water Article
Optimization-Based Agricultural Water-Saving Potential Analysis in Minqin County, Gansu Province China Qiong Yue, Fan Zhang
ID
and Ping Guo *
Centre for Agricultural Water Research in China, China Agricultural University, Beijing 100083, China;
[email protected] (Q.Y.);
[email protected] (F.Z.) * Correspondence:
[email protected]; Tel.: +86-106-273-8496 Received: 3 July 2018; Accepted: 8 August 2018; Published: 23 August 2018
Abstract: To deal with the contradictions that are caused by natural conditions and unreasonable water allocations in Minqin County, which are located downstream of the Shiyang River basin in arid northwest China, an optimization-based multi-scale calculation method was proposed for analyzing agricultural water-saving potential. Firstly, an optimization model was developed for allocating water and land resources legitimately with the conjunctive use of surface water and groundwater. Secondly, the groundwater equilibrium was fully considered in developing optimization model to achieve the ecological value of agricultural water savings. Then, multi-scale agricultural water-saving potentials were analyzed based on optimal results under different water-saving levels. These results provide local water managers with satisfactory economic benefit with higher water use efficiency. With reasonable management strategies of water and land resources, the ecosystem of Minqin County could gradually recover in the future. The results of the multi-scale water-saving potential analysis can help decision makers to identify desired water-saving plans that consider the coordinated development of the local economy, society, and ecology. Keywords: water and land optimal allocation; conjunctive use of surface water and groundwater; multi-scale agricultural water-saving potential analysis
1. Introduction Due to the rapid urbanization and industrialization in China, the increasing conflict between limited water resources and increased water demands calls for reasonable water allocation schemes. As the biggest consumer in most areas, especially in arid and semi-arid areas that are characterized by low rainfall and high evaporation, irrigation water plays an important role in agriculture production [1,2]. For example, in the arid regions of northwest China, irrigation water accounts for approximately 90% of the total water use [3], while the irrigation water use efficiency is far lower than the advanced regions [4]. That is, enormous potentials exist in the conservation of limited agricultural water resources. The accurate assessment of agricultural water-saving potential can not only help local sustainable development by affecting local food security, ecological security, and water security, but also provide specific water-saving direction to local decision makers. Minqin County, which is located downstream of the Shiyang River Basin, is a typical arid area suffering serious ecological deterioration due to the long-term exploitation of groundwater and unguided water management. It is of great significance to study the water-saving potential of Minqin County as a typical case. The agricultural water-saving potential was measured using various techniques, engineering projects, and water-saving management, and involved four scales, including crop, field, irrigation area, and regional/basin [5]. Many researchers have tried to measure the agricultural water-saving
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potential [6–9]. Different measures have been adopted, including efficient irrigation technology, deficit irrigation regulation, and irrigation scheduling improvement [10]. However, comprehensive analysis would be obtained by multiple scales of water-saving measures and considering multiple effects [11]. Although every method has its appropriate applications, according to its advantages and disadvantages, unreasonable results might be obtained when overlooking the important effects that are created by agricultural water and land allocation. Zhang and Guo [12] integrated water management optimization model and agricultural water-saving potential analysis methods to analyze local irrigation water-saving potential based on optimization results. However, how to combine multiple scales of water-saving measures with water and land resource optimal allocation has rarely been considered and needs to be addressed. Decreasing of available irrigation water and the increasing of water demand call for the reasonable allocation and effective management of agricultural water resources. Additionally, crop planting structure optimization, which allocates the optimum planting proportion to each crop to achieve the goals of increasing agricultural economic benefits and decreasing irrigation water use [13], is equally essential for sustainable agricultural development and to address serious water shortage problems. Therefore, irrigation water and planting structure optimization models were extensively employed to provide theoretical guidance for decision makers [14–16]. The interaction and exchange between surface and groundwater resources were considered throughout the conjunctive use of surface water and groundwater [17,18], in which the available groundwater depths were introduced [12,19]. In addition, some studies tried to combine various optimization models with a water-saving potential analysis, and thus contribute to the optimal management of water and land resources [12,20]. However, with more attention being paid to optimizing agricultural water and land resources separately via different optimization models, the interactions between them were not fully considered and few researchers realized the ecological value in agricultural water savings. Therefore, to deal with the multiple contradictions that are caused by natural conditions and unreasonable water allocation, which manifest in ecological harm and further affect agricultural water and land resource management, the objectives of this study are to (1) establish an integrated optimization model for water and land resource allocation based on the conjunctive use of surface water and groundwater; (2) introduce the groundwater equilibrium constraint to obtain several irrigation water schemes and planting areas under different water-saving levels; and, (3) conduct the analysis of multi-scale irrigation water-saving potential with water-saving measures. Finally, an optimization-based multi-scale research method for agricultural water-saving potential is developed. The method was applied to Minqin County, which is located downstream of the Shiyang River basin in northwest China, to demonstrate its effectiveness and practicality for agricultural water and land management and agricultural water-saving potential analysis. The results may support local decision makers formulate better allocation schemes, and the study methods should be helpful for managers to identify the desired water and land allocation plans in similar areas. 2. Study System 2.1. Study Area Minqin County is a typical arid region, located downstream of the Shiyang River basin, northwest China, within the east longitude 102◦ 520 –103◦ 500 and the north latitude 38◦ 220 –39◦ 60 . As the green barrier of ecological security in northwest China with the title of the desert oasis, Minqin County is surrounded by the Badain Jaran desert and Tengger desert (Figure 1), and is located in an arid climate zone, with an average annual precipitation and potential evaporation of about 110 mm and 2623 mm, respectively. Precipitation varies greatly among different seasons, with precipitation from July to September, accounting for nearly 66% of the whole year’s precipitation [2]. The average annual precipitation from 1985–2014 in Minqin County is shown in Figure 2, and the precipitation distribution in 2014 of Minqin County is expressed in Figure 3. Minqin County has three main irrigation areas:
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Hongyashan, Huanhe, and Changning, among which Hongyashan’s irrigation area occupies 92.8% of theoccupies whole irrigation area [21]. All of the surface water for Minqin Countyfor comes from 92.8% the whole Minqin irrigation area [21]. All ofsupply the surface water supply Minqin Water Minqin 2018, 10, xof FOR PEER REVIEW 3 of 19 theCounty Hongyashan and agriculture is the and highest water use sector. comes reservoir, from the Hongyashan reservoir, agriculture is the highest water use sector. occupies 92.8% of the whole Minqin irrigation area [21]. All of the surface water supply for Minqin County comes from the Hongyashan reservoir, and agriculture is the highest water use sector.
Figure 1. Sketch map of the Shiyang River basin in northwest China. Figure 1. Sketch map of of thethe Shiyang in northwest northwestChina. China. Figure 1. Sketch map ShiyangRiver Riverbasin basin in 250
150
Precipitation/mm
Precipitation/mm
200
250 200 150
100 100 50 50 0
1985 1986 1985 1986 1987 1987 1988 1988 1989 1989 1990 1990 1991 1991 1992 1992 1993 1993 1994 1994 1995 1995 1996 1996 1997 1997 1998 1998 1999 1999 2000 2000 2001 2001 2002 2002 2003 2003 2004 2004 2005 2005 2006 2006 2007 2007 2008 2008 2009 2009 2010 2010 2011 2011 2012 2012 2013 2013 2014 2014
0
Time/year Time/year
Figure 2. Precipitationfrom from1985–2014 1985–2014 in in Minqin County. Figure Precipitation Figure 2.2.Precipitation from 1985–2014 in Minqin MinqinCounty. County.
Because of economic developmentand andpopulation population growth growth in in the Because of economic development the upper upperand andmiddle middlereaches reachesofof Because of economic development and population growth in the upper and middle reaches of the the Shiyang River, the available surface water in Minqin County is rapidly decreasing. Meanwhile, aa the Shiyang River, the available surface water in Minqin County is rapidly decreasing. Meanwhile, more serious problem is the over-exploitation of the groundwater. In the pastdecreasing. few decades, the overShiyang River, the available surface water in Minqin County is rapidly Meanwhile, more serious problem is the over-exploitation of the groundwater. In the past few decades, the overexploitation of the groundwater has caused a series of ecological problems,Insuch the continuous a more seriousofproblem is the over-exploitation ofof the groundwater. theasas past decades, exploitation the groundwater has caused a series ecological problems, such the few continuous decline of the groundwater level (Figure 4), deterioration of the groundwater environment, and the thedecline over-exploitation of the groundwater has caused a series of ecological problems, such of the groundwater level (Figure 4), deterioration of the groundwater environment, as and further leads to serious desertification, increased salinity, and oasis shrinking [22]. To restore the continuous decline of the groundwater level (Figure salinity, 4), deterioration the groundwater further leads to serious desertification, increased and oasisofshrinking [22]. To environment, restore the ecological system of the Minqin oasis, local water managers limited the groundwater exploitation, andecological further leads to serious desertification, increased salinity, and oasis shrinking [22]. exploitation, To restore the system the Minqin oasis, local managers limited the groundwater and thus led to aofcorresponding reduction ofwater agricultural production. Although the government has ecological system of the Minqin oasis, local water managers limited the groundwater andtaken thus action led to by a corresponding reduction of agricultural production. Although the government has transferring water from other areas in recent years, the water availability for exploitation, the study action by transferring water from other areas in recent years, the water availability for the study andtaken thus led to a corresponding reduction of agricultural production. Although the government area is far less than the water requirement. Thence, both the water utilization efficiency and water has areaaction is farby less than the need water Thence, both the water utilization efficiencyfor andthe water taken transferring water from other areas in recent years, the water availability study production efficiency torequirement. be improved. production be improved. Thence, both the water utilization efficiency and water area is far lessefficiency than the need watertorequirement. production efficiency need to be improved.
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45 45 40 40 35 35 Precipitation/mm
Precipitation/mm
30 30 25 25 20 20 15 15 10 10 5
5
0
0 1
1
2
2
3
3
4
4
5
5
6 7 7 8 Time/month Time/month 6
8
9
9 10 10 11 11 12 12
Figure Precipitation distribution for2014 2014 inMinqin Minqin County. Figure 3. Precipitation distribution forfor 2014 in in Minqin County. Figure 3.3.Precipitation distribution County. 7
Groundwater avalibility/108m3
Groundwater avalibility/108m3
6
0
0
5
5
Groundwater Groundwater depthdepth
6
5
5
4
4
3
3
2
2
1
1
0
0 25 25 20002000 20012001 20022002 20032003 20042004 20052005 20062006 20072007 20082008 20092009 20102010 20112011 20122012 20132013 20142014 20152015 Time/year Time/year
10 10
15 15
Groundwater depth/m
Groundwater availability Groundwater availability
Groundwater depth/m
7
20 20
Figure Groundwater availability and groundwater depth from 2000–2015 inMinqin Minqin County. Figure 4. Groundwater availability and groundwater depth from 2000–2015 in in Minqin County. Figure 4.4.Groundwater availability and groundwater depth from 2000–2015 County.
economic development of Minqin County mainly depends on agricultural production, TheThe economic development of Minqin County mainly depends on agricultural production, andand The economic development of Minqin County mainly depends on agricultural production, the main crops in Minqin include spring wheat, maize, cotton, vegetables, melons, and oil crops. the main crops in Minqin include spring wheat, maize, cotton, vegetables, melons, and oil crops. TheThe and the main crops in Minqin include spring wheat, maize, cotton, vegetables, melons, and oil current irrigation quotas of the typical crops in Minqin County displayed in Figure 5. The current irrigation quotas of the typical crops in Minqin County are are displayed in Figure 5. The crops. The current irrigation quotas of the typical crops in Minqin County are displayed in Figure 5. statistical of Minqin County in recent years seen in Table 1, which shows statistical datadata of Minqin County in recent years cancan be be seen in Table 1, which shows thatthat the the The statistical data of Minqin County in recent years can be seen in Table 1, which shows that the proportion of economic crops increased gradually, cultivated areas of water-consuming proportion of economic crops hashas increased gradually, andand the the cultivated areas of water-consuming proportion of economic crops has increased gradually, and the cultivated areas of water-consuming crops increasing indicating fierce contradictions exists between crop planting crops are are increasing too,too, indicating thatthat fierce contradictions exists between the the crop planting crops are increasing too, indicating that fierce contradictions exists between the crop planting structure structure limited available water. contradiction a great challenge faced by local managers structure andand limited available water. ThisThis contradiction is aisgreat challenge faced by local managers and limited available water. This contradiction is a great challenge faced by local managers in practice. in practice. in practice. 2 ). Table 1. Summary of crop acreage from 2011–2015 in Minqin County (unit: hm Table 1. Summary of crop acreage from 2011–2015 in Minqin County (unit: Table 1. Summary of crop acreage from 2011–2015 in Minqin County (unit: hm2hm ). 2).
20112011 20122012 20132013 20142014 20152015
Spring Wheat Spring Wheat Spring Wheat 7400.04 7400.04 2011 7400.04 3853.35 2012 3853.35 3853.35 3353.35 2013 3353.35 3353.35 2014 4473.33 4473.33 4473.33 2015 4673.57 4673.57 4673.57
Maize Maize Maize 5386.69 5386.69 5386.69 9560.05 9560.05 9560.05 9373.38 9373.38 9373.38 9646.67 9646.67 9646.67 9967.17 9967.17 9967.17
Cotton Cotton Cotton 12,826.73 12,826.73 12,826.73 11,480.06 11,480.06 11,480.06 8880.04 8880.04 8880.04 7553.33 7553.33 7553.33 5000.25 5000.25 5000.25
Oil Seed Melon Oil Melon OilFlex FlexFlex Seed Seed Melon 7160.04 2106.68 7160.04 2106.68 7160.04 2106.68 7773.37 2173.34 7773.37 2173.34 7773.37 2173.34 9666.72 2633.35 9666.72 2633.35 9666.72 2633.35 10,666.67 10,666.67 2320.00 2320.00 10,666.67 2320.00 11,513.91 11,513.91 2473.46 2473.46 11,513.91 2473.46
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450 400
Irrigation quota/mm
350 300 250 200 150 100 50 0 Spring wheat
Seed melon
Cotton Crops
Maize
Oil flax
Figure 5. Current irrigation quotas of typical crops in Minqin County. Figure 5. Current irrigation quotas of typical crops in Minqin County.
With the the improvement improvement of of agriculture agriculture water water management, management, great great water-saving water-saving potentials potentials exist exist in in With local irrigation irrigation water water allocation allocation system. system. Therefore, Therefore, itit is isof ofgreat greatsignificance significance to toanalyze analyzethe theagricultural agricultural local water-saving potentials potentials with with the the efficient efficient utilization utilization of of limited limited water water and and land land resources resources in in order order to to water-saving improve agricultural promote socioeconomic development, and helpand ecological improve agriculturalproduction, production, promote socioeconomic development, help restoration ecological in Minqin County. restoration in Minqin County. 2.2. Data Data Collection Collection 2.2. Natural environmental environmental data, data, socio-economic socio-economicinformation, information,and andtypical typicalcrops cropsdata data were were used used in in Natural this study. Meteorological data from 1985–2014 in the Minqin County were obtained from the China this study. Meteorological data from 1985–2014 in the Minqin County were obtained from the China Meteorological Data Data Sharing Sharing Service Service System; System; crops crops data data [23], [23], including including the the typical typical crop crop sensitivity sensitivity Meteorological index, crop growth stages division, crop coefficient, and maximum crop yield, were used in thisinstudy; index, crop growth stages division, crop coefficient, and maximum crop yield, were used this crop planting data and population were obtained from The Statistical Yearbook of Wuwei City; streamflow study; crop planting data and population were obtained from The Statistical Yearbook of Wuwei City; data, water supply data, supply and current waterirrigation distributions originated in theoriginated survey of Wuwei streamflow data, water data,irrigation and current water distributions in the City; the groundwater management objectives were available in The Shiyang River Basin Governance survey of Wuwei City; the groundwater management objectives were available in TheKey Shiyang River Projects; current irrigation were irrigation obtained from Thewere Wuwei Industry Water Quota; and finally, Basin Keythe Governance Projects; quotas the current quotas obtained from The Wuwei Industry the value range the parameters in Table 2 [24] were references of the coefficient including Water Quota; andoffinally, the valueexpressed range of the parameters expressed in Table 2 [24] were references the regional specific yield, the leakage coefficient of the canal system, the precipitation infiltration of the coefficient including the regional specific yield, the leakage coefficient of the canal system, the coefficient, and the field leakage coefficient. All were for the optimization precipitation infiltration coefficient, and the field used leakage coefficient. All model. were used for the
optimization model.
Table 2. The range of the main parameters.
Table 2. The range of the main parameters. Precipitation Canal Leakage Specific Yield Specific Precipitation Infiltration Leakage Infiltration Coefficient Canal Coefficient Yield Coefficient Coefficient Clay 0.02–0.05 0.08–0.15 0.08–0.15 Clay 0.02–0.05 0.08–0.15 0.08–0.15 Silt 0.05–0.10 0.12–0.18 0.12–0.18 Silt sand 0.05–0.10 0.12–0.18 0.12–0.18 Fine 0.06–0.20 0.15–0.22 0.15–0.22 Sand 0.12–0.25 0.18–0.25 Fine and sandgravel 0.06–0.20 0.15–0.22 0.15–0.22Sandstone 0.04–0.10 0.05–0.10 Sand and 0.12–0.25 0.18–0.25 Basalt 0.05–0.08 0.08–0.15 gravel Sedimentary 0.03–0.10 0.05–0.08 Sandstone rocks 0.04–0.10 0.05–0.10 Basalt 0.05–0.08 0.08–0.15 Sedimentary 0.03–0.10 0.05–0.08 rocks
Irrigation Supply Irrigation Supply Coefficient Coefficient 0.08–0.12 0.08–0.12 0.12–0.18 0.12–0.18 0.15–0.20 0.15–0.20 - - -
-
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3. Study Study Methods 3.1. System System Framework Framework 3.1. land resource resource This paper attempts to combine the optimization model for agricultural water and land allocation with agricultural water-saving potential. This This section section contains contains two two major components allocation (Figure 6): (1) integrated optimization model for agricultural water and land resource allocation and two parts is (2) multi-scale multi-scale agricultural agricultural water-saving water-savingpotentials. potentials.The Thejoint jointconnect connectpoint pointbetween betweenthese these two parts the total available water of Minqin County. is the total available water of Minqin County.
Allocation optimization model for agricultural water and soil resources Evapotranspiration constraints Food security constraints Groundwater equilibrium constraint
Groundwater Equilibrium Transformation Precipitation infiltration
Planting structure optimization
Field infiltration Well irrigation return Canal leakage
Conjunctive
Lateral recharge
allocation
Planting area constraints Water supply constraints
evapotranspirat ion Well water exploitation Lateral excretion
Underground aquifer Optimal solutions Study area and data Water Resources Overview
Agricultural water-saving potentials
Precipitation Crop scale
Field scale
Irrigation area scale
Area scale
Groundwater
Data preparation Crops
Decision alternatives
Surfacewater
Social economy
Nature
Figure 6. System framework. Figure 6. System framework.
The optimization model is developed to achieve the efficient utilization of water resources and The optimization developed achieve efficient utilization of water resources and optimally allocate themodel land isresources oftocrops viathe adjusting the crop planting acreage. The optimally allocate the land resources of crops via adjusting the crop planting acreage. The appropriate appropriate adjustments of agricultural water resources and cultivated areas are explored to support adjustments of agricultural water resources and cultivated areas are explored to support decision decision makers for better agricultural water management. makers for better agricultural water management. When considering the realities of the study area, analysis with the multi-scale water-saving When which considering theofrealities of the study area, analysis with the multi-scale water-saving potentials, consists the crop-scale, field-scale, irrigation area-scale, and region-scale, are potentials, The which consists ofpotentials the crop-scale, field-scale, irrigation area-scale, and region-scale, conducted. water-saving of different scales are calculated by corresponding waterare conducted. water-saving potentials of different scales aare calculated by corresponding saving measures. The By comparing the results among different scales, relatively reliable analysis could water-saving measures. By comparing the results among different scales, a relatively reliable analysis be obtained to help local managers to alleviate agricultural water shortage. could be obtained to help local managers to alleviate agricultural water shortage. 3.2. Groundwater Equilibrium Analysis 3.2. Groundwater Equilibrium Analysis Irrigated groundwater balance refers to the relationships between the total groundwater Irrigated groundwater balance refers to the relationships between the total groundwater recharge, recharge, the total discharge, and the storage during a period of time. That is, the difference between the total discharge, and the storage during a period of time. That is, the difference between the the groundwater recharge and the discharge in a certain period is equal to the change of the groundwater recharge and the discharge in a certain period is equal to the change of the underground underground reservoir inventory, which can be expressed as follows [25]: reservoir inventory, which can be expressed as follows [25]:
ΔQ = Qs − Qe
∆Q = Qs − Qe
(1) (1)
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where ∆Q is the change in the amount of groundwater during the certain period; Qs is the total amount of groundwater recharge during the certain period; and, Qe is the total amount of groundwater discharge during the certain period. The main sources of recharge in the study area include precipitation infiltration, field infiltration, canal leakage, well irrigation seepage feedback, and lateral recharge. The main discharges include well water exploitation, evapotranspiration, and lateral discharge. The change of the groundwater level indicates the amount of variation in the stored groundwater. The amount of lateral supply and lateral discharge is so small as to be ignored. The limit depth of groundwater evaporation is 5 m [26]. Minqin’s groundwater depth is far more than 5 m, so the evapotranspiration is assumed to be zero in this study. Therefore, the groundwater equilibrium function can be expressed, as follows: µA∆H = Q p + Qc + Q f + Qw − Qo
(2)
where the µ is the regional specific yield; A (hm2 ) is the area; ∆H (m) is the increase of the groundwater level during the certain period; Qp (104 m3 ) is the precipitation infiltration during the certain period; Qc (104 m3 ) is the canal leakage during the certain period; Qf (104 m3 ) is the field infiltration during the certain period; Qw (104 m3 ) is well irrigation seepage feedback during the certain period; and, Qo (104 m3 ) is the amount of well water exploitation during the certain period. 3.3. Integrated Optimization Model for Water and Land Resource Allocation When considering the transformation of surface water resource, the Jensen model (which Jensen M.E. first proposed in 1968) is introduced in this study to optimize the farmers’ net economic benefit with an objective formulation. The decision variables are the allocation of the surface water and groundwater for each crop in each period, and the crops’ cultivated areas. The objective function is expressed as: m
n
max = ∑ Ai Bi Ymi ∏ i =1
j =1
ij + EPij + GWij
SW
ETmaxij
λij
m
T
i =1
t =1
− C1 ∑ Ai ∑
SWit η1
m
T
i =1
t =1
− C2 ∑ Ai ∑
GWit η2
m
− ∑ A i Di
(3)
i =1
T
SWij =
∑ qijt SWit
(4)
t =1 T
GWij =
∑ qijt GWit
(5)
t =1
where i is the crop type; j is the number of crop growth stage; t is the index of time periods; Ai is the crop area of crop i (hm2 ); Bi is the price of the crop per unit (CNY/kg); Ymi is the maximum yield of crop i per unit under full irrigation (kg/hm2 ); SWij is the irrigated surface water of crop i during stage j (mm); GWij is the irrigated groundwater of crop i during stage j (mm); EPij is the effective precipitation of crop i during stage j (mm); ETmaxij is the maximum evapotranspiration of crop i during stage j (mm); λij is the water sensitivity index of crop i within stage j; SWit is the irrigated water for surface water of crop i during period t (mm); GWit is the irrigated water from the groundwater of crop i during period t (mm); C1 is the price of surface water per unit (CNY/m3 ); C2 is the price of groundwater per unit (CNY/m3 ); η1 is the utilization coefficient of surface water; η2 is the utilization coefficient of groundwater; Di is the planting cost of crop i (CNY); and, qijt is the proportion of crop i during stage j in the period t. There are some constraints in this model, as described below. 3.3.1. Groundwater Equilibrium Constraint In order to prevent land salinization and desertification, respectively, caused by a high groundwater level and low groundwater level, the groundwater equilibrium constraint was introduced into the model. The available upper and lower bounds of the groundwater depth were used to control
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the groundwater level in a reasonable range in order to protect the groundwater resources. The specific constraint is expressed as follows:
Ht = Ht−1 −
αPt − µ
h m SW SW ∑ Ai β η1it + β0 η 0 it + β00 GWit −
i =1
GWit λη2
i
+ EGt
µA Hmin ≤ Ht ≤ Hmax
(6) (7)
where Ht is the groundwater level depth in the period t (mm); Ht−1 is the groundwater level depth in the period t − 1 (mm); Pt is the effective precipitation in the period t (mm); α is the precipitation leakage coefficient; β is the canal leakage coefficient; β0 is the irrigation return flow coefficient; η 0 is the field water use coefficient; λ is the ratio of agricultural water consumption to the total amount of well water exploitation; β00 is the well regression coefficient; EGt is the groundwater evaporation in the period t (m3 ); Hmin is the groundwater level upper limit (m); and, Hmax is the groundwater level lower limit (m). 3.3.2. Water Supply Constraints The amount of surface water and groundwater calculated in the model cannot exceed the total available agricultural water supply, thus the following constraints are proposed. Surface water supply constraint m
∑
i =1
T
Ai ∑ SWit ≤ Qη1
(8)
t =1
Groundwater availability constraint m
T
i =1
t =1
∑ Ai ∑ GWit ≤ Gη2
(9)
where Q (104 m3 ) is the surface water supply and G (104 m3 ) is the groundwater availability. 3.3.3. Evapotranspiration Constraint In the model, the sum of precipitation and irrigation water during each growth stage should be less than the maximum water requirement, and satisfy the minimum evapotranspiration of crops during the corresponding growth stage. Therefore, the constraint is formed, as follows: ETminij ≤ SWij + EPij + GWij ≤ ETmaxij
(10)
where ETminij is the minimum evapotranspiration of crop i during stage j (mm). 3.3.4. Food Security Constraint In Minqin County, the main grain crops are spring wheat and maize. The sum of the two outputs should meet the minimum food requirements of the total population (420 kg/person) [20]. This constraint can be expressed, as follows: k
n
i =1
j =1
∑ Ai Ymi ∏
SWij + EPij + GWij ETmaxij
!λij
≥Y×S
where Y is the per capita minimum food weight (kg/person) and S is the local population.
(11)
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3.3.5. Planting Area Constraints In the process of optimization, the total acreage of the crops should less than the maximum area. The maximum and minimum planting area constraints are formed to avoid the extreme cultivation adjustment of certain crops. Constraints are expressed, as follows m
∑ Ai ≤ Amax
(12)
Ai ≤ Ai,max
(13)
Ai ≥ Ai,min
(14)
i =1
where Amax is the total crop acreage (hm2 ); Ai,max is the maximum acreage of the crop i (hm2 ); and, Ai,min is the minimum acreage of the crop i (hm2 ). 3.3.6. Non-Negative Constraints The total amount of irrigation water in each crop growth period must be greater than zero, and the expression of these constraints are as follows: SWit ≥ 0, GWit ≥ 0
(15)
Spring wheat, maize, cotton, seed melon, and oil flax are selected for model optimization in this study. The number of crop growth stages is five, and the number of time periods is 12. The average prices of spring wheat, seed melon, cotton, maize, and oil flax are 2.5 CNY/kg, 1.6 CNY/kg, 14.5 CNY/kg, 1.6 CNY/kg, and 6.5 CNY/kg, respectively. The precipitation meteorological data multiplied by the effective utilization factor is calculated as the effective precipitation. The values of ET0 and ETmax are obtained from the FAO-56 (Food and Agriculture Organization) Penman-Monteith method. The available water supply is calculated when considering the proportion of planting and the irrigation water use coefficient based on the total agricultural irrigation water consumption in 2014. Other relevant parameters are shown in Tables 3–5. The optimization model is programmed by LINGO mathematical optimization software (LINDO Systems Inc., Chicago, IL, America), and the results are the optimization schemes of the comprehensive model of water and land resource allocation. Table 3. Typical crops’ water product function parameters. Growth Stage
1
2
3
4
5
Spring wheat
Date EP (mm) ETmax (mm) λ
3.21–4.29 5.55 72.69 −0.223
4.30–5.12 2.72 61.04 0.330
5.13–6.1 4.39 113.49 0.094
6.2–6.13 4.03 78.76 0.590
6.14–7.16 22.72 233.29 0.329
Seed melon
Date EP (mm) ETmax (mm) λ
5.1–6.20 12.79 97.14 0.108
6.21–7.5 8.67 40.02 0.062
7.6–7.31 27.64 115.60 0.256
8.1–8.20 18.52 42.77 0.200
8.21–9.17 14.63 50.35 −0.038
Cotton
Date EP (mm) ETmax (mm) λ
4.21–6.22 15.28 55.63 0.245
6.23–7.16 19.69 89.08 0.172
7.17–9.14 48.32 170.12 0.469
9.15–10.20 10.58 16.59 0.063
maize
Date EP (mm) ETmax (mm) λ
4.14–5.20 7.01 71.13 0.193
5.21–6.28 11.56 139.54 0.115
6.29–7.26 28.32 151.58 0.005
7.27–9.13 37.44 209.26 0.100
Oil flex
Date EP (mm) ETmax (mm) λ
4.17–6.9 11.64 176.99 0.172
6.10–6.18 3.02 49.06 0.084
6.19–7.5 9.34 95.16 0.102
7.6–8.27 52.65 196.99 0.011
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Table 4. Growth stage-time period transformation variable (qijt ). January
February
March
April
May
June
July
August
September
October
November
December
Spring wheat
1 2 3 4 5
0.00 0.00 0.00 0.00 0.00
0.00 0.00 0.00 0.00 0.00
0.35 0.00 0.00 0.00 0.00
0.97 0.03 0.00 0.00 0.00
0.00 0.39 0.61 0.00 0.00
0.00 0.00 0.03 0.40 0.57
0.00 0.00 0.00 0.00 0.52
0.00 0.00 0.00 0.00 0.00
0.00 0.00 0.00 0.00 0.00
0.00 0.00 0.00 0.00 0.00
0.00 0.00 0.00 0.00 0.00
0.00 0.00 0.00 0.00 0.00
maize
1 2 3 4 5
0.00 0.00 0.00 0.00 0.00
0.00 0.00 0.00 0.00 0.00
0.00 0.00 0.00 0.00 0.00
0.57 0.00 0.00 0.00 0.00
0.65 0.35 0.00 0.00 0.00
0.00 0.93 0.07 0.00 0.00
0.00 0.00 0.84 0.16 0.00
0.00 0.00 0.00 1.00 0.00
0.00 0.00 0.00 0.42 0.00
0.00 0.00 0.00 0.00 0.00
0.00 0.00 0.00 0.00 0.00
0.00 0.00 0.00 0.00 0.00
Cotton
1 2 3 4 5
0.00 0.00 0.00 0.00 0.00
0.00 0.00 0.00 0.00 0.00
0.00 0.00 0.00 0.00 0.00
0.33 0.00 0.00 0.00 0.00
1.00 0.00 0.00 0.00 0.00
0.73 0.27 0.00 0.00 0.00
0.00 0.52 0.48 0.00 0.00
0.00 0.00 1.00 0.00 0.00
0.00 0.00 0.47 0.53 0.00
0.00 0.00 0.00 0.65 0.00
0.00 0.00 0.00 0.00 0.00
0.00 0.00 0.00 0.00 0.00
Seed melon
1 2 3 4 5
0.00 0.00 0.00 0.00 0.00
0.00 0.00 0.00 0.00 0.00
0.00 0.00 0.00 0.00 0.00
0.00 0.00 0.00 0.00 0.00
1.00 0.00 0.00 0.00 0.00
0.67 0.33 0.00 0.00 0.00
0.00 0.16 0.84 0.00 0.00
0.00 0.00 0.00 0.65 0.35
0.00 0.00 0.00 0.00 0.57
0.00 0.00 0.00 0.00 0.00
0.00 0.00 0.00 0.00 0.00
0.00 0.00 0.00 0.00 0.00
Oil flex
1 2 3 4 5
0.00 0.00 0.00 0.00 0.00
0.00 0.00 0.00 0.00 0.00
0.00 0.00 0.00 0.00 0.00
0.47 0.00 0.00 0.00 0.00
1.00 0.00 0.00 0.00 0.00
0.30 0.30 0.40 0.00 0.00
0.00 0.00 0.16 0.84 0.00
0.00 0.00 0.00 0.87 0.00
0.00 0.00 0.00 0.00 0.00
0.00 0.00 0.00 0.00 0.00
0.00 0.00 0.00 0.00 0.00
0.00 0.00 0.00 0.00 0.00
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Table 5. Variable declaration. Variable
Variable Meaning Description
Value
η1 η2 C1 C2 α β β0 β” η0 λ EGt µ A H1 Q G Y S
Surface water irrigation utilization coefficient Groundwater irrigation utilization coefficient Surface water price, CNY/m3 Groundwater price, CNY/m3 Precipitation leakage coefficient Canal leakage coefficient Irrigation backflow coefficient Well irrigation compensation coefficient Field water delivery efficiency Agricultural water consumption ratio Evapotranspiration, mm Specific yield Area, 106 hm2 Initial value of groundwater depth, m Surface water supply, 104 m3 Groundwater availability, 104 m3 Per capita minimum food weight, kg/person Population, 104
0.61 0.85 0.24 0.26 0.20 0.16 0.20 0.20 0.85 0.44 0.00 0.16 1.59 18.75 8511.39 2794.85 420.00 24.11
3.4. Multi-Scale Agricultural Water-Saving Potential Analysis Method The agricultural water-saving potentials mainly include four scales: crop, field, irrigation area, and region/basin. There are notable differences in the agricultural water-saving potential in each scale, and the implementation of water-saving measures in the corresponding scales inevitably affects the others [5]. The researches on the multi-scale agricultural water-saving potentials are as follows: 1.
2.
3.
4.
The crop physiological process is taken into account in the process of the crop-scale agricultural water-saving potential calculation. To obtain satisfactory crop yields, the irrigation mode of water deficit is applied to save irrigation water. Different degrees (5%, 10%, 15%, 20%, 25%, and 30%) of water deficit are divided and implementation proportions of 10%, 20%, and 40% are set up, from which the most advisable scheme can be picked out. The difference between the previous irrigation water value and the current irrigation water value is calculated accordingly as the crop-scale water-saving potential. The field-scale water-saving potentials are obtained by calculating the differences between the previous irrigation water consumption and the optimized irrigation water allocation with crop planting structure adjustments. As displayed in Figure 5, the irrigation water consumption of spring wheat and maize is larger than the other crops. Thus, reducing the planting area of these two crops and increasing the area of cotton and oil flax appropriately could save more irrigation water. However, to ensure regional food security, the area of spring wheat and maize cannot be reduced too much. The irrigation area-scale water-saving potentials are reflected by the leakage and the loss of water in the process of irrigation water delivery, which relate to engineering measures, including canal system seepage, pipeline water supply, sprinkler irrigation, and drip irrigation. Currently, in Minqin County, the surface water irrigation utilization coefficient, and the groundwater irrigation utilization coefficient reach 0.61 and 0.85, respectively, and could be further enhanced. Assuming that the head flow is constant, the irrigation water delivered to the field is increased, while the water utilization coefficient is improved. The increased amount of water is calculated as the water-saving potential in the irrigation area. The optimal combination of water-saving measures in crop, field, and irrigation areas is considered in the calculation of the region-scale water-saving potential. The detailed calculation steps (Figure 7) are as follows: (1) the optimization scheme of the integrated optimization
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Water 2018, 10, xfor FOR PEER REVIEW model agricultural water
12 of 19 and land resource allocation is used as the optimal combination of crop-scale and field-scale water-saving measures; (2) the agricultural net irrigation water scale and field-scale water-saving measures; (2) the agricultural netcoefficient irrigation water is calculated; is calculated; (3) water savings with different water utilization improvements are (3) water savings with different water utilization coefficient improvements are calculated with calculated with engineering water-saving measures applied; (4) the amount of gross irrigation engineering water-saving applied; (4) are thecompared amount to of the gross irrigation water is water is calculated; and, (5)measures the calculation results current irrigation water calculated; and, (5) the calculation results are compared to the current irrigation water consumption, and the reduction of the headwater diversion is calculated as the region-scale consumption, and the reduction of the headwater diversion is calculated as the region-scale agricultural water-saving potential. agricultural water-saving potential.
Integrated optimization model of agricultural water and land resource allocation Optimize net irrigation quota
Crop scale
Optimize the planting structure Net irrigation water demand
Field scale
Canal water use coefficient Field water use coefficient Current status of irrigation water
Gross irrigation water
Agricultural watersaving potential
Engineering measures Irrigation area scale
Region scale
Figure chart. Figure 7. 7. Region Region water-saving water-saving potential potential calculation calculation flow flow chart.
4. Results Analysis and Discussion 4. Results Analysis and Discussion 4.1. Water and and Land Land Resource Resource Allocation Allocation 4.1. Integrated Integrated Optimization Optimization Model Model for for Agricultural Agricultural Water The land resource allocation is used in The integrated integrated optimization optimizationmodel modelfor foragricultural agriculturalwater waterand and land resource allocation is used order to obtain the distribution of water and crop area during the crop growth period (Table 6, Figure in order to obtain the distribution of water and crop area during the crop growth period (Table 6, 8). As expressed in Table 6, the6, optimization results increase thethe water that Figure 8). As expressed in Table the optimization results increase water thatisisallocated allocatedto to maize maize because the maize has the highest water productivity among five crops. Simultaneously, the because the maize has the highest water productivity among five crops. Simultaneously, the irrigation irrigation water of other four crops is reduced when compared with the current scheme. After water of other four crops is reduced when compared with the current scheme. After optimization, 4 m3 irrigation water can be saved. As shown in Figure 8, excessive optimization, 8.66 × 10water 8.66 × 104 m3 irrigation can be saved. As shown in Figure 8, excessive groundwater exploitation groundwater exploitation in the period of water supply canconjunctive be effectively avoided by the in the peak period of water supplypeak can be effectively avoided by the allocation of surface conjunctive allocation of surface water and groundwater. Thence, the groundwater depth can water and groundwater. Thence, the groundwater depth can maintain a safe level with a slight degree maintain safe level with a slight degree lifting,ecological which contributes positively to regional ecological of lifting, awhich contributes positively to of regional restoration. The optimal total net benefit restoration. The optimal total net benefit is 0.4 billion CNY, and the net benefit per unit water is 3.3 CNY. is 0.4 billion CNY, and the net benefit per unit water is 3.3 CNY. When considering maximum economic When considering maximum economic benefit, land resources are allocated to the crops with benefit, land resources are allocated to the crops with high yield and low water demand to makehigh full yield and low water demand to make full use of limited resources. For local water managers, use of limited resources. For local water managers, adjusting the local crop irrigation quota and crop adjusting the local cropwater irrigation quota and cropincreases, acreage could promote water savings, production acreage could promote savings, production and expanded revenue. For local farmers, increases, and expanded revenue. For local farmers, reducing the cultivation of spring wheat and oil reducing the cultivation of spring wheat and oil flax, and replacing them with maize, cotton, and seed flax, and replacing themchoice with maize, cotton, and benefits. seed melon, may be a better choice and bring greater melon, may be a better and bring greater benefits.
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Table 6. Comparison of the indicators before and after optimization.
Table 6. Comparison of the indicators before and after optimization. Spring Wheat Seed Melon Cotton Spring Wheat Seed Melon Cotton Present 390 360 300 Irrigation (mm) Present 390 360 300 Optimization 366 237 238 Irrigation (mm) Optimization 366 237 238 Present 4473 2320 7553 2) Present 4473 2320 7553 Crop area (hm Optimization 3353 2633 8880 Crop area (hm2) Optimization 3353 2633 8880 Present 7321 2250 1476 2 Present 7321 2250 1476 Yield(kg/hm (kg/hm 2) ) Yield Optimization 7201 2809 1530 Optimization 7201 2809 1530 Present 2.08 0.61 0.44 Present 2.08 0.61 0.44 3) 3 ) Waterproductivity productivity(kg/m (kg/m Water Optimization 1.78 0.88 0.46 Optimization 1.78 0.88 0.46 3) 3 Water Waterbenefit benefit(CNY/m (CNY/m )
600
Present Present Optimization Optimization
5.00 5.00 4.27 4.27
4.86 4.86 7.04 7.04
6.41 6.41 6.69 6.69
Surface water
Groundwater
Precipitation
Measured groundwater depth
Maize Oil Flax Maize Oil Flax 420 300 420 300 473 286 473 286 9647 10,667 9647 10,667 9970 9823 9970 9823 10,143 2200 10,143 2200 14,614 3150 14,614 3150 2.41 0.69 2.41 0.69 2.62 0.87 2.62 0.87 5.56 4.52 5.56 4.52 6.03 5.65 6.03 5.65
17 17.5
Optimized groundwater depth
500
400
18.5
300
19 19.5
200
Groundwater depth/m
Water applied/mm
18
20 100
20.5
0
21 1
2
3
4
5
6 7 Time/month
8
9
10
11
12
Figure8.8.Water Waterdistribution distributionand andgroundwater groundwaterlevel levelchanges changesinineach eachmonth monthof of2014. 2014. Figure
4.2.Multi-Scale Multi-ScaleAgricultural AgriculturalWater-Saving Water-SavingPotential Potential 4.2. 4.2.1.Crop-Scale Crop-ScaleWater-Saving Water-SavingPotential Potential 4.2.1. TheJensen Jensenmodel modelwas was introduced to calculate yield of crops in each scenario 7). The introduced to calculate the the yield of crops in each scenario (Table(Table 7). The The highest yields minus the actual yields are regarded as the reduction of the crop yields. highest yields minus the actual yields are regarded as the reduction of the crop yields. The The relationships between the reduction of crop yields degree water deficitare areexpressed expressedinin relationships between the reduction of crop yields andand thethe degree of of water deficit Figure 9. As shown in Figure 9, 15% is a desirable degree of water deficit while considering boththe the Figure 9. As shown in Figure 9, 15% is a desirable degree of water deficit while considering both water-saving potential and crop yields. Based on the water-saving schemes, water-saving potentials water-saving potential and crop yields. Based on the water-saving schemes, water-saving potentials were obtained obtained by by reducing different implementation proportion of 10%, 20%, were reducing the theirrigated irrigatedwater waterbybyinin different implementation proportion of 10%, and 40% (Figure 10). In this study, 100% represents totally applying water-saving measures to save 20%, and 40% (Figure 10). In this study, 100% represents totally applying water-saving measures to water in the Although more water hashas been saved with 40% implementation, assurance save water in region. the region. Although more water been saved with 40% implementation, assurancein and high costcost jointly hinder its application. Thus, a scheme of 20% with a intechnology technology and high jointly hinder its application. Thus, a scheme of implementation 20% implementation 15% water deficit seems more recommendable for local managers, in which the water-saving potential with a 15% water deficit seems more recommendable for local managers, in which the water-saving 5 m3 . In addition, as shown in Figure 9, reducing the irrigation water of oil flax and reaches 9.68 × 109.68 potential reaches × 105 m3. In addition, as shown in Figure 9, reducing the irrigation water of oil seed melon firstly contribute to minimizing farmers’farmers’ losses. losses. flax and seed melon firstly contribute to minimizing
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Water PEER Water 2018, 2018, 10, 10, xx FOR FOR PEER7.REVIEW REVIEW Table Summary of typical crop yields at different levels of water deficit.
14 14 of of 19 19
Table of Table 7. 7. Summary Summary of of typical typical crop crop yields yields at at different different levels of water water deficit. deficit. 2) Yield (kg/hmlevels 2 Water Deficit Yield (kg/hm 2)) Spring Wheat Seed MelonYield (kg/hm Cotton Maize Oil Flax Water Water Deficit Deficit 5% 5% 10% 10% 15% 15% 20% 20% 25% 25% 30% 30%
5% 10% 15% 20% 25% 30%
Spring Spring Wheat Wheat 7158.1 7158.1 7158.1 6738.0 6738.0 6738.0 6320.6 6320.6 6320.6 5906.1 5906.1 5906.1 5494.7 5494.7 5494.7 5086.5 5086.5 5086.5
Seed Seed Melon Melon 2676.5 2676.5 2676.5 2592.7 2592.7 2592.7 2507.0 2507.0 2507.0 2419.3 2419.3 2419.3 2329.2 2329.2 2329.2 2236.6 2236.6 2236.6
Cotton Cotton
1457.3 1457.3 1457.3 1384.4 1384.4 1384.4 1311.3 1311.3 1311.3 1238.0 1238.0 1238.0 1164.4 1164.4 1164.4 1090.6 1090.6 1090.6
Maize Maize
14,045.5 14,045.5 14,045.5 13,319.8 13,319.8 13,319.8 12,593.3 12,593.3 12,593.3 11,866.0 11,866.0 11,866.0 11,137.9 11,137.9 11,137.9 10,408.9 10,408.9 10,408.9
Oil Oil Flax Flax
3322.13322.1 3322.1 3256.73256.7 3256.7 3188.93188.9 3188.9 3118.53118.5 3118.5 3045.3 3045.33045.3 2968.9 2968.92968.9
The The degree degree of of water water dificit dificit
Thereduction reductionproportion proportionof of crops' crops'yields yields The
0% 0%
5% 5%
10% 10%
15% 15%
20% 20%
25% 25%
30% 30%
5% 5% 10% 10% 15% 15% 20% 20% 25% 25% 30% 30%
Spring Spring wheat wheat
Seed Seed melon melon
35% 35%
Spring Spring maize maize
Oil Oil flax flax
Cotton Cotton
Figure The relations between the degree of water deficit. Figure the reductionof ofcrop cropyields yieldsand and the degree water deficit. Figure9.9. 9.The Therelations relationsbetween between the the reduction reduction of crop yields and the degree ofof water deficit. 250 250 193.5 193.5
Watersavings/10 savings/1044m m33 Water
200 200
150 150 96.8 96.8
100 100
50 50
00
48.4 48.4
10% 10%
20% 20% Implementation Implementation proportions proportions
40% 40%
Figure savings bydifferent differentimplementation implementation proportions. Figure 10. The water proportions. Figure10. 10.The The water water savings savings by by different implementation proportions.
4.2.2. Field-Scale Water-Saving Potential 4.2.2. Field-ScaleWater-Saving Water-SavingPotential Potential 4.2.2. Field-Scale this section, assumed that growth period is In thissection, section,itititisis isassumed assumed that that water water quantity quantity in the typical crop growth period is constant constant InIn this water quantityin inthe thetypical typicalcrop crop growth period is constant with the change of planting area. The saved water consumption was calculated, and the results were with the change of planting area. The saved water consumption was calculated, and the results were with the change of planting area. The saved water consumption was calculated, and the results were shown in Figure 11, where 100% represents that the planting structure is totally adjusted. Although shown Figure11, 11,where where100% 100%represents represents that that the Although shown ininFigure the planting plantingstructure structureisistotally totallyadjusted. adjusted. Although the the water water consumption consumption of of grain grain crops crops is is high, high, itit is is inadvisable inadvisable to to decrease decrease grain grain crops’ crops’ acreage acreage too too
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2018, 10, x FOR PEER of REVIEW 15 of 19 theWater water consumption grain crops is high, it is inadvisable to decrease grain crops’ acreage too much, due therestriction restrictionof ofregional regional food food security. the current proportion much, due totothe security. From Fromthe thedata dataininTable Table1,1, the current proportion much, due to the restriction of regional food security. From the data it in Table 1, the current proportion grain crops, including spring wheat and and maize is that 15% is ais a of of grain crops, including spring wheat maize is 41%. 41%.Therefore, Therefore, itcan canbebeconcluded concluded that 15% of grain crops, including spring wheat and maize is 41%. Therefore, it can be concluded that 15% 6 m 3. is a proper adjustment proportion, with the field-scale water-saving potential reaching 2.34 × 10 6 proper adjustment proportion, with the field-scale water-saving potential reaching 2.34 × 10 m3 . proper adjustment proportion, with the field-scale water-saving potential reaching 2.34 × 106 m3.
4 3 Water savings/10 4 m3 m Water savings/10
350 350 300 300 250 250 200 200 150 150 100 100 50 50 0 0
321.0 321.0 234.0 234.0 156.0 156.0 78.0 78.0
5% 5%
10% 15% 10% The degree of planting structure15% adjustments The degree of planting structure adjustments
20% 20%
Figure different planting plantingstructure structureadjustments. adjustments. Figure11. 11.Water Water savings savings of of different Figure 11. Water savings of different planting structure adjustments.
4.2.3. Irrigation 4.2.3. IrrigationArea-Scale Area-ScaleWater-Saving Water-Saving Potential Potential 4.2.3. Irrigation Area-Scale Water-Saving Potential From the differentindustries industriesinin2014, 2014,the the current surface irrigation From thedata dataofofwater waterconsumption consumption of different current surface irrigation 8 3 7 3 From the data water consumption of different industries in 2014, the current surface irrigation 8m 3of 7m 3. In water is 1.40 × 10 , and the gross ground irrigation water is 3.49 × 10 this study, the available water is 1.40 × 10 m , and the gross irrigation water is 3.49 × 10 m . In this study, the available 8 m3, and the 7 m3. In this study, the available water iswater 1.40 × is 10is1.74 ground irrigation water is 3.49 ×the 10total 33,, which irrigation water 1.74××10 1088 mgross isisthe calculation result ofofthe agricultural water supply irrigation which the calculation result total agricultural water supply 8 m3, which is the calculation result of the total agricultural water supply irrigation water 1.74 × 10proportion multiplied bythe theis planting (70%). plenty of leakage water couldcould be saved by multiplied by planting proportion (70%).Although Although plenty of leakage water be saved multiplied by the planting proportion (70%). Although plenty of leakage water couldimprove be savedthe by irrigation area-scale water-saving engineering measures, it is difficult to substantially by irrigation area-scale water-saving engineering measures, it is difficult to substantially improve irrigation area-scaleof water-saving engineering measures, itcoefficient. is difficultBased to substantially improve the useuse coefficient system and field water usewater on previous thewater water coefficientthe ofcanal the canal system and field use coefficient. Based researches on previous water use coefficient of the canal system and field water use coefficient. Based onwater previous researches [5], reasonable targets are set up, and the water-saving potentials under different use efficiency researches [5], reasonable targets are set up, and the water-saving potentials under different water [5], reasonable targets are set up, and the water-saving potentials under different water use levels are calculated (Figure 12). When the surface water utilization coefficient is 0.66 efficiency and the use efficiency levels are calculated (Figure 12). When the surface water utilization coefficient is 0.66 levels are calculated When the surface water utilization is 8.62 0.66×and 3. groundwater utilization(Figure is 0.90, 12). the water-saving potential of the irrigationcoefficient area-scale is 106 mthe andgroundwater the groundwater utilization is water-saving 0.90, the water-saving potential of the irrigation area-scale 6 m3. is utilization is 0.90, the potential of the irrigation area-scale is 8.62 × 10 Therefore, it is necessary to choose suitable regional development water-saving targets, when 8.62Therefore, × 106 m3 .itTherefore, it is to necessary to chooseregional suitable regional development water-saving targets, is necessary choose suitable water-saving targets, when implementing irrigation area-scale water-saving measuresdevelopment in irrigation areas. when implementing irrigation area-scale water-saving measures in irrigation areas. implementing irrigation area-scale water-saving measures in irrigation areas. groundwater groundwater
surface water surface water
4 3 Water savings 4 m3 m Water savings /10/10
1000.0 1000.0 900.0 900.0 800.0 800.0 700.0 700.0 600.0 600.0 500.0 500.0 400.0 400.0 300.0 300.0 200.0 200.0 100.0 100.0 0.0 0.0
0.01 0.02 0.03 0.04 0.05 0.01 The improvements 0.02 of irrigation 0.03water utilization 0.04coefficient 0.05 The improvements of irrigation water utilization coefficient Figure 12. Irrigation area-scale water-saving potentials of different levels. Figure Irrigationarea-scale area-scalewater-saving water-saving potentials potentials of Figure 12.12.Irrigation of different differentlevels. levels.
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4.2.4. Region-Scale Water-Saving Potential Water 2018, 10, x FOR PEER REVIEW
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From the results of the optimization model, it can be found that the net irrigation water requirement 7 m3 , of which Region-Scale Water-Saving Potential is 9.984.2.4. × 10 the surface water is 8.26 × 107 m3 , and the groundwater is 1.71 × 108 m3 . Based on From the optimization the water-saving under different the results ofresults, the optimization model, itpotentials can be found that the net improvement irrigation waterlevels 7 m3, and of water use efficiency 13). water As shown 13,the thegroundwater possible region-scale requirement is 9.98 ×are 107 calculated m3, of which(Figure the surface is 8.26 in × 10Figure is 1.71 × 108 m3. potential Based on the results, water-saving potentials under different improvement water-saving canoptimization theoretically reachthe 1.69 × 107 m3 . The region-scale water-saving potential is levels of any waterother use efficiency are calculated (Figure considering 13). As shownthe in Figure 13, the possible regiongreater than scales, with harmoniously regional ecological, social, and 7 m3. The region-scale water-saving scale water-saving potential can theoretically reach 1.69 × 10 economic stable development. Furthermore, it can be concluded that region-scale water-saving potential is greater than any other scales, with harmoniously considering the regional ecological, measures can effectively alleviate the contradiction between agricultural water supply and demand and social, and economic stable development. Furthermore, it can be concluded that region-scale waterimprove the irrigation water use efficiency. For local managers, it is meaningful to execute a suitable saving measures can effectively alleviate the contradiction between agricultural water supply and water-saving via thethe optimal combination water-saving measures in crop, and irrigation demand scheme and improve irrigation water use of efficiency. For local managers, it isfield, meaningful to areas.execute It is equally important to choose suitable development goals on of thewater-saving basis of local reality. in a suitable water-saving scheme via the optimal combination measures To sum up,and multi-scale water important conservation measures candevelopment save considerable irrigation crop, field, irrigationagricultural areas. It is equally to choose suitable goals on the waterbasis and ease thereality. regional water pressure. Through saving water intuitively, the water-saving measures of local Toscale sum have up, multi-scale water conservation canfield-scale save considerable at the crop a negative agricultural impact on certain crops’ yields,measures while the water-saving irrigation water and ease the regional water pressure. Through saving water intuitively, the watermeasures can increase crop yields for farmers with saving a large amount of irrigation water. Irrigation savingwater-saving measures at the crop scale impact on certainleakage. crops’ yields, while fieldarea-scale measures canhave saveaanegative large quantity of water Finally, thethe region-scale scale water-saving measures can increase crop yields for farmers with saving a large amount of water-saving measures embody the greatest advantage in saving water to ameliorate the status quo irrigation water. Irrigation area-scale water-saving measures can save a large quantity of water of water shortages. That is, a reasonable scheme can contribute to ecological restoration, effectively leakage. Finally, the region-scale water-saving measures embody the greatest advantage in saving alleviating theameliorate deterioration of the groundwater in Minqin County.toThus, water to the status quoecology of waterand shortages. That is, aenvironment reasonable scheme can contribute the preferred choice is to adopt the region-scale water-saving measures for local ecological environment ecological restoration, effectively alleviating the deterioration of the ecology and groundwater protection and normal agricultural production. Several recommendations are presented, as follows: environment in Minqin County. Thus, the preferred choice is to adopt the region-scale water-saving
2.
3.
measures for local ecological environment protection and normal agricultural production. Several
To ensure the reasonable allocation of limited water resources, an optimal amount of irrigation recommendations are presented, as follows: water for five crops, which not include fallow or non-growing-season periods, should be allocated, 1. To ensure the reasonable allocation of limited water resources, an optimal amount of irrigation with 366 mm for spring wheat, 237 mm for maize, 238 mm for cotton, 473 mm for seed melon, water for five crops, which not include fallow or non-growing-season periods, should be and 286 mm for oil flax. allocated, with 366 mm for spring wheat, 237 mm for maize, 238 mm for cotton, 473 mm for seed To achieve the286 efficient of limited land resources, moderate adjustments of crop melon, and mm forutilization oil flax. cultivated areasthe could be made, including a 25% reduction spring wheat, 13.5%of increase 2. To achieve efficient utilization of limited land resources,ofmoderate adjustments crop of seedcultivated melon, 17.5% cotton, 3.35% areduction of maize, and wheat, 7.9% increase of oil flax. areas increase could be of made, including 25% reduction of spring 13.5% increase of seed melon, 17.5% increase of cotton, 3.35% reduction of maize, and 7.9% increase of oil flax. To obtain a larger degree of water-saving, stronger financial support should be provided 3. To obtain a larger degree of water-saving, financial should be provided for for engineering water-saving renovation stronger measures fromsupport the government’s investment. engineering water-saving renovation measures from the government’s investment. The The comprehensive promotion of efficient water-saving irrigation technology should be the comprehensive promotion of efficient water-saving irrigation technology should be the development target for local agriculture water managers. development target for local agriculture water managers. 1800.0 1600.0 Water savings / 104 m3
1.
1400.0 1200.0 1000.0 800.0 600.0 400.0 200.0 0.0 0
0.01 0.02 0.03 0.04 0.05 The improvement of irrigation water utilization coefficient
0.06
Figure Region-scalewater-saving water-saving potentials levels. Figure 13.13. Region-scale potentialsofofdifferent different levels.
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Furthermore, the entire region was considered in the optimization, to deal with the unreasonable water allocation of the Minqin County. These efforts would support local managers to develop more rational water allocation schemes. However, water resource managers face complicated problems in the practice of optimization. For example, China’s agricultural land is decentralized. Farmers have the right to choose the growing crops based on individual interests, rather than overall interests, which makes the implementation of the optimization scheme difficult. These factors would be fully considered in future research. 5. Conclusions This study proposed an integrated optimization model for agricultural water and land resource allocation, and an optimization-based multi-scale water-saving potential analysis framework for support irrigation water management. It can offer alternative water-saving schemes in corresponding conditions for decision makers. The characteristics of this study are as follows: 1.
2.
3.
The integrated optimization model incorporated planting structure optimization and water resource allocation with the conjunctive use of surface water and groundwater, and it was applied to Minqin County in Gansu Province, China. The optimization results provide comprehensive and efficiency water-saving solutions with higher economic benefits. The groundwater equilibrium constraint was used in the model. Taking full advantage of the interaction between surface water and groundwater, this model keeps the groundwater environment a stable and safe condition with a lifting tendency of the groundwater depth. Through balancing agricultural water and ecological water, optimal solutions contribute to both irrigation water saving and ecological restoration. Four scales were chosen in order to analyze the agriculture water-saving potential via adopting water-saving measures, including cutting down the water supply, adjusting the planting structure, improving the utilization coefficient of irrigation water, and the optimal combination of above measures based on integrated optimization model. By analyzing the results of the four scales, it was found that region-scale water-saving scheme not only demonstrates a huge potential of water-saving and agricultural benefit, but also contribute to balancing agricultural prolificacy and ecological restoration.
In addition, the framework, the allocation optimization model and the analysis method in this study can also be applied to other similar regions to help better water-allocation scheme formulation. However, some limitations exist in the proposed method. For example, local policies, year-to-year variability in supply and demand, as well as precise relationships in model formulation could be taken into account in further research. Author Contributions: Conceptualization, Q.Y.; Methodology, Q.Y. and F.Z.; Software, Q.Y.; Writing-Original Draft Preparation, Q.Y.; Writing-Review & Editing, Q.Y., F.Z. and P.G.; Supervision, P.G.; Project Administration, P.G.; Funding Acquisition, P.G. Funding: This research was supported by the National Key R&D Program of China (No. 2017YFC0403201) and National Natural Science Foundation of China (No. 51621061). Acknowledgments: The authors are grateful to the editor and the anonymous reviewers for their insightful comments and suggestions. Conflicts of Interest: The authors declare no conflict of interest.
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