Spatial and short-term temporal variations in runoff ...

Catena 33 Ž1998. 123–138

Spatial and short-term temporal variations in runoff, soil aggregation and other soil properties along a mediterranean climatological gradient C. Boix-Fayos a,b,) , A. Calvo-Cases b, A.C. Imeson a , M.D. Soriano-Soto b, I.R. Tiemessen a a

Department of Physical Geography and soil, UniÕersity of Amsterdam, Nieuwe Prinsengracht 130, 1018 VZ Amsterdam, Netherlands b Department of Geography, UniÕersity of Valencia AÕenida Blasco Ibanez ˜ 28, 46010 Valencia, Spain Received 22 July 1997; accepted 25 March 1998

Abstract Physical and chemical soil properties were measured along a mountainous climatological gradient in the province of Alicante ŽSpain.. The objective was to evaluate how the climate affects certain soil properties at different temporal and spatial scales. These properties include infiltration, runoff and sediment concentrations resulting from rainfall simulation experiments performed in winter and in summer. Chemical soil properties like carbonate content, organic matter content and CEC were analysed in reference soil profiles along the gradient. Physical soil properties like soil moisture content, macroaggregation and waterstable microaggregation were measured at monthly intervals during a year. The comparison of the results was done at different spatial Žsite, slope and patch. and temporal Žmonthly and seasonal. scales by means of some statistical tests. It can be concluded that there are some soil properties positively related to the gradient, like organic matter, clay content and CEC which increase with the annual rainfall. However, runoff coefficients and erosion are higher when the climatic annual rainfall. However, runoff coefficients and erosion are higher when the climatic conditions become more arid. Aggregation and infiltration capacity are higher on north-facing slopes and in vegetated patches than in south-facing slopes and in bare patches. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Runoff; Erosion; Soil aggregation; Soil properties

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Corresponding author. Fax: q31-20-5257431; e-mail: [email protected].

0341-8162r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 3 4 1 - 8 1 6 2 Ž 9 8 . 0 0 0 4 8 - 4

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C. Boix-Fayos et al.r Catena 33 (1998) 123–138

1. Introduction Some models have predicted an increase in the global temperature of between 28 and 38 due to a doubling of the carbon dioxide content of the atmosphere ŽManabe and Wheterald, 1975; Hansen et al., 1981.. It is expected that air temperature will increase in general, although with monthly variations, and that precipitation will decrease during the summer months and increase in other seasons ŽBultot et al., 1988; Lough et al., 1983.. These climatic changes will probably induce changes in the vegetation cover, which will be associated with severe soil erosion in some areas, which will in turn become a serious economic problem in the long term ŽKirkby, 1989.. Any change in climate will affect key processes of land degradation which will have repercussions on the soils and landscape. Climatic change will affect processes such as sediment and solute production, infiltration, overland flow ŽLavee et al., 1991. and will have an impact on hydrological systems, affecting variables like effective evapotranspiration, soil moisture, groundwater storage and stream flow ŽBultot et al., 1988.. Eybergen and Imeson Ž1989. discuss the importance of identifying and studying key processes which are sensitive to climatic conditions. Later, Imeson and Emmer Ž1992. recognise that these key soil processes, which are likely to be influenced by the predicted climatic change, are those involving: Input and output of water, the input and output of calcium carbonate and the input, decomposition and output of organic matter. The problem is how to study the potential changes caused by the climatic change on present day slope processes. One approach is to consider processes along climatic gradients, associated with altitudinal or latitudinal transects ŽLavee et al., 1991.. In this way data on how processes are affected by different microclimatic conditions can be obtained. In practice, this approach has many difficulties because there are many more variables Žland-use, vegetation, lithology, soils, etc.. affecting the transects along climatic gradients. The objectives of this paper are to investigate how certain soil properties and processes are influenced by climatic conditions and how these condition the spatial and temporal distribution of soil properties. For this purpose, a fairly homogeneous Mediterranean climatological and altitudinal gradient in terms of lithology, soils and vegetation was selected. In order to get a general view of how the climatic and microclimatic conditions affect the soil response along this gradient, some variables were taken as indicators Žrunoff, soil moisture, aggregate distribution and stability. of different processes Žinfiltration, soil water regimes, aggregation.. These variables were measured at different temporal Žmonthly, seasonally. and spatial Žsite, slope, patch. scales.

2. Study area The study area is located in the north of the Alicante province ŽSpain.. Three sites ŽBenidorm, BE; Callosa, CS; Cocoll, CC. were selected to carry out this study ŽFig. 1.. This area is characterised by a strong climatological gradient from north to south, especially pronounced with respect to the annual rainfall, with a maximum in the northern site ŽCocoll. with more than 800 mm and a minimum to the southern site of


125

Fig. 1. Location map of the study sites ŽBE: Benidorm, CS: Callosa, CC: Cocoll..

less than 400 mm ŽBenidorm. ŽGaussen, 1954; Tames, 1949. ŽTable 1.. All three sites are located on Upper Cretaceous limestone rocks and on the South-facing side of the Prebetic range near the Mediterranean coast. During the period of study Žfrom October 1992 to October 1993., the maximum rainfall events occurred in December 1992, February 1993 and October 1993. Mean values of soil moisture Žfor the 0–3 cm of the soil surface. for each slope are shown in Fig. 3. The warmest month for Benidorm and Callosa was July and whilst for Cocoll it was August. The coldest month for the three sites was January. The temperatures for the three sites were higher in autumn ŽOctober, November, December. than in spring ŽMarch, April and May..

3. Methods To evaluate the influence of climate on the spatial Õariability of soil properties at different scales the following three scales of study were established. Ži. Site scale; for this purpose a meteorological station recording rainfall, Žamount and intensity. air temperature and soil temperature ŽBoix et al., 1994. was installed on

Table 1 Main characteristics of the study sites Slopes

Aspect Ž8.

Altitude Žm.

Annual rainfall Žmm.

Average annual temperature Ž8C.

North-facing Benidorm ŽBEn. Callosa ŽCSn. Cocoll ŽCCn.

75 10 10

74–90 280–360 994–1026

383 457 853

18 16 13

South-facing Benidorm ŽBEs. Callosa ŽCSs. Cocoll ŽCCs.

210 240 120

74–106 282–344 850–910

383 457 853

18 16 13

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the south-facing slopes of each site along the gradient, in order to compare the results within a framework of different climatological conditions. Žii. Slope scale; north-facing and south-facing slopes were selected at each site to carry out the sampling and the experiments, in order to evaluate the influence of aspect Žsix slopes in total: three north-facing slopes and three south-facing ones.. Žiii. Patch scale; sampling points and plots for experiments were established on different parts of the slope covering different microenvironments Žbare patch, vegetated patch, patch with high superficial stoniness, etc.. in order to evaluate the different microclimatic conditions within the same slope. Fig. 2 gives an overview of the field experimental setup in the study site of Benidorm. To evaluate the temporal Õariability of soil properties the sampling was undertaken on an annual, seasonal or monthly basis, depending on the soil characteristics to be measured. The experimental work consisted of three types of sampling or experiments. Ži. First, a soil description and classification was made ŽFAO, 1977; FAO–UNESCO, 1988.. On average four soil profiles were selected from the upper to the lower part of each slope, following the catena concept. Chemical and physical analysis were performed as follows: granulometrical analysis and grain size fractions were determined after removing the organic matter with H 2 0 2 30%. ŽMinisterio de Agricultura, 1986., pH was measured in a soil–water extract and a soil CaCl 2 extract Ž1:2.5 solution., with a pH meter, salinity was measured in a water–soil extract 1:5 solution with a conductivity meter ŽRichards, 1954.. Total calcium carbonate content was measured according to Jackson Ž1958.. Organic matter was measured by wet oxidation ŽK 2 Cr2 O 7 . ŽWalkley and Black, 1934. and cation exchange capacity was measured according to Peech Ž1945.. Žii. Second, a total of 117 rainfall simulation experiments were carried out on the six slopes. The rainfall simulator used is described by Calvo et al. Ž1988. and Cerda` et al. Ž1997.. The intensity of the rain was 55 mm hy1 and the nozzle of the rainfall simulator was located at an altitude of 2 m. The plots where the experiments were performed were round with a surface area of 0.24 m2 . An average of ten plots were located on each slope covering different morphological positions Župper, medium and lower slope. and different microenvironments Žshrubs, herbs, crusted surface, stony surface, bare surface etc... The rainfall simulation experiments undertaken on each plot had a duration of 60 min. A steady runoff rate was achieved in most of the cases. The experiments were conducted in winter ŽFebruary 1993. and in summer ŽJuly 1992 and July 1993. to characterise the hydrological response of the soil under wet and dry conditions. During the experiments, the runoff was measured every minute or every 30 s and a runoff sample was taken every 15 min to calculate the sediment concentration. Runoff coefficients and erosion rates were calculated from the data collected. Runoff coefficients, sediment concentrations and the coefficients of variation of both parameters were used as indicators to evaluate the response of soil to simulated rain. Žiii. Third, a total of 42 sample points were selected on the six slopes in order to characterise the condition of the soil surface aggregation. These sample points covered


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Fig. 2. Example of the experimental setup in one of the study sites ŽBenidorm..

different morphological positions on the slopes and represented different vegetation units including bare patches. Monthly sampling was carried out at these points over a 13-month period. Soil moisture samples and samples for studying soil aggregation were taken from the upper 0–3 cm of the soil. Volumetric soil moisture was calculated for each month and aggregation was studied at three levels. Ø First, the aggregate size distribution for 4 months Žseasonally, October, February, May and August., was determined using the following fractions: ) 10, 10–5, 5–2, 2–1, 1–0.105 and - 0.105 mm, by dry sieving by hand and taking especial care of not destroying the aggregates at bigger fractions. Stones and litter ) 2 mm were removed.


128

Ø Second, the aggregate stability of macroaggregates Ž4–4.8 mm. for air-dried aggregates and wet aggregates at pF1 was determined using the water drop test ŽCND, counting number of drops required to destroy the aggregate up to a maximum of 200 drops. ŽImeson and Vis, 1984.. Ø Third, the percentage of water stable microaggregation was determined, for each month, in the - 0.105 mm fraction using a Microscan II Quantachrome particle analyzer ŽEdwards and Bremner, 1967; Cammeraat and Imeson, 1998.. To determine the water-stable microaggregates two runs using the Microscann particle analyzer are required. In the first run, soil sieved at - 105 m m in a distilled water solution is introduced in the Microscann and a first particle size distribution is obtained. In a second place, the same sample is attacked with sodium pyrophosphate Ž1 ml. and ultrasound at an energy level of 1800 J during 1 min with the objective of breaking down existing bonds between the particles. From this treatment a second size distribution of primary particles is obtained. The difference between both particle size distributions Žthe one resulted from the first run and the one resulted from the second run. is used as an indicator of the waterstable microaggregates existing in the soil sample. 4. Results The description of the results is focused on the variation of soil chemical and physical properties along the gradient ŽSection 4.1., the spatial and temporal variation of soil aggregation ŽSection 4.2. and the results of the rainfall simulation experiments ŽSection 4.3.. 4.1. Soil conditions along the gradient At the most arid site ŽBenidorm. the soils are shallow and poorly developed Žlithic Leptosols. while in the intermediate ŽCallosa. and most humid ŽCocoll. areas the soils are deeper and better developed Žlithic Leptosols, haplic Calcisols and chromic Luvisols.. Mean values for some chemical properties of the Ah horizons on each slope are shown in Table 2. Although differences are small, slightly higher pH values are found at the lowest site ŽBenidorm. on the south-facing slope. Slightly higher electrical conduc-

Table 2 Some chemical characteristics of the superficial soil horizons in the three study sites Žaverage values of five samples for each slope. BE a nd

pH H 2 0 Ž1:2.5. pH Cl 2 Ca Ž1:2.5. EC g ŽmS. Ž1:5. CaCO 3 Ž%. a

CS b nd

CC c nd

BE a s e

CS b s e

CC c s e

Mean

c.v.f

Mean

c.v.f

Mean

c.v.f

Mean

c.v.f

Mean

c.v.f

Mean

c.v.f

8.18 7.70 0.29 42.26

40 40 34 38

8.33 7.45 0.16 38.45

1 0 31 25

7.90 7.34 0.30 1.76

1 1 16 36

8.27 7.73 0.30 42.53

0 12 2 11

8.10 7.61 0.21 12.11

2 0 0 8

7.83 7.32 0.25 15.72

3 2 16 84

Benidorm, b Callosa, c Cocoll, d north-facing slope, e south-facing slope, f coefficient of variation Ž%., g electrical conductivity.


129

Fig. 3. Average values of volumetric soil moisture in all the slopes from October 1992 to October 1993 ŽBE: Benidorm, CS: Callosa, CC: Cocoll; n: north-facing slope, s: south-facing slope..

tivity values are also found for the lowest site and for the north-facing slope of the highest site ŽCocoll.. The pH values seem to decrease with altitude, although the EC values are higher for the most arid and for the most humid sites ŽBenidorm and Cocoll, respectively. with lower values at the intermediate site ŽCallosa.. The carbonate content clearly decreases with the altitude, taking into account the variation of the climate, showing maximum values for both aspects in Benidorm, while much lower contents are found at the other sites. On all of the slopes the soils are saturated with Ca2q. Magnesium cations are mainly found on the north-facing slope at Cocoll. The highest values of exchangeable sodium are found at the intermediate site ŽCallosa.. All of the profiles show a 100% base saturation. The organic matter and clay content increases with altitude as well as the CEC, which shows also a slightly increase with altitude along the gradient, with higher values on the north-facing slopes ŽFig. 4.. These three parameters are directly related to the gradient. 4.2. Dynamics of soil aggregation Mean values for waterstable microaggregation Ž- 0.105 mm., for the stability of macroaggregates Ž4–4.8 mm. and for the proportion of aggregates in different fractions

Fig. 4. Organic matter content Ž%., clay content Ž%. and CEC Žcmc kgy1 . of the Ah soil horizons Žmean values. in the three sites along the gradient ŽBE: Benidorm, CS: Callosa, CC: Cocoll: n: north-facing slope, s: south-facing slope..

130


Table 3 Mean values of the aggregate fraction statistically different between bare and vegetated patches Percentage of aggregates

Vegetated patches

Bare patches

F-test

p-level Ž - .

- 0.105 mm

9

12

F Ž1,156. s11.34

0.001

were calculated and compared at the different scales Žsite, slope, patch and temporal.. Analysis of variance was used to test the significance Ž a s 0.05. of the differences in aggregate stability Žfor macroaggregates between 4–4.8 mm and microaggregates 0.105 mm. and aggregate size between the sites. The Spjotvoll and Stoline test Žor Tukey honest significance difference test. was used to perform post-hoc comparisons of means. Significant results are shown in Tables 3–9. 4.2.1. Spatial scale At the patch scale, only the proportion of aggregates in the - 0.105 mm fraction is significantly higher in bare than in vegetated patches ŽTable 3.. Although more aggregates in the larger fractions Ž10–5 mm and 5–2 mm. were found in vegetated patches Ž8 and 25%, respectively. than in bare patches Ž5 and 21%., the differences were not statistically significant. The mean percentage of waterstable microaggregation Ž- 0.105 mm. was also insignificantly higher in vegetated Ž34%. than in bare patches Ž32%.. The stability of the macroaggregates in the fraction 4–4.8 is higher for vegetated than for bare patches, but the differences are not statistically significant for both air-dried Žaverage median disruption value of 84 raindrop impacts for vegetated and 62 for bare patches. and moist ŽpF1. aggregates Žaverage median disruption value of 138 for vegetated and 129 for bare patches.. At the slope scale, north-facing slopes show significant greater aggregation in the 10–5 mm fraction than south-facing slopes ŽTable 4.. The stability of the macroaggregates Ž4–4.8 mm. is also significantly higher on north-facing than on south-facing slopes, for both dry ŽpF 6.1. and moist ŽpF1. aggregates ŽTable 4.. The 1–0.105 mm and - 0.105 mm fractions are significantly higher on the south-facing than on the north-facing slopes ŽTable 4..

Table 4 Mean values of the aggregate parameters statistically different between north and south-facing slopes Percentage of aggregates 10–5 mm 1–0.105 mm - 0.105 mm Average of the median disruption value Air-dried aggregates pF1 aggregates

North-facing slopes

South-facing slopes

F-test

p-level Ž - .

9 36 9

5 41 11

F Ž1,156. s17.05 F Ž1,156. s 5.85 F Ž1,156. s8.15

0.000 0.016 0.004

97 179

56 100

F Ž1,40. s 4.47 F Ž1,38. s14.38

0.040 0.000


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Table 5 Mean values of the aggregate parameters statistically different between the sites Percentage of aggregates a

W. micro. )10 mm 10–5 mm 5–2 mm 2–1 mm 1–0.105 mm - 0.105 mm a

Benidorm

Callosa

Cocoll

F-test

p-level Ž - .

36 2 14 31 19 27 9

32 0.43 2 21 23 43 11

29 1 4 17 21 46 12

F Ž2,341. s 35.17 F Ž2,155. s9.76 F Ž2,155. s 79.19 F Ž2,155. s 45.19 F Ž2,155. s9.95 F Ž2,155. s 73.07 F Ž2,155. s6.29

0.000 0.000 0.000 0.000 0.000 0.000 0.002

Waterstable microaggregates -105 m m.

At the site scale, differences in aggregation values are statistically significant ŽTable 5.. The highest water stable microaggregation appears in the most arid and lowest site ŽBenidorm., a medium value appears in the intermediate site ŽCallosa. and finally, the lowest value appears in the highest and most humid site ŽCocoll.. Aggregation in larger fractions Ž) 10, 10–5 and 5–2 mm. is dominant in Benidorm. Aggregation in the fraction 2–1 mm is dominant at the intermediate site ŽCallosa. and aggregation in the smaller fractions Ž1–0.105 and - 0.105 mm. is clearly dominant at the highest site ŽCocoll. ŽTable 5.. Differences in aggregate stability at pF1 between the six slopes appeared significant ŽTable 6., but not differences in aggregate stability at pF 6.1. Aggregates at pF1 are much more resistant to breakdown on the Benidorm north-facing slope than on the rest of the slopes. In general, Benidorm, the lowest and most arid site, shows greater aggregates and very stable in wet conditions probably due to the high bioactivity of earthworms, taking into account that most of the aggregates at this site are worm casts. 4.2.2. Temporal scale Over 1-year period, the highest water stable microaggregation corresponds to the wettest months: October 1992, February 1993 and October 1993 ŽTable 7 and Fig. 3.. January shows a low soil moisture but a very high waterstable microaggregation, coincidentally after a period of very high soil moisture in December. The combination of humid and relatively warm months Ždepending on the site, for the last October Ž1993. in the studied period: 400 mm of rain and 98C in Cocoll and 130 mm of rain in Benidorm and 218C of temperature. results in a very high waterstable microaggregation.

Table 6 Mean values statistically different between all the slopes along the transect Average of the median BE a nd disruption value

BE a s e

CSb nd

CSb s e

CC c nd

CC c s e

F-test

p-level Ž - .

pF1 aggregates

120

159

130

167

28

F Ž5,34. s6.65

0.000

a

200

Benidorm, b Callosa, c Cocoll, d north-facing slope, e south-facing slope.

132

October 1992

November

December

January

February

March

May

June

July

August

September

October 1993

F-test

p-level

34

31

29

35

38

32

30

33

33

33

32

35

F Ž11,331. s 3.22

0.000


Table 7 Mean values of waterstable microaggregation of the three sites during 12 months


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Table 8 Mean values of aggregate fractions statistically different between months representing every season Percentage of aggregates

October 1992

February 1993

May 1993

August 1993

F-test

p-level Ž - .

)10 mm 1–0.105 mm

2 32

1 41

1 39

0.5 41

F Ž3,153. s8.58 F Ž3,153. s 4.66

0.000 0.004

The seasonal variability of the aggregate size distribution is only significant for the fractions: ) 10 mm and 1–0.105 mm ŽTable 8.. Large aggregates Ž) 10 mm. are destroyed during rainfall events Žcase of February. and incorporated into smaller fractions Ž1–0.105 mm.. 4.3. Hydrological response of soils to simulated rain The results of the rainfall simulation experiments demonstrate differences in the hydrological response of soils along the climatic gradient, between north-facing and south-facing slopes and between vegetated and bare patches. The small size of the plots used to perform the rainfall simulation experiments permits the study of the spatial variability of runoff at the patch, slope and site scales. Similar type of plots to perform rainfall simulation experiments were already used by Bork and Bork Ž1981. and Cerda` i Bolinches Ž1995. in Spain. In summer Žwith low antecedent soil moisture. runoff coefficients are very low on all the slopes. The south-facing slope in the most arid and lowest site ŽBenidorm. shows the highest runoff coefficient and sediment concentration, the values of both parameters decrease with altitude ŽTable 9.. In winter, under wet soil conditions, the runoff

Table 9 Runoff coefficients and sediment concentrations resulted from the rainfall simulation experiments carried out in the six slopes along the gradient in winter and in summer Slopes a d

BE n BE a s e CS b nd CS b s e CC c nd CC c s e BE a nd BE a s e CS b nd CS b s e CC c nd CC c s e a

Season

summer

winter

Runoff coefficient

Sediment concentration Žg ly1 .

Mean

Coefficient of variation Ž%.

Mean

Coefficient of variation Ž%.

0.04 0.21 0.10 0.16 0.01 0.06 0.12 0.33 0.48 0.54 0.23 0.30

125 86 110 138 100 133 108 73 58 63 157 87

0.43 0.84 0.12 0.37 0.17 0.24 0.44 0.99 0.44 2.01 0.10 0.93

84 50 225 70 283 92 102 109 96 201 167 227

Benidorm, b Callosa, c Cocoll, d north-facing slope, e south-facing slope.

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coefficients and the sediment concentration are maximum in the intermediate site ŽCallosa. as a result of a combination of high soil moisture with high superficial stoniness. Imeson et al. Ž1998., using the same type of plots and rainfall simulator, studied the hydrological response of soils in a climatological gradient close to our study area and found also a general trend of increasing runoff coefficients and erosion rates at lower altitudes. North-facing slopes always show lower runoff coefficients and sediment concentrations. The infiltration capacity is always higher on vegetated than on bare plots. The coefficients of variation show a high spatial variability of runoff on all the slopes. The maximum coefficient of variation of runoff in winter is 157% in the North-facing slope of Cocoll. In summer the maximum coefficient of variation of the runoff is 138% on the south-facing slope of the intermediate site ŽCallosa.. The coefficient of variation of the sediment concentration reaches a maximum value Ž284%. on the north-facing slope of the most humid site ŽCocoll. for the summer experiments. For the winter experiments the south-facing slope of Cocoll shows the highest coefficient of variation of sediment concentration Ž227%..

5. Discussion The spatial variability of soil properties is evaluated taking as a starting point the variation of climatic conditions at different scales. In this way, three scales were considered: Site scale Ždifferences between the sites along the climatological transect., slope scale Žinfluence of aspect, differences between north-facing and south-facing slopes. and patch scale Žinfluence of microclimatic conditions, differences between bare and vegetated patches.. At the site scale, the results obtained demonstrate that there are certain soil parameters which follow inversely or directly the trend of the climatological gradient. The organic matter content, the clay content and as a consequence of that the CEC increase with altitude along the gradient ŽFig. 4.. On the contrary, the runoff and sediment concentration in summer Žwith very dry soil conditions. increase towards the lowest site in the gradient ŽTable 9.. The climate seems to control some soil conditions and its erosional and hydrological response. However, the degree of aggregation does not follow the logical trend shown by the climatological conditions at the site scale. The proportion of large aggregates, the water stable microaggregation Ž- 0.105 mm. and the stability of large aggregates or macroaggregates Ž4–4.8 mm. under wet conditions ŽpF1. is higher in Benidorm, the lowest site with the most arid climatological conditions. This contradiction seems to reflect the effects of an important biological factor at this site, namely, earthworm activity that produces large aggregates and waterstable microaggregates. The high stability of worm casts has been reported by Swaby Ž1949. and Lee Ž1985.. In this case the worm casts or large aggregates at Benidorm were found to be very stable under wet conditions but very unstable under dry conditions, and similar results have been found by other authors ŽPanabokke and Quirk, 1957; Le Bissonnais et al., 1989.. These large, but unstable, dry aggregates are responsible for the formation of


135

a soil crust ŽFarres, 1978; Freebairn et al., 1991. when rain occurs after a period of drought. This crust is composed, in the case of Benidorm, of a significant proportion of microaggregated particles, very similar to the situation found by Imeson and Verstraten Ž1989. in highly calcareous soils in Spain and Collis-George and Greene Ž1979. for an agricultural irrigated soil. In this last case, a new surface layer was formed by microaggregates, derived from the slaking of large aggregates under raindrop impact. This layer was shown to have a very low hydraulic conductivity. In addition a negative relationship between the stability of dry aggregates and sediment concentration Ž r s y0.72, p s 0.000, n s 43. has been found, explaining partially the higher sediment concentrations found in the places where the aggregate stability is lower. Thus, this crust at the Benidorm site reduces the infiltration capacity of the soils and increases the runoff and erosion rates compared to the other sites, as can be seen from the results of the summer rainfall experiments ŽTable 9.. Analysing the data at the slope scale, it is seen that a small increase in the moisture content of the aggregates and a more homogeneous vegetation cover increase the aggregate stability Žmoister aggregates with more roots on north-facing slopes are more stable, Table 4. and the soil surface is not crusted so rapidly, directly increasing the infiltration capacity of the soil. This is the reason why the north-facing slopes, in general show lower mean values for the runoff coefficient and sediment concentration in both the summer and winter rainfall experiments ŽTable 9.. At the patch scale, the soil aggregation condition and the infiltration rates are higher for vegetated patches, where the soil has favourable conditions Žmoderate temperatures, higher moisture, higher organic matter content, etc.. ŽBoix et al., 1994.. Many authors have demonstrated that soil aggregates are stabilised by roots and by an increase in the organic matter content ŽTisdall and Oades, 1979, 1982; Blackman, 1992; Zhang, 1994.. Although in this case a higher aggregate stability in the fraction 4–4.8 mm has been found in the soil under vegetated patches, this was not found to be statistically significant. The runoff coefficients are always higher for bare than for vegetated patches. Cerda` i Bolinches Ž1995. reports that on limestones runoff appears in bare patches between shrubs under the same rainfall intensities. Vegetated patches have higher rates of infiltration and therefore no runoff appears. In this case the high spatial variability of runoff on all the slopes, especially in summer, is shown by the values of the coefficient of variation ŽTable 9. and is caused by the presence of a patchy vegetation distribution on all the slopes. The temporal variability of the soil properties seems to be also conditioned by the climate. The waterstable microaggregation seems to increase with high soil moisture conditions, showing higher values after rainy events. The aggregate size distribution of macroaggregates also changes on a seasonal scale: large aggregates break down into smaller fractions during rainfall events. And, finally, the soil infiltration capacity seems to be influenced, among others, by the soil moisture. The results of the rainfall simulation experiments undertaken in winter show a very different trend compared to those observed in summer. In summer the lowest infiltration capacity was found together with the highest runoff coefficient in the most arid site. The intermediate site ŽCallosa. showed the highest runoff coefficients in winter due partly to the influence of the high stoniness ŽCalvo Cases et al., 1994..

136


6. Conclusions Based on the different scales investigated, a general trend is apparent: when the climatic or microclimatic conditions become less arid, resulting in a higher water availability over a range of moderate temperatures, the values of some important soil properties which prevent the erosion of the soils Žclay, organic matter, CEC, aggregation and infiltration rates. increase. However, some exceptions are apparent due to the action of very specific factors or processes. For instance, an apparently high degree of soil aggregation in the most arid site appears due to the earthworm activity. Nevertheless, this apparently high aggregation found at Benidorm does not produce an increase in the infiltration capacity of the soil when the conditions are very arid, due to the low stability of the dry large aggregates and the consequent formation of crusted surfaces. The study of geomorphological processes along climatological gradients, seems to be a good approach to investigate the influence of climate on present-day processes, however, certain limiting factors, like the soil disturbance due to land use or especially biological activity, have to be taken into account. The provided data can be very useful to calibrate models of climate change impact on geomorphological processes.

Acknowledgements The authors wish to thank the Commission of the European Communities for the financial support provided by the projects ERMES I ŽEV5V-CT91-0023. and ERMES II ŽENV4-CT95-0181.. The first author also wishes to thank the Commission of the European Communities for the award of an Human and Capital Mobility fellowship ŽEV5V-CT-94-5228. which allowed her to produce the final version of this paper. Thanks are also extended to J.M. Schoorl for his suggestions and for producing Fig. 1 and to B. van Wesemael, A. Cerda` and the two anonymous referees for their constructive comments which helped to improve the quality of the manuscript.

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