| The piedmont plain of Taihang Mountains in Hebei province is facing severe groundwater depletion,which is mainly caused by the well-irrigation for winter wheat-summer maize double-cropping system.It is a typical region that is facing a dilemma between groundwater depletion and crop production demand.Quantitatively estimating the spatiotemporal variability and sustainability of shallow groundwater table depth and shallow aquifer storage is an important basis for proper groundwater management in this well-irrigated area.Meanwhile,quantitatively simulating the variation of shallow groundwater balance,soil water balance,crop yield,and crop water productivity under the specific scenarios of limited irrigation schedule and bare fallow could provide reasonable estimation for evaluating the effects of control methods on groundwater overexploitation.Because of the heterogeneity of the underlying surface,it is important to establish a distributed hydrological model,in which the crop growth processes can be considered,to provide reasonable estimated data with time and space.The main research contents and results for this study are as follows:(1)Based on the modification of relative subroutines in the source code,a modified SWAT model was established in the study area to simulate the agro-hydrological process under the winter wheat-summer maize double-cropping system.Firstly,four new parameters were added into the groundwater module in order to calculate and output the shallow groundwater table depth in different spatial and temporal scales.They are the bottom bed burial depth of the shallow aquifer,the porosity of the shallow aquifer,the specific yield of the shallow aquifer,and the lateral groundwater recharge amount.Then the 11 parameters in SWAT groundwater module were initialized on basis of abundant hydrogeological investigations and groundwater assessment results.Based on the SIFU-2 method,the model was satisfactorily calibrated and validated using shallow groundwater table data from 16 national wells monitored monthly from 1993 to 2010 and 148 wells investigated yearly from 2006 to 2012.To further demonstrate the model's rationality,the multi-objective validation was conducted by comparing the simulated groundwater balance components,actual evapotranspiration,and crop yields to the Groundwater Resources Assessment results,remote sensing data and statistical material,respectively.Overall,a distributed SWAT model that could reasonably simulate the agro-hydrological processes in the study area was established by calibrating parameters and validating model using multiple sources data.(2)The calibrated model was used to estimate both shallow groundwater table fluctuation and shallow aquifer water storage change in time and space under the current irrigation schedule during 1993-2012.The results showed that the shallow groundwater table declined at a rate of 0.69-1.56 m/a on average,which depleted almost 350×108 m3 of shallow aquifer water storage in the well-irrigated cropland during the past 20 years.Because of the heterogeneity of the underlying surface and precipitation,these variations were spatiotemporally different.Generally,the shallow groundwater table declined 1.43-1.88 m during the winter wheat growing season,while it recovered 0.28-0.57 m during the summer maize growing season except when precipitation was exceptionally scarce.According to the simulated decrease rate of groundwater storage,the shallow aquifer in the study area may face a depletion crisis within the next 80 years.Meanwhile,the shallow aquifer would still be unsustainable under the optimal irrigation scheduling with sustainable crop yield.Therefore,the winter wheat yield must be sacrificed to prevent the exacerbation of the groundwater crisis.(3)The shallow groundwater balance,soil water balance,crop yield and crop water productivity under winter wheat-summer maize double-cropping system was simulated with the limited irrigation schedule for winter wheat(scenario 2).And the results were parallelly compared to those with the current irrigation schedule(scenario 1).The limited irrigation schedule for winter wheat is one-time irrigation in the jointing growth period.The results showed that the annual shallow groundwater exploitation and recharge in the study area were 118 and 85 mm,respectively,and the shallow groundwater table correspondingly declined at a rate of 0.28 m/a on average.Compared to the scenario 1,the decline of shallow groundwater table during the winter wheat growing season was reduced obviously.However,because of the less recharge,in the scenario 2,the recovery of shallow groundwater table during the summer maize growing season was reduced at the same.In the year with a precipitation exceedance probability(PEP)of 50%,the annual change of shallow groundwater table presented a rising trend or a little declining with less than 0.05 m/a in ten subbasins in the northern and southern of the study area.It could be considered that the shallow groundwater of these ten subbasins achieving the exploitation and replenishment balance in the scenario 2.The water balance in the root zone(2m deep)was changed too.Compared to the scenario 1,the variation of soil water storage during the winter wheat growing season was changed from 4.4 mm to-44.1 mm;and the variation of soil water storage during the summer maize growing season was changed from-6.7 mm to 42.6 mm.The variation of soil water storage was stable during the 20 years,and there was no trend of decreasing or increasing continually.In the limited irrigation schedule scenario,the contribution of rainfall to evapotranspiration was raised,and the ratio of leakage to the sum of rainfall and irrigation was reduced in comparison with scenario 1,indicating that the scenario 2 was better for the well-irrigated cropland to use the rainfall and irrigation effectively.In the scenario 2,the reduction rate of winter wheat was 24.9%,37.9%and 38.4%in the growing season with PEPs of 50%,75%and 95%,respectively.The crop water productivity of winter wheat increased 0.1 kg/m3 averagely.And the crop yield and crop water productivity of summer maize have not changed much in scenario 2.Generally,in comparison with the scenario 1,under the limited irrigation schedule for winter wheat,the annual shallow groundwater exploitation would reduce 29.59×108 m3,and the annual filed water consumption would reduce 12.62×108 m3.The shallow aquifer storage depletion rate would change from 17.5×108 m3/a(scenario 1)to 4.0×108 m3/a(scenario 2),and the decline rate of shallow groundwater table would be reduced to one quarter of the value in scenario 1.But the effect of groundwater overexploitation control method would be achieved at the cost of losing 28.1%of the winter wheat yield annually.(4)The spatiotemporal variations of shallow groundwater table depth and shallow aquifer storage were simulated in the scenario of leaving the winter wheat-summer maize rotation bare fallowed for 20 years(scenario 3).The results showed that in this bare fallow scenario,the shallow aquifer storage would increase 33-120 mm and 37-103 mm in one winter wheat and one summer maize growing season,respectively.Averagely,the shallow aquifer storage would recover at a rate of 138 mm/a,equal to conserve 14.64×108 m3/a shallow groundwater resources in the well-irrigated area.Moreover,the shallow groundwater table would present a rising trend,and the average rate was 1.0 m/a in the scenario 3.Compared with scenario 1 and scenario 2,although all the yields of winter wheat and summer maize would be sacrificed,the effect of shallow groundwater conservation was obvious under the bare fallow scenario,especially in the Plain of the Ziya River basin.Generally,the variation of soil water storage in the root zone(2m deep)was quite small during the 20 years,indicating that the reservoir dynamic of topsoil in the study area was almost stable in the scenario 3. |
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