when irrigation schemes have been undertaken without any provision for obviously needed drainage. Land improvement schemes have also failed by applying measures in the wrong order; for instance, building irrigation canals before installing the drainage system;in such cases, it is necessary to begin by constructing the main collectors and the pumping stations (if provided for in the plan). Installation of the secondary drains before the water collectors may cause the drains to be flooded,the drain-cuttingsto crumble away, and the ground around them to become saline. It is important,too, that all the preliminary operations for land improvement-building of settlements, roads,communications,levelling of the land-go together with the construction of the irrigation drainage system. Leaching by itself,unless followed by agricultural exploitation,may well have little effect.

C.

RECLAMATION OF NON-SALINE ALKALI SOILS AND SODA SALINE SOILS (SOLONETZ)

1. Chemical methods Various chemicalcompoundshave been tried for the improvementof this type of solonetz soil and evaluated in terms of effectiveness and cost.The countries in which most of the experimental work has been done since the 1920’s are the USA and the USSR. As mentioned above,the calculation of the dosage of the chemical substances used for soil improvement is based on the theory of the equivalent of exchangeable sodium, i.e. the dosage of the chemical compound used (gypsum, calcium chloride, etc.) must be equivalent to the quantity of exchangeable sodium to be removed.In making this calculation,a final exchangeable sodium percentage (ESP)of 10 is considered as not resulting in any noticeable peptisation of the soil. The gypsum requirement can be calculated from the following formula:

(ESP 100

CEC=gypsum requirement in me/100g of soil

ESP initial is calculated before reclamation; ESP final is the desired value after reclamation;and CEC is the cation exchange capacity in me/100g of soil.If,for example,ESP initial= 30,ESP final= 10 and CEC =24 then (3z)24=4.8 me of gypsum/100 g of soil Since 1 me ofgypsum/lOO g of soil equals860parts per million ofgypsum,for one hectare to a depth of 20 c m (roughly 3000000 kg), the amount of gypsum required will be: 860. lovG.3*10G. 4.8 # 12400 kg

This calculation is based upon 100% replacement of sodium by calcium. Because of the presence in some alkali soils of free soda,the actual efficiency is lower.Thus it is recommended that the amount of applied gypsum be increased in accordance with equivalents of free sodium carbonate and bicarbonate. The amount of amendments to be applied can be calculated by comparison with pure gypsum as follows:

436

RECLAMATION OF S A L I N E AND ALKALI SOILS Amendment

Tons

Gypsum (CaSO, .2Hz0) Calcium chloride (CaCl, .2H20) Limestone (Caco,) Sulphur Sulphuric acid Iron sulphate (FeS04.7H,O) Aluminium sulphate (A12(S04)3.18H20) Calcium polysulphide (Cas,) 24%sulphur

1-00 0.85 0.58 0.19 0.57 1.62 1 *29 0.77

The properties of sulphur as a soil-improvingagent depend on how finely it is ground and how effectively it is injected, as determined by special experiments on the kinetics of the oxidation of sulphur in soil (cf. THOMAS, 1936). The quality of gypsum,too,depends largely on its fineness. GABALY at the VI1 Congress of SoilSciencesdemonstrated that the optimum particle sizefor gypsum is about2mm.(Inthe USA,the gypsum powder is passed on a 100-meshscreen,giving a particle size of 2 mm.) O n the basis of general conclusions drawn from many experiments, the manual issued by the Salinity Laboratory ofthe US DepartmentofAgriculture recommends the following chemicalsubstancesforimprovement of solonetz-solonchaksand typical sodic-sulphaticsolonetz soils and describes their interaction with the two sub-typesof solonetz in question (solonetz-solonchaksand typical solonetz). In the first case,there are alkaline-earthcarbonates at some depth from the soil surface,while in the second,the arable stratum contains no calcium carbonates. For thejirst sub-type of solonetz: Gypsum: Na,-soil +CaSO,~Ca-soil+Na,SO, Elementary sulphur: Microbial oxidation: 2s+30, -+2SO,

SO,+H,O=H,S04 H,SO, +2CaCO,%CaSO, +Ca (HCO,), Na,-soil+CaSO,=Ca-soil +Na,SO, Lime-sulphur (calcium polysulphide) :

+4H,S04 Cas,+80, +4H,O%K!aSO, HzS04+CaCO,sCaSO, +CO,+H,O Na,-soil+CaSO,%Ca-soil+ Na,SO, Iron Sulphate:

2 FeSO,+3H,0%2H,S04 +FeO/Fe(OH), H2S04+CaCO,%CaSO, +CO,+H,O Na,-soil+CaSO,%Ca-soil+ Na,SO,

A similar reaction occurs with aluminium sulphate with sulphuric acid. In the second case, typical solonetz (saturated paste p H > 8.5) Gypsum: as in the above formula. Sulphur: producing sulphuric acid,reacts with the soil:

Na,-soil+H,S04S2H-soil+Na,SO, Lime sulphur: ditto,via H,S04

For this sub-typeand more especially for solodised solonetz,the liming with fine ground lime,waste of sugar mill,slag of nicotine,has been widely used by DE SIGMONDand ARANY(1924, 1926): the application and BROWN.The reaction is then as follows: CaCO, was also recommended in 1934 by KELLEY

NaH-soil+CaCO,%Ca-soil +Na HC03 According to a paper submitted by H.JENNY, farmers in the western states of the USA (California, Arizona, etc.) use concentrated sulphuric acid as a means of improving calcareous solonetz soils with irrigation.It reacts very quickly with Na-soils,in addition to which its cost oftransport (being concentrated) is much lower than that of other chemicalsubstances (such as gypsum). In calculating the cost,the reduction

437

IRRIGATION,DRAINAGE A N D SALINITY of the time taken to improve the soil is also an extremely important factor,since expenses only begin to be defrayed when the exploitation period is reached,i.e. when crops can be cultivated in the area. The comparativeeffectiveness ofthe many difrerent chemicals used for soil improvementis usually assessed on the bases of the three following indices: (1) improvement of the experimental crop yield;(2) degree of desalinisation of the soil profile; (3) degree of de-alkalisationof the soil (as shown by the dynamics of the exchangeable sodium content in soils). Effective results were obtained in the experiments carried out by Kelley and his colleagues during 10 years on the soda solonetz-solonchaks of California after application of chemical substances. Table 13.1 contains the data for these experiments. Table 13.1. Changes over 10 years in the chemical composition of soda solonetz-solonchaks as a result of soil iiriprovement measures and irrigation-(After KELLY, 1951)

Before improvement,1921 Depth Exchange base (in me/100g) ESP

(cm)

Ca-tMg K

0-30 30-60 60-90 90-120

1.08 0.42 1-78 2.57

0-30 30-60 60-90 90-120

1.35 1.21 3.19 3.61

pH

Na

After improvement,1931 Exchange base (in me/lOOg) ESP Ca+Mg K Na

pH

I. Experiments at Fresno with gypsuming (25, 30 or 37.5 tons/ha) 0.23 0-98 0-28 0.34

3.13 2.87 2.41 1-59

70 67 54 35

9.7 9.4 9.6 9.1

5.03 459 4.63 4.13

O O O O

0.27 0.40 0.43 1 -00

8 19

7.5 8.1 8.3 8 -7

0.21 0.44 0.38 0.85

5 10 8 17

7.0 7.5 8.8 8.3

5

8

II. Experiments with ‘sulphuramendment (up to 2.5 tonslha) 0.44 0.34 0.20 0.13

2.54 2.90 2.00 1.26

58 65 37 25

9.7 9.2 9.0 9.4

4.06 3.75 4.05 3.66

0.20 0.15 0.37 0.30

As may be seenfrom Table 13.1,ten years of application of chemicalscombined with leaching in irrigated areas,has brought soils into the weakly alkaline category.At the same time,the crop yield,only nine years after the beginning of the process, equals the maximum stable figure for alfalfa (1643 to 20 tons/ha). The necessity of combining chemical treatment with irrigation includingleaching is pointed out by ARANG who has observed that the effect of gypsum disappears after 3 to 4 years in non-irrigatedsoils. POWERSgave in 1946 data from long-term experiments (conducted over 20 years) on the comparative effectiveness of various treatments.As expected,at the end of the period in irrigated areas,the exchangeable sodium content in plots where chemicals have been applied is about half that of the soil in control plots, while the lucerne crop yield is from two to three times larger-see figures below: Control plot Manure +sulphur Manure +gypsum Sulphur Gypsum Alum +manure

24.1 centals/ha 147.9 centals/ha 78.9 centals/ha 74.6 ceutals/ha 84.0 centals/ha 100.08 centalslha

Other interesting research was carried out by OVERSTREET,MARTIN and KING(1951)on soda solonetzsolonchaks at Fresno to determine the comparative effectiveness of gypsum,sulphur and sulphuric acid on a highly alkaline soil (ESP 60 to 100). After seven months of experiments, including leaching of the soils, alkalinity was again determined,with the results shown in Table 13.2. These figures indicate the effectiveness of sulphuric acid amendment,which quickly reduces soil alkalinity (bringing the ESP to 10). As regards cost,the prices of the different chemicals used compare as follows: MARTIN and KING consigypsum 854 a ton,sulphur $48 and €PzSQ, (93 concentration) $1 13. OVERSTREET, der that the amount of sulphuric acid used coirld be reduced to between 3-5and 7 tons/ha (depending on the level of alfalfa productivity). This explains why farmersand soil-improvementspecialistsin the USA are becoming increasingly interested ín using concentrated sulphuric acid for improving cstlcareous solonetz soils in irrigated areas. Large scale amelioration programme of soda-solonetzand soda-salinesoils is being executed in Armenia 438

R E C L A M A T I O N OF S A L I N E AND ALKALI SOILS Table 13.2. Change in the alkalinity of the uppermostfoot of the irrigated soil afer application of chemical amendments-(OvERsTREET, MARTIN and KING)

Exchangeable sodium in me/100g

Plots

pH

Yield of hay in second

(1:5)

year of experiments (in cental/ha)

-

Initial soil Gypsum (25-31a25 tons/ha)

9.1 2.0

9.0 8.6

129

Initial soil Sulphur (4.71 tons/ha)

8.7 3.6

9.1 7.7

191.2

Initialsoil Sulphur (4.71 tons/ha)

856 8-42

9.60 9.45

Initialsoil soil untreated

10.39 7.69

84

-

9-55 9.65

90.5

by application of 1 % sulphuric acid solution,heavy leaching (15000 to 30000 m3/ha)sometimes combined with rice cultivation, and extensive deep drainage (horizontal drains combined with tube wells). In the Ararat valley in the Armenian SSR, there are large areas of meadow-type solonetzand soda solonchaks which it would be economically advantageous to reclaim and ase for cotton,fruit and vine growing. Since the alkaline horizons ofthese soils are very thick (as much as 60-100 cm), the sulphuric acid is applied layer by layer, with multi-tierploughs, or using a specialripper.The standard application of sulphuricacid is 30 tons/ha (expressed as 80% acid), and the crop yield for the fist two years is sufficient to pay for the improvement cost. Data of a reclamation are given in Table 13.3.In 1963,the yield of winter wheat was 3200 kg/ha,in 1964 the yield of alfalfa hay was 12 tons/ha.The time required for this type of amelioration is not less than 3-4 years (A.CHITCHAN). 2. Possibility of improving alkaline solonetz soils in irrigated areas by making use of the calcium salts (carbonates) in the soil itself

It is well known that the following relationship exists between the solubility of CaCO, in water and the p H value (cf. SIGMOND,1938). PH

Solubility of CaCO, (in me/l)

6.21 6.50 7.12 7.85 8.60 9.20 10.12

19.3 14.4 7.1 2.7 1.1 0.82 0.36

The most practical means of reducing p H and permanently increasing the CO2concentration in the soil is to intensify the microbiological processes of decomposition of organic substances ploughed into the soil in the form of green or farmyard manure. In this way the exchange of Ca against adsorbed N a is promoted. Alkaline-resistantgreen manure crops must be selected for this purpose. In West Pakistanthey grow Sesbunia aculeatu (ASGHAR and HAFEEZ, 1955) :Experiments have shown that a substantialreduction of soil alkalinity may be obtained by this method. The research studies indicate that Sesbunia aczrlenta should be introduced in the process reclamation at two distinct stages: 439

IRRIGATION, D R A I N A G E AND SALINITY Table 13.3. Water extracts analysis of soda-solonchakof Armenia before and after applications of siilphuric acid and leaching-(After A.CI-IITCHAN)

Time

Depth

in

%of dry soil

(cm)

alkalinity

Before the

0-25

ameliora- 25-50 tion (1959) 50-100 During the amelioration (1962) After the amelioration (1964)

100-150 150-220 0-25 25-50 50-100 100-150 0-25 25-50 50-100 100-150

salts

CO,” totalHC0, C1

so4

Ca

Mg

Na calculated

1.878 1.506 1.267 0.744 0.308 0.659 0.152 0.442 0.594 0.088 0.151 0.143 0.072

0.076 0.046 0.047 0.050 0.019

0.007 0.007 0.005 0.004 0.002 0.109 0.008 0.005 0.004 0.008 0.011 0.006 0.004

0.004 0.004 0.001 0.002 0~001 0.010 0.003 0.001 0-001 0.003 0.002 0.003 0.002

0.605 0.504 0.442 0-253 0.101 0.092 0.035 0.120 0.182 0.013 0.031 0.040 0.015

6-319 22.670 1-277

0.068

0.184 0-087 0.026

0.337 0.537 0.012

1.508 7.113 0.338

-

-

0.072 0.099

-

-

0.345 0.147 0.136 0.149 0.079 0.025 0.038 0.242 0.283 0.040 0.055 0,066 0.032

0.010 0.007 0.007

0.606 0.485 0.254 0.073 0.027 0.416 0.062 0.059 0.149 0.015 0.042 0.021 0.012

0.692 1.410 0.620

2.580 8.566 0.102

0.978 4.533 0.179

0.311 0.359 0.429 0.263 0.098 0.007 0.006 0.008 0.016 0.009

Underground waters 1959 1962 1964

220 150 150

Time

-

0.029

Depth

in m e per 100g of dry soils

(cm)

alkalinity CO3

Before the

amelioration (1 959)

During the amelioration(1962)

After the amelioration (1964)

0-25 25-50 50-100 100-150 150-220 0-25 25-50 50-100 100-150 0-25 25-50 50-100 100-150

2.55 1.52 1.58 1.68 0.65

2.40 3.30

-

-

total HC03 C1

5.65 2-41 2.23 2.45 1.30 0-41 0.62 4.08 4.64 0.66 0.90 1-08 0.52

8.76 10.12 12.08 7.42 2.75 0.20 0.17 0-22 0.45 0.25 0.27 0.20 0.20

Na calculated

so4

Ca

Mg

12.60 10.09 5-28 1.52 0.56 8.65 1.92 1-23 3-10 0-3 1 0.87 0.44 0-25

0.33 0.33 0-24 0.21 0.12 4.45 0.32 0.21 0-16 0.41 0.56 0.25 0.16

0.36 0.36 0-11 0.16 0.11 0.80 0-25 0.08 0.08 0.25 0.15 0.25 0.17

26.32 21.93 19-24 11.02 4.38 4.00 1.51 5.24 7-95 0.56 1.33 1a72 0-64

20.34 94.38 3.72

7.52 3.56 1.06

31-40 46.14 0.99

65.56 309-26 14-71

Underground waters 1959 1962 1964

220 226 150 150 0.93

11.34 23.12 10.16

72.80 241.46 2.88

-After a successful crop of rice to reduce the residual alkalinity which develops as a result of the action of sodium salts on the clay during leaching and growth of rice.

-As a first crop in land under reclamation where rice is not likely to give a successful yield. For Californian conditions,KELLEYand his colleagues(1937,1951) recommend growing the followingplants: Cynodon dactylon (Bermuda grass) ;Lolium perennium (ray grass) ;Sorghum vulgare Sudanense,Chlorisgayana (Rhodes grass); Melilotus alba and a mixture of ray grass, clover (alfalfa clover) and sweet clover. KELLEY (1937) reports the following results of experiments on the biological improvement of irrigated solonetz soils in Fresno by growing Bermuda grass (2 years), barley (1 year), alfalfa (4years), and oats (1 year). 440

R E C L A M A T I O N O F SALINE A N D ALKALI SOILS Table 13.4. Composition of exchangeable cations in soclic-sulphaticsolonetz at Fresno-(After KELLY)

(inme/100g)

Depth (cm) (1)

0-30 30-60 60-90 90-120

K

Ca+Mg

1.27 0.0

0.33 2.70

(2) 3-99 3.97 3.85 4.69

Na

ESP

(1)

(2)

0.57

(1) 2-44

(2) 0.05

0.18

0.96 0.59 0-77

4.47 4.60

0.19 0.69

(1) 57 97 90

0.34

0.11

2.29

0.20

48

0.14

(2)

1 4 13 4

(1) =before improvement (1930) (2)= after improvement (1937)

Another way of inducing the biological activity in solonetz soils and the production of carbonic acid is by applying organic fertilisers (manure,peat, etc.) combined,if necessary, with calcium salts. The data from specialised research on soil improvement done in the Kirgizian SSR show that good results were obtained in the sodic-sulphaticsolonetz sierozem soils of the Chuish valley by using a mixture of oxidised coal (with a high content of humic acids) and sugar-millwaste (in the proportion of 1 :1). The improved soils produced substantially heavier crops of sugar beet and tomatoes.

3. Improvement of sodic solonetz soils in non-irrigated areas

Sodic solonetz of the meadow type (hydromorphic) and meadow-steppetypes in non-irrigated areas can be improved either by applying chemical amendments or by making use of the calcium salts in the soil (CaCO, CaSO, .2H,O).The choice ofthe method and the time needed forthe process selected depend on the amount of atmospheric precipitation in the particular bioclimatic zone and on the solubility of the chemical substances used. The choice ofchemical substances depends,in turn,on economic factors.Thus,the duration of the process will vary under different climatic conditions even though the same chemicals (gypsum for instance) are used;e.g. the process will be quicker in the case of solonetz in Hungary (with a mean annual rainfall of about 520-600 mm)than in that of the central chernozem belt of the European part of the USSR (including the Ukraine), where the mean annual rainfallis about 450-500 mm. It will require even more time in western Siberia,where the rainfall is only 300-400mm.These differences occur despite the fact that in all three cases we are dealing with solonetz soils in chernozem forest-steppeareas. The most detailed and continuous research on the improvement of solonetz and solonetz-solonchakshas been carried out on the soil of the Hungarian Plain,where large stretches of sodic-solonetz soils occur: superficially solodised solonetz;typical non-carbonatesolonetz; carbonate solonetz-solonchakssoils with surface salinity (soda-solonchaks). For the Ca-deficientalkaline soils Hungarian specialists apply two methods of soil improvement: (1) liming;and (2)covering the soil surface with the yellow carbonate subsoil.With the first method, 40-80 tons of manure and 40-150 tons of calcium carbonate or,more often,sugar-millwaste (70 to 100 tons), are used per hectare. All these substances are then worked into the topsoil by mixing tillage to a depth of 7.8 cm. With the second method,the liming substanceis taken from the lower horizon of a non-alkalisoil and spread on the surface in amounts of about 400 tons/ha together with the dark-colouredsoil layer accompanying the carbonate layer. The covering layer thus formed is ploughed into the top layer of the original soil. This method ofimproving superficially solodised solonetzis the oldest,having been proposed and extensively For the second category of solonetz with a neutral or weakly alkali used some 160 years ago by THESSEDIK. reaction,gypsum has been widely tried out in Hungary,as have (under experimentalconditions)iron sulphates, and ANTIPOValuminium sulphatesand sulphuricacid (see SIGMOND).During the past decade,PRETTENHOFFER KARATAEV (1960) have shown that this group of solonetz soils can be improved by means of a mixture of gypsum and calcium carbonate (waste from sugar-mills), used in various proportions depending on the alkalinity of the soil. In reclamation work on the carbonate solonetz-solonchaksbetween the Danube and the Tisza the main substancesusedaregypsum and powdery lignite;the latter,as a sulphursource,containsup to 3 %of sulphur

441

I R R I G A T I O N , DRAINAGE A N D SALINITY

which produces sulphuric acid by oxidation.The lignite is, of course,used in large quantities (70-280 tons per hectare) mostly in irrigated areas sown to rice (HERKE). The effect of soil improvement measures on soil fertility is apparent from the figures given in Table 13.5; while Table 13.6 shows the rate of de-alkalisation;as can be seen,the soil is de-allcalised to a depth of 40-60 cm relatively quickly.

R.HERKE)

Table 13.5. Change in the yield on iinproved solonetz soils in Hungary-(After ~~

Crop

Yield (in ton/ha)

Increase in yield (in ton/ha)

1. In irrigated areas Control plot Rice Gypsum Rice Treated with lignite Rice

1*23 1 e74

-

2. In non-irrigated areas Control plot Grass Gypsum Grass

0.33 5-26

0.51 0.70

1.93

4.93

Table 13.6. Change in the alkalinity of soils in Hungary as a result of cheniical soil-improvementmeasures

Treatment

(cm)

Liming

Gypsum Lignite

ESP

Depth before

Author

after improvement

1. Experiment in non-irrigatedarea at Karczag after4 years 0-20 56.8 6.5 20-40 51.8 15.1 40-60 49.0 18.4 60-80 23.7 80-100 15.0 18.8 after 3 years 0-20 68.8 3.6 20-30 90.4 40.0 30-40 96.8 42.7 0-20 60.5 4.6 20-30 85.7 58.1 30-40 90.9 72.7

DE

SIGMOND

HERICE

Successful work has been done in Hungary on composite methods for the improvement of solonetz. It consists of surface application of chemicals with deep tillage to break up the compacted solonetzhorizons Another line of research has concentrated on (Southern Agricultural Institute,Szeged, PRETTENHOFFER). achieving a substantial reduction in the dosage of calcium salts applied for improvement of solonetz soils by using a small quantity of these salts mixed in with the seeds. (Institute of Soil and Agrochemistry of the Hungarian Academy of Sciences; SZABOLCSand ABRAHAM). However,the rainfall is considerably less (450-500mm) in the chernozem steppes of the European part of the USSR (Central chernozem belt, and Ukrainian SSR) where, as stated earlier, there are large areas of sodic-sulphatic solonetz and which in the 'twenties was the scene of the first experimental work on soil in 1928)-work which has developed considerably improvementto be done inthe USSR (by R.I

Cl (%)

Depth Total salts (ton/ha) (in cm)

Spring Autumn Spring Autumn 0-2 2-10 10-25 25-40 100-120 160-180

1.3 1.5 1.6 1-1 0.5 0.5

15.1 1.7 1.5 0.8 0.3 0.3

0.03 0.05 0.10 0.11 0.11 0.80

3-96 0.14 0.14 0.16 0.06 0.04

Approximate seasonal coefficient of accumulation

Spring Autumn Spring Autumn Total CI 0-25

65

112

4

18

2

4

0-80

123

172

14

29

1.5

2

0-350

307

315

40

47

1

1

The salinisation of the arable horizon during the dry irrigation season,not only causes a drop in the yield for the current year,but affects the crop in the following year as well. All the above considered measures for prevention of salinisation of irrigated soils are also designed to regulate the salt regime,in that they prevent or greatly attenuate seasonal salt accumulation in soils. Ifthese measures are not sufficient,the salt regime ofirrigated saline soils,in conditions such as prevail in Central Asia and Transcaucasia,will have to be regulated by irrigation during non-vegetationperiods,i.e. in late autumn,winter or early spring,to intensify the seasonal desalinisation of the arable and root horizons. This is essential in order to ensure germination of the seeds and normal development of the plants in the current vegetation year. (b) MetJiods for regulation of the salt regime (1) Tilluge Deep ploughing of saline soils,particularly during the wet season,usually results in a substantialintensification of the seasonal desalinisation process. This is particularly marked when winter ploughing is done in the damp winter of a Mediterranean climate.

(2) Grasses in crop rotation

It is a well-knownfact that the cultivation of a thick cover ofgrasses(or especially oflucerne)incroprotation, with copious irrigation has an excellent effect on the salt regime. W e have observed this at first hand in Uzbekistan,where cotton is grown in rotationwith lucerne.Irrigated lucerne crops,as already said,improve the water regime and within 2-3 years cause the salts to move from the arable and root horizons into the deeper sub-arablesoilstratum.Such a cropping pattern is a very importantmeans ofregulating the saltregime and decreasing or eliminating seasonal salinisation in weakly and medium saline soils.

451

IRRIGATION, D R A I N A G E A N D SALINITY (3) Intensive irrigation regime In the USSR it has been found essential to keep the soil moisture at not less than 75-85 %ofthe field capacity in order to regulate the seasonal salt regime of strongly saline irrigated soils and to obtain good crops of cotton and grass. Accordingly, in a given period and for a year,two or three water applications have to be made in supplement for saline soils,in comparison with non-saline soils, the moisture of which may sink as low as 60-65 % of field capacity. In Central Asia during the months of July and August when transpiration,evaporation and salt accuniulation attain their maximum,the time between irrigations on saline soils should not be longer than 10-12days: the desalinising effects do not last more than 5-6 days. The effect of the irrigation frequency on the salt regime may be judged from the data obtained by the Vaksh SoilImprovementStationin Tadjikistan.In order to keep the soilmoisture above a given percentage of the field capacity,the quantities ofirrigation water used during the vegetation period and the resultswere as follows: Lower moisture as %of F.C.

Water requirements m3/ha

Effects

90 80 70 60 50

9150 6400 5180 4200 3790

desalinisation up to 2m soils are leached slight seasonal salinisation salinisation strong salinisation

Table 13.13. Fluctuations in the contents of chlorides in saline soils with various watering intensity (N.A.NOGINA’S figures)(Clin tonslha)

Moisture levelnot lowerthan: Season

spring autumn

60%

70%

0-45 cm

0-105 cm

0-200

0-45 cm

0-105

cm

cm

0-200 cm

0-45 cm

0-105 cm

0-200 cm

1a 2 4 10.04

14.42 17.21

19.87 22.43

2-99 9.57

8.99 11.01

18.05 17-23

3.93 3.40

9.15 3.88

12.40 4.96

(4) Winter preventive waterings In order to leach the easily soluble salts out of the arable and sub-arablehorizon, winter irrigations are practised (‘preventive watering’). In the climate ofthe Caucasus and CentralAsia, such preventive waterings intensify the natural process of desalinisation caused by autumn and winter atmospheric precipitations. The overall salt reserves in the soil remain unchanged and the easily soluble salts simply move from the top layers into the lower horizons and the groundwaters: this type of watering desalinises the arable and subarable horizons fairly adequately, so that seeds can sprout normally and agricultural plants produce a good yield before summer salinisation takes place. According to the data produced by Soviet scientific research stations in Central Asia and Transcaucasia, autumn and winter preventive waterings result in substantialdesalinisation when amounts of water between 1500 and 3000 m3/haare used (Table 13.14). Preventive waterings on reclaimed saline soils are necessary only for a time. Table 13.14. Seasonal salt regime in soils treated with preventive watering (salt content in

Depth

(4

23.1 after

9.x

%)

20.v

18.VI

15.VII

1O.IX

0.5 0.4 0.4

0.7 0-6 0-7

0.9 0.5 0.6

1.5 0.7 0.8

preventive watering ~

0-5 5-10 10-20

452

~~

~~

1.8 0.8 0.7

-

0-3 0.4 0.3

RECLAMATION O F SALINE A N D ALKALI SOILS

It is important that autumn and winter preventive waterings be confined to areas where soil salinity necessitates this treatment.In the USSR,such areas are to be found in the Amu-Daryaand Syr-Daryadeltas, along the lower reaches of the Zeravshan,and in the Kura-Araxesdepression, The seepage due to these irrigations raises the water table and the salts migrate to patches of fallow and uncultivatedland which thus becomes saline;therefore,to obtain lasting resultsand to avoid such salinisation, a deep drainage system has to be installed throughout the whole of the irrigation project. The drainage will lower the level and decrease the salinity of the groundwaters,thereby reducing the need for autumn and winter leaching and,eventually, attenuating seasonal soil salinisation. (5) Winter leaching of solonchaks as a means of regulating the salt regime Solonchaks and strongly saline soils can be cultivated only if the arable and sub-arablehorizons are leached heavily before sowing. The water depth used for this operation will be much larger than for preventive waterings: soils with a salt content of 1-2%will require leaching with 5000-6000 m3/hain order to obtain an agricultural crop (cotton,beet,lucerne); such leachingswill be necessary every year,owing to the intensive resalinisation occurring every autumn (mineralised groundwater). Practical experienceacquired by Sovietspecialistsshowsthat solonchaksmay be used for irrigated farming without deep horizontal or vertical drainage ifvery smallpatches-no more than 15-25 %ofthewhole areaare developed at a time: in such cases,the leaching and groundwaters flow off on to unreclaimed territory, where they gradually evaporate,and deposittheir salts.Inthisway,it is possible to produce crops onrelatively small areas by condemning the rest of the land (75-85 %)to a gradual salinisation. Leaching without drainage does not change the salt regime ofthe area as a whole;groundwater and easily soluble saltsremain near the surface.Therefore the soils either revert to their original salinity or become even more saline: drainage has to be installed,even with efficient tilling and frequent autumn waterings. Where salinity increases,the yields are small and uneven, or else plants wither,repeated leachings become necessary thus impeding farming operations and increasing the salinity of the surrounding fallow land. In the long run,the accumulation of easily soluble salts is a danger not only for the undeveloped part of the territory but also for the part reclaimed by leaching. Therefore, when dealing with solonchaks,leaching should not be done without the installation of a collector-drain network designed to desalinise the soil permanently and deeply. Table 13.15. Salt regime of irrigated solonchaks after leaching (%of salts)

Without drainage,Tadjikistan,Vaksh river valley, depth 0-50 cm Section No.

Autumn 1936

Spring 1937

Autumn 1937

35 30

1.31 1.29 1-57 1.29 0.57

0.60 0-76 0.29 0.52 0.34

1-13 1.36 1.50 1.50 0.90

44 41 70

Table 13.15 shows that solonchaks revert entirely to their original conditionwithin one year after leaching without drainage. Similar results have been found for the Indus Valley soils (ASGHAR and HAFEEZ, 1961)

(6) Regulating the salt regime of oases by deep drainage

At the present stage in the technique of irrigation,substantial quantities feed the groundwaters.With each cubic metre of water, a certain amount of easily soluble salts enters the irrigation system.With an average application depth of 10000 m3/hapel:year, 3 to 5 tons of salts are brought every year per hectare. These salts together with those contained in the seepage water will gradually migrate, via the groundwaters,to the edges of the irrigated area and the low-lyingparts of the relief, where they will accumulate. In irrigated oases where substantial areas of saline soils are developed the water table will be raised by the necessary leachings and preventive waterings. Many irrigationprojects are located in strongly saline territories(deltas,terraces along the lower courses of rivers, outlying parts of dry deltas), where the groundwaters are both stagnant and highly concentrated. In irrigation projects of this kind,having no natural groundwater drainage,the preventive measures described in the foregoing chapters will not suffice,by themselves,to halt the salinisation.In such territories,a network of deep draining collectors will have to be installed.

453

IRRIGATION, DRAINAGE A N D SALINITY 3. Keeping of the salt balance and leaching requirement in conditions of perfect drainage*

The principles of keepingthe ‘saltbalance’and leaching requirement when applied to steady-statewater flow rates or to the total equivalent depths of irrigations and drainage water over a considerable period oftime serve as a useful background for discussing salinity control. Originally the term ‘saltbalance’was applied by SCOFIELDto the tons per year of salt either removed or deposited in an irrigated area. Under conditions where it is possible to measure the amount of salt added to an area in irrigation water and the amount ‘saltbalance’controlis a usefulindicatorof removed in drainage water with reasonable accuracy, SCOFIELD’S year-to-yeartrends in salinity conditionsin the area.The principle of ‘saltbalance’can be applied to the root zone of crops. The leaching requirement(LR),related to keeping the salt balance, has been defined by the US Salinity Laboratory as the ratio ofthe equivalent depth of drainage water to the equivalent depth of irrigation water, Dd/Di, thatis requiredto maintain a given soil solution concentration at the bottom of the root zone.Because the concentration of the soil solution at the bottom of the root zone equals the concentration of the drainage water, one may write LR=Dd/Di=CJCd Thus, the leaching requirement may be calculated from knowledge of the concentration of the irrigation water and the permissible concentration of the drainage water. If, in addition to irrigation water, salt-free rainwaterisinvolved,an adjusted concentration for applied water must be calculated. The permissible concentration of the drainage water depends upon the salt tolerance of the crop. More particulars on the calculations of the leaching requirement are to be found in Chapters 2 and 11 of this book. Uniform areal iiifiltration of water is tacitly assumed in using the calculationsfor the depth of irrigations. However, because of difficulties in obtaining uniform water distribution and the marked variations in soil permeability within a field,this assumption is rarely valid. In practice, an additional depth of water must be applied to meet the leaching requirement at nearly all points in the field.The need for additional water is directly related to the degree of variance in soil permeability over the field. The principles of keeping ‘saltbalance’in the sense of equilibrium and leaching requirement are of great importance and value. The role of underground water as a main source of soluble salts in salinisation of soils on the one hand and as a recipient ofsalts (during the leaching) on the other hand,should be recognised. Permanently existing intrusions ofcapillary salt solutionsrising from an underground-watertablenecessitates the application of additionalirrigation water over the amount calculated on the basis of simplified equations (see Chapter 2).

E. AMELIORATION

OF SALINE SOILS BY LEACHING

World experience has confirmed the effectivenessofdrainage and leaching althoughthere have been numerous cases of failure due to various technical defects,insufficient care and, even now,insufficient experience. For reclamation and permanent effective development of solonchaks,the following measures are worked out by Soviet experimental stations and scientific institutes.

(i) (ii)

(iii)

Keep the water below the critical depth (2.5-3.0 m) by drainage Reduce the content of easily soluble salts in soils to 0-3-0*4 %and in the top strata of the groundwaters to 2-3 g/1by irrigation,leaching and drainage Avoid renewed accunidation of toxic salts in the root horizon by irrigation and also, if necessary, by repeated leachings,in combination with drainage

In the USA leaching and drainage are planned as far as possible,to keep the upper five feet of soil free of excess salt. Where it is not feasible to maintain the water table below five feet, special precautons must be taken to prevent the upward movement of salt into the root zone. Sometimesthis can be done by frequent * Reference is made to the paper submitted by Dr.C.A. BOWER,Director of the US Salinity Laboratory, to the symposium on Salinity Iield in Lahore, Palcistan (December 1964)

454

RECLAMATION OF SALINE A N D ALKALI SOILS over-irrigationto maintain a net downward movement of water but successis ordinarily dependent upon the water table not rising higher than about 3 feet below the soil surface. 1. Timing and methods of leaching solonchaks

The best period for effective leaching of solonchaksunder the conditionsprevailing in CentralAsia and in the Caucasus of the USSR is in late autumn or winter when the soil moisture is low and the water table deep. Summer leaching is, as a rule,the least effective because a large proportion of the applied water is lost by evaporation, and because there is a very strong tendency to secondary salinisation at this season.There are grounds for statingthat summerleachingisjustified in cases ofclayey soilcontaininglargequantitiesofsodium carbonate and sodium sulphate,or when leaching is combined with rice cultivation. Spring leaching is likewise not very effective because of the danger of recurring salinisation at this season. Before leaching,solonchaks should be cleared of the overgrowth of halophyte shrubs with which they are frequently covered,they should be levelled with an accuracy of 5 cm.The field is then ploughed to a depth of 25-30 cm-preferably with a subsoiler which will break up the deep subsoil horizons-and the surface levelled again.After this,they should be divided into leachingplots,each measuring between 0.3 and 1 hectare, and separated from the others by ridges. When the meso-reliefis complicated and the water permeability high, the size of the plots has to be reduced (e.g. in the Indus plain, plots are about 0.1 ha). Water is poured on to the field quickly, and distributed by means of a system of small auxiliary canals,the amount being controlled by weirs. O n drained fields,leaching operations should be begun in the plots lying inthecentralparts oftheintervalsbetween drains,whereit is advisable,moreover,to use ratherlargeramounts of water, since the salts here are more difficult to wash away than in the immediate vicinity of the drains where the groundwaters are at a lower level, and circulate more freely. Leaching should begin on lower-lying areas and work up gradually on to higher ground.The leaching water is not poured on all at once,but in applications of approximately 1500-2000 m3/haat a time.The first water application should not exceed the deficit in field capacity-which, in late autumn,in the top 1 m soil stratum,will be, on the average,approximately 800-1000 m3/ha.This water spreadsand soaksinto soil horizons having virtually no downward flow,and gradually dissolves allthe solublesalts contained inthe soil. Even with the most strongly saline soils (containing about 500-700 ions of salts per hectare in the top metre stratum) the water depth,calculated as above,is sufñcient to fully dissolve the salts. Subsequently (after two or three days) further leachings will have to be done,with applications of 15002000 m3/ha,for removal ofthe salt solutions;each application should be given timeto soak in before the next is done. Ifpossible, arrangementsshould be made during leaching to check the amounts of salts washed out: the best methods are to determine chloride content or electrical conductivity. As the water soaksin,the easily solublesalts are gradually removed:fìrst,sodium and magnesium chlorides and magnesium sulphates,while sodium sulphates remain longer particularly in cold weather. Gypsum is practically unaffected by leaching-but in any case,is harmless. After leaching of solonchaks,transitional plants are grown-grasses or alfalfa are used especially as the first crop in correct crop-rotation.However,when drainage is inadequateand there is a danger of renewal of salinity,many specizlistsprefer îoleachsoilsespecially forcottonorbeets,In thiscase,intensiverow-cultivation for loosening the soil,together with heavy irrigations,increases and maintains the effects of leaching.The following year after preventive winter watering,perennial grasses are sown,as a beginning of regular croprotation. It is difficult,when using waterings of more than 5000-7000m3/ha,to complete the leaching operations in a single autumn-winter season;in such cases, they will be spread over two seasons.After the first year of leaching, the fields are sown with transition crops (barley, pulses, beets). After harvesting,in autumn, a second year’s seasonal leaching is done, in order to complete the soil desalinisation. Then normal crop rotation is introduced starting preferably with grasses. 2. Determination of leaching doses U p till now,the question of the amount of water to apply for leaching solonchakshas been settled,mainly, by empirical criteria,on the assumption that first-handexperiencein the field provides the most reliable data.

455

IRRIGATION, D R A I N A G E A N D SALINITY

At the same time,theoretical principles for leaching are gradually being evolved. L.P. Rozov proposed in 1936 the following empirical formula for calculating the leaching application required for radicalreclamation of solonchaks: M=FC-m+n. FC where

M=amount of water (in m3/ha) FC=field capacity (in m3/ha) m =water reserve in the soil before leaching (in m3/ha) n =coefficient.

The coefficientn, according to Rozov,varies between 0.5 and 2,depending on the salinity,and the mechanical composition of the soil.The higher the salinity,the worse the mechanical composition of the soil,the larger n will be, and the larger also,will be the amounts of water necessary for leaching. the process of leaching can be illustrated by equation: According to V. R.VOLOBUEV,

Q = QI.+Qz where

3- Q3

Q is the leaching depth QIis the quantity of water required to penetrate the soil up to the field capacity,i.e.QI=M-m, where M is the field capacity and m is the reserve of natural moisture Qzis the quantity of water required to bring the soil from the field capacity up to full capacity; i.e. Qz= P -M,where P is the full water capacity Q3is the quantity of water filtrating through the soil after its complete saturation.It may be expressed as a multiple of P.The coefficient y1 depends on the salinity and hydrophysical properties of the soil,i.e.Q3= nP

Types of leaching may be classified as follows (VOLOBUEV):

I. Leaching with drainage into own capillary

1.

moisture capacity of ground Q=Q1,or

Q=M-m

II. Leaching with drainage into capillary or

2. 1.

non-capillary moisture capacity of soils Q=Ql+Qz, or Q=P-m

2.

III. Leaching using drainage to carry off the water Q=Q1+Q,+Q3or Q=P-m+nP

1.

2. 3.

IV. Leaching by means of ‘squeezing out’ or washing out: Q=R where R is the surface loss by transpiration,evaporation or surface off-flow

4. 1. 2. 3.

Leaching into moisture capacity in zone of surface desiccation Leaching into deep-layer moisture capacity Leaching into moisture capacity of leached layer only Leaching into both own moisture capacity and that of neighbouring plots Drainage by means of shallow horizontal drainage Drainage by means of a deep horizontal drainage: (a) dense,(b) widely spaced out Drainage by means of a Californian drain Drainage by means of wells By scraping off the surface By washing off the surface By leaching the surface

determination of the amount of leaching water required depends,ultimately,upon According to DREGNE, soil or drainage water analyses in order to know when reclamation has been accomplished to the desired depth. A n approximation of the amount of water needed can be made by using the US Salinity Laboratory’s guide for leaching by flooding.Under that condition,a depth of 5 cm of low-saltwater is sufficient to leach out about one-halfof the salt in 10 c m depth of soil.If the root zone is 200 cm deep,an application depth of 100 cm is required.To remove 80%of the salt, 10 c m depth of water per 10 cm depth of soil is required, For 90% removal,20 c m of water per 10 cm of soil is needed. If a soil has an electricalconductivity of the saturation extract of 8 millimhos in a root zone of 200 cm, 100 c m of water passing through the root zone would reduce the electrical conductivity to about 4 millimhos. In practice, the amount of water used will depend upon the quality of the leaching water and soil conditions.A soil test is required to know how effective the leaching has been. Continuous flooding is frequently less effective per unit ofwater applied than is intermittentflooding,which permits salts to diffiuse out of the fine pores while the soil is still wet.When

456

R E C L A M A T I O N O F SALINE A N D ALKALI SOILS

leaching alkali soils,alternate wetting and drying is desirable for fine-texturedsoils which crack upon drying, since cracking brings an improvementin permeability. One essential factor in calculating the amount ofwater to use for leaching is the prediction of the field soil moisture in spring,when farming operations begin.This depends on the level ofthe water table:if this level, in March-April, lies above a certain level (generally 1-3-1.8m),the soil moisture will be so great as tdrender any farmingoperationsimpossible.On the other hand,waiting until the field dries out involvesevaporationof water and reaccuniulation of salts so that the effects of leaching are lessened.Hence leaching must always be carefully timed; otherwise,the groundwaters will rise too much and there will be flooding. Leaching doses, m3/ha

O

0.8

1.6

24 32 Salinity %

Fig. 13.8. Connection between leaching applications and salinity,according to the data of various different 1 :Zolotaya Orda (MALYGM, 1932) 2:Kara Chala, maxiresearch workers (from V. VOLOBUEV). m u m (SHOSHIN,1937) 3: Kara Chala, average (SHOSHIN,1937) 4: theoretical leaching norms 5: Kara Chala, minimum (SHOSHIN,1937) 6: Ferghana, Hungry Steppe (FEDEROV, 1932) 7: Djafarkhan (SHOSHIN,1935)

V. R.VOLOBUEV, in 1947,produced a series ofcurves (Fig.13.8) based on generalisation ofdata previously obtained in the course ofnumerous experiments.Subsequently V.A.KOVDA, in 1957,established thefollowing formula:

where:y=depth of leaching water (in mm) x=mean salt content in the 2 m soil profile (in

%)

n, = coefficient depending on mechanical soil composition in sand= O 5 loams= 1.0,clays=2.0 nz=water table depth 1.5-2 m = 3 2-5.0 m = 1.5 7-10 m = 1.0 123

=groundwater salinity weak or medium= 1.0 strong= 2.0 very strong (brines) = 3.0 457

IRRIGATION, DRAINAGE A N D SALINITY e.g. : loamy soil with a salinity of 2%, groundwater of medium salinity at 8-00m y = l . 1. 1 .400.2-1-100=800+100mm

If the groundwaters were highly saline (50 g/l) at a depth of 4 mythe leading requirement would be (instead of 7000 to 9000m3/ha)35000 to 37000 m3/ha.Such a water depth has to be given in several applications at 2000 to 2500 m3/ha. Leaching depths exceeding 15000 to 20000 m3/hahave to be used in combination with rice growing: waterings will be carried out,using 30000 m 3to 35000 m3/ha(withdrainge in operation), and thiswillensure rapid and effective desalinisation of soils. Subsequently facts and figures amassed-on the basis, mostly, of laboratory material-showed that the effectiveness of leaching, expressed in terms of the quantity of salts removed by a given quantity of water from a given unit of soil,increaseswith the salinity:for salt contentsabove 1.5-2.0 %, in the soil,this effectiveness increases sharply and progressively. Hence,in the view of A.T.MOROZOV and,later,V. R.VOLOBUEV, the relation between the leaching dose and the soil salinity is not rectilinear but logarithmic. VOLOBUEV’S equation (1960) is as follows: N=Klog ($)a where N = leaching dose (in m3/ha) Si=soil salinity (in % or tons/ha) So =tolerated residual soil salinity (in % or tons/ha) K=coefficient of proportionality, reckoning m3per hectare as equal to 10000 a=parameter depending on soil salinity and on the proportion of chlorides in its salt For chloride salinity,a is equal to 0.90-0.95; for sulphato-chloridesalinity,approximately 1 ;for sulphatosodium salinity,between 1.1 and 1.2;and for sulphato-sodium-calciumcomposition around 1-50. Table 13.16. Leaching depths (in m3)calculated on the basis of the degree and type of salinity and the mechanical composition of the soil being leached-@. R.VOLOBUEV)

Salt composition (%of solid residue in the 0-100 cm layer) S i

Chloride (CI=40-60% of solid residue) SO= 0.3

Sulphato-chloride (Cl=25-35% of solid residue) So = 0.3

Sulphato-sodium (CI=10-20 %of solid residue) So = 0.4

Sulphato-sodium calcium

so=1.0

Medium clayey soils or soils with heterogeneous,stratified structure,having a similar salt-extractionrate 0.2-0.5 0.5-1.0 1.0-2.0 2.0-3’0 3.0-4.0

a= 0.92 4000 6500 9500 11000 12O00

a= 1.02 3000 3500

a=1.12 1000 4000 7500 9500 11 O00

a= 1.48

a= 0.72 a= 0.82 a=0.62 2500 1500 1000 4500 4000 3300 5500 6500 6000 6500 7500 7000 7500 8500 8000 Loamy or clayey soils with low salt extraction rate a= 1.22 a= 1.32 a= 1-42 5000 3000 1500 8500 7000 5500 12O00 11 O00 10000 13 O00 12O00 14500 15 O00 14000 15500

a=1.18

8500

1o O00 11500

-

4500 7000 9000

Soils with light mechanical composition

0-2-0.5 0.5-1*0 1.0-20 2.0-3.0 3.0-4.0 0.2-0.5 0.5-1.0 1-0-2.0 2.0-3.0 3.0-4.0

458

-

-

4000 5500 7000 a= 1.78

-

5500 8500 11 O00

R E C L A M A T I O N OF SALINE A N D ALKALI SOILS

Allowance must however be made for the fact that laboratory tests on monoliths omit completely the influence of mineralised groundwaters when studying the process of leaching. It is well known that groundwaters lying close to the soil surface and having a high concentration (over 25-30 g/l) are chloride in type and cause chloride soil salinity.The presence of concentrated groundwaters,especially when they lie close to the surface,substantially reduces the effects of leaching carried out under practical conditions,so that the waterings have,in fact,to be two or three times the theoreticalamount calculated. In actual leaching operations, therefore, it is necessary to increase the application depth when soils are strongly saline and contain a high proportion of chlorides.A great deal depends of course on the drainage in each particular area. Illustrations of this will be given below. The Laboratory of Saline Soils of the Soil Institute of V. V. DOKUCHAEV carried out experiments,in the fifties,on the leaching of dry takyr-typesolonchaksin Southern Turkmenistan.These takyrs were extremely saline,on average 2-3 %,down to a depth of 2 m;the groundwaters occurred at a depth of more than 25 m. Owing to the extremely dry climate,the top 10-15 m stratum was air dry.Various leachings were applied: the results are shown on Fig. 13.9. A

B 800

400

O

400

800

800

O

400

400

800

O

t 40

c

D 800

400

O

400

800

800

400

O

400

800

O

40 80 120

160 200 240 280

Fig. 13.9. Salt content in soil solutions of solonchak-typetakyrs in Turkmenistan. A: before leaching B: after leaching with 4000 m3/ha C:after leaching with 7000 ma/haD:after leaching with 10000 ma/ha after leaching with (in ma/ha)

salt content CI content in the soil moisture solution (in g/l) in the stratum 0-50cm (in g/O

leaching of the salts: in the stratum the salinity has (in cm) increased from to (in g/l)

4000 7000 10000

5-6 2-6 3-5

0.5-1.4 1 1

90-100 cm

25-4

110-120 150-190

29.4

-

50 53 51

(illustration 0-1 m) GI

459

IRRIGATION, D R A I N A G E A N D SALINITY Leaching did not cause increased alkalinity.Neither water-extractsnor soil solutions were found to contain carbonates(the amount of HCO,’in water extracts did not exceed 0-03%).According to theoretical calculations,good results could be obtained with leachings of 6000-7000 m3/ha;in practice,applications as small as 4000 m3/haproduced excellent results;while applications of 7000 and 10000 desalinised the soil right down below the root zone.The extreme effectiveness of leaching in this particular case was due to the absence o€ groundwaters,and the fact that the soil profile was completely dried out to a great depth. at the Kara-Kalpakexperimental station Another, similar,experiment was carried out by A.A. KIZILOVA in the Amu-Darya delta,on crust solonchaks with sulphate salinity. Groundwater occurred at a depth of 3 mytheir salt content was 6.2 g/l;SO,”content was 3.6 g/l; N a content, 1.3 g/l. The quantity of soluble salts in the 0-20 c m layer attained 4-7%,the SO,”content was about 3 %,and that of Cl’, 0.38 %.Lower down the soil profile, a gradual decrease in the quantities of these soluble salts was observed (Fig. 13.10,Table 13.17). The concentration of the soil moisture solution in the surface layer attained very high values, such as a general content of 130 g/l;SO,” content,86 g/l;CI’content,12g/l.In the lower soil layers, the salt concentration in soil solutions approached that of the groundwaters (6-7 g/l). Owing to the high level of the groundwater table and the poor drainage of the areas,the various leachings produced practically no effect at all: slight desalinisation occurred only in the very topmost soil layers (0-40 cm). Shifting of salts through the profile was observed down to a depth of 80-100 cm (Fig. 13.10). B 1200 O

O

8

.E!

.%

58 160

5@160 CI

240

240

n

-

8

1200

80

80

C 1200 O

400 0400

D 400

o

400

1200

12.00

400

o 400.

1200

O

g

80

80

.-c

C .. ..

5 160

z u 160

n8 240

240

B

Na’

Mg”

Ca”

so:

Fig. 13.10. Salt content in soil solutions of crust solonchak (Kara-KalpakExperimental Station). A:before leaching B; after leaching with 4000 m3/ha C: after leaching with 6000 m3/ha D:after leaching with 8000 m3/ha Thus, when 4000 m3/haleachings were applied,the soil remained strongly saline:the SO,” content in the 0-20 cm layer decreased to 1.08%;in the 20-40 c m layer it hardly changed; and in the 40-80 cm layer it increased slightly. When 6000and 8000m3/ha leachingswere applied,the SO,”content in the 0-40cm layerdecreased slightly, while salts shifted to a depth of 60 cm;the behaviour of chlorides followed a similar pattern. O n the oplts where experiments were carried out, salt reserves were calculated on half-metre soil layers, both in absolute quantities (tons per hectare) and in percentages, taking the initial salt content (Cl’and SOc)as 100%. 460

R E C L A M A T I O N OF SALINE AND ALKALI SOILS

The amount of salts leached out in the top metre layer of the soil was extremely small (2-7%ofthe initial amount) as against 70-96% obtained with the same leachings when no groundwaters existed (Tables 13.17 and 13.18). Table 13.17. Influence of leaching depth on desalinisution of solonchak (soils having direrent groundwater fables)

Place of investigation and soil

Content before Depth leaching (cm) (tons/ha)

c1

Remaining quantities as % of initialcontent after leaching of (in m3/ha): 4000

7000

10000

14000

so,

c1

so*

CI

SO4

C1

SO,

Kizyl0-50 27.4 Arvat 50-100 38.4 experi0-100 65.8 mental 100-150 23.9 station 150-200 22.5 (solonchak takyr)* (water table deeper than 25 m)

-

4.6 51.6 32.1 234.8 102.6

-* -

4.1 16.2 11.2 2644 135.0

-* -

3.2 3.6 3.5 57.7 353.8

-* -

Tashaouz 0-50 experi50-100 mental 0-100 station 100-150 (crust solonchak) (water table 6 m deep)

63.0 54.6 117.6 68.6

42.1 20.9 63.0 27.5

71.7 110.2 88.4

71.3 163.0 99.4

-? -

-

4.8 32.2 18.5 171.9

37.5 84.2 52.0 140.7

-

-

Kizyl-Arvat Tashaouz

0-100 0-100

-

-

-

-

67.9 11-6

-

-

CI

SO,

-

-

-

4.6 18.3 11.0 76.7

34.4 73.7 47-5 102.1

-

Salts washed out as %of initial content 88.8

-

0.6

-

96.5 81-5

-

-

-

-

47.0

89.0

52-5

* No calculation was made for SO,,reserves, sincetheir salinity is of chloride type 7 These leaching depths were not applied

Table 13.18. Influence of leaching applications on desalinisafion of crust solonchaks with high groundwater table

Place of investigation and soil ~

~~

Depth Content before (in cm) leaching (in t/ha) CI SO,

Remaining quantitiesas %of initialcontent after leaching with (in ma/ha):

0-50 20-9 50-100 9.0 0-100 29.9

57.4 141-1 97.6

CI

4000 SO4

8000

6000

CI

so,

CI

so4

44.5 210.0 94.3

63.2 184.9 95.9

24.9 250.0 92.6

55.6 202.0 95.0

~~

Kara-Kalpaxexperimental station (crust solonchak) (water table 3 m deep)

161.9 59.5 221.4

72.7 161.7 96.6

Salts washed out of 0-100 cm layer,as percentage of initial quantity 0-100

2.4

3.4

5.7

4.1

7.4

5.0

Thus,it is clear that the depth and concentration of the groundwater has a decisive effect on the results of leaching.In order to obtain good results in the experiment carried out at the Kara-KalpakStation,efficient deep drainage would have to be installed,and leaching doses in the range of 30000-40000 m3/haapplied. In other words,either rice would have to be grown on these solonchaks,or else leaching operations would have to be continued for two or three years. 3. Leaching without drainage

Leaching without drainage can be effective only when the water table is sufficiently deep and the leaching requirementslow.The result is that leaching without drainage requires specific conditions.

461

IRRIGATION, DRAINAGE A N D SALINITY

The first and most important of these is that the amount of water used must never cause the groundwaters to rise to a dangerous level.During the period of intense salt accumulation-i.e. in spring-the water table must never rise above the ‘criticallevel’. It is essential,therefore,that leaching without drainage should be done when the level of the groundwaters is the lowest-i.e. in most regions,in the autumn. The second point is that the leaching application must be calculated on the basis of the free porosity of the soil. If the soil salinity is such that the permissible leaching thus calculated provcs insufficient for desalinisation, then operations will have to be spread over 2-3 seasons. Accelerated leaching under these conditions is possible only when the coefficientof land usage is low (20-30 %and less), and when the surplus leaching water can be taken up by the spare moisture capacity of surrounding,unused, fallow soils. (This will,however,increase the salinity of those soils.) Hence,leaching without drainage is really expedient only if calculated on the basis of the moisture capacity of the soil and subsoil of the plot being treated; as this is not very large, the amount of water that can be used will be limited, and this type of leaching cannot be really effective for dealing with strongly saline soils. For determination of the leaching norm permissible in places without drainage, I.F.MUSICHUC suggests the following formula: H-h N = M - m + -.10000 20

where N is the leaching norm (in m3/ha) M is the limit moisture capacity of the horizon being leached (in m3/ha) m is the water reserve in the horizon to be leached, before leaching (in m3/ha) H i s the depth of the water table before leaching (in m) h is the permissible water table level after leaching (in m) 20is the ratio between the height to which the groundwaters rise (H-h) and the water depth applied The free soil and subsoilmoisture capacity with an initial groundwater level between 3.0-2.4 m and 2.01.5 m are respectively: 3200-2700 and 2300-1600 m3/ha.Thismeans that,for leachingwithout drainage, 1600 to 3200 m3/ha can be used,i.e. usually sufficient to desalinise only weakly saline soils. In such cases,only a small percentage ofthe irrigated area (10-15 %)can be irrigated at a time (withthe groundwater table at 2m). A n example of leaching without drainage is the experiment in the Murghaf Valley (Turkmep SSR)from 1936-39. The specified requirements for this type of operation were fulfilled: leaching was done in the autumn on selected plots a few hectares in area; the leaching norm (2500-3000 m3/ha) was calculated in accordance with the spare moisture capacity of the subsoil,taking account ofthe granulometric composition of the soil and the water table depth. With this norm, it was possible to wash out up to 80-85% of the chlorides contained in the top soil layer. Desalinising operations were also extended deeper down but it was only in soils with a total soluble salts of about 0.5 %that the 80 cm profile could be desalinised sufficiently to permit the normalgrowthofcotton (permissiblechloride content in soil-0-02 %).Soilswith a high salinity required higher leaching norms,so that they could not be treated completely in less than two years.The water table rose sharply not only in the leached areas,but also in surrounding areas. Towards spring,however, the groundwaters in the leached plots returned to the level usual in this area. Another example of leaching without drainage is given in an experiment conducted by R. C. REEVE, A.F.PILLSBURY and L.V. WILCOX of the US Salinity Laboratory and the University of California. The soilwas highly micaceous,light coloured,and irregularly stratified,being derived from unconsolidated transported materials. There was no drainage problem since the water table was more than 6 m below the surfacethroughout the experiment.The soil was highly saline at all depths,as well as very high in boron to a depth of 80 cm.Gypsum was present in large amounts in the upper part of the soil. Leaching treatments consisted of applications of 3000; 6000; 12000;18000;24000;30000;and 36000 m3/hawith an oat crop being grown on all leaching levels except 18000 and 24000 m3/ha. Leaching effectively removed salt (Fig. 13.11) and boron (Fig. 13.12) from the soil;the rate of removal of salts other than boron salts was about three times greater than it was for boron, since approximately one metre of applied water per metre of soil depth was adequate to reduce the salt content to 20%of the original level but about 3 metres of water per metre of soil was required to make the same reduction in boron. Experience in the irrigated areas of mallee country in south-easternAustralia however, has shown that very much larger quantities of water are required especially for comparable reduction of boron content on land with heavy-textured and highly sodic subsoils,even with artificial drainage.

462

R E C L A M A T I O N OF SALINE A N D ALKALI SOILS Depth of profile (inches) 0-

20

'

I I l l I I I 10 20 30 40 50 60 70 80 E.C.of saturation extracts (millimhos per cm)

60-

O

Fig. 13.11. Distribution of saltin soilprofile before and a€terleaching,as measured by electricalconductivity of the saturation extract (1 ft water= 3000 m3/ha) Depth of profile (inches) Or

I

I

I

1

I

l

O 10 20 30 40 50 60 Boron concentration in saturation extract PPM

Fig. 13.12. Distribution of obron in soil before and after leaching as measured in saturation extract (1 ft water= 3000 m3/ha)

4. Leaching with drainage

By using drainage,very lasting and stable results can be obtained,and even highly saline soils can be fairly rapidly desalinised. The leaching norm is calculated with a view to complete desalinisation, first of the root zone and then subsequently of the underlying subsoil and groundwater profile. Experience has shown that the root zone can often be desalinised satisfactorily by applying a depth equal to that of the profile to be treated;but as there are great differences in local reclamationconditions,a whole series of local factors will have to be taken into account when calculating the leaching required in order to guarantee satisfactory desalinisation. Work done at the Mughan,Zolotaia-Ordaand other experimental stations shows that,after the root zone has been leached,and the land reclaimed by sowing agricultural crops,desalinisation of the groundwaters continues successfully thanks to the effects of higher watering norms,filtrationfrom various kinds of canals, and so on. Hence one way of achieving complete desalinisation of soils and groundwaters is to proceed by stages.The first step is to leach the root zone after which the fields are sown with crops which greatly reduce the danger of resalinisation, and make land use economically feasible.In the course of the years following leaching,the lower-lying subsoil and groundwater horizons are desalinised by copious irrigations. Due attention must be paid to the effects of leaching in combination with different types of drainage. At the experimental station of Ferghana (1949-51) leaching operations were carried ont in combination

463

IRRIGATION, DRAINAGE A N D SALINITY with shallow drains (1.2-1-5m) spaced at 100 metres. Leaching was done in autumn and winter,with 40004500 m3/ha. The initialsoil salinitywas very high (approximately 1.5-2.0%totalsoluble salts) risinginthelowerhorizons of the top metre layer to as much as 4.5 %.As a result of three years’leaching,the salt content in the top 30 c m layer was reduced by more than 86 %; lower down,to a depth of 114 cm, it was reduced by 60%. The mineralisation ofthe groundwaters on the reclaimed plots likewise dropped considerably: from 23.8 to 9.5 g/l,In view of the shallowness of the groundwaters,it was liable to provoke resalinisation. During the first year,the leaching was not sunicient to ensure a normal growth of cotton plants, but in the second year, after leaching,the cotton crop was already 3000 kg/ha,and by the third year, it had risen to 4000 kg/ha. OZTAN gives an example of amelioration of saline soils at Tarsus (Turkey). The experiment was established in 1958 to determine the effects of various amendments,under different amounts of leaching water, on the removal of salts. The experiment was set up as a random block with three replications for heavy (10000 m3/ha) leaching study and two replications for the light leaching (6000 m3/ha). Each plot size was 4 square metres. To prevent lateral movement of water all sides of each plot were protected by wooden boards. The plots were drained by a tile drain (1.50 m deep) running through the middle of the experimental area. The amendments were selected in taking into account that the soil contained alkaline earth carbonates. kg/decare Check (leaching alone) Gypsum

Sulphur Barnyard manure

O 460 70 2800

The amountsofthe applied amendments were approximatelyequivalentto their chemicalvalue as replacing agents compared with the gypsum requirement method of SCHOONOVER. The chemicalamendments produced practically no visible effect.The results ofleaching are shown in Fig. 13.13. Depth of soil (cm) O

30.

60 .

90 .

120

1,

I l I I I 150 225 300 375 450 Content of CI- ion in saturation extract (meil) Heavy leaching alone Before leaching, 1958 After 1 crop year, 1960 I

75

I -

+ manure

-a.-

-I-

siilpliur

-e.-

-j-

gypsum

---------.

.-I-.

---..-.------.-. -.-

Fig. 13.13. Removal of CI-ionby leaching and amendments (data by V.OZTAN) Extremely successful operations for reclamation of saline soil with deep drainage were carried out at the Mughan experimental station (Azerbaijan SSR).From the beginning of irrigation on the Mughan Steppe (1903) a tendency to strong and widespread secondary salinisation was noted; therefore,a deep collector,

464

R E C L A M A T I O N OF SALINE A N D ALKALI SOILS

six closed drains,one open drain and a pumping station were installed at the Mughan experimental station in 1930-31. The drains were 2.5 to 4.0 m deep and 350 to 580 m apart. All the plots were carefully levelled out. Leaching was carried out with 3000 to 12000 m3/ha (the average being a little less than 8000 m3/ha). With a soil salinity of about 1-2% a leaching of 6000 m3/haresulted in a desalinisation sufficient to permit normal growth ofcrops;but total applicationsbetween 12000and 18000m3/hawith an efficientdrainagewere necessary in order to obviate all danger ofsecondary salinisation.The best resultswere obtained with autumn and winter leaching;summer leaching required much larger applications. By this method, soil with 1-2% salinity in the horizon above the groundwaters could be put under cultivation at the end of one year;only more strongly saline and heavy clayey soilshave required two years’treatment before use. The reclamation crops included barley (Hordeumvulgure), shadbar (Trifolium resupinutum) and so on. The lasting character of the desalinisation achieved must be stressed: there were virtually no cases of secondary salinisation, either in the first year or later; moreover, the desalinisation of the subsoil and groundwaters continued,extending to depths one-and-a-halfor even twice that of the drains. The drainage system installed at the Mughan experimental station in 1930-31 has been working uninterruptedlyever since,and the results obtained there constitute the scientific basis for soil reclamation of land under cultivation throughout large areas of the Mughan and Salyansk Steppes (150000hectares). An example of leaching with a combination of deep and shallow drainage is provided by the reclamation work done at the Bukhara experimental station. Thethin drainage systeminstalled in the region ofthe Bukharaexperimentalstation,at a depthof2.7-3.0 my has proved very effective (mean annual discharge: 0.19-0.26 I/s/ha), and has made its influence felt over a wide area,i.e. up to between 350 and 500 m on either side of its axis. With this drainage, it has been proved possible to control salinity and produce good yields of the main agricultural crops in all the lands of the station,except for a few strongly saline depressions with a shallow water table. In one such place which had, origially,been the bed of a lake,the groundwater was only 0-3-1.0m below the soil surface,and rose,in spring,to the surface. The collector-drainsrunning through this area did not suffice to drain it properly. A shallow system was therefore installed over an area of approximately five hectares: ditchers were used, and the drains laid at a depth of 70-80 cm and from 45 to 85 m apart. In the course oftwo years,14980 m3/haofwater were poured on to this plot,while 24380m3were drained. In other words,the system drew off large quantities of groundwaters both from the plot itself and from the higher-lyingland around it. In the course of two seasons, also, more than 392 tons of salt per hectare were removed in the drain water. The average salinity in the top metre horizon dropped from between 0.9 and 1.7% to between 0.33 and 0.42%.In the second year,the redaimed pIot produced a cotton harvest more than 2000 kg/ha. After completion of leaching operations,shallow drains are filled up. Thanks to a combination of widely spaced deep drains supplemented,temporarily, by shallow drains, land-levelling,and efficient water and agro-technicalmanagement, the land of the station was completely reclaimed.The saline patches (where no crops would grow at all) which,prior to 1950,represented 30-35 % of the territory had, by 1954,been entirely eliminated. The salt content in the top metre layer is now only 0.6-0.3 %, whereas the topsoil previously had a salt content of 1.0-13 % and even more. 5. Deep temporary drainage

A rapid,simple method of reclaiming comparatively small areas of saline heavy soils has given good results in the riverine plain of northern Victoria. This method is applicable where water in an aquifer underlying the clays,which often drain well vertically,may rise in a bore to such a level that it may be pumped away to drains by an inexpensive centrifugal pump. Ponding of irrigation water on the surface before and during pumping and the lowering of water table associated with the relief ofupward pressure from the aquifer,facilitate substantia1leaching of salt. Soil salts on the main site examined consist largely of sodium chloride.During ten days ofponding,sodium chloride in the surface 15 c m of soil decreased from 1.6 %to 0.05%, with only 56 hours of pumping by a 3.8 cm or 5 cm pump. Corresponding changes in the top 30 c m were from 1.82% to 0.32%. Such leaching was not 465

IRRIGATION, DRAINAGE A N D SALINITY attainable by other means because of the highly saline (about 5% sodium chloride) water table from 30 to 92 c m below the soil surface in this area, which in 1960 carried no worthwhile vegetation, only scattered samphire.Since leaching,the area of one hectare concerned has grown millet followed by successfulestablishment ofperennial pasture which carried 1 1 sheep per hectare between August 1961 and April 1962.Pumping has not been necessary since 1960. The bore for pumping was put down 7-3metres by hand. This method, which seeks to reduce pumping costs,leaching water and discharge salty water to minimum figures may be particularly good for intractable salt-agected areas in otherwise productive farms. Continuous pumping from a suitable aquifer is now under trial, and a discharge of only 5 I/s has shown water table recessions over an area of 50 hectares. 6. Methods for improving the efficiency of leaching on solonchak soils in practical conditions

The experience gained in the USSR has shown that this treatment is less effective, in practice, than either theoreticalcalculations or results obtained in experimental stations would indicate.There are several reasons for this: -incorrect leaching schedules (leaching not done at the optimal moment) -inadequate levelling of the field surface -failure to spread the leaching water over the fields with sufficient care and accuracy -mistakes made in assessing the salinity of soils and particularly of groundwaters -leaching too small -main collectors overflowing, and field-drainsoverflooded -drainage system technically faulty -drains neither deep enough nor close enough together

All these shortcomings can be avoided by a more thorough preliminary study of soils before leaching, betteï technical qualifications of the staff and improvement of the organisation and of the inspection of operations. In many cases,however,the disappointing results given by leaching are due to the high salinity and low permeability of the soils. In such cases,the following measures should be taken in order to make leaching more effective: (a) Mechanical measures prior to leaching Deep ploughing,in some cases to a depth of 40-50 cm,mole-ploughingof heavy clay soils;sand (700-1000 tons/ha) or manure (50-70tons/ha) ploughed in heavy unstructured soils (e.g. takyrs). (b) Physico-chemicalmeasures The first two or three leachings can be carried out with waters containing a certain amount of soluble salts (5-10 g/l) in order to produce coagulation of the soil colloids and increase their water permeability. Bare fallowing,to dry the soil out, may cause thermic coagulation of the soil colloids and soil aggregation. (c) Hydrological and hydrotechnicalmeasures On saline soils with high saline groundwaters (50-80 g/l), the leaching applicationsmust be increased (using one-and-a-halftimes or twice as much water). The number and frequency of leaching can be varied and leaching can be carried out when the water table is deep. Extra deep drains (3-3.5 m) can be installed and also an additional close network of shallow drains. In dealing with especially bad solonchaks, irrigated rice cultivation can be introduced,and the number of deep drains increased.Small holes can be drilled in the bottom of the open drain or collector in order to increase the outflow of underground waters (particularly to increase effectiveness and speed if they have hydrostatic pressure). There is a suggestion (E.WARUNTIAN) of leaching by means of so-called‘forcedleaching’,applying as much as 60000-70000 m3/haper-year. 7. Leaching saline soils in combination with rice growing

Rice cultivation results in a marked soil desalinisation owing lo the vast quantities of water it requires for

466

RECLAMATION OF S A L I N E AND ALKALI SOILS irrigation and flooding during the vegetation period-30 000-40O00 m3/hain arid regions. Rice growing is very useful as a means of reclaiming extremely saline soils,or soda soils,with poor permeability and crust and fluffy solonchaks,while at the same time obtaining a large crop the first year. Saline soils after rice can be used successfully for other crops; provided they are skilfully farmed and correctly irrigated, they will remain non-saline. Depending on the climate and water resources,rice cultivation can be combined with cultivation of grain or clover as the second crop during the same year; or rice can be in the regular 2-3 years’rotation with cotton,beets, etc. With respect to soils containing mainly soda or sodium sulphate salts there is an additional reason for growing rice-namely that autumn-winter leaching,done when both the air and soil temperaturesare low, usually has practically no effect on either Nazco3or Na,SO, while in summer the solubility of these salts increases. Leaching in combination with rice growing desalinises soils right down to the water table. Moreover, rice growing,combined with drainage, causes a sharp drop in the mineralisation of the groundwaters (as much as 5-8 times), owing to the fact that irrigation water dilutes them,pushes them down and forces them to spread out sideways (Table 13.19). Table 13.19 shows that,even with shallow drains, the great water depths applied reduce in the top 2 m layer,the total soluble salts and the CI contents respectively to 35-50 % and 1.6-2.5 % of the initial values. Table 13.19. Change in soil salinity andgroundwater mineralisation as a result of vice growing (Uzbekistan,plot drained by drainage system 1 m deep)

Flooding applications (in m3/ha) total soluble

Soil Layers

before after

before

100-200 nn Groundwater CI (%) total soluble salts CI (%) total soluble salts ( %> (g/U after before after before after before after

3*2

1-2 1.4 1.1

0.7 0.5 0-5

0.013 0.009 0.007

1.8 1.7 1.7

0.7 0.6 0.6

0.4 0.3 0.4

0.015 0.012 0.009

44.8 33.5 39.5

5.6 4.3 6.8

1.3 1.4

0.7 0.6

0.020 0.025

2.1 1.8

1.5 1.1

0.3 0.3

0.016 0.053

38.1 29.1

23.8 13.2

0-100 c m

salts (%)

(Constant) 51 O00 45O00 39O00

2.7 2.5

(Intermittent) 29 500 22500

3-1

3.0

Both the effect of leaching under rice growing and the stability of the desalinised soils will depend directly on the drainage intensity,the level and mineralisation of the groundwaters,the soil permeability,the type of plants subsequently cultivated and their irrigation regime. The heavier the mechanicalcomposition ofthe soils,the weaker the desalinising effects of rice growing will be. As an example,Table 13.20 shows the results obtained on the clayeyloam alkaline strongly saline soils of southern Azerbaijan. Table 13.20. Changes in salinity of soil due to rice cultivation

Horizons in c m (salinity in %)

Observation stage

Before sowing After harvesting Increase or decrease

0-10

10-20

1 -22 0.73 -0.49

2.07 1-02 -1.05

20-30 2.91 1.69 -1.22

30-50 3-53 2.20 -1.23

50-70 3.73 2.83 -0.90

70-100 3-64 3.00 -0.64

100-150 3.80 3.69 -0.11

150-200 3.66

I

Amount of salt removed (in %of initial content) 40.0

50.7

42.0

35.0

24.2

17.5

3.0

467

IRRIGATION, D R A I N A G E A N D SALINITY Leaching under rice was done in combination with deep (2.5 in) and supplementary shallow (1 in) drains, main drains being 20-50m apart.During the rice growing season,up to thirty-threewaterings (flooding) were done at two to three days’ interval,with seasonal water allocation of approximately 35000-40000 m3/ha. Even during the first year,the desalinisation was marked and during the second or third year ofrice growing the desalinisation of these soils,though originally strongly saline,was completed. Nevertheless,great care needs to be taken, and local conditions thoroughly studied when growing rice for desalinising and reclaiming solonchak soils. The great amount of water required for this cropping causes radical changes in the water and salt regime not only on the rice field but in the surrounding areas as well.The growing of rice desalinises the groundwaters as well as the whole of the soil profile, this effect will be greater,and the improvementmore lasting, when the area is well drained, and the soil has a good permeability.On undrained plots with strongly saline soils,the desalinisation so obtained is neither marked nor lasting-and it has the additional disadvantage of causing waterlogging and salinity in the adjacent lands. When rice is grown without drainage on soils with a high water table (1.5-2.0 m) and low permeability, only the top layer of the soil is desalinised, the groundwater mineralisation remains practically unchanged, and the land rapidly becomes saline again-in some cases even more saline than before. Hence, rice growing and irrigation on developing land, without drainage, cannot be used as a method for prevention of salinity.

If solonchaks are desalinised by rice growing combined with deep drainage,then sown with cotton plants, beet and alfalfa and treated with winter preventive waterings,their groundwaters will,as a rule, also become desalinised. This means that there are good prospects of lasting results,so that the land can be economically developed.Thus the main means for maintenance of the effects of rice growing on strongly saline soils with high (1-5-2.0m)mineralised groundwaters is a comprehensive network of deep drains. Other points,equally imperative,are that rice growing must be followed up in subsequentyears by correct summer irrigation application (1000 to 1200 m3/ha), and that the soil of the arable horizon must be kept constantly loosened by thorough and regulartilling.When all these measures are taken,there will be a process of gradual desalinisation of both soils and groundwaters. The experience of UAR shows that by utilising rice growing, leaching and adequate deep horizontal drainage,the most saline soils ofthe Nile delta have been successfully developed into flourishing rich oases.

F. CROPS DURING RECLAMATION 1. Advantages of cultivation during reclamation -

1

Cropping during reclamation of saline and alkali soils is desirable since it facilitates amelioration and provides income to defray in part the cost of reclamation.Ploughing for annual crops can be beneficial by loosening and drying the surface soil thus temporarily improving permeability. Resalinisation has been most frequent when successful leachings have not been followed by management and cropping.Leaching tends to make soils more compact,thereby promoting the development of capillary processes and, if there are no plants to provide shade,salt may rise in large quantities to the surface of the treated soil.The danger of recurring salinisation is especially acute when there is a large amount of residual salinity,as happens with strongly saline soils. When dealing with strongly saline soils which cannot be desalinised in one leaching season, or in cases where leaching deteriorates the soil, it is advisable to make use of special reclamation crops in order to eliminate resalinisation,alkalinity,and improve the soil structure. There are several advantages of cropping. Root development opens up channels and improves aggregation in the surface and subsoil,thereby increasing permeability.Root respiration and decomposition,because of the carbonic acid that is formed,increase the soluble calcium in calcareous soils and aid the reclamation of alkali soils. Organic matter is added to the soil when plant remains are ploughed under after harvest.Crops also shade the soil and reduce evaporation which in turn reduces water loss and the accumulation of salts on the surface. Although crops are advantageous in reclamation, it may be better to do the initial leaching on bare soil because standing water will injure most plants and further land levelling may be required.

468

R E C L A M A T I O N O F SALINE A N D ALKALI SOILS

However,in Australia it is considered that ifthe salinity situation will allow it, it is best to make the grading of the land as good as possible by rapid procedures (to minimise the time the land is in fallow), sow the crop and irrigate to leach the upper soil and grow the crop. If the crop is an annual, further close attention is given to grading before each of the succeeding crops. In northern Victoria leaching during the growth of annualcropshas promoted reclamation ofsaline-sodicsoilsprovided that cultivationbetween cropsis limited. This means that the land is left bare for only a few days in a cool time of the year-autumn or spring-so that no salt concentration occurs at the surface. Another practice which has been successfulis the use of pasture clippings from other areas as a mulch on the reclamation site. After the high salt or exchangeable sodium level has been reduced,growth of a tolerant crop like barley can be combined with heavy irrigations to further reduce soil salinity.If this is followed by heavy winter leaching when no crop is on the ground, a wider selection of crops will be permitted with resultant soil improvement and greater income from the land. Since leaching removes soluble plant nutrients, especially nitrates, along with undesirable salts,fertiliserswill probably have to be added to obtain good yields after reduction of the initially high salt or exchangeable sodium level. Research in Hungary,the USSR, and the USA indicates that crops respond to the same plant nutrients in non-saline and saline soils but the higher the salinity,the less is the response. 2. Crops resistant to waterlogging

Combiningcrop productionwith reclamation ofsaline and alkali soilsrepresents the most desirablesituation. Unfortunately,few crops will tolerate both flooding and high soil salinity.Waterlogging resulting from ponding leaching water (flooding) on the undrained soil surface has adverse effects on most cultivated crops, principally because of the poor aeration that results. The tolerance or sensitivity of crops to waterlogging depends upon (1) plant species, (2) air temperature, (3) duration of flooding,and (4) stage of plant growth. Alfalfa is one of the most sensitive crops, since permanent damage may result from flooding for periods of 24 hours or less on hot summer days. Paddy rice is one of the most tolerant plants. Table 13.21 lists the tolerance to flooding of several forage and cultivated crops as compiled from published sources.The list is based upon floodingtolerancewhilethecropisgrowing actively.N o exact limits can be set upon the number of days that crops in the three categories can be flooded without suffering serious damage because rapidity, stage of growth, and sudden increases in temperature affect tolerance. A rough guide might be less than 10 days of flooding for sensitive crops, 10 to 30 days for semi-tolerantcrops, and more than 30 days for tolerant crops. If flooding occurred during the winter or late in the growing season,the period of flooding probably could be increased considerably for most crops. For many crops, such as barley, the period when they are most sensitive to flooding damage is on hot summer days when the plants are in the flowering stage. Crops would be most tolerant on cool days when they are in the fruit-stage of growth. During their dormant stage,plants can tolerate considerably longer periods of flooding than they can during rapid growth. Similarly,winter wheat is more tolerant in winter when it is growing slowly than it is in spring when warm weather speeds growth. Tolerance to flooding damage is generally greatest under the same conditions that favour salt tolerance (older plants, cool weather, short exposures to flooding or salinity). While some crops which are resistant to flooding damage are also salt tolerant,paddy rice seems to be the one which best toleratesprolonged flooding with saline water. However,upland rice cannot tolerate flooding as well as paddy rice. The ability of rice to excrete oxygen into the soil accounts for its tolerance to flooding. Other cultivated plants can sometimes adjust to waterlogged conditions by producing adventitious roots that are better adapted to poor aeration than the original roots. Flooding saline soils subjects plants to both salinity and waterlogging,so tolerance to both is essential if flooding is prolonged. Other particulars on the influence of waterlogging are to be found in Chapter 9. 3. Crops resistant to salinity or alkalinity

Crop tolerance (or resistance) to soil salinity and exchangeable sodium is an important factor to consider during reclamation because of the value of obtaining income from the land while it is being reclaimed.

469

I R R I G A T I O N , D R A I N A G E A N D SALINITY Table 13.21. Relative tolerance of crop plants to waterlogging

Sensitive

Semi-tolerant

Tolerant

Alfalfa (Medicagosativa) Apricot (Prunusarmeniaca)

Apple (Malussylvestris) Bromegrass (Brornus inermis) Cotton (Gossypiumhirsutum) Fescue,meadow (Festucaelatoir) Orchardgrass (Dactylisglomerata) Plum (Prunus domestica)

Canarygrass,Reed (Phalaris

Barley (Hordeum vulgare) Bean,green (Phaseolusvulgaris) Clover,ladine (Trifoliumrepens var.) Clover,strawberry (Trifolium fvagiferum) Clover,swcet (Melilotusalba) Lettuce (Lactucasativa) Oats (Avena sativa) Peach (Prunuspersica) Potato (Solanum tuberosum) Tomato (Lycopersiconesculentum) Wheatgrass,crested (Agropyron cristatum)

-

arundinucea) Clover,alsike (Trifoliumhybridum) Clover,White (Trifoliumrepens) Dallisgrass (Paspalumdilatatum) Fescue,tall (Festucaaruizdinacea) Pear (Pyrus communis)

Rice, upland (Oryzasativa) Rye (Secalecereale) Rice,paddy (Oryza sativa) Raygrass,perennial (Loliumperenne)Trefoil,big (Lotusuliginosus) Sorghum,grain (Sorghum vulgare) Timothy (Phleumpratense) Trefoil,birdsfoot (Lotus corniculatus) Trefoil,narrowleaf (Lotus tenuis) Wheat (Triticumvulgare) Wheatgrass,slender (Agropyrontrachycaulum)

Tables of salt tolerance of many crops have been prepared, such as that of the U S Salinity Laboratory (Handbook 60,1954). With respect to salt tolerance,the ability of a crop to withstand excess soil salinitymust be evaluated for at least three stages of growth: germination, seedling stage, and maturation. In general, plants will tolerate more salinity as they grow older, but there may be some variation after germination. Rice,for example, is very salt tolerant during germination but sensitive as a seedling. It becomes more tolerantwhile tilling,sensitive again in the flowering stage, and quite salt tolerant as it matures. Another factor involved in evaluation of salt tolerance is the difference among some crops in the effect of salt on vegetative growth and grain production. In saline soils,rice plants may grow fairly tall but produce little grain,whereas barley and cotton plants may be stunted while still producing good grain and lint yields. The difference in grain production is due probably to differences in sensitivity to salt during the critical flowering stage of growth. Climate and plant variety may introduce additional variables in salt tolerance. In general,crops are more tolerant in a cool climate than in a hot one. Varietal differences in salt tolerance have been observed for cereal crops and for rootstocks of fruit trees, among others. The dependence of crop tolerance on crop variety,soiland water management,and environmentalfactorsmakes it di%cult tojustify numerouscategories of salt tolerance in lists prepared for world-wide or continental use. Useful refinement in such lists can be made on an area or regional basis if adequate information on differences in tolerance is available. Adverse effects ofsoil salinityongerminationcan be minimised by heavierthan normal seeding.Ifgermination is reduced by 50%at the salt level existing in a soil,a full stand can still be obtained by using a seeding rate of twice the amount required on a non-saline soil.High seeding rates also aid in reducing the harmful effectof the delay in germination and emergence on crop stand. This delay can cause greater exposure to soil-borne seedling diseases which is typical of saline soils. The greater number of seeds will not reduce disease infection but rather increase the chances that a satisfactory number of plants will survive the hazards of germination and emergence. Differences among various crops are compared by determining the soil salinity level (measured as electrical conductivity of the saturation extract) at which crop yields are reduced by 50% from yields on non-saline soils under comparable growing conditions. Some investigators have used a 20 or 25% reduction,or other criteria,for making similar comparisons.In spite of the differences in methods of evaluating salt tolerance, there is a high degree of agreement among most lists (see Table 13.22). Evaluation oftolerance to exchangeable sodium is complicated by the direct and indirect effects of sodium on soils and plants. Adsorbed sodium can be troublesome because of direct toxicity of sodium, strong alkalinity of the soil solution, a sodium-induceddeficiency of calcium, or because of the deterioration in soil structure,with consequent reduced aeration and restricted root growth which usually accompanies

470

RECLAMATION OF SALINE AND ALKALI SOILS Table 13.22. Yield decreasesof certain cropsdue to variable salt levels in soilsolution. L., Salt tolerance ofplants,USDA Ag. Id. Bul. 283,1964) (Datatakenfrom BERNSTEIN,

EC x lo3 of saturation extract causing indicated Crop 10%

yield decreases 25 %

50%

Forage Crops Bermudagrass (Cynododactylon)" Tall wheatgrass (Agropyronelongatum) Crested wheatgrass (Agropyrondesertorum) Tall fescue (Festucaarimdinaceae) Barley,hay (Hordeumvulgare)? Perennial ryegrass (Loliumperenne) Hardinggrass (Phalaristuberosastenoptera) Birdsfoot trefoil (Lotuscorniculatus tenuifolius) Beardless wildrye (Elymustriticoides) Alfalfa (Medicagosativa) Orchardgrass (Dactylisglomerata) Meadow foxtail (Alopccurus glomerata) Alsike clover (Trifoliumhybridum) Red clover (Trifoliiim pratense)

13 11 6 7 8 8 8

6 4 3 2.5 2 2 2

16 15

11 10.5 11 10 10

18 18 18 14.5 13.5 13 13

8

10

7 5 4-5 3.5 2.5 2.5

11

16 13 12 11 10 9 7 5-5 5 6 6 4.5 4.5 2

18 16 16 12 14 12 9 9 8.5 8 7 6.5 6.5 3.5

10 7 6.5 6

12 8 8 8

8 8 6.5 4

4

Field Crops Barley, grain (Hordeumvulgare)? Sugarbeet (Betavulgaris) Cotton (Gossypiumhirsutum) Safflower (Carthamustinctorius) Wheat (Triticumvulgare)? Sorghum (Sorghumvulgare) Soybean (Sojamax) Sesbania (Sesbaniamacrocarpa)? Sugarcane (Saccharumoficinarum) Rice, paddy (Oryzasativa)? Corn (Zeamays) Broadbean (Viciafaba) Flax (Linumusitatissimum) Field bean (Phaseolusvulgaris)

12 $0 10 8 7 6 5.5 4 3 5 5 3.5 3 1.5

Vegetable Crops Beet, garden (Eetavulgaris)$ Spinach (Spinaciaoleracea) Tomato (Lycopersicumesculentum) Broccoli (Brassicaoleracea italica) Cabbage (Brassicaoleracea capitata) Potato (Solanunztuberosum) Corn, sweet (Zeamays) Sweet potato (Ipomoeabatatas) Lettuce (Lactucasativa) Bell pepper (Capsicumfrutescens) Onion (Alliumcepa) Carrot (Daucuscarota) Green bean (Phaseolusvulgaris)

g.

8 5.5 4 4 2.5 2-5 2-5 2.5 2 2 2 1.5 1.5

4 4 4 3.5 3 3 3.5 2.5 2

7 6 6 6 5 5 4 4 3.5

Average for different varieties

p Less tolerant during seedling stage.Salinity at this stage should not exceed 4or 5 mmhos/cm,EC, $ Sensitive during germination.Salinity during germination should not exceed 3 mmhos/cm EC,

471

IRRIGATION, DRAINAGE A N D SALINITY high exchangeable sodium levels in low-saltsoils.Table 13.23 representsan attempt to show how some crop plants respond to excess exchangeable sodium,with both direct and indirect effects taken into consideration. The table can be considered as tentative,at best,but it is hoped that it will serve as a useful guide, Sensitive crops are injured directly at exchangeable sodium levels which are too low to cause appreciable indirect effects; tolerant crops are largely unalfected by direct sodium toxicity but are injured by indirect effects on soil structure,induced calcium deficiency,or high p1-I.The semi-tolerantcrops may be only moderately tolerant to either direct or indirect sodium effects. Several investigators believe that tolerance to exchangeable sodium is related to the amount of sodium that plants are able to acctmulate in their tops without injury or in the ratio of the amount in the tops to the amount in the roots.Plants which normally accumulate considerable sodium in their tops or which have a wide ratio (3 or 4 to 1) of sodium in the tops to sodium in the roots generally appear to be sodium tolerant;whereas plants that normally tend to exclude sodium from their tops are frequently sodium sensitive. Further particulars on sensitivity of crops to salinity are given in Chapter 9. Table 13.23. Relative tolerance of crop plants to exchangeable sodium*

Sensitive Avocado (Perseaamericana) Bean,green (Phaseolus vulgaris) Corn (Zeamays) Grapefruit (Citrusparadisi) Orange (Citrzisspp.) Peach (Prunuspersica) Tangerine (Citrusreticulata)

Semi-tolerant

Tolerant

Carrot (Daucuscarota) Alfalfa (Medicagosativa) Clover,Ladino (Trifolium repens var.) Barley (Hordeum vulgare) Dallisgrass (Paspolum dilatatum) Beet,garden (Beta vulgaris) Fescue,tall (Fetsucaarundinacea) Beet, sugar (Betavulgaris) Lettuce (Lactucasativa) Bermudagrass (Cynodon dactylon) Oat (Avenasativa) Cotton (Gossypium hirsutum) Onion (Allium cepa) Rhodegrass (Chlorisgayana) Radish (Raphanussativus) Wheatgrass,crested (Agropyron cristatum)

Rice (Oryzasativa)

Wheatgrass,tall (Agropyron elongatum)

Rye (Secalecereale) Ryegrass,Italian (Lolium multiforum) Sorghum (Sarghcimvulgare) Spinach (Spinaceaoleracea) Tomato (Lycopersiconesculentum) Vetch (Viciasativa) Wheat (Triticumvulgare) * Approximate levels ofexchangeablesodium percentage(ESP)corresponding to the three categories of toleranceare:sensitive, less than 15 ESP;semi-tolerant,15-40 ESP;tolerant,more than 40ESP

4. Crop selection

Selecting crops to be grown during reclamation of saline and alkali soils involves more than studying tolerance lists and choosing the most tolerant crop. Economic necessity may require selecting a high-value tolerant crop like cotton as soon as possible after initial soil treatment and leaching. However,perennial close-growingforage crops would be a better choice,from the standpoint of reclamation,because they shade the soil and reduce evaporation as well as permeate the upper soil with roots that add organic matter and increase soil permeability and microbial activity.Pasture and hay grasses are also good choices for similar reasons. Soil building crops are more important during reclamation of saline and alkali soils than for most normal soils because of the adverse physical, chemical and microbiological conditions resulting from the excess salt and exchangeable sodium in the soil. After reclamation has been achieved,crop yields generally will be relatively poor unless fertilisers,manures, and soil building crops are utilised to improve the soil and restoreits productivity.Reclamation only serves to reduce a limitation in crop growth;it does not automatically ensure good yields.

472

R E C L A M A T I O N OF SALINE A N D A L K A L I SOILS

When soils are moderately affected by salinity and exchangeable sodium,careful crop selection,combined

with other practices that minimise adverse soil effects,can permit successful farmingwithout further reclamation. Sodium and salt effects, in this case, certainly restrict the choice of crops but they do not necessarily prohibit farming.Growing fieldbeans may be out of the question,whereas tomatoes or beets may produce quite satisfactory yields.Emphasis should be placed upon what can be grown,rather than upon what cannot be grown. The south-western United States is a good example of an irrigated area where salt and sodium are commonly present at moderate levels in both soils and irrigation water in many places where complete reclamation is impossible or uneconomical. In Central Asia (USSR)two crops which have been used fairly frequently,and with good results,during reclamation operations are millet and barley. They have, however, the disadvantage of a short growing period;thus, after harvesting,the field has to be worked so as to keep the soil well loosened. An extremely useful crop is djugara (a kind of millet) which has a high salt tolerance and a long growing period when irrigated;in addition,it produces large quantities of green manure. As mentioned earlier,a plant suitable for growing on soils which have been leached but are still characterised by residual salinity and clayey mechanical composition is lucerne:it has a fairly good salt resistance and, when planted on solonetz-typesoils,it promotes the substitution of calcium for sodium. Another plant widely used in Central Asia during leaching operations is the sunflower,which has a high salt resistance and provides good shade for the soil surface thereby preventing resalinisation.It makes heavy clayey soil friable and permeable.

5. Soil treatment

After the intially high salt and sodium hazard has been reduced during reclamation,attention should be directed toward soil treatment in order to increase crop yields and continue soil improvement. Leaching, an essential step in reclamation,removes both undesirable and desirable saltsin the soil.The most important of the desirable salts lost are the nitrates since they are highly water soluble. Also, nitrogen is the plant nutrient most likely to be deficient in soils of the arid regions for non-leguminouscrops. Some loss of other nutrient salts undoubtedly occurs but to a less significant degree than the loss ofnitrates.Addition ofnitrogen can be made by use of fertilisers,manure, or forage and hay legumes as green manures. If phosphorus fertilisers are necessary for good yields on similar normal soils,that nutrient,too,probably will be required on the soils being reclaimed. Present indications are that the kind of fertiliser required on saline soils is approximately the same as required on similar non-salinesoils.The situation with respect to sodium-affected soils is less clear. The purpose of producing good crop growth during reclamation is twofold: to increase profits and to hasten reclamation.It is especially beneficial on alkaline soilsbecause the presence of a vigorous root system improves soil permeability and reduces the need for soil amendments as a result of the carbon dioxide released during respiration and decomposition. Itis extremely importantthat soil shouldbe thoroughly treated after leaching in order to prevent resalinisation and improve its physical properties. Deep ploughing and subsoiling are particularly recommended, especially in places where there is a dense subsoil layer. For decreasing the density and improving the structure of leached clayey soils,there is a special method, known as scorching or thermic fallowing:after the grain has been harvested,the field is deep ploughed,and then left to ‘scorch’by exposure to the sun during the hottest season of the year. This causes lumpy soil to break up into small particles thereby improving its structure and its water and physical properties. Besides eliminating harmful salts which either prevent the growth or seriously decrease the yield of agricultural plants, leaching has the advantage of increasing the root zone. The creation of a deep root zone is one ofthe most vital prerequisites for the effective exploitation of reclaimed lands. One of the best methods for producing a deep root zone is thorough loosening ofthe soil,a process which acquires particular importance in places with a dense subsoil.If loosening is not done before leaching, it is essential that it be done afterwards.

473

IRRIGATION, DRAINAGE A N D SALINITY The method used is either ploughing to a depth of 35 cm without turning the sod over or else subsoiling. Deep subsoiling on the land of the Chardzhousk experimental station (Turkmen SSR) produced a cotton crop 40%larger than that obtained with ordinary ploughing which turns the sod over. Subsoilers are particularly effective for breaking up the subsoil. One of the most useful methods for preventing the resalinisation of leached soils and making them more fertile is efficient and thorough autumn ploughing; however, in order to give good, even results,the soil must contain sufficient moisture. If deep ploughing is done, certain conditions pertaining to the regime of nutritive substances must be observed. In order to increase the effects of deep hoeing, it is essential to supply organic substances in the form of manure mixed with non-mobilenitrogeneous mineral fertiliser, since the deep soil horizons as a rule contain large quantities of phosphorus, but little nitrogen.

6. Cultivation practices

Use of cultivation practices designed to minimise salt and sodium effects can make the difference between success and failure on affected soils.Most of these practices are only modifications of usual practices. They are based on the premise that salts move with the irrigation water,plants are most sensitive to injury during their seedling stage,salt concentration in the soil solution increases as the soil dries out,and soil permeability usually is low in non-saline alkali soils. For alkali soils,the major cultivation requirement is to provide a very slightslope,or no slope at all,to the fieldin the direction of water flow.This is necessary because of the slow rate of water penetration into such soils.In some places,fields are flat-levelledand provision is made to drain off the irrigation water if it stays on the fieldmore than one or two days. Obviously,only crops having some tolerance to waterlogging can be grown under that condition. The important consideration is to apply irrigation water in such manner that the root zone is thoroughly wetted over the entire field rather than only at one end of the field. To reduce the danger of waterlogging injury to plants, fields in which annual crops are to be grown can be heavily watered prior to planting, and perennial crops in the period of dormancy or slow-growth.A good supply of stored water in the subsoil then permits lighter irrigations during the growing season. Allowing the soil to crack deeply before irrigating may cause some root-breaking but it will improve soil structure by alternate wetting and drying and also permit deeper penetration of water. In general, cultivation practices useful on non-alkali,fke-textured, slowly permeable soils are equally useful on slowly permeable alkali soils. This includes planting row,crops on high beds to reduce the danger of waterlogging injury On saline soils,severalpractices are helpful in reducing salt damage to crops.All ofthe practices suggested here are predicated upon soil drainage being adequate. For furrow irrigated row crops, one of the most effective practices is to plant in a position where salt accumulation will be low. Since salt accumulation is greatest on the top of a single-rowbed, seeding there can cause a problem during germination even though the soilas a whole may be only slightly saline.Planting near the edges ofa double-rowbed reduces the hazard because salt movement will be towards the centre of the bed and away from the seed rows. Planting on the side of a sloping bed further reduces the salt hazard. Under conditions of high salinity,it is sometimes possible to grow salt-tolerantcrops satisfactorily by planting in the bottom ofthe furrow,where the salt level immediately after irrigation will be about the same as the salt content of the irrigation water. The majar hazard associated with furrow planting is the problem of soil crusting that may seal off the surface before the seedlings emerge. Figure 13.14shows the relative salinityhazard ofplanting in four differentpositions on row crop land. Planting in the middle of a double-row bed has the same effect as planting on the top of a single-row bed. A modification of the furrow planting technique is to plant in the furrow, then, in later cultivations after the plants are established, make a new furrow between the original furrows,pile up soil around the plants in the row, then irrigate in the new furrow. By this means, the salinity hazard can be minimised during germinationand seedling periods,after which the usualcultivation and harvesting practices can be used. As the soil gradually dries out after an irrigation, the salt concentration of the soil solution increases, then drops again following the next irrigation.The greater the degree of soil drying before water is applied, the greater will be the salt concentration of the soil solution and the more difficult it will be for plants to absorb enough water to enable them to grow.Although the total amount of salt in the soil solution does not

474

R E C L A M A T I O N OF SALINE A N D ALKALI SOILS

usually changevery much,the concentrationdoes,and thatisthe important consideration.Thisconcentrating effect of soil dehydration can be reduced by scheduling frequent irrigations rather than infrequent ones, thus helping to avoid excessive concentration of salts in the soil solution. Light rains during the growing season can have an adverse effect on plant growth on saline soilsby washing salt from the soil surface into the upper part of the root zone.That part of the root zone is normally less saline than the soil surface,particularly in the case of row crops.A light rain (or light sprinkler irrigation) provides just enough water to cause some downward movement of salts from the surface into the root zone, bringing about a rapid increase in salinity of the soil solution and causing injury to the crop.This harmful effect of rain can be minimised by following the rain with an irrigation heavy enough to leach the salts beyond the root zone.

L / k . . >. . .. .... ... : : ..: : ,.... ..> .:,:.x,:,:::::Y

..../ ..... .. .. . .. . .. . .; . .. . . ; .. .. ;. ... . ; . ; ....; .. .$ : . ?; ..; .. ? ..

..............

2 L ...s..................... ?.....

.............. : ... ..... : . .: . >.

. : . : . . : . .. .. ...... . . . .: . .. .

Fig. 13.14. Relation of salinity hazard to position of seed (e)row A. Single-rowbed, seed row on top of bed: greatest salinity hazard to germination B. Double-rowbed, seed rows near edges of bed: salinity hazard less than in A C. Sloping bed, seed rows on side of bed: salinity hazard less than in B D. Seeds planted in irrigation furrow: salinity hazard least, soil crusting may restrict seedling emergence

Creation of a soil mulch by surface tillage after an irrigation has been advocated as a means of conserving moisture and slowing the upward movement of salts. Present thinking is that little or no advantage results from this practice because by the time the soil is dry enough to till, a natural mulch of dry surface soil is already present. Although the practice is not c o m o n for salinity control, some farmers plant row crop seeds deeper than usual, then remove the excess soil soon after germination has occurred.The objective is twofold:to reduce the danger of the soil around the seed drying out before germination and to avoid having the seedling grow through the salt layer that builds up on top of the row. Large-seeded crops like cotton are better suited to this practice than are small-seededcrops because the timing of the removal of the excess soil is less critical.

HI

475

IRRIGATION, DRAINAGE A N D SALINITY

G.

POST-R EC LAMATl O N P R O G RAM M E

1. Importance of good irrigation and management

Some reclamation processes can be effected very quickly-for instance,the removal of salts from the top root profile of the soil.Other methods take longer,extending over a number of years;for instance,the process of general desalinisation of the deep soil and subsoil horizon and groundwaters.As long as the lower lying subsoils and groundwaters have not been desalinised there will always be a danger of resalinisation. In view ofthe above,the bringing ofreclaimed lands under cultivation is to be regarded as the first and most important stage of soil improvement operations although the process of agricultural management of such lands should affect long-termfactors also. Resalinisation and alkalisation can however occur on drained and leached lands also, when unfavourable changes take place during exploitation. The installation of drainage does not eliminate the need to avoid overwatering: experience shows that a reduction ofthe water application to between 2000 and 4000m3/hamay cause the groundwater level to drop by as much as 0.5-1.0 m, even when there is no drainage. It is obvious that the use of excessive quantities of water will lower the efficiency of the drains. Unless the irrigation network is fitted with regulator-devices and operated correctly, the surplus water will flow out of the canals into the open drains and collectors, causing them to silt up and overflow. Correct operation of the drainage system is of particular importance. Open drains which are overgrown and silted up, or systems functioning with a faulty run-off,are not effective in removing the groundwaters and do not give satisfactory results. It is possible that with a permeable soil,good water, and an effective drainage system, salt and sodium will never again accumulate in the soil,but this is true only as long as these conditionsas well as the importance of good irrigation,drainage, soil and crop management are recognised. Practices that are good for salinity and alkalinity control also help to assure high crop yields on soils where salt problems are no longer acute.They include land levelling,efficient water application methods to assure even water penetration, crop rotations with deep rooted and fibrous rooted legumes and grasses, judicious use of fertilisers and manures, maintenance of drains,preventing canal seepage,selecting adapted crops, disease and insect control, and others. Additional special practices include applying enough excess irrigation water each year to prevent salt accumulation,using the leaching requirement calculated from the salinity ofthe irrigation water as a guide,and treating high sodium,low saltwaters with gypsum periodically to avoid formation of free soda and to increase the exchangeable sodium in the soil.

2. Continwation of operation of drainage and leaching for prevention of resalinisation

The desalinisation ofthe root zone,though an important stagein soil reclamation,does not solvethe problem of salinity completely;thus it is essential,while planting crops on the surface layer of the soil, to take measures for the complete desalinisation of the lower soil horizons,including also the groundwaters. Desalinisation,when extended sufficientlydeep (down to 5-7m),greatly reduces the chances ofresalinisation,provides a deep root zone for plants and creates conditionsfor a more even water regime în that plants are able to draw directly on the groundwaters. At this stage in reclamation operations, work for desalinising the subsoil and groundwaters can be carried on while at the same time using the topsoil for irrigation agriculture. It is essential to maintain the moisture content on reclaimed fields,by watering,at between 100and 80%of the field moisture capacity.In order to leach the salts,autumn and winter waterings with norms of 10003000 m3per hectare are necessary. The drainage system, at this stage, drains approximately 25-30% of the irrigationwater.With this reclamation and irrigation system,the desalinisation ofthe groundwaters takes between 7 and 15 years;but the process can be accelerated considerably by applying larger leaching norms when available water resources permit. The results achieved in desalinising the groundwaters also depend directly on efficient land management. A rise of salts to the surface leading eventually to secondary salinisation results from soil left untreated or inadequately treated,or leached soil left fallow without crops to provide shade.

476

R E C L A M A T I O N O F SALINE A N D ALKALI SOILS

Thus,the time required for complete reclamationdepends to a great extent both on the correct use of the collector drain network and on appropriate agrotechnical management. There are several ways to detect the serious reappearance of a salt problem: the first, and most common, is to observe the field and the uniformity of the crops;the second is to sample the soil at intervals of three to fiveyears to test for salt; the third is to maintain data on the amount of salt passing the irrigationdrainage system each year. When salinity is again threatening,leaching and drainage becomes essential once more. Now, however, there are more opportunitiesto accomplishtheseprocesses at times which do not interferewith usual cropping. Drains can be cleared and deepened and leaching done between cropping seasons (winter, in most cases) rather than during the season in order to avoid the complete loss or reduction in crop yields that occurs from excessive salinity during the cropping season. However, salinity control can be accomplished this way only if the salt build-up has been discovered before it becomes a limitation to crop growth. If crop yield is used as the criterion ofsalinity,chances are that therewill be a major reduction in yields before ameliorative measures are taken. 3. Prevention of secondary alkalinity (free soda formation and exchangeable sodium)

Accumulations offree soda and exchangeable sodium in the soil can be detected before they become excessive only by soil analyses,except for those rare cases when sensitive crops are grown. As with salinity,soil tests should be made at intervals ofoneto threeyears,to determinewhetherfreealkalinityand exchangeablesodium areincreasinginthe soil.Ifthere isan increase,treatmentmust be done by direct soil applications ofan amendment such as gypsum or by application in the irrigation water. Other than gypsum,calcium chloride,polysulphides, and sulphuric acid,most soil amendments are not sufficiently soluble to be dissolved in the irrigation water and applied that way. The solubility ofgypsum (about 2200ppm) is enough to reduce the content offree sodium bicarbonate and sodium carbonate of most irrigation waters to a satisfactory level but only if gypsum is added in large quantities during every irrigation. If the soda content and exchangeable sodium percentage in the soil are high, soil application of gypsum is preferred;if the sodium level is moderate or low,water applications are effective.The disadvantage of applying soil amendments in irrigation water is that their distribution over the field will be no better than the distribution of the irrigation water: if distribution is poor, there will be a concentration or loss of amendments in the same place where the irrigation water is concentrated or lost as run-offfrom the end of the field. Preventionof exchangeablesodium build-up in calcareous soils can be assisted by planting fibrousrooted, close growing grasses and legumeswhich produce considerable amounts ofcarbon dioxide during respiration and decomposition.Just as reclamation of alkali soils is facilitated by production of large amounts of carbon dioxide in calcareous soils through manure applications or crop growth, secondary alkalisation is minimised by the same practices that increase soluble calcium in the soil solution. Re-alkalisationprobably is due more to poor quality irrigation water than to upward movement ofhigh sodium groundwater,althoughthe latter may causethe same effectiffollowed by leachingwith low salt water. In either case,small amounts of soil amendments can prevent re-alkalisationwhen applied regularly orbefore the sodium problem becomes acute. For this reason, single superphosphate (16 to 20% available P,O,) containing about 50% gypsum is preferable to concentrated superphosphate (no gypsum) as a phosphorus fertiliser on soils where re-alkalisationis a threat.

4. Observation on water table, salinity and alkalinity of soils and groundwater For efficient irrigation and drainage,it is essential to install an extensive network of hydrometric measurements on the irrigation canals and to equip the collector-drainagesystem with control points. There should be control and measurements at least on the head sections of all main canals and at outlets of the main collectors. With this apparatus,regular checks of the irrigation and drainage water should be made. In addition,salts of the drain water should be analysed every month or two. There should also be a network of bore holes for checking the level and mineralisation of the groundkm along waters on irrigated lands. They should be distributed throughout the area at intervals of 1*0-1*5

411

IRRIGATION, DRAINAGE A N D SALINITY lines about 5-6 k m apart.All the wells along one line should be connected by levelled passages and also with the intersecting canals and collectors.If the drainage system is operating effectively,the water table and the composition of the groundwater will return each year to approximately the same position they held the previous year. As long as this occurs,there probably will be little change in salinity or exchangeable sodium percentage of the soil. If,however, the water table level or the composition of the groundwater change for the worse by 10% or more, trouble may be coming. The groundwater regime is checked every ten days at the same time as hydrometric checks ofthe irrigation and collector drainage networks. Three times a year-before the growing season in the middle of it, and between the end and leachingchemical analyses are made of the groundwaters and of the water in the collector-drainagesystem. Halo-hydrometric data and data for the water-salt regime o€ the groundwaters should be examined regularly by the irrigation authorities in order to check the progress of reclamation operations and the effectiveness and correct operation of the drainage installations. In order to control soil salinity and alltalinity,samples of soils should be taken trom several ley points on irrigated territory.Useful results can be obtained by making repeated precision salt tests of a number of key plots measuring 500-2000 hectares each combined with bulk analysis ofsoils,subsoils and groundwaters. The results of such tests, made after 3-5 years,provide material for a detailed scientific assessment of the process of reclamation and the state of the reclaimed lands when under cultivation. 5. Crop rotation After drainage and leaching,soils formerly barren or having only limited fertility acquire great potential fertility.World experiencedemonstratesclearly that crops grown on reclaimed land can,with proper management,produce large and stable yields. Thus,for example,the reclaimed lands ofthe Mughansk experimental station (Azerbaijan) produced the following crops, taking the average of the total area year after year: cotton,37-43, wheat and barley, 27-32, grass for hay, 80-110 metric centners per hectare; the average for the Bukharsk experimental station was 35-49 metric centners of raw cotton per hectare. These high figures were obtained thanks not only to the desalinisation of the soil,but also to effective soil treatment and correct crop rotation. In each case,the selection of crops for rotation on reclaimed and irrigation lands has to be made on the basis ofthe specialfeatures ofthe land.Irrigationcauses important and frequently undesirable changesin the structure ofthe soil;and the danger ofsecondarysalinisationoften remains.Therefore,crop rotationpatterns selected should enrich the soil in nutritive elements and also ensure periodical renewal of its structure, Apart from this,the subsidiarycropsused in the rotationcycleshould also promote desalinisation and thereby help to protect the soil against resalinisation. Numerous theoretical and practical experiments demonstrate the advantages of growing lucerne in alternation with cotton. Cotton grown after lucerne produces a larger crop than cotton grown on old arable soil; the benefits of lucerne become apparent 18 months after it has been introduced into the rotation cycle (when sown in autumn). Lucerne is effectivetoo,for reclamation purposes:fields sownwith good lucernearecapable oftranspiration at the rate of 10000-18000 ma/haand frequently have a groundwater level 70-100 c m lower than that of surrounding fields sown with cotton plants;thus this crop promotes the leaching of salts. In crop rotation,provision also has to be made forpost-harvestcrops.At present,fieldsremain unprotected by vegetation from the time the spring crops are harvested until the autumn.Thisleads to secondary salinisation especially in dry years. Meanwhile, experience in many countries indicates that useful results can be obtained,in warm climates,by growing second crops on irrigated lands.Apart from producing valuable food or fodder,the second crop has the advantage of using large quantities of fresh groundwaters,the level of which is thus lowered;in addition,it protects the soil against evaporation and salt accumulation in the arable layer, and increases the plant residues in the soil,with beneficial consequences to both structure and humus reserves. With pasture, reclamation may have been performed with considerable irrigation water supplies for leaching,whereas after reclamation considerably less water may be available. Under such conditions,hardy types of perennials give the best protection against return of salts to the root zone. Suitable species include lucerne,Rhodes grass,Phalaris tuberosa and strawberry clover.

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6. Forestation of canals

The importance ofplant transpirationfor reclamation purposes has already been stressed in connection with the principles of crop rotation on irrigated lands.This process which assumes large proportions on leached soils,might be regarded as a kind of ‘biologicaldrainage’in that it uses up considerable quantities ofgroundwaters, and so lowers their level. Research shows that a similar effect is produced by trees growing on irrigated lands,especially when they are planted along the irrigation canals. According to observations made in Central Asia, one tree causes an annual evaporation of 50-90 m3, i.e. as much as a drain usually collects and discharges for every metre of its length. It has also been noted that trees planted along irrigation canals cause a drop of more than 1 m in the groundwater level during the growing season, thus changing the groundwater slope towards the canal, instead of from the canal.The depression curve was traced over a distance of200 metres from the double row of trees along the conveying channel. In view of the above it is recommended,as a useful reclamation measure, to plant two or three rows of trees along the permanent sections of the irrigation system.Trees eminently suitable for this purpose include the eucalyptus, willow, poplar and mulberry, all of which have an extremely high transpiration capacity. Fruit trees, such as banana, mango,apricot,may also be used for this purpose. It is also useful to plant trees along collectors and drains,In this case,of course,salt resistant species will be used (elaeagnus,pseudo-acacia,mulberry,tamarisk). The planting of trees must not prevent the cleaning of canals,collectors and drains. The following factors must be considered when selecting trees: the degree of soil salinity, the rate of growth,and economic value of the trees. For planting along irrigation networks on non-saline or slightly saline soils,the following aïe recommended :Populus Bachotenii, Robinia pseudoacacia,Fraxinus pubescaras, Morus alba, etc. On saline soils where there are discoloured salt patches on the surface and salt waters in the vicinity (strips along ditches, collectors) the following may be planted : Eleagnus angustifolia, Arthrophytum haloxilon, Tamarix ramosissima and Tamarix hispida. In oases surrounded by sandy desert,trees are planted to serve also as protection against wind and sand displacement.

REFERENCES ABRAHAML. and SZABOLCS I. (1964), Improvement of alkali soils with small doses of reclamation materials, VIIIth Iíat. Congress of I.S.S.S.,II,875. On solonetzes and saline soils of the Hungarian People’s Republic and means for their reclamation,Acta Agronomica Acad. Scientiarum Hungarical. t. 10. ARANY S. A.(1926), The reclaiming effect of waste lime of sugar mill on the Hungarian alkali soils,Hung. Ac. of Sci., X X M (Hungarian,long summary in English). ASGHAR A.G.and HAFEEZ KHANM.A. €3.(1955), Behaviour of saline-alkalinePunjab soils under reclamation,Proc. Pb. Eng. Congress,Lahore,XXXN. ASGHARA.G.and HAFEEZ KHANM.A.H.(1958), Field studies on prevention ofsoil salinisation,Agronomy Journal, USA,50. ASGHAR A.G.and HAFEEZ KHANM.A.H.(1961), Re-appearanceof salts after reclamation of saline soils by leaching,Res. Pub., II,5. AVERIANOV S. F. (1 958), Demineralizing effects by drainage, International Commission on Irrigation and Drainage,Ann. Bull. 4. BESEDNOV N.A.(1935), Experimental drainage in the Mugan. Ed. MYHAILOV K.A., Zakgyz, TyJEis. CHANG C.W., DOERING E.J. and REEVE R.C.(1965), Engineering aspects of the reclamation of sodic soils with high-salt-waters,ASCE Proc. Irr. and Drg. Div.,91 (IR4): 59-72. GRABOVSKA O.A.,Academy of Sciences,Tadjik SSR. GRABOVSKA O.A. (1961), Processes of soil leaching under melioration in Valleys of South Tadjikistan, V,i, Academy of Sciences,Tadjik SSR. GRACIE D.S., RIZK M.,MOUKHTAR A. and MOUSTAFA A. H.L.(1934), The nature of soil deterioration in Egypt. Ministry of Agricult. Egypt. Techn. and Sci. Series Bull., 148, 1-22. ANTIPOV-KA~?S,TAEV I. N.(1960),

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KABAEV V. E.(1958), Results of the radical amelioration of saline soils in the Bukhara region;In: The application of drainage in the development of saline soils,Acad. of Sciences, USSR, Moscow,

KELLEY W.P. (1937), The reclamation of alkali soils,Calif. Agr. Exp. Sta. Bull.,617. KELLEY W.P.(1951), Alkali soils. Theirformation,properties and reclamation, Reinhold, New York. KELLEY W.P.and THOMAS E.E.(1928), Reclamation of the Fresno type of black-alkalisoil,Calif.Agr. Exp. Sta. Bull,455.

KERZUM P. A. (1958), The collector and drainage network in the Vaksha Valley. In: The application of drainage in the development of saline soils, Acad. of Sci.,USSR,Moscow. KOVDA V. A. (1964), Chapter 2, 1947. Origin and regime of saline soils; Acad. Sci., USSR,MoscowLeningrad.

KOVDA V. A. (1958), Drainage for desalinization of irrigated lands. In: The application of drainage in the development of saline soils, Acad, of Sci., USSR, Moscow.

KOVDA V. A.and EGOROVV. V. (1958), Soil and ameliorative conditions of application of drainage for desalinization of irrigated lands;In: The application of drainage in the development of saline soils, Acad. of Sci., USSR., Moscow. LEGOSTAEV V. M.(1951), Drainage on the irrigated area,Mag. Hlopcovodstvo,9. MALYGUIN V. S. (1939), Deep subsurface drainage,Sojuz Nikhy, Tashkent. MAIERHOFER C.R.(1951), The drainage of irrigated lands,Agric. Eng., 32,II. MAIERHOFER C.R. (1948), Drainage problems in the Rio Grande Valley. The reclamation era, 34,6. MAIERHOFER C.R.(1953), S o m e aspects of drainage in reclamation, Bureau of Reclamation,Colorado. OVERSTREET ROY, MARTIN J. C. and WNG H . M.(1951), Gypsum,sulphur and sulphuric acid for reclaiming an alkali soil of the Fresno series, Hilgardia, 21,5. PEARSON GEORGE B. and AYERS ALVIN D. (1960), Rice as a crop for salt affected soil in process of reclamation, USDA,Production Research Report, 43. RABOTCHEV I. S. (1953), Amelioration of saline soils of Turkmenistan,Acad. Sci. Turkm. SSR,Ashhabad. RABOTCHEV I. S. and EPHIMOV G.S. Horizontal Drainage of irrigated area in Turkmenian SSR.Acad. Sci Turkm. SSR,Ashhabad. RASMUSSEN W.W., LEWIS G.C. and FOSBERG M . A. (1964),Improvement of the Chilcott-Sebree(SolodizedSolonetz), Slick Spot Soils in Southwestern Ida, US.Dept. Agr., ARS,41-91. REEVE R.C.and BOWERC.A.(1960), Use of high-salt-watersas a flocculant and source of divalent cations for reclaiming sodic soils,Soil Science, 90,139-44. REEVE R.C. and DOERING E. J. (1966), High salt-water dilution method for reclaiming sodic soils, Soil Science Soc.Proc.,30(4), 498-504. RESHETKINA N.M.(1964), Effectiveness of vertical drainage in the central complex of reclamativemeasures on an example of the Golodnaya Steppe, Transactions of the meeting on economics, of investments into irrigation,Institute for People’s Economy,Tashkent. ROZONOVA. A. (1958), Salinization and reclamation of irrigated soils: In Application of drainage in the development of saline soils, Academy of Science,USSR, Moscow. SHOSHIN A. A.(1954), Improvement and leaching of saline soils in the Delta Kura and Arax, Socialist Selskoje hozjaistvo Azerbaidgana. I,Baku. THORNE D . W.and PETERSON H.B. (1964), Irrigated soils, 2nd edition N.I.The , Blakiston Company. us SALINITYLABORATORY(1954), Diagnosisand improvement of saline and alkali soils,Agriculture Handbook 60,U S D A . VOLOBUEV V. R.(1948), Leaching of saline soils,Arerneshp. Baku, VOLOBUEV V.R.(1949), Effective distance of action of Dgafarhan draining collector,Trudy Azniigim, T.I. Baku.

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14. Design and Operational Recommendations for the Establishment or Improvement of Irrigation and Drainage Projects* INTRODUCTION 1. National policies on land and water use

THElife of all persons and all societies depends on water and land, but different ways of life are reflected in a wide variety of national policies concerning these resources. Particularly in arid regions development of these resources is so important that the problems are given national attention and development is mostly carried out by the government in order to ensure an optimum use of resources with respect to the present and the future. A clear statement of national objectives is needed and the relative importance of water resources development inrelationto other developmentprogrammes has to be reflected in the yearly investmentprogramme and the yearly national budget. In principle it is important that the basin or sub-basinshould be regarded as an entity for planning,construction and operation, even if the performance would be realised in successive stages. Collection of facts should be the basis for action programmes. Only then can a systematic procedure for making choices,and arrangementsto keep the programme going,be developed. (a) Purpose of the project Water utilisation on a national or even on a regional scale is a much wider subject than irrigation.In general, with increasingeconomic developmentmulti-purposeprojects become more complex and of growing importance involving purposes such as protection against flooding,hydro-electricpower,navigation,fish and wildlife preservation and recreationalfacilities. The risingpublic interestinthese aspects may sometimes successfully oppose economic development of water, lands or reservoir sites for irrigation. National policy has to reflect the competition between different purposes, considering present needs and methods of use and potential changes in needs. (b) Water and land rights In many poorly developed countries,water rights are vague,defined by custom rather than law.In developing such regions,laws should be enacted to preserve existing rights to use water for irrigation.If local customs or laws are conflicting or inadequate to meet changing development,new laws should be advised,contemplating future need. The riparian doctrine of equal rights of owners of property abutting a stream to an undiminished flow, common to northern Europe and carried to the humid eastern USA,for example, was inadequate for the arid West and was supplanted there by the appropriative doctrine of first in time,first in right,subject to loss of right if not continued in beneficial use. Preferential rights were recognised with right of condemnation for higher use in the order of domestic,municipal, industrial,irrigation,hydro-electricpower,etc.,and in some states water was made appurtenantto land.The pressures for inter-basindiversion to supply growing populations and economically favourable irrigated agriculture have transcended both riparian and appropriative conceptsand raise questions ofregionaland nationaldevelopment.Inturn,this has stimulated recognition of need to reserve water supplies within basins of origin for potential but undeveloped uses. Generally, water laws should be administered by executive determination subject to court adjudication. (c) Development policies Optimum economic development of irrigation is greatly affected by the relative scarcity of one or more of the production factors:labour,capital,water,and land. This chapter was edited by C. VAN DEN BERG from the manuscripts submitted by F. E. DOMINY and B. P. BELLPORT, G.DROUHIN, and S. A. GUIRSHKAN as authors with contributions by W.N.ALLANand A. FRANKE

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Where water and capital are scarce in comparison with population (not uncommon features in developing countries in arid zones), new holdings will generally be small in order to satisfy as inany families as possible. The lack of capital prevents the rapid development of other economic activities such as industry which could give sufficientotherjob opportunities.The mechanisation offarm work which could lead to a low labour/land ratio,will also be limited by lack of capital. As a consequence of this, demands for land reform may be followed by government purchase or expropriation to facilitate resettlement.Other regulations may be the limitation of acreage to be irrigated per landowner. The scarcity of water will further require a careful distribution and use. It follows that irrigation charges in such cases can best be dctermined on the basis of water used,in order to avoid waste by over-irrigation. In general,scarcity ofwater and a relative dense population willtend to enlarge the influence ofthe government on the development project. Where on the other hand labouris scarce but capital is sufficientlyavailable,the development:allows more opportunity for private developers, who may obtain approval from the government to utilise a national resource.As capital is no longer a limitation for farm mechanisation, the man/land ratio should be lower, i.e.the farms will be luger. Where water and land are relatively more abundant (due to low population density or a low percentage of i'zrm workers) greater freedom in water use will be allowable. It may for instance lead to irrigation charges on the basis of land watered. The policy of irrigation development should take these economic considerations into account as far as public acceptance permits. In so doing, the implications of providing all necessary items of infrastructure, including those which are commonly provided to communities as public services (e.g. roads, water supplies, drainage, public security,education, health services,etc.), should be fully taken into account.

2. Importance of improvement of existing projects as compared with establishment of new projects

Any comprehensive programme of project planning should consider the social and economic aspects of existing development.This aspect of planning is essential,particularly in a regional programme. The questionwhether to improve an existing project usually comes up as a result ofpoor or even decreasing productivity of a project area,the need for protection against flooding, or the possibility of supplementing water from holdover storage. The answer can vary greatly according to circumstances and only a few considerations are given below:

If the limiting factor is the land and if the available surfzce has been brought into use, however imperfectly, the only solution is to improve what is already available in such a way as to increase total productivity.This is fairly often the situation in small basins within the arid zone. The work will very often consist in improving,sometimes in entirely refashioning,the irrigation network so as to obtain the best water supply,in improving or installing a drainage system,in re-establishingsoils spoiled by excess water or possibly by salinisation or alkalisation Where lands are not limited, it will usually be worthwhile to start improving the existing systems and eventually later transform them completely.The reasons for this are: -an old,technically unsatisfactory system is often to be found on the best land in the area,where the ground is easy to work and drain -the water supplies used are often easy to develop and will remain so when the planned improvement has been carried out -lastly and mainly,a long-standingtradition may have accustomed an entire community to the skilled farming imposed by irrigation, so that there is no need to start from the beginning, but, instead, a source of wealth is present which is relatively easy to develop Even with the same discharge it will be possible to economise water by improving the existing facilities (peak discharge and consumption per hectare) and to enlarge the cultivated area.When water supplies are increased,the reconstruction of the existing system may be a prerequisite for the extension of the area under cultivation, In some cases, however, there will be powerful a priori arguments against improving the existing network and a preference for shifting to a new project: 482

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-when, for example, it is realised that an old irrigation system was laid down as a result of a mistaken appreciation (for instance,poor soils) or that in view ofthe technical possibilities at the time there was no alternative and the solutionhad been almost one of despair -when it is found that the shortcomings of the system and of the ways in which it has been used in the past are so serious as to have reduced the land to a state of almost irreversible sterility. This is not uncommon in countries where the soil is very clayey and saline,as in parts of North Africa; the rehabilitationof such soils is theoretically not impossible,but the unproductive time involved and the high costs(diEcultdrainage,improvement,water and so forth) can make the operation uneconomical (c) It may also be more economical to develop a virgin water supply for deep fertilevirgin soilsthan to seek the diminishing incremental gain from holdover storage, on a marginal developed mea, or to invest millions in flood control works to protect a meagre investment when the alternative developmentneeds little or no flood protection. The wise alternative is to favour the new development,but to afford full legal protection to the old

Where a considerable agricultural area is abandoned in favour of another area the net effect on agricultural production must be evaluated,Loss ofcurrentproduction in the one area may be an appreciable offset to the new production in the other. In all cases of alternatives the ratio ofbenefits to costs should be decisive for the determination ofpriority, taking into account the social aspects of the different solutions.

Part I. Establishment of Irrigation and Drainage Projects A. GENERAL CONSIDERATIONS Developingirrigationand drainageprojects requiresthesynthesisofmany factsderivedfrom social,economic, agrooomic and engineering studies.Thetype and detailofbasic data needed willvary with the stageofdevelopment of the project.Once the attention is focused on a particular region,minimum data are directed towards the identification of problems and particular limiting factors of the area concerned. At least three stages of development may be recognised after this brief period of general survey until construction.They are: (a) Reconnaissance (b) Feasibility (c) Design and construction During veconnaissance investigations are limited and as much use as possible is made of available data in order to arrive at a preliminary evaluation of the potential of a project. A rough plan formulation and cost-benefit determination will determine if more detailed investigation is justified and when and how that investigation shoald be carried out. A reconnaissance study may result in a masterplan for development in which various projects are identiiied and formulated following a preliminary priority rating. The purpose offeasibility investigation is to secure authorisation for construction.Its scope and detail should be sufficient to present an essentially complete plan as to purposes, scope, facilities,costs,benefits and financing.It will lead to a definite project report. The design ari% coizstvuction stage includes supplemental investigations necessary to fix locations and specifications in accordance with the conclusions reached on the recommendations of the definite project report. Sometimes there may still be a stage between reconnaissance and feasibility, called ‘tentativereport’, ‘avant-projet’.In such a case the cost and benefit appraisal is usually postponed to this stage. In this chapterthe data concerningthe reconnaissance and the feasibility stagewill be discussed and only a fewremarksmade on design and construction. Constructionaldetailsparticularly are beyond the scopeofthis book. 483

IRRIGATION, DRAINAGE A N D SALINITY Next to the variation in the intensity of data collecting due to the stage of the project,there is also a variation in the required duration of data collection.Some physical datasuch as climatic and hydrologicalrecords are dependent upon the vagaries ofnature and it may take many years to demonstratenear maxima, minima and means.In such cases,critical periods,deviating from normal,may for instance determine whether water shortages during a drought period are tolerable or project structures are adequate to store or pass a flood without destruction. The minimum length of record required is then dependent on climatic characteristics,particularly on the range of maximum and minimum conditions.A short period of record may indicate a reasonable possibility of stable climatic and hydrological conditions and support dependence upon a record of 10 years or less. Under unstable conditions of climate even a much longer record may only show that instability is the rule and that predictions are uncertain. Under such conditions,every effort should be made to obtain available information of unusual events such as: ‘bucket-survey’data on high intensity storms,high water marks and so on. The data on topography,geology, soils and land use on the other hand are much more stable than climatic and hydrologic measurements and maps, once prepared, may be good for many,years. In between these two groups the more or less fluctuating data on population growth,demand for agricultural commodities, prices of labour and materials and yields of crops and livestock can be placed. A reliable mean value has to be built from records over several,and sometimes over many,years. In general,project studies are conducted to diagnose present conditions in an area and to predict future conditions with the project. Mostly, however,there are different possibilities for the set-upof a project and several possible variations in its realisation, Usually alternative solutions with cost-benefitdeterminations for the different alternatives are compared to find the best alternative. The technique can be applied on different levels of importance of the scheme, for example: (a) The project as a whole. In a case of limited water supply the project may be split up into small farms having an intensive cropping pattern or into bigger farms with less intensive cropping patterns (b) Important parts of the project. When fairly high water losses from irrigation canals are expected,the irrigation canals may be lined. The costs of lining have then to be compared with the cost of the alternative: loss of water and increase of artificial drainage (c) Details of the project. If a drainage system is needed the depth of drains will vary with varying drain distances. In this case a whole range of alternative solutions,giving the same effect,may be compared

Thenumber ofexamples can easily be increased.In general,investigations in a project have to be intensified ifalternative solutionsare to be compared on a reliablebasis. Moreover,in order to test alternative solutions, sensitivity tests based on slightly different but realistic assumptions may be recommended.Such tests while made during the study usually provide importantindications as to the relative importance ofthe assumptions made for individual elements of the study.

B.

PRELIMINARY STAGE

1. Information required in the reconnaissance stage (a) Quantity and quality of available wuter

This investigation, if not the most difficult, may take the longest time,except in cases where water of good quality is abundant,relative to the availableland area,as for instance in the case of a large river with a heavy permanent discharge. Normally, daily discharge records, in cubic metres per second or litres per second with monthly totals in cubic metres, are the basic data of water supply studies.Ifpossible,there should be a continuing programme ofstream gauging and gauge height recordingfor certain key stationsunder a national or regionalprogramme.Such recordsmay be used for estimates during the reconnaissancestage supplemented by gauge readings and periodic current meter discharge measurements at points of particular interest. If little information is available at the outset,records of at least five years should be made available and

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preferably longer. As often a fairly long series of climatological observations can be made available, the discharge data obtained should be correlated as carefully as possible with the long-timerecords of climatological data. It should be emphasised that only fully reliable data are of use. In regard to the amount of water available,peak discharge and the amount of water during the irrigation season are of particular importance.In arid regions regulation of water supply by storage may often be the best solution. Complete chemical analyses of the composition and concentration of dissolved constituents in the water supply are essentialin order to appraise the suitability of the water supply for intended uses.These determinations should at all times be made with reference to discharge rate in streams.In arid zones there is often a great difference in salinity between the first flood of the wet season and later floods and even considerable differences between the beginning of a flood,its peak, and its final phase. At least some determinations of minor elements are essential. The determinations will enable computations of leaching,salt balance and predictions of mean quality of anticipated storage. Water quality determinations should be started early in the investigation. Suspended sediment and bedload must also be measured in relation to discharge rate in streams. At least miscellaneous sediment samples should be obtained during reconnaissance. In dealing with irregular water courses the preliminary investigation must include an estimate of the possibilities ofregulating water supply.The construction of storage dams accounts for a considerablepart of the cost of an irrigation project and it is necessary to make at least a rough estimate of this item at the very beginning. As far as well systems are considered in combination with surface systems,at least some well drilling will be justified with measurements of water level drawdowns and yields. The chemical composition of this water has to be evaluated,similar to the surface water requirements.When test wells are sampled,water should be evacuated until constant values are obtained as indicated by measurements of electrical conductivity. (b) Climate Climatic data are essential in planning projects, even in the preliminary stage. Precipitation,temperature, humidity, wind and evaporation, as well as determination of land use and yield are basic to appropriate design of structures.Ifnot available,their recording should start at an early stage.Daily precipitation records for a well-distributednetwork of stations together with monthly and annual summaries will serve for reconnaissance purposes whereas some rainfall intensity records should be made available. In the reconnaissance stage daily maximum and minimum temperature data are basic for computing water requirements,frost-free periods for plant growth and so on.Wind velocity and direction are important in considering such factors as erosion hazards and efficiency of sprinkler irrigation. For evaporation data, dependence during the reconnaissance stageis usually placed on existing records with estimates extended to project conditions.Humidity and solar radiation are becoming increasingly importantin estimating water requirements,but generally their recording at climatological stations will start in a later stage. (c) Land, geology, topography

A variety of complex and interrelated land data are required in planning an irrigation and drainage project. Systematic appraisal of the soil and substrata,topography,and drainage factors are essential.The systematic appraisal, conducted as an integrated study with economics, hydrology, engineering and other disciplines, resultsinthe selection oflands suitableforirrigation and their relative degree ofsuitability.Land classification survey for irrigation and drainage provides a systematic and efficient means of data collection and decisionmaking. The aim ofsuch surveysis to predict thefuturesoil-water-crop interactionsunder irrigationfor units ofland having comparable soil,substrata,drainage,and topographical conditions within the area of investigation. The land classification survey will relatethe soil,topographical,and drainage characteristics of the land to irrigation suitability.Many factors need to be observed and measured in accomplishing such a synthesis. The importance and relevance ofthe characteristics will vary depending upon the time,place,economic,and social environment ofa project setting.Typical characteristicsofthe natural soil bodies involved are:texture, structure, depth, stoniness, horizon arrangement and layering, exchange capacity, exchangeable cations, soluble salts,lime,gypsum,indurated caliche,hardpans, clay pans, clay mineral type,pH, density, colour, organic matter, hydraulic conductivity,infiltration rate,moisture characteristics,and fertility level. Micro and macro topography are evaluated with respect to degree and direction of slope,land grading, and land 485

IRRIGATION, D R A I N A G E A N D SALINITY development requirements. Water quality data, both chemical and suspended load, must be assessed and correlated to specific soil and substrata units to assure that the selected irrigable lands and water are compatible for irrigation use. The drainage possibilities of the area as a whole need to be considered in relationship to the drainage characteristics of the soil.For this,at least some data are needed regarding (a) position and fluctuationof groundwater levels,(b) groundwater contours and main directions of groundwater flow, (c) identification ofbarrier layers and horizontal and vertical permeability of subsoils,(d) natural drainage of thearea and eventualseepagefrom surroundingareas,(e)drainagepattern ofthe area and eventualsub-division of the area into sub-areasof differing hydrological characteristics. In the preliminary stage a reconnaissance or semi-detailedland classification performed on suitable base maps with a scale of about 1 :25000 can be used. For sub-reconnaissancepurposes or for obtaining a very rough preliminary appraisal of irrigation possibilities,scales as low as 1 :50000to 1:250000 may sometimes be used.Soil surveys showing the distribution ofnatural soilbodies are useful in making the land classification survey. The basis upon which such soil surveys are made needs to be carefully studied and results thereof carefully interpreted to derive appropriate land class designations in an irrigability survey.Soils series,types and associations are seldom directly and uniquely correlated to irrigation suitability Also a topographical and geological reconnaissance is needed to arrive at reasonable estimates of the drainage characteristics of the area or sub-areas. It is clear that the topographical survey will also serve to appraise degree and direction of slope,land grading and land development requirements,whereas the geologicalsurvey may supply data on the probable volume of the groundwater basin and its possible use. It is again emphasised that the data obtained must be directed towards prediction of the effects of the water on the land as related to sustained production under irrigation.Thus the data have to be combined in a land classification,the integrating tool for making predictions of the interactionsofwater-land-crop. In the reconnaissance stage the land classification should indicateroughly the potential possibilities ofthe land,the difficulties to be expected and the limitations of certain land classes. Use of aerial photos greatly facilitatesthe collection and interpretation of data.

(d) Crops, value, etc. The land classification,supplemented by overall agricultural studies, should malte it possible to assess the crop suitability of the soils. Not until these determinations of the physical possibilities have been made can the economic and social desiderata be taken into account.In determining crop values, within the framework of a trading economy,it will generally be necessary to take into account not only the gross value of the products, but also the conversion factor to be used for indirect national revenues connected with services,transport,processing and so on. Similarly, an estimate should be made of the farm costs;in the reconnaissance stage at least a rough estimate of the benefits should be made. (e) PeopIe, social and economic conditions The reconnaissance survey should include figureson the population ofthe area and their social and economic condition. Some impression must be obtained on the paying capacity of the farmers depending on farm sizes at normative incomes, on people’s abilities for irrigation farming,on economic activities other than agriculture, on the requirement of settlers from outside the area,etc. These calculations may give some indications as to the credit requirements. 2. Preliminary evaluation of irrigation and drainage requirements Once a general insight into the possible cropping pattern has been obtained,it will not be very difficult to get a fairly precise idea ofthe water requirementofthe crops (see Chapter 8 ofthis book). Evenin the preliminary stage,the crop water requirement should includenot only total annualneeds,but also a seasonal breakdown. To arrive at irrigation water requirementssome estimation has to be made of losses on the farm,from the conveyance system and eventual leaching requirements over the deep percolation losses on the farm in order to maintain favourable salinity levels in the root zone. Transfer of experience from projects having comparable soils, crops and climate provides an effective means of estimating farm and conveyance losses. Where appropriate experience is not available, then some experimental determinations at typical sites,together with the collected information on the subsoil,should

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DESIGN A N D OPERATIONAL R E C O M M E N D A T I O N S give the minimum information required. The field tests should at least include surface ponding tests and permeability determinations of the layers of importance for the expected irrigation canals. The leaching requirement depends on the salinity of the irrigation water and the required salinity level of the root zone and can be calculated with the help of simple formulae (cf. Chapter 2 and Chapter 11 of this book.) To determine the drainage requirement the total water losses,the expected rise in the groundwater table, the required water table and the natural drainage have to be considered. It is particularly necessary to have an idea about the question whether in the project water storage in the soil and natural drainage will be sufficientto account for water losses and leaching water. It can be safely assumed that apart from exceptional cases (high terraces near a deep valley forinstance) it will nearly always be necessary to provide a drainage system in order to avoid the risk of salinisation ofthe soil. The depth of the groundwater table and the salinity of the groundwater as found during the reconnaissance survey may already give an indication of the intensity of the drainage system and the time of its installation after the beginning of irrigation. 3. Cost and benefit estimates

Once an approximation of the quantity of available water and irrigation water requirements is known,the approximate size of the project can be found. Rough estimations of the construction costs can now be made,including eventual clearing with respect to vegetative cover and (eventually) levelling of the soil. If the construction of a storage dam is considered,the cost estimate of its construction should be based on a topographical and geological reconnaissance survey. This is important as the construction of a storage dam accounts for a considerable part of the costs of an irrigation project. Together with the calculation of the benefits (the increasein crop value, together with a conversion factor as described under Section Id), the elements are now available to decide what steps have to be considered next. 4. Conclusion

Theapproximations mentioned earliershouldnowleadto thedecisionwhether the project under consideration

is sufficiently promising to continue the studies in order to arrive at feasibility and economic justification. The data availableafter reconnaissancewill rarely permit definite decisions on such importantitems as size of the project, size of farms,lining of canals and so on.But the collected data will have to show the possibility ofa favourable development ofthe area due to available people,land and water,before feasibility studies are started, detailed specific studies (e.g. model, analogue) may be usefully suggested. The following Sections CyD and E refer mainly to the feasibility stage. As indicated before, the investigations are then directed towards authorisation for construction so that the final result presents a complete detailed plan.

C.

DETAILED DATA COLLECTION

1. Collaboration of specialists The complex relationship of social,economic,agronomic and engineering data requires a coordinated team approach to irrigation development.Apart from irrigation and drainage engineers specialists in the field of hydrology,soil science,geology, soil mechanics,survey, agronomy and economics are needed. The coordination and timing of the work of these specialists is one of the important responsibilities of the planning administrator. In a large organisation,particularly in highly developed countries,it may be possible to attach central service specialists to a regional or central headquarters office and make them available as need requires for individual project study. But when the magnitude of the work in the various 487

IRRIGATION, D R A I N A G E A N D SALINITY specialised fieldsdoes not warrant employment of all these staffspecialists,or where the number of specialists is insufficient as in most developing countries,another solution has to be found. In the first place, it may be possible to arrange for performance of work by other agencies on a local, regional or nationallevel, competent to collect the necessary data for the study of a project. Secondly,part of the work could be contracted to private firmswho have the specialists and the special equipment to perform the work. Thirdly, for less developed countries it becomes essential to call in a team of specialists or an organisation from outside.A selection ofa balanced team of specialistsfrom an appropriate organisationhas the advantage ofmaking availablespecialistsused to working togetherand ableto draw on all theinformation and resources of that organisation. The tasks should be clearly defined and the contract drafted with care. In other cases,a request for the regular assistance of a fewwell-qualified expertsmay suffice.This will have the advantage of making the training of local personnel easier. Finally, consultants may be employed on highly technicalproblems to provide knowledgeorjudgement beyond that ofthe regular staffor the specialists working in the project. Whatever the circumstances, it will always be found advantageous to sub-contract with agencies or organisations for the carrying out of certain highly specialisedwork if the documentation involved does not exist.This applies for instanceto topographical studies,to geophysicalinvestigations when needed and so on. It is always highly advisable to bring all this work under a single authority,competent to direct and to arbitrate between differing viewpoints in order to avoid dissipation of effort and sterile controversies.

2. Programmes and schedules of data collection Programming and time schedules are important throughout project planning and development;they permit the orderly staging of interdependent work. Much depends on the existing organisation and the basic data available. In a large organisation having a continuing responsibility for project development,it is possible to carry on a number ofinvestigationsof dserent projects and in varying detail more or less simultaneously. In that case individual assignments have to be timed next to the time schedule of every particular project so that staffspecialists are continuously and effectively employed in their own specialities.In small organisations the flexibility of many projects may be missing and it is essential that staff members be experienced or adaptable in many specialities or that greater use must be made of the capabilities of other agencies or consultants. Programmingis furtherstronglyinfluenced by the existence or non-existenceofbasic data.Iflittleis known of a particular project area,the time needed to obtain climatologicaland hydrological data will be long and their collection will be continued during feasibility studies.The collection ofthe non-statisticaldata may then be started considerably later. In general, the time needed for collection of detailed data cannot be indicated as this depends on the factors mentioned. It should,however,be pointed out that the time allowed must be adequate to give results that can be used with confidence.The possibility offailure for lack ofreliable data is far more serious than an apparent delay. A wise governmental precaution is to provide the means for collecting all manner of basic data essential to the development of its resources. Government sponsored data collection should be initiated wherever representative potentials for development exist without waiting until need requires feasibility planning for early construction. Such programmes are expensive and to be of value they must have continuity. The availability of funds will,of course, govern the density of observations and the extent of the programme.

3. M a p s and aerial photographs

As mapping is often an excellent means of presenting information in an easily usable form, the greatest possible use should be made of clear maps. Although planimetric maps are adequate for many purposes, topographic maps representing a third dimension are preferable for planning purposes. The detail required determines the scale of maps to be used. Aerial photography is being used increasingly for photo interpretation to expedite the field surveys and the 488

DESIGN A N D OPERATIONAL RECOMMENDATIONS preparation ofmaps.Photo-mosaicson the same scale as the photographs are extremely useful as field maps. Inthe feasibilitystage,suchfieldmaps should be preferably ofthe scale 1 :5000 or 1 :IO000for suchpurposes as: soil survey,land use, land classification,natural vegetation, and so on, and also for paper locations of irrigation distribution systems,drainage and roads. Usually, the report maps will have a smaller scale, for instance 1 :25000. For structure sites and more detailed route location,the scale of the field maps should be 1:lOOO or 1 :2000.These scales (showing contour intervals of 1 metre or 0-5metre) are generally adequate for design and operation stage work also,but in these latter stages ground surveys commonly supplement maps. Standardised skeleton topographical maps are valuable as the basis for recording and presenting various kinds of specialised data, e.g.land classifications,climatic conditions,vegetation, population, communications,etc.

4. Type and detail of data needed

The resources ofwater and land delineated roughly by the reconnaissance survey are studied in much greater detail during feasibility investigations.Ofcourse,uniformity of some conditions as soils and substratawould permit less detailed investigations.But iflittle is known of an area,many observations are needed to demonstrate this uniformity. As has been said before, the variation of climatological and hydrological data requires recording over a sufficiently long period. It would be desirable to have records approaching the life of project facilities to demonstrate average conditions and particularly the severity of critical periods. However,such a record can seldom be made available,especially in undeveloped areas. A basis of about a ten years’record at actual points of consideration is highly desirable for planning in the feasibility stage. The frequency of soil profile and substrata observations required for any survey will depend primarily on landscape conditions and the purpose of the survey. For instance, on loessial plains, high uniformity of soil conditions may be expected, permitting wide spacing of sampling points; whereas soils on alluvial plains or low terraces may require very intensive examination. Thus no general rules can or should be specified regarding the number of observations and measurements required. Recording of observable soil characteristics in pits, conducting field measurements, and sampling soils for laboratory analysis should be done at a frequency required to make sound judgements regarding the irrigability of the area.The data thus collected on soils need to be integrated with the topographic and drainage conditions for interpretation into land classes reflecting various degrees of irrigation suitability.Details of land classification can be found in specialised publications. It is particularly important for the future success of a project to have a good understanding of the existing groundwater relations and the expected changes as a result ofirrigation.For this the existing drainage pattern has to be mapped. Further the groundwater levels have to be determined, for instance,with a density of one observationwell per 100 ha if the existing groundwater table is not too deep. Repeated observations on selected wells are needed in the course of a year in order to register the seasonalvariation in level. From the contourlines ofall observationsin the sameperiod,the directionofflow can be derived.In Chapter 11 simple methods are described to find the amount of ‘natural’drainage. As irrigationwill raise the groundwater table,an idea should be obtained ofthe storagecapacity of the soil between the existing groundwater level and the ultimately permitted level (say 1-2metres below soil surface) in the operational stage of the completed project. Where the construction of a storage dam forms part of the project, a thorough study of the probable dam-siteis needed,particularly to evaluate the consistency ofrock foundations,groundwater conditions and estimated seepage losses. In the area to be irrigated the geological investigations should be carried out to provide data on the groundwater conditions of deeper layers. Thickness and permeability of aquifers are important for the calculation ofgroundwater movement. Mostly pump tests are needed to arrive at a reliable value ofthese properties. Piezometer readingsin different deeper soil layers will demonstratethe Conditions of the groundwater at different depths. Such data are needed to demonstrate seepage of deep groundwater to the area or loss of groundwater to surrounding areas. Quality of water determinations as described under Section B(1) should be carried out in surface water as well as in groundwater.

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IRRIGATION, D R A I N A G E A N D SALINITY 5. Crops and holdings

It is evident that if the land under consideration is already partly cultivated,it will be necessary to map the existing land'distributidnto see how far the existing situation can be preserved. The enquiry should also cover the gross and net income derived in the existing state of cultivation without irrigation,in order to assess the realbenefit,both socialand economic,which will accrue from the introduction of irrigation. The results will be more satisfactory if data can be obtained from a few small areas already under irrigation. On the basis of the data collected,the conditionsofclimate,land and water will determine the suitability of differentcrops.Usually the cropping pattern will be determined by the experienceand skill ofthe farmers and the prevailing economic climate.When economically desirable,the government may influence the cropping pattern,for instance by price policies for certain crops. Apart from the question of cooperative or individual farming,it is desirable that the size of holdings be determined by the cropping pattern, the available manpower and machinery, and by the desirable family income. Usually farm sizes will have to vary with the quality of the soil,the smallestholdings with the most intense cropping pattern being planned on the best soils.

6. Field experiments Field experiments are conducted to provide critical data needed for sound planning which cannot be obtained with satisfactoryprecisionfrom available data,computation or correlation,ortransfer ofexperience.Logically they should precede the establishment of a project,but usually this is not possible owing to the delay entailed in obtaining verified and usable results. It will always be an advantage,however, to start such experiments as early as possible in the feasibility stage and if possible to continue during exploitation.Experiments can be approached as follows: (a) On experimental stations.Such stations are useful to determine how soils will evolve under irrigation, what dressings and fertilisers should be used,how to improve plant strains and introduce new plants and so on. Frequently arising problems such as effectiveness of leaching on different soil types, consumptive use of water by different crops and so on should all be investigated in such a centre. Since such stationsmake considerable demands on scientific manpower and materials, their number should be kept as low as possible and the experiments at the start should be selected carefully so as to obtain practicable solutions for the most important problems of project design and operation. It may be possible to find an existing research station in a region similarand close to that under study, but if this is not the case it will be valuable to set up a fully equipped station in the area to be developed. The zone selected should be representative of the different soil types to be encountered (b) On test plots of the same size as the holdings envisaged. Here an attempt can be made to apply on a practical scale the information obtained at the experimental station.Avery importantaspect ofthe work on these test farms is the investigation of the best and most advantageous crop rotation,the production costs,the farm profits,and so on,The plots should be run as closely as possible to practical farming conditions and without unnecessary expenditures (c) In larger experimental zones. Problems concerning the distribution of water, flow of surface and groundwater and so on,require a larger area of,for instance,a few hundreds of hectares. Jn the experimental area as a whole the water and salt balance could be studied,considering the quantity of water supplied by irrigation water and precipitation,the amount ofwater drained,and the development ofthe groundwater table. Salt quantities in irrigation water, drainage water and the soil should also be compared.The area would also permit investigations on distribution and measurementofwater,comparison of open and covered drains,the water losses in irrigation canals,the methods of maintenance,etc. j

I

Of course there are different ways to obtain the information needed by field experiments if only it is kept in mind that these experiments are essential to fill the gaps of knowledge needed to make sound predictions in a given locale. 490