Quantitative Estimation of Soil-Ground Water Storage Utilization During the Crop Growing Season in Arid Regions with Shallow Water Table Depth

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Article Quantitative Estimation of Soil-Ground Water Storage Utilization during the Crop Growing Season in Arid Regions with Shallow Water Table Depth

Tianxing Zhao 1 , Yan Zhu 1,*, Jingwei Wu 1, Ming Ye 2, Wei Mao 1 and Jinzhong Yang 1

1 State Key Laboratory of Water Resources and Hydropower Engineering Science, Wuhan University, Wuhan 430072, China; [email protected] (T.Z.); [email protected] (J.W.); [email protected] (W.M.); [email protected] (J.Y.) 2 Department of Earth, Ocean, and Atmospheric Science, Florida State University, Tallahassee, FL 32306, USA; [email protected] * Correspondence: [email protected]; Tel.: +86-27-6877-5432

Received: 23 October 2020; Accepted: 27 November 2020; Published: 29 November 2020

Abstract: Water storage in unsaturated and saturated zones during the crop non-growing season is one of the important supplementary water resources to meet crop water requirements in arid areas with shallow water table depth. It is necessary to analyze utilization of the soil-ground water storage during the crop growing season and its attribution to irrigation during the non-growing season. To facilitate the analysis, a new method based on measurements of soil moisture content and water table depth is developed. The measurements used in this study include (1) 15-year data of soil moisture content within a depth of 1 m from the land surface and water table depth measured in Jiefangzha, including its four subareas and (2) 4-year data of the same kind in Yonglian, located in arid northern China. The soil-ground water storage utilization is calculated as the difference of water storage between the beginning and end of the crop growing season in the whole computational soil profile. The results of average soil-ground water storage utilization in Jiefangzha and its four subareas and Yonglian are 121 mm, 126 mm, 113 mm, 124 mm, 185 mm and 117 mm, and the corresponding average utilization efficiencies in the non-growing season are 32.2%, 32.5%, 31.5%, 31.6%, 57.3% and 47.6%, respectively. Further, the water table fluctuation method was used to estimate the variation in water storage. The coefficients of soil-ground water storage utilization, soil-ground water storage utilization below 1 m soil depth and ground water utilization are defined, and their average values are 0.271, 0.111 and 0.026 in Jiefangzha, respectively. Then, the contribution of soil-ground water storage utilization to actual evapotranspiration is evaluated, which are over 23.5% in Jiefangzha and Yonglian. These results indicate that the soil-ground water storage plays an important role in the ecological environment in arid areas with shallow water table depth.

Keywords: soil-ground water storage utilization; coeﬃcient of soil-ground water storage utilization; ground water utilization; shallow water table depth; non-growing season irrigation

1. Introduction Agricultural irrigation has consumed the largest amount of fresh water in most countries over recent decades due to the rapid increase in agricultural areas [1], which may aggravate water resources scarcity and lead to a challenge for socio-economic development [2], especially in arid regions [3]. Water consumed by plants is not only supplied by irrigation water, but also by water storage in the unsaturated and saturated zones in arid areas with shallow water table depth [4,5]. The irrigation or precipitation water during the non-growing season can be stored as soil water and ground water, which

Water 2020, 12, 3351; doi:10.3390/w12123351 www.mdpi.com/journal/water Water 2020, 12, 3351 2 of 19 can be utilized by crops during the crop growing season. Therefore, it is important to quantitatively analyze the utilization of the soil-ground water storage during the crop growing season. This paper focuses on quantitatively estimating soil-ground water storage usage during the crop growing season and analyzing its impact factors. The study site of this research is the Hetao Irrigation District located in the upper reaches of the Yellow River, which is one of the three largest irrigation districts in China. Autumn irrigation after crop harvest happens in late autumn to leach salt and to reserve soil water and ground water for the next crop growing season [6,7]. The autumn irrigation consumes approximately one-third of the annual amount of water diverted from the Yellow river [8]. There are plans to decrease the amount of autumn irrigation water, due to the shortage of surface water resources of the Yellow river [9]. However, considering that part of autumn irrigation water is used by crops during the crop growing season, decreasing autumn irrigation water may result in water shortage [10]. Thus, estimating the soil-ground water storage utilization in the autumn irrigation (non-growing season) is of significant importance for adjusting a suitable irrigation quota during the crop growing season and maintaining crop production in this area. What is more, analyzing the influence mechanism of the impact factors (including the autumn irrigation scheme and planting structure) on soil-ground water storage utilization is useful for water management in arid areas with shallow water table depth. Therefore, these impact factors should be discussed. Many authors pay attention to the ground water used by crops in areas with shallow water table depth [11–13]. Yang et al. [14] monitored a vegetated lysimeter during a growing period of winter wheat and found that ground water contributed 16.6% of the total evapotranspiration when water table depth ranged from 1.6 m to 2.4 m. Wu et al. [15] found that ground water contribution was about 50% of crop evapotranspiration using a soil water balance method. Luo and Sophocleous [16] compared the ground water evaporation under different water table depth scenarios with lysimeter experiments. In these works, the consumption of soil water was ignored, because the soil moisture contents were similar at the beginning and end of the crop growing season due to rich irrigation and precipitation or sufficient water supply from ground water. These conditions, however, do not exist when the water supply is insufficient during the crop growing season in arid areas with shallow water table depth. The study of Bandyopadhyay and Mallick [17] revealed that the soil water provided more water for crop growing than ground water did, 50.80 mm and 6.37 mm respectively, for the growing season of winter wheat in West Bengal, India, where the water table depth fluctuated between 0.8 m and 1.5 m. Luo et al. [18] reported that the soil water storage utilization in the upper 60 cm and the ground water evaporation were about 100 mm and 40 mm of winter wheat, respectively, in an irrigation district of the Yellow River Basin, where the water table depth ranged from 0.5 m to 3.0 m. Zhang and Wegehenkel [19] found that the capillary rise and the change of the soil water storage were 132.7 mm/year and 11.5 mm/year in an arable land in Müncheberg, Germany, where the water table depth was from 0.2 m to 3.0 m. Soppe and Ayars [20] found that the ground water use and the change of soil water storage were 170 mm and 43 mm by lysimeter experiment with a water table depth of 0.9 m. Hence, soil water and ground water are both important parts of crop water consumption and neither can be ignored in areas with shallow water table depth. Since soil water and ground water have an intensive hydraulic connection in agricultural areas with shallow water table depth [21], it would be highly laborious to measure the soil moisture content from top soil to ground water, which are necessary data for estimating the quantity of soil-ground water storage utilization [22,23]. Two methods are usually used to calculate the soil-ground water storage utilization in practice. The first is to treat the phreatic evaporation or the ground water utilization as the quantity of the soil-ground water storage used by crops, due to the lack of soil moisture content data. Qu et al. [24] calculated the phreatic evaporation by empirical formula considering the relationship between the phreatic evaporation and the water table depth. Yang et al. [25] and Yu et al. [26] estimated the ground water utilization using the water table fluctuation method by multiplying the specific yield and changes of water table depth during the crop growing season. This method is reasonable Water 2020, 12, 3351 3 of 19 when the soil moisture content has slight changes at the beginning and the end of the crop growing season. Otherwise, it may underestimate the quantity of water storage used by crops. The second method is to divide the soil profile into two zones at a certain depth. In the zone above the depth, the difference of the soil water storage at the beginning and end of the crop growing season is calculated by measured soil moisture data, since it is easy to be obtained. In the zone below the depth, the ground water utilization is obtained by using lysimeter experiments, tracer experiments, or numerical modelling approaches when there are no measured water table depth data [18,27–30], or by the water table fluctuation method where water table depth data are available [31,32]. However, the ground water utilization obtained by this experimental method can only represent a point or a small region [3], and the numerical modelling approaches have limitations because of the lack of data [33]. The crux of using the water table fluctuation method is to determine the specific yield, which is an important hydrogeological parameter connecting flow processes in the unsaturated and saturated zones [34]. This is defined as the volume of water that an aquifer releases from or takes into storage, per unit aquifer area, per unit change in water table depth [35]. Pumping tests are usually carried out to obtain the specific yield. However, the value obtained by pumping tests is usually lower than that obtained by water balance calculations [36], which is caused by the non-instantaneous drainage of pores in the unsaturated zone. Nwankwor et al. [37] found that the specific yield obtained by the type-curve method of Boulton was 0.07, which was significantly smaller than the value of 0.25 obtained by the volume-balance method for a shallow sandy aquifer. The specific yield estimated by Malama [38] was smaller than 0.10 using the pumping test data of the Neuman model [39] at the Boise Hydro-geophysical Research Site in Boise, Idaho (US), while the value was 0.23 when using an alternative linearization of water table kinematic condition theory with the same pumping test data at this site. It is usually arbitrary to determine the value of specific yield when calculating the storage variation of ground water. The values of 0.15, 0.07 and 0.046, were adopted by Gao et al. [40], Chen et al. [41] and Yang et al. [25], respectively, when using the water table fluctuation method for the same area, resulting in the large variation of calculated ground water utilization. The specific yield is critical to the calculation of soil-ground water storage utilization, while its determination is usually arbitrary, resulting in an inaccurate soil-ground water storage utilization. Therefore, it is important to determine the reasonable specific yield when using the water table fluctuation method. To obtain the soil-ground water storage utilization during the crop growing season, 15 years’ soil moisture content within a depth of 1 m, and the water table depth dataset for Jiefangzha and its four subareas and a four year dataset from Yonglian, both located in arid northern China, are used in this study. The impact factors of soil-ground water storage utilization are discussed for helping in water management strategy. Then, to help in using the water table fluctuation method accurately, coefficients of soil-ground water storage utilization, ground water utilization and soil-ground water storage utilization below 1 m soil depth are analyzed. Finally, the contributions of soil-ground water storage utilization, ground water utilization and soil-ground water storage utilization below 1 m soil depth to the actual evapotranspiration (ET) are evaluated.

2. Materials and Methods In this section, the observed data of the study areas used for calculating the soil-ground water storage utilization is presented in Section 2.1. The conception and calculation methods for soil-ground water storage utilization are then described in Section 2.2. The moving average method used for reducing random error and eliminating the noise of the soil-ground water storage utilization is shown in Section 2.3. To obtain the actual evapotranspiration for an analysis of the contribution of soil-ground water storage utilization to actual evapotranspiration, the water balance method is described in Section 2.4. Specifically, the soil-ground water storage utilization is calculated in Jiefangzha and Yonglian. The soil moisture content is extended to the whole computational soil profile by a linear interpolation method, and then the soil-ground water storage utilization is calculated directly by the difference Water 2020, 12, 3351 4 of 19 of water storage in the whole computational soil profile between the beginning and end of the crop growingWater 2020, season.12, 3351 Then the soil-ground water storage utilization is divided into soil water storage4 of 20 utilization and ground water utilization. The soil-ground water storage utilization below 1 m soil depth is also estimated since the maximum sampling depth is usually 1 1 m m in in many many agricultural agricultural areas. areas. Next, coecoefficientsfficients of soil-groundsoil‐ground water storage utilization, ground water utilization and soil-groundsoil‐ground water storage utilization below 1 m soil depth are analyzed. Finally, the contributions of soil-groundsoil‐ground water storagestorage utilization,utilization, ground ground water water utilization utilization and and soil-ground soil‐ground water water storage storage utilization utilization below below 1 m 1 soilm soil depth depth to theto the actual actual evapotranspiration evapotranspiration (ET) (ET) are are calculated. calculated.

2.1. Study Area and Data Description The Hetao Irrigation District is located in the west of the Inner Mongolia Autonomous Region, China, as as shown shown in in Figure Figure 1.1. The The average average annual annual precipitation precipitation and and evaporation evaporation (measured (measured by 20 by cm 20‐ cm-diameterdiameter evaporation evaporation pans) pans) are 152 are mm 152 mmand 2400 and 2400mm, mm,respectively. respectively. The evaporation The evaporation is obtained is obtained from fromthe China the China Meteorological Meteorological Data Data Service Service Center Center (http://data.cma.gov.cn). (http://data.cma.gov.cn). Agricultural Agricultural irrigationirrigation heavily reliesrelies onon surface surface water water resources resources diverted diverted from from the the Yellow Yellow River. River. The The water water table table depth depth of the of Hetaothe Hetao Irrigation Irrigation District District varies varies from from 0 m to 0 3.5 m mto over3.5 m space over and space time, and with time, an with average an average annual value annual of 2.03value m. of The 2.03 water m. The table water depth table increases depth increases during the during crop growingthe crop seasongrowing in season this area, in becausethis area, the because water consumptionthe water consumption is larger than is larger the irrigation than the and irrigation precipitation. and precipitation. It is assumed It that is assumed the study that area the does study not havearea does lateral not hydraulic have lateral connection hydraulic with connection surrounding with regions, surrounding due to regions, the gentle due land to surfacethe gentle and land the smallsurface variation and the ofsmall water variation table depth of water in space table [ 42depth], and in that space the [42], hydrological and that the processes hydrological (e.g., inﬁltration, processes evapotranspiration(e.g., infiltration, evapotranspiration and deep percolation) and mainly deep percolation) occur in the verticalmainly direction.occur in the There vertical is a freeze-thaw direction. periodThere is from a freeze early‐thaw December period to from late early April December in the Hetao to Irrigationlate April District.in the Hetao Irrigation District.

Figure 1. The location of the two study areas and the monitoring sites.

There are two experimental areas in thethe district,district, the JiefangzhaJiefangzha sub-districtsub‐district (JFZSD) and the Yonglian irrigationirrigation area (YIA). The Jiefangzha sub-districtsub‐district can be divided into four subareas, Wula, Wula, Yangjia, HuangjiHuangji andand Qinghui,Qinghui, by thethe trunktrunk diversiondiversion canals,canals, asas shownshown inin FigureFigure1 a.1a. TheThe areaarea ofof Jiefangzha is is 2287 2287 km km2.2 The. The major major soil soil textures textures in Jiefangzha in Jiefangzha sub‐ sub-districtdistrict are silt are loam silt loam and sandy and sandy loam loam[9], and [9], the and saturated the saturated moisture moisture contents contents are areclose close in inspace. space. Therefore, Therefore, the the soil soil moisture moisture content content is treated as the the same. same. The The saturated saturated soil soil moisture moisture content content is is 0.46 0.46 cm cm3/cm3/cm3 in3 inJiefangzha Jiefangzha as measured as measured by bythe the Water Water Resources Resources Research Research Institute Institute of Inner of Inner Mongolia. Mongolia. Wheat Wheat and andmaize maize are the are main the main crops, crops, and andtheir their growing growing seasons seasons are arefrom from late late March March to tolate late July July and and from from the the end end of of April April to to the the end of September, respectively. The number of ground water and soil moisture content observation sites in each studystudy area area are are shown shown in Tablein Table1. There 1. There are 57 are ground 57 ground water observationwater observation wells and wells 22 soil and moisture 22 soil contentmoisture observation content observation sites distributed sites acrossdistributed Jiefangzha, across as Jiefangzha, shown in Figure as shown1a. All in soil Figure moisture 1a. contentAll soil observationmoisture content sites are observation adjacent to sites the ground are adjacent water observationto the ground wells. water The observation major crops arewells. sown The after major the crops are sown after the freeze‐thaw period after the end of April in Hetao Irrigation District. Only wheat is sown at the end of March, and consumes little water, which can be ignored. So the end of April is chosen as the beginning of crop growing season in this study. The soil moisture content and water table depth at the end of the crop growing season are observed at the end of September. Five soil samples are collected at one monitoring site at the depths of 0~0.1 m, 0.1~0.2 m, 0.2~0.4 m, 0.4~0.7 Water 2020, 12, 3351 5 of 19 freeze-thaw period after the end of April in Hetao Irrigation District. Only wheat is sown at the end of March, and consumes little water, which can be ignored. So the end of April is chosen as the beginning of crop growing season in this study. The soil moisture content and water table depth at the end of the crop growing season are observed at the end of September. Five soil samples are collected at one monitoring site at the depths of 0~0.1 m, 0.1~0.2 m, 0.2~0.4 m, 0.4~0.7 m and 0.7~1.0 m. The average soil moisture content within 1 m soil depth of Jiefangzha and its four subareas at the beginning and end of the crop growing season from 2003 to 2017 are displayed in Figure2a,c. The average water table Water 2020, 12, 3351 5 of 20 depth of Jiefangzha and its four subareas at the beginning and end of the crop growing season from 2003 to 2017m are and displayed 0.7~1.0 m. The in average Figure soil2b,d. moisture The content water within diversion 1 m soil during depth of Jiefangzha the crop and growing its four season and subareas at the beginning and end of the crop growing season from 2003 to 2017 are displayed in non-growing season (which is also referred to as autumn irrigation) of Jiefangzha and its four subareas Figure 2a,c. The average water table depth of Jiefangzha and its four subareas at the beginning and from 2003 toend 2017 of the was crop provided growing season by thefrom Hetao 2003 to 2017 Administration are displayed in Figure as shown 2b,d. The in Figurewater diversion2e,g, respectively. Five weatherduring stations the crop are distributedgrowing season in and the non Hetao‐growing Irrigation season (which District is also from referred west to to as east. autumn There are small irrigation) of Jiefangzha and its four subareas from 2003 to 2017 was provided by the Hetao di erences of the meteorological data in the ﬁve weather stations when we analyze the data from 1961 ﬀ Administration as shown in Figure 2e,g, respectively. Five weather stations are distributed in the to 2013. TheHetao previous Irrigation study District also from showed west to east. that There the are representative small differences distance of the meteorological of one weather data in station can be longer thanthe 44five km weather in Hetao stations Irrigation when we analyze District the data [43]. from Therefore, 1961 to 2013. one The weather previous stationstudy also can represent showed that the representative distance of one weather station can be longer2 than 44 km in Hetao the meteorologicalIrrigation condition District [43]. Therefore, of the whole one weather area station in Jiefangzha can represent (2287 the meteorological km ). The condition precipitation of data for Jiefangzha duringthe whole the area crop in Jiefangzha growing (2287 season km2). and The non-growing precipitation data season for Jiefangzha comes during from the the crop Hangjinghouqi meteorologicalgrowing station season (shown and non in‐growing Figure season1b), which comes canfrom bethe foundHangjinghouqi in Figure meteorological2f,h, respectively. station (shown in Figure 1b), which can be found in Figure 2f,h, respectively.

Figure 2. The measurements in Jiefangzha and its four subareas, and Yonglian. Measurements of (a) Figure 2. The measurements in Jiefangzha and its four subareas, and Yonglian. Measurements of (a) soil moisture content within 1 m soil depth and (b) water table depth at the beginning of the crop soil moisturegrowing content season. within Measurements 1 m soil of depth(c) soil moisture and (b content) water within table 1 m depthsoil depth at and the (d) beginning water table of the crop growing season.depth Measurementsat the end of the crop of growing (c) soil season. moisture (e) The contentwater diversion within and 1 (f m) precipitation soil depth during and the (d ) water table depth at the end of the crop growing season. (e) The water diversion and (f) precipitation during the crop growing season. (g) The water diversion (autumn irrigation) and (h) precipitation during the non-growing season. Water 2020, 12, 3351 6 of 19

Table 1. The number of ground water and soil moisture content observation sites in each study area.

Annual Average Number of Soil Number of Water Irrigation Study Area Water Table Moisture Content Table Depth Area (km2) Depth (m) Monitoring Sites Monitoring Wells Jiefangzha 1243 1.634 22 57 Wula 195 1.392 5 10 Yangjia 430 1.451 6 17 Huangji 497 1.787 9 23 Qinghui 121 2.042 2 7 Yonglian 20 1.461 10 10

The location of Yonglian irrigation area can be found in Figure1c. The area of Yonglian is 29.7 km2. The major crop is sunﬂower, the growing season of which is from the end of May to late September. There are 10 soil moisture content observation sites in Yonglian, which are adjacent to the 10 ground water observation wells, as shown in Figure1c. In Yonglian, there are only four years’ dataset (2005, 2006, 2007 and 2009) available for calculation. The soil moisture content is measured at the depths of 0~0.1 m, 0.1~0.3 m, 0.3~0.5 m, 0.5~0.7 m and 0.7~1.0 m. The average soil moisture contents within 1 m soil depth of Yonglian at the beginning and end of the crop growing season are displayed in Figure2a,c. The average water table depths at Yonglian at the beginning and end of the crop growing season are shown in Figure2b,d. The water diversion during the crop growing season and non-growing season was provided by the Hetao Administration as shown in Figure2e,g, respectively. The precipitation data during the crop growing season and non-growing season comes from the Yonglian experiment station and the Wuyuan meteorological station (shown in Figure1b), which can be found in Figure2f,h, respectively.

2.2. Calculation of Soil-Ground Water Storage Utilization Figure3a shows the typical soil moisture profile at the beginning and end of crop growing season in the study area. The soil moisture profile at the beginning of the crop growing season is higher due to the water storage caused by the autumn irrigation. The water storage is consumed by transpiration and evaporation during the crop growing season, which leads to a lower soil moisture profile after harvest. The difference in water storage between the beginning and end of the crop growing season is regarded as the soil-ground water storage utilization during the crop growing season, which can be calculated by the following equation, ∆W = W W , (1) 1 − 2 where ∆W is the soil-ground water storage utilization, mm; and W1 and W2 are the water storages at the beginning and end of the crop growing season, mm. Water 2020, 12, 3351 7 of 19 Water 2020, 12, 3351 7 of 20

Figure 3. The schematic diagrams of ( a) the the soil moisture profiles proﬁles at the beginning and end of the crop growing season,season, ( b()b the) the three three zones zones of the of verticalthe vertical soil proﬁle soil profile for calculating for calculating the water the storage water (marked storage as(markedW), (c) as the W soil), (c water) the soil storage water utilization storage utilization (marked as(marked∆Ws) and as Δ theWs) ground and the water ground utilization water utilization (marked as(marked∆Wg) andas Δ (Wd)g) the and soil-ground (d) the soil water‐ground storage water utilization storage utilization below 1 m below soil depth 1 m soil (marked depth as (marked∆W1) and as aboveΔW1) and 1 m soilabove depth 1 m (marked soil depth as ∆ W(marked01). Note: as Δ∆WW01is). the Note: soil-ground ΔW is the water soil storage‐ground utilization, water storage mm. Wutilization,2 is the water mm. storages W2 is the at water end of storages the crop at growing end of the season. crop wtdgrowingis the season. water table wtd is depth, the water mm. tablewtd1 anddepth,wtd mm.2 are wtd the1 waterand wtd table2 are depths the water at the table beginning depths and at the end beginning of the crop and growing end of season, the crop mm. growing∆h is theseason, variation mm. Δ ofh water is the table variation depth of during water thetable crop depth growing during season, the crop mm. growingZs is the season, maximum mm. sampling Zs is the

depth,maximum mm. samplingZb is the depth, maximum mm. computational Zb is the maximum depth, computational mm. depth, mm.

The per unit area water storagestorage WW of the vertical soilsoil proﬁleprofile atat aa speciﬁcspecific timetime isis calculatedcalculated as,as,

Z ZZ mm bb X Wdzd, W = 0 θdz = iiθi di,(2) (2) i1 × 0 i=1 where Zb is the maximum computational soil profile depth, mm; θ is the soil moisture content at the where Zb is the maximum3 −3 computational soil profile depth, mm; 3θ is−3 the soil moisture content at the soil depth z, cm3 cm 3; θi is the soil moisture content of layer i, cm3 cm 3; di is the thickness of the layer soil depth z, cm cm− ; θi is the soil moisture content of layer i, cm cm− ; di is the thickness of the layer i, mm; and m is the number of calculation layers. The Zb value is set as 3.5 m in this study since the i, mm; and m is the number of calculation layers. The Z value is set as 3.5 m in this study since the maximum water table depth in Jiefangzha and Yonglian bduring the crop growing season is shallower maximum water table depth in Jiefangzha and Yonglian during the crop growing season is shallower than 3.5 m. The θi value is the average soil water content of the soil moisture content monitoring sites than 3.5 m. The θ value is the average soil water content of the soil moisture content monitoring sites in the study area.i in the study area. The vertical soil profile, as shown in Figure 3b, is divided into three zones from top to bottom The vertical soil profile, as shown in Figure3b, is divided into three zones from top to bottom for for calculating the water storage W. In the first zone, the calculation uses measured soil moisture calculating the water storage W. In the first zone, the calculation uses measured soil moisture content content data, the second zone uses soil water content data obtained by a linear interpolation method, data, the second zone uses soil water content data obtained by a linear interpolation method, and and the third zone below the water table depth uses the saturated soil moisture content. The soil‐ the third zone below the water table depth uses the saturated soil moisture content. The soil-ground ground water storage utilization (marked as ΔW) can be divided into two parts by the initial water water storage utilization (marked as ∆W) can be divided into two parts by the initial water table depth, table depth, which are the soil water storage utilization marked as ΔWs and the ground water which are the soil water storage utilization marked as ∆Ws and the ground water utilization ∆Wg as utilization ΔWg as shown in Figure 3c. Besides, the soil‐ground water storage utilization below 1 m shown in Figure3c. Besides, the soil-ground water storage utilization below 1 m soil depth (marked as soil depth (marked as ΔW1) is estimated as shown in Figure 3d, since the maximum sampling depth ∆W1) is estimated as shown in Figure3d, since the maximum sampling depth is usually 1 m in many is usually 1 m in many agricultural areas and researchers usually consider ΔW1 as the ground water agricultural areas and researchers usually consider ∆W as the ground water utilization [31]. To help utilization [31]. To help in using the water table fluctuation1 method accurately, three coefficients are in using the water table fluctuation method accurately, three coefficients are defined as follows for defined as follows for estimating the soil‐ground water storage utilization, the ground water estimating the soil-ground water storage utilization, the ground water utilization and the soil-ground utilization and the soil‐ground water storage utilization below 1 m soil depth: water storage utilization below 1 m soil depth: W =,∆W (3) η = h , (3) ∆h

Wg g  ∆Wg, (4) ηg = h , (4) ∆h W   1 , (5) 1 h Water 2020, 12, 3351 8 of 19

∆W η = 1 , (5) 1 ∆h where η is the coefficient of soil-ground water storage utilization, ηg is the coefficient of ground water utilization, which is used to quantify the variation of water storage in the ground water fluctuation zone with the water table depth during the crop growing season, η1 is the coefficient of soil-ground water storage utilization below 1 m soil depth, and ∆h is the increase of water table depth during the crop growing season, mm. Equations (3)–(5) are applied to arid and semi-arid regions where the water table depth at the end of the crop growing season is larger than that at the beginning of the crop growing season. Since the soil and ground water storage at the beginning of the crop growing season is mainly from the autumn irrigation in the study area, a coefficient referred to as the utilization efficiency of autumn irrigation (λ) is proposed to quantify the contribution of the autumn irrigation to the water consumption of crops in the next year, ∆W λ = , (6) Qa where Qa is the amount of autumn irrigation, mm.

2.3. Moving Average Method The moving average method is used to reduce random errors and eliminate the noise of temporal soil-ground water storage utilization [44]. For time series data x, the moving average value is calculated as follows [45], j+k 1 X f = x , (K = 2k + 1, j = k + 1, k + 2, , N k) (7) j K i ··· − i=j k − where fj is the moving average soil-ground water storage utilization, mm; K is the length of memory window, which is expressed as K = 2k + 1 and k is an integer. In this study, K is adopted as 5 years. xi is the original time series of soil-ground water storage utilization, mm, and N is the length/number of the time series x.

2.4. Water Balance Analysis to Estimate the Actual Evapotranspiration The water balance method is used to estimate the actual evapotranspiration during the crop growing season as follows [46], ET = ∆W + Is + Ps, (8) where ET is the actual evapotranspiration in the ﬁeld during the crop growing season, mm; Is is the amount of ﬁeld irrigation water during the crop growing season, mm; and Ps is the precipitation during the crop growing season, mm. The contributions of soil-ground water storage utilization, soil-ground water storage utilization below 1 m soil depth and ground water utilization to the actual evapotranspiration (marked as RW, RW1 and RWg) can be calculated as, ∆W RW = , (9) ET ∆W RW = 1 , (10) 1 ET

∆Wg RW = , (11) g ET

These contributions (RW, RW1 and RWg) can quantitatively distinguish the contribution of the soil-ground water storage utilization within diﬀerent depths to the actual evapotranspiration. The soil-ground water storage utilization, soil-ground water storage utilization below 1 m soil depth and ground water utilization play diﬀerent roles in actual evapotranspiration. So it is helpful to calculate WaterWater2020 2020, ,12 12,, 3351 3351 99 of of 19 20

different contributions for determining the contribution of every example of water storage utilization ditoff theerent actual contributions evapotranspiration. for determining the contribution of every example of water storage utilization to the actual evapotranspiration. 2.5. Statistical Indicators 2.5. Statistical Indicators Classical statistical indicators, including average, maximum, minimum, standard deviation and Classical statistical indicators, including average, maximum, minimum, standard deviation and coefficient of variation, are used to characterize the features of soil‐ground water storage utilization coefficient of variation, are used to characterize the features of soil-ground water storage utilization and other variables. The coefficient of variation (Cv) is used to explain the extent of the variability of and other variables. The coefficient of variation (Cv) is used to explain the extent of the variability of variables, which can be calculated by [47], variables, which can be calculated by [47],

Sx Cv Sx , (12) Cv = X , (12) X where Sx is the standard deviation of the dataset, and 𝑋 is the average value of the dataset. where Sx is the standard deviation of the dataset, and X is the average value of the dataset. 3. Results and Discussion 3. Results and Discussion 3.1. Results of the Soil‐Ground Water Storage Utilization 3.1. Results of the Soil-Ground Water Storage Utilization The soil‐ground water storage utilizations at Jiefangzha and its four subareas (2003~2017) and The soil-ground water storage utilizations at Jiefangzha and its four subareas (2003~2017) and Yonglian (2005, 2006, 2007 and 2009) during the crop growing season are shown in Figure 4a,b, and Yonglian (2005, 2006, 2007 and 2009) during the crop growing season are shown in Figure4a,b, and the the statistical results are listed in Table 2. The average soil‐ground water storage utilization of statistical results are listed in Table2. The average soil-ground water storage utilization of Jiefangzha Jiefangzha in 2003~2017 is 121 mm, varying from 57 mm to 229 mm and the standard deviation is 49 in 2003~2017 is 121 mm, varying from 57 mm to 229 mm and the standard deviation is 49 mm. The mm. The average soil‐ground water storage utilizations in Jiefangzha, Wula, Yangjia, Huangji, average soil-ground water storage utilizations in Jiefangzha, Wula, Yangjia, Huangji, Qinghui and Qinghui and Yonglian are 121 mm, 126 mm, 113 mm, 124 mm, 185 mm, and 117 mm, respectively. Yonglian are 121 mm, 126 mm, 113 mm, 124 mm, 185 mm, and 117 mm, respectively. The Qinghui The Qinghui subarea has the largest average soil‐ground water storage utilization (185 mm), standard subarea has the largest average soil-ground water storage utilization (185 mm), standard deviation deviation (86 mm) and variation range (307 mm). One reason is that the Qinghui subarea obtained (86 mm) and variation range (307 mm). One reason is that the Qinghui subarea obtained smaller smaller water diversion and precipitation during the crop growing season, the average value of water diversion and precipitation during the crop growing season, the average value of which is 357 which is 357 mm from 2003 to 2017, smaller than those in Jiefangzha (381 mm) and other subareas. mm from 2003 to 2017, smaller than those in Jiefangzha (381 mm) and other subareas. The average The average utilization efficiencies of autumn irrigation (λ) in Jiefangzha, Wula, Yangjia, Huangji, utilization eﬃciencies of autumn irrigation (λ) in Jiefangzha, Wula, Yangjia, Huangji, Qinghui and Qinghui and Yonglian are 32.2%, 32.5%, 31.5%, 31.6%, 57.3% and 47.6% (shown in Table 2), Yonglian are 32.2%, 32.5%, 31.5%, 31.6%, 57.3% and 47.6% (shown in Table2), respectively. respectively.

FigureFigure 4. 4. ((aa)) TheThe resultsresults andand ((bb)) thethe boxbox plotsplots ofof soil-groundsoil‐ground water water storage storage utilization utilization ( (∆ΔWW)) during during the the cropcrop growing growing season season in in Jiefangzha Jiefangzha and and its its four four subareas, subareas, and and Yonglian. Yonglian.

Table 2. Statistical results of soil‐ground water storage utilizations and utilization efficiency of autumn irrigation of Jiefangzha and its four subareas, and Yonglian.

Soil‐Ground Water Storage Utilization (mm) Average Utilization Study Efficiency of Standard Area Average Maximum Minimum Autumn Irrigation Deviation (%) Jiefangzha 121 229 57 49 32.2 Wula 126 243 68 50 32.5 Water 2020, 12, 3351 10 of 19

Table 2. Statistical results of soil-ground water storage utilizations and utilization eﬃciency of autumn irrigation of Jiefangzha and its four subareas, and Yonglian.

Soil-Ground Water Storage Utilization (mm) Average Utilization Study Area Standard Eﬃciency of Autumn Average Maximum Minimum Deviation Irrigation (%) Jiefangzha 121 229 57 49 32.2 Wula 126 243 68 50 32.5 Water 2020Yangjia, 12, 3351 113 202 18 64 31.5 10 of 20 Huangji 124 250 69 49 31.6 QinghuiYangjia 185113 202 357 18 5064 8631.5 57.3 YonglianHuangji 117124 250 149 69 9549 2431.6 47.6 Qinghui 185 357 50 86 57.3 Yonglian 117 149 95 24 47.6 To analyze the temporal variation of soil-ground water storage utilization, only the soil-ground water storageTo analyze utilization the temporal in Jiefangzha variation of and soil its‐ground four subareaswater storage from utilization, 2003 to only 2017 the are soil used‐ground since the datawater series storage of Yonglian utilization is insu in Jiefangzhaﬃcient. Theand its moving four subareas average from soil-ground 2003 to 2017 water are used storage since utilizationthe data in Jiefangzhaseries of and Yonglian its four is subareas insufficient. is shown The moving in Figure average5a, in soil which‐ground an obvious water storage increasing utilization trend in can be found.Jiefangzha The soil-ground and its four water subareas storage is shown utilization in Figure is 5a, increased in which by an 7.00obvious mm, increasing 7.33 mm, trend 12.01 can mm, be 4.85 mm,found. and 13.32 The soil mm‐ground per year water in storage Jiefangzha, utilization Wula, is Yangjia,increased Huangji by 7.00 mm, and 7.33 Qinghui, mm, 12.01 respectively. mm, 4.85 The datamm, of Jiefangzha and 13.32 mm is taken per year as an in example Jiefangzha, to Wula, analyze Yangjia, the reasons Huangji for and the Qinghui, increasing respectively. trend. The The water storagedata at of the Jiefangzha beginning is taken of the as crop an example growing to seasonanalyze increasesthe reasons by for 3.8 the mm increasing per year, trend. contributing The water 54.6% storage at the beginning of the crop growing season increases by 3.8 mm per year, contributing 54.6% to the increase of the soil-ground water storage utilization, and the water storage at the end of the crop to the increase of the soil‐ground water storage utilization, and the water storage at the end of the growing season decreases by 3.2 mm per year, contributing 45.4% to the increase of the soil-ground crop growing season decreases by 3.2 mm per year, contributing 45.4% to the increase of the soil‐ waterground storage water utilization, storage utilization, as shown as in shown Figure in5b. Figure 5b.

FigureFigure 5. The 5. The moving moving average average results results of of (a ()a) the the soil-ground soil‐ground water utilization utilization (Δ (∆WW) in) in Jiefangzha Jiefangzha and and its fourits subareas, four subareas, (b) the ( waterb) the storages water storages at the beginningat the beginning and end and of end the cropof the growing crop growing season season in Jiefangzha, in Jiefangzha, (c) the autumn irrigation time in Jiefangzha, (d) the area of spring irrigation in Jiefangzha, (c) the autumn irrigation time in Jiefangzha, (d) the area of spring irrigation in Jiefangzha, (e) the area (e) the area of summer crops in Jiefangzha, and (f) the area of autumn crops in Jiefangzha. of summer crops in Jiefangzha, and (f) the area of autumn crops in Jiefangzha. 3.2. The Impact Factors on Soil‐Ground Water Storage Utilization The correlations between the soil‐ground water storage utilization in Jiefangzha and the eight impact factors are listed in Table 3. It can be found that there are five impact factors that have significant correlation with the soil‐ground water storage utilization at the significance level of 0.01 or 0.05. The factors are the autumn irrigation time (Tai), the area of the spring irrigation (Si), the area Water 2020, 12, 3351 11 of 19

3.2. The Impact Factors on Soil-Ground Water Storage Utilization The correlations between the soil-ground water storage utilization in Jiefangzha and the eight impact factors are listed in Table3. It can be found that there are five impact factors that have significant correlation with the soil-ground water storage utilization at the significance level of 0.01 or 0.05. The factors are the autumn irrigation time (Tai), the area of the spring irrigation (Si), the area of summer crops (Ss), the area of autumn crops (Sa) and the increase of water table depth during the crop growing season (∆h). The autumn irrigation time has a significant positive relationship with the soil-ground water storage utilization (R = 0.647, P = 0.009). The autumn irrigation occurs in 2003~2007 and 2013~2017 from 23 September~31 October and 3 October~9 November as shown in Figure5c. The delayed autumn irrigation time leads to the higher soil moisture content at the beginning of the next crop growing season, so the water storage at the beginning of crop growing season would increase causing the increase of soil-ground water storage. The area of the spring irrigation has a positive relationship with the soil-ground water storage utilization (R = 0.641, P = 0.010). The spring irrigation area increases from 42 km2 in 2003~2007 to 497 km2 in 2013~2017 as shown in Figure5d, and the larger area of the spring irrigation leads to the larger water storage at the beginning of crop growing season. The area of the summer crops has a significant negative correlation with the soil-ground water storage utilization (R = 0.572, P = 0.026), while there is a significant positive correlation between the area − of the autumn crops and the soil-ground water storage utilization (R = 0.604, P = 0.017). As shown in Figure5e,f, the area of summer crops is decreasing and the area of autumn crops is increasing in Jiefangzha, causing the increase of water consumption in August and September and the decrease of water storage at the end of the crop growing season. This indicates that the adjustment of planting structure can significantly impact the soil-ground water storage utilization. There is a significant positive relationship between the increase of water table depth during the crop growing period and soil-ground water storage utilization (R = 0.554, P = 0.032), which also can be found in Figure6a.

Table 3. Correlation between the soil-ground water storage utilization and the eight impact factors.

¯ Impact Factors Tai Si Ss Sa ∆h Ia + Pa Is + Ps h Correlation coeﬃcient (R) 0.647 ** 0.641 * 0.572 * 0.604 * 0.554 * 0.479 0.403 0.333 − − Signiﬁcance level (P) 0.009 0.010 0.026 0.017 0.032 0.071 0.136 0.225

Note: (1) Tai is the autumn irrigation time. Si is the area of the spring irrigation, mm. Ss is the area of summer crops, 2 2 km . Sa is the area of autumn crops, km . ∆h is the increase of water table depth during the crop growing season, mm. Ia + Pa is the total water supply during the non-growing season, mm; Ia is the field irrigation water during the non-growing season, mm; Pa is the precipitation during the non-growing season, mm. Is + Ps is the total water supply during the crop growing season, mm; Is is the field irrigation water during the crop growing season, mm; Ps is the precipitation during the crop growing season, mm. h is the average water table depth during the crop growing season, mm. (2) * Correlation is significant at the 0.05 level (2-tailed), and ** Correlation is significant at the 0.01 level (2-tailed).

The correlation of the other three impact factors (the total water supply during the non-growing season Ia + Pa, the total water supply during the crop growing season Is + Ps and the average water table depth during the crop growing season h) with the soil-ground water storage utilization are not signiﬁcant at the 0.05 level. It can be found that there is a positive relationship between the total water supply during the non-growing season and soil-ground water storage utilization (R = 0.479, P = 0.071). When the total water supply during the non-growing season increases, the water storage at the beginning of crop growing season will increase. However, as shown in Figure6b, the total water supply during the non-growing season is very stable at 200 mm–210 mm from 2006 to 2014, which caused the poor correlation with the increase of soil-ground water storage utilization in these years. The total water supply during the crop growing season has a negative relationship with the soil-ground water storage utilization (R = 0.403, P = 0.136).The linear negative correlation is obvious − when the total water supply during the crop growing season is larger than 400 mm, close to the crop water requirement of major crops in this area, which means that crops can obtain suﬃcient water by Water 2020, 12, 3351 11 of 20

of summer crops (Ss), the area of autumn crops (Sa) and the increase of water table depth during the crop growing season (Δh). The autumn irrigation time has a significant positive relationship with the soil‐ground water storage utilization (R = 0.647, P = 0.009). The autumn irrigation occurs in 2003~2007 and 2013~2017 from 23 September~31 October and 3 October~9 November as shown in Figure 5c. The delayed autumn irrigation time leads to the higher soil moisture content at the beginning of the next crop growing season, so the water storage at the beginning of crop growing season would increase causing the increase of soil‐ground water storage. The area of the spring irrigation has a positive relationship with the soil‐ground water storage utilization (R = 0.641, P = 0.010). The spring irrigation area increases from 42 km2 in 2003~2007 to 497 km2 in 2013~2017 as shown in Figure 5d, and the larger area of the spring irrigation leads to the larger water storage at the beginning of crop growing season. The area of the summer crops has a significant negative correlation with the soil‐ground water storage utilization (R = −0.572, P = 0.026), while there is a significant positive correlation between the area of the autumn crops and the soil‐ground water storage utilization (R = 0.604, P = 0.017). As shown in Figure 5e,f, the area of summer crops is decreasing and the area of autumn crops is increasing in Jiefangzha, causing the increase of water consumption in August and September and the decrease of water storage at the end of the crop growing season. This indicates that the adjustment of planting structure can significantly impact the soil‐ground water storage utilization. There is a significant positive relationship between the increase of water table depth during the crop growing period and soil‐ground water storage utilization (R = 0.554, P = 0.032), which also can be found in Figure 6a. Water 2020, 12, 3351 12 of 19 Table 3. Correlation between the soil‐ground water storage utilization and the eight impact factors. irrigationImpact and precipitation Factors causingTai less soil-groundSi Ss water storageSa utilization,Δh Ia + as Pa shown Is + P ins Figure 𝒉 6c. Correlation coefficient In the works of Chen et al. [310.647] and ** Liu0.641 et al. * − [3],0.572 there * is0.604 also * a negative0.554 * relationship0.479 −0.403 between 0.333 the ground water contribution(R) to evapotranspiration and the total water supply during the crop growing season.Significance The positive level relationship (P) 0.009 between 0.010 the average0.026 water0.017 table 0.032 depth during0.071 the0.136 crop growing0.225 seasonNote:h and (1) the Tai soil-groundis the autumn water irrigation storage time. utilization Si is the area is notof the signiﬁcant, spring irrigation, with R mm.= 0.333 Ss is andthe areaP = of0.225. 2 2 The relationshipsummer crops, between km . Sa the is the soil-ground area of autumn water crops, storage km . utilization Δh is the increase and the of water average table water depth table during depth is quadraticthe crop as growing shown season, in Figure mm.6d. Ia + A Pa critical is the total water water table supply depth during (2080 the mm) non‐growing exists, before season, which mm; Ia the is the field irrigation water during the non‐growing season, mm; Pa is the precipitation during the relationship between the soil-ground water storage utilization and the average water table depth is non‐growing season, mm. Is + Ps is the total water supply during the crop growing season, mm; Is is positive, and then it becomes negative. The critical water table depth was also found by Chen et al. [41]. the field irrigation water during the crop growing season, mm; Ps is the precipitation during the crop The existence of the critical water table depth indicates that the soil-ground water storage utilization growing season, mm. ℎ is the average water table depth during the crop growing season, mm. (2) * would increase ﬁrst, and then decrease in the future with a decrease of water diversion from Yellow Correlation is significant at the 0.05 level (2‐tailed), and ** Correlation is significant at the 0.01 level river, which is considered to increase the water table depth [48,49]. (2‐tailed).

Figure 6. The relationships of (a) the increase of the water table depth during the crop growing season (∆h), (b) the total water supply during the non-growing season (Ia + Pa), (c) the total water supply during the crop growing season (Is + Ps), and (d) the average water table depth during the crop growing season h with the soil-ground water storage utilization (∆W).

In general, the soil-ground water storage utilization is significantly affected by the autumn irrigation time, the area of the spring irrigation, the area of summer crops, the area of autumn crops, and the variation of water table depth during the crop growing season. Changing the autumn irrigation scheme and planting structure would be effective ways to increase soil-ground water storage utilization. When the water supply during the crop growing season is insufficient, delaying the autumn irrigation time, increasing the amount of autumn irrigation and increasing the area of spring irrigation are useful strategies to help increase the soil-ground water storage before the crop growing period. Meanwhile, our suggestions are to adjust the planting structure, increase the area of autumn crops, and reduce the area of summer crops, which can help crops use more soil-ground water resources for water consumption during the crop growing season.

3.3. Results of Soil Water Storage Utilization, Ground Water Utilization, Soil-Ground Water Storage Utilization below and above 1 m Soil Depth and the Three Coefficients In this section, the two components of soil-ground water storage utilization are estimated, including the soil water storage utilization ∆Ws and the ground water utilization ∆Wg, and the soil-ground water storage utilization below 1 m soil depth ∆W1 and above 1 m soil depth ∆W01. The water table depth at Water 2020, 12, 3351 13 of 19 the end of the crop growing season is larger than that at the beginning of the crop growing season in Jiefangzha and Yonglian, and the ∆h is larger than 0, so Equations (3)–(5) can be used in this research. Then, the three coefficients, the coefficient of soil-ground water storage utilization η, the coefficient of ground water utilization ηg and the coefficient of soil-ground water storage utilization below 1 m soil depth η1, are calculated. The results of ∆Ws and ∆Wg and their contributions to the soil-ground water storage utilization ∆W in Jiefangzha and Yonglian are shown in Figure7, the statistical results of Water 2020, 12, 3351 13 of 20 which are listed in Table4. The average soil water storage utilization ∆Ws is 112 mm, which accounts for 92.6% of the soil-ground water storage utilization ∆W, while the ground water utilization ∆Wg in Figure 7, the statistical results of which are listed in Table 4. The average soil water storage is 9 mm (7.4% of the ∆W) in Jiefangzha. In Yonglian, the average ∆Ws is 98 mm, which accounts for utilization ΔWs is 112 mm, which accounts for 92.6% of the soil‐ground water storage utilization ΔW, 83.0% of the ∆W, and the average ∆W is 19 mm (17.0 % of the ∆W). The contribution of soil water while the ground water utilization Δg Wg is 9 mm (7.4% of the ΔW) in Jiefangzha. In Yonglian, the storageaverage utilization ΔWs is during 98 mm, the which crop accounts growing for season 83.0% isof larger the ΔW than, and 80% the average in this study, ΔWg is which 19 mm proves (17.0 % that the soilof waterthe ΔW storage). The contribution utilization of is soil the water main storage part utilization of the soil-ground during the watercrop growing storage season utilization. is larger The resultsthan illustrate 80% in thatthis study, the variation which proves in water that the storage soil water in the storage ground utilization water fluctuationis the main part zone of the during soil‐ the crop growingground water season storage is small. utilization. due to theThe capillary results illustrate siphoning that phenomenon, the variation in which water leads storage to slightin the soil ground water fluctuation zone during the crop growing season is small. due to the capillary moisture content variation near the saturated zone. The ground water utilization ∆Wg has a significant positivesiphoning relationship phenomenon, (R = 0.961, whichP leads= 0.000) to slight with soil the moisture increase content of water variation table near depth the saturated during zone. the crop The ground water utilization ΔWg has a significant positive relationship (R = 0.961, P = 0.000) with growing season in Jiefangzha, as shown in Figure8a. The coe fficient of ground water utilization η is the increase of water table depth during the crop growing season in Jiefangzha, as shown in Figure g shown in Figure9, the statistical results of which are listed in Table4. The coe fficient of ground water 8a. The coefficient of ground water utilization ηg is shown in Figure 9, the statistical results of which utilizationare listedηg ranges in Table from 4. The 0.008 coefficient to 0.04 of with ground the water standard utilization deviation ηg ranges of 0.01, from and 0.008 the to average 0.04 with value the is 0.026 instandard Jiefangzha. deviation In Yonglian, of 0.01, and the the average average coe valuefficient is 0.026 of groundin Jiefangzha. water In utilization Yonglian, isthe 0.028. average In the workscoefficient of Xu et al. of [ground9] and water Yue et utilization al. [50], the is 0.028. coeffi Incients the works of ground of Xu wateret al. [9] utilization and Yue et were al. [50], 0.039 the and 0.086 forcoefficients estimating of theground variation water ofutilization the ground were water 0.039 storage and 0.086 in Jiefangzha for estimating and Yichang,the variation both of of the which are locatedground in water the Hetao storage Irrigation in Jiefangzha District. and Yichang, They chose both of these which values are located according in the to Hetao the specific Irrigation yield basedDistrict. on the hydrogeological They chose these values studies. according However, to the it canspecific be foundyield based that on the the adopted hydrogeological values in studies. the works of Xu etHowever, al. [9] and it can Yue be et found al. [50 that] would the adopted overestimate values thein the variation works of Xu ground et al. water[9] and storage. Yue et al. Therefore, [50] would overestimate the variation of ground water storage. Therefore, in calculating the variation of in calculating the variation of ground water storage, the coefficient of ground water utilization should ground water storage, the coefficient of ground water utilization should be carefully selected, which be carefullyis smaller selected, than the which specific is smalleryield. than the specific yield.

FigureFigure 7. The 7. results The results of soil of water soil storagewater storage utilization utilization (∆Ws ),(Δ groundWs), ground water water utilization utilization (∆W (gΔ),W soil-groundg), soil‐ ground water storage utilization below 1 m soil depth (ΔW1), soil‐ground water storage utilization water storage utilization below 1 m soil depth (∆W1), soil-ground water storage utilization above above 1 m soil depth (ΔW01) and their contributions to the soil‐ground water storage utilization (ΔW) 1 m soil depth (∆W01) and their contributions to the soil-ground water storage utilization (∆W) in in Jiefangzha and Yonglian. (a) and (c) The results of these water storage utilizations in Jiefangzha Jiefangzha and Yonglian. (a) and (c) The results of these water storage utilizations in Jiefangzha and and Yonglian, respectively. (b) and (d) The results of the contributions of water storage utilizations Yonglian, respectively. (b) and (d) The results of the contributions of water storage utilizations to to soil‐ground water storage utilization (ΔW) in Jiefangzha and Yonglian, respectively. soil-ground water storage utilization (∆W) in Jiefangzha and Yonglian, respectively.

Table 4. Statistical results of ΔWs, ΔWg, ΔW01, ΔW1, ηg, η1 and η in Jiefangzha and Yonglian.

Jiefangzha Yonglian Min Variables Mini Standard Maxi Standard Average Maximum Cv Average imu Cv mum Deviation mum Deviation m WaterWater 20202020,, 1212,, 33513351 1414 ofof 2020

0.2 ΔWs 112 201 43 45 0.399 98 129 73 24 0.2 ΔWs 112 201 43 45 0.399 98 129 73 24 Water 2020, 12, 3351 14 of4545 19 0.1 ΔWg 9 28 0 9 0.957 19 22 17 2 0.1 ΔWg 9 28 0 9 0.957 19 22 17 2 2525 0.2 ΔW01 Table 4.74Statistical 134 results of ∆17W s, ∆Wg, ∆33W 01, ∆W0.4421, ηg , η1 and74 η in Jiefangzha96 and57 Yonglian.17 0.2 ΔW01 74 134 17 33 0.442 74 96 57 17 3333 Jiefangzha Yonglian 0.1 ΔW1 46 95 25 21 0.451 44 52 39 6 0.1 ΔW1 46 95 25 21 0.451 44 52 39 6 Variables Standard Standard 4747 Average Maximum Minimum C Average Maximum Minimum C Deviation v 0.02 Deviation v0.0 ηg 0.026 0.040 0.008 0.010 0.368 0.028 0.029 0.02 0.001 0.0 ηg 0.026 0.040 0.008 0.010 0.368 0.028 0.029 7 0.001 35 ∆Ws 112 201 43 45 0.399 98 129 737 24 0.24535 ∆Wg 9 28 0 9 0.957 19 22 170.05 2 0.1250.1 η1 0.111 0.152 0.069 0.027 0.243 0.065 0.072 0.05 0.010 0.1 ηW1 0.111 0.152 0.069 0.027 0.243 0.065 0.072 0.010 ∆ 01 74 134 17 33 0.442 74 96 5700 17 0.2336161 ∆W1 46 95 25 21 0.451 44 52 390.12 6 0.1470.2 η 0.271 0.497 0.104 0.114 0.419 0.174 0.201 0.12 0.036 0.2 η ηg 0.0260.271 0.0400.497 0.0080.104 0.0100.114 0.3680.419 0.028 0.174 0.029 0.201 0.0272 0.0010.036 0.03508 η1 0.111 0.152 0.069 0.027 0.243 0.065 0.072 0.0502 0.010 0.16108 Note:η ΔW0.271s is the soil 0.497 water storage 0.104 utilization, 0.114 mm. 0.419 ΔWg is 0.174 the ground 0.201 water utilization, 0.122 mm. 0.036 ΔW01 0.208is Note: ΔWs is the soil water storage utilization, mm. ΔWg is the ground water utilization, mm. ΔW01 is Note:the soil∆W‐groundis the soilwater water storage storage utilization utilization, above mm. 1∆ Wm soilis the depth, ground mm. water ΔW utilization,1 is the soil mm.‐ground∆W wateris the the soil‐grounds water storage utilization above 1 m gsoil depth, mm. ΔW1 is the soil‐ground01 water soil-groundstorage utilization water storage below utilization 1 m soil abovedepth, 1 mmm. soil η depth,g is the mm. coefficient∆W1 is theof ground soil-ground water water utilization. storage utilization η1 is the storage utilization below 1 m soil depth, mm. ηg is the coefficient of ground water utilization. η1 is the below 1 m soil depth, mm. ηg is the coefficient of ground water utilization. η is the coefficient of soil-ground coefficient of soil‐ground water storage utilization below 1 m soil depth.1 η is the coefficient of soil‐ coefficientwater storage of utilization soil‐ground below water 1 m soil storage depth. utilizationη is the coe ffibelowcient of 1 soil-groundm soil depth. water η storageis the coefficient utilization. Cofv issoil the‐ coegroundfficient water of variation. storage utilization. Cv is the coefficient of variation. ground water storage utilization. Cv is the coefficient of variation.

Figure 8. The correlation between (a) ground water utilization (ΔWg) and (b) soil‐ground water Figure 8.8. TheThe correlation correlation between between (a )( grounda) ground water water utilization utilization (∆W g()Δ andWg) ( band) soil-ground (b) soil‐ground water storage water storage utilization below 1 m soil depth (ΔW1) with the increase of the water table depth during the storageutilization utilization below 1below m soil 1 depthm soil (depth∆W1) ( withΔW1) the with increase the increase of the of water the water table table depth depth during during the crop the cropgrowingcrop growinggrowing season seasonseason (∆h) in ((ΔΔh Jiefangzha.h)) inin Jiefangzha.Jiefangzha.

FigureFigure 9. 9. TheThe results results of of the the coefficient coecoefficientfficient of of soil soil-groundsoil‐‐groundground water water storage storage utilization utilization ( (ηη),), the thethe coefficient coecoefficientfficient of of soil-groundsoil‐ground water storage utilization below 1 m soilsoil depthdepth ((η1) and the coecoefficientfficient of groundground waterwater soil‐ground water storage utilization below 1 m soil depth (η1) and the coefficient of ground water utilization (ηg)) of ( a) Jiefangzha from 2003 to 2017 (except 2012) and ( b) Yonglian inin 2005,2005, 2006, 20072007 utilization (ηgg) of (a) Jiefangzha from 2003 to 2017 (except 2012) and (b) Yonglian in 2005, 2006, 2007 andand 2009. 2009.

The results of average soil-ground water storage utilization above and below 1 m soil depth TheThe resultsresults ofof averageaverage soilsoil‐‐groundground waterwater storagestorage utilizationutilization aboveabove andand belowbelow 11 mm soilsoil depthdepth ((∆ΔWW0101 and Δ∆WW11) andand theirtheir contributions contributions to to soil-ground soil‐ground water water storage storage utilization utilization∆W ΔinW Jiefangzha in Jiefangzha and (ΔW01 and ΔW1) and their contributions to soil‐ground water storage utilization ΔW in Jiefangzha Yonglian are shown in Figure7, the statistical results of which are listed in Table4. The average ∆W andand YonglianYonglian areare shownshown inin FigureFigure 7,7, thethe statisticalstatistical resultsresults ofof whichwhich areare listedlisted inin TableTable 4.4. TheThe averageaverage01 isΔW 7401 mm, is 74 accounting mm, accounting for 60.6% for of60.6% the soil-groundof the soil‐ground water storage water storage utilization utilization∆W, and Δ theW, and∆W 1theis Δ 47W mm,1 is ΔW01 is 74 mm, accounting for 60.6% of the soil‐ground water storage utilization ΔW, and the ΔW1 is accounting47 mm, accounting for 39.4% for of 39.4%∆W in of Jiefangzha. ΔW in Jiefangzha. In Yonglian, In Yonglian, the average the average∆W01 and ΔW∆01W and1 are Δ 74W1 mm are 74 and mm 44 47 mm, accounting for 39.4% of ΔW in Jiefangzha. In Yonglian, the average ΔW01 and ΔW1 are 74 mm mm, accounting for 62.5% and 37.5% of ∆W. The contribution of soil-ground water storage utilization andand 4444 mm,mm, accountingaccounting forfor 62.5%62.5% andand 37.5%37.5% ofof Δ ΔWW.. TheThe contributioncontribution ofof soilsoil‐‐groundground waterwater storagestorage above 1 m soil depth is larger than 60%, which indicates that the utilization of soil-ground water utilizationutilization aboveabove 11 mm soilsoil depthdepth isis largerlarger thanthan 60%,60%, whichwhich indicatesindicates thatthat thethe utilizationutilization ofof soilsoil‐‐groundground storage is mainly from the soil depth above 1 m. There is a signiﬁcant positive relationship (R = 0.842, waterwater storagestorage isis mainlymainly fromfrom thethe soilsoil depthdepth aboveabove 11 m.m. ThereThere isis aa significantsignificant positivepositive relationshiprelationship ((RR P= =0.842,0.000) P between = 0.000) thebetween soil-ground the soil water‐ground utilization water below utilization 1 m soil below depth 1 ∆mW soil1 and depth the increase ΔW1 and of the = 0.842, P = 0.000) between the soil‐ground water utilization below 1 m soil depth ΔW1 and the water table depth during the crop growing season in Jiefangzha, as shown in Figure8b. The coe ﬃcient Water 2020, 12, 3351 15 of 19

of soil-ground water storage utilization below 1 m soil depth η1 is shown in Figure9, the statistical results of which are displayed in Table4. The coe fficient of the soil-ground water storage utilization below 1 m soil depth η1 ranges from 0.069 to 0.152 with the Cv value of 0.243 and the average value is 0.111 in Jiefangzha. In Yonglian, the average η1 is 0.065. The maximum sampling depth is usually 1 m in many agricultural areas. So this coefficient is an important parameter to estimate the variation of the water storage resource below 1 m soil depth. The value used for calculating the soil-ground water storage utilization below 1 m soil depth by Chen et al. [41] and Liu et al. [3] was 0.07 in Jiefangzha. It can be found that the calculated soil-ground water storage utilization below 1 m soil depth would induce a 37% error. The coefficient of soil-ground water storage utilization η is shown in Figure9, the statistical results of which are listed in Table4. The coe fficient of the soil-ground water storage utilization η ranges from 0.104 to 0.497 with a standard deviation of 0.114. The coefficient of the soil-ground water storage utilization η varies greatly over the years. The average coefficients of soil-ground water storage utilization are 0.271 and 0.172 in Jiefangzha and Yonglian, respectively. This value is consistent with the work of Steinwand et al. [51], where the coefficients were 0.18 for the sandy site and 0.28 for the clay loam site. This value is much larger than the specific yield used for calculating the variation of the soil-ground water storage during the crop growing season, which is obtained by pumping tests due to the non-instantaneous drainage in the vadose zone and aquifer compressibility [36,52,53]. Therefore, the specific yield from the pumping test cannot be used directly for calculation of the soil-ground water storage utilization, which could underestimate the results. In general, the three coefficients are ranked as η > η1 > ηg, and there is a great difference among them. So one should be careful to choose suitable coefficients when calculating the variation of soil-ground water storage.

3.4. Contributions of Ground Water Utilization, Soil-Ground Water Storage Utilization below 1 m Soil Depth, Soil-Ground Water Storage Utilization to the Actual Evapotranspiration The results of the actual evapotranspiration (ET) during the crop growing season in Jiefangzha (2003~2017) and Yonglian (2005~2009) are shown Figure 10a,c, the statistical results of which are listed in Table5. The average calculated actual evapotranspiration during the crop growing season is 502 mm from 2003 to 2017 in Jiefangzha, ranging from 413 mm to 591 mm. In Yonglian, the average calculated actual evapotranspiration is 485 mm from 2005 to 2009, ranging from 431 mm to 561 mm. The contributions of soil-ground water storage utilization (RW), soil-ground water storage utilization below 1 m soil depth (RW1) and ground water utilization (RWg) to the actual evapotranspiration in Jiefangzha and Yonglian are shown in Figure 10b,d, the statistical results of which are listed in Table5. In Jiefangzha, the average contribution of soil-ground water storage utilization to the actual evapotranspiration (RW) is 23.7%, which demonstrates that the soil-ground water storage utilization is an important source for crop water consumption. The average contribution of soil-ground water storage utilization below 1 m soil depth to the actual evapotranspiration (RW1) is 9.2% and the average contribution of ground water utilization to the actual evapotranspiration (RWg) is 1.8% in Jiefangzha. The RW in Jiefangzha has an increasing trend from 2013 to 2017, which increases from 19.1% in 2003~2007 to 29.3% in 2013~2017. The RW has a signiﬁcant negative relationship with precipitation during the crop growing season (R = 0.621, P = 0.013). Due to the larger precipitation during the − crop growing season in 2012 and 2014, the RW of 2012 and 2014 are smaller than that in 2011~2017. In Yonglian, the average RW, RW1 and RWg are 24.2%, 9.0% and 4.0%, respectively. It can be found that the average contribution of soil-ground water storage utilization below 1 m soil depth to the actual evapotranspiration is over 9.0% and the contribution of soil-ground water storage utilization to the actual evapotranspiration is over 23.5%. The results of Gao et al. [40] are consistent with the results of this study; the average contribution of soil-ground water storage utilization below 0.5 m soil depth to the actual evapotranspiration were 25.6%, 12.0% and 15.2% in upper, middle and lower reaches of Hetao Irrigation District, respectively, which is within the range of RW1 (9.0%) to RW (23.5%) in this Water 2020, 12, 3351 16 of 19

Water 2020, 12, 3351 16 of 20 study. These results indicate that storing water resources during the non-growing season by irrigation or precipitation is a useful way to alleviate the water shortage during the crop growing season in arid season by irrigation or precipitation is a useful way to alleviate the water shortage during the crop agricultural regions with shallow water table depth. growing season in arid agricultural regions with shallow water table depth.

FigureFigure 10. 10.Results Results of of the the actual actual evapotranspiration evapotranspiration (ET ),(ET precipitation), precipitation and contributionsand contributions of ground of ground water

utilizationwater utilization (RWg), ( soil-groundRWg), soil‐ground water storage water storage utilization utilization below 1below m soil 1 depth m soil ( RWdepth1), and (RW soil-ground1), and soil‐ waterground storage water utilization storage utilization (RW) to the (RW actual) to the evapotranspiration actual evapotranspiration in Jiefangzha in Jiefangzha and Yonglian. and ( Yonglian.a) and (c) Results(a) and of(c) the Results actual of evapotranspiration the actual evapotranspiration (ET) and precipitation (ET) and precipitation in Jiefangzha in and Jiefangzha Yonglian, and respectively. Yonglian, (respectively.b) and (d) Results (b) and of the (d) contributions Results of the of thecontributions water storage of the utilizations water storage to the actualutilizations evapotranspiration to the actual inevapotranspiration Jiefangzha and Yonglian, in Jiefangzha respectively. and Yonglian, respectively.

Table 5. Statistical results of ET, RW, RW1 and RWg in Jiefangzha and Yonglian. Table 5. Statistical results of ET, RW, RW1 and RWg in Jiefangzha and Yonglian. Vari Jiefangzha Yonglian able JiefangzhaStandard Yonglian Standard Average Maximum Minimum Average Maximum Minimum Variabless StandardDeviation DeviationStandard Average Maximum Minimum Average Maximum Minimum ET 502 591 413 Deviation53 485 561 431 Deviation58 ETRW 23.75%502 40.00% 591 12.31% 413 8.31% 53 24.21% 485 29.93% 561 21.15% 431 4.04% 58 RW 1 9.20%23.75% 40.00%16.55% 12.31%4.20% 8.31%3.69% 24.21%9.03% 10.55% 29.93% 7.93% 21.15% 1.12% 4.04% RWRW1g 1.75%9.20% 16.55%4.86% 4.20%0.02% 3.69%1.53% 4.03% 9.03% 4.96% 10.55% 3.06% 7.93% 0.77% 1.12% RWNote:g ET1.75% is the actual 4.86% evapotranspiration, 0.02% mm. 1.53% RW is the 4.03%contribution 4.96% of soil‐ground 3.06% water storage 0.77% Note:utilization.ET is the RW actual1 is the evapotranspiration, contribution of mm. soil‐RWgroundis the water contribution storage of soil-ground utilization water below storage 1 m soil utilization. depth. RW1g is the contribution of soil-ground water storage utilization below 1 m soil depth. RWg is the contribution of ground wateris the utilization. contribution of ground water utilization.

4. Conclusions 4. Conclusions In this study, a long‐term soil moisture content and water table depth dataset are used to In this study, a long-term soil moisture content and water table depth dataset are used to estimate estimate soil‐ground water storage utilization during the crop growing season and its attribution to soil-ground water storage utilization during the crop growing season and its attribution to irrigation irrigation of the non‐growing season in arid agricultural areas with shallow water table depth; of the non-growing season in arid agricultural areas with shallow water table depth; coeﬃcients coefficients of soil‐ground water storage utilization, ground water utilization and soil‐ground water of soil-ground water storage utilization, ground water utilization and soil-ground water storage storage utilization below 1 m soil depth are also analyzed. The contributions of soil‐ground water utilization below 1 m soil depth are also analyzed. The contributions of soil-ground water storage storage utilization, soil‐ground water storage utilization below 1 m soil depth and ground water utilization, soil-ground water storage utilization below 1 m soil depth and ground water utilization to utilization to actual evapotranspiration are further evaluated. The major conclusions are as follows: actual evapotranspiration are further evaluated. The major conclusions are as follows: (1) The soil‐ground water storage is critical to crop water consumption during the crop growing (1) The soil-ground water storage is critical to crop water consumption during the crop growing season, which accounts for over 23.5% of the actual evapotranspiration. season, which accounts for over 23.5% of the actual evapotranspiration. (2) More than 30% of the autumn irrigation water during the non‐growing season can be used as soil‐ground water utilization in arid areas with shallow water table depth. (3) Changing the autumn irrigation scheme and the planting structure can significantly impact the soil‐ground water storage utilization. Water 2020, 12, 3351 17 of 19

(2) More than 30% of the autumn irrigation water during the non-growing season can be used as soil-ground water utilization in arid areas with shallow water table depth. (3) Changing the autumn irrigation scheme and the planting structure can significantly impact the soil-ground water storage utilization. (4) The utilization of soil-ground water storage is mainly from the soil depth above 1 m, which contributes more than 60% of the soil-ground water storage utilization. (5) For using the water table fluctuation method to estimate the variation of the soil-ground water storage, one should be careful in choosing the suitable coefficient. The commonly used specific yield obtained by pumping tests would overestimate the ground water utilization and underestimate the soil-ground water utilization.

Author Contributions: Conceptualization, T.Z., Y.Z. and J.Y.; methodology, T.Z., Y.Z. and J.Y.; software, T.Z.; validation, T.Z., Y.Z., and W.M.; formal analysis, T.Z.; investigation, Y.Z., J.W., W.M., and J.Y.; resources, Y.Z. and J.Y.; data curation, Y.Z. and J.Y.; writing—original draft preparation, T.Z.; writing—review and editing, Y.Z., M.Y., and W.M.; visualization, T.Z. and Y.Z.; supervision, Y.Z.; project administration, Y.Z.; funding acquisition, Y.Z. and J.Y. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by Natural Science Foundation of China through Grants 51779178, 51790533, and 51629901, the National Key Research and Development Program of China (2017YFC0403301), and the project of water conservancy science and technology plan in Inner Mongolia Autonomous Region (Grant No. 213-03-99-303002-NSK2017-M1). Conﬂicts of Interest: The authors declare no conﬂict of interest.

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