Assessment Of Water Exchange Between Soil Water And Groundwater In Irrigation Area Based On SWAT-MODFLOW

The change of irrigation water source from surface water to groundwater resulted in profound changes of the hydrological cycle processes in irrigation areas,and caused problems such as overextraction of shallow groundwater and continuous decline of groundwater level.In this paper,the People's Victory Canal Irrigation District is selected as study site,the integrated SWAT-MODFLOW model was built to simulate the hydrological cycle processes in the irrigated area under the influence of human activities.Obejectives of this study were to clarify the spatial and temporal variation characteristics of soil water and groundwater,and quantify the water exchange between soil water-groundwater in the irrigated area.Research results of this study can provide theoretical basis for rational utilization of water resources and regulation of groundwater in irrigated areas.The main research contents and conclusions are as follows:(1)Based on the observed data and spatial interpolation analysis of Arc GIS,the characteristics and influencing factors of the temporal and spatial variation of groundwater level and soil moisture content in the irrigated area were analyzed.The groundwater level in the western part of the irrigated area is the highest,following by the central part,and the groundwater level in the eastern part of the irrigation area is the lowest.Groundwater flows from southwest to northeast in the irrigated area.The groundwater level at most sampling points changed obviously during the study period,but the monthly variation was less than 2 m,and the interannual variation was small.The spatial distribution characteristics of monthly soil moisture content in the irrigated area changed greatly compared with that of groundwater level.Soil moisture content is greatly affected by irrigation,evapotranspiration and soil texture.As a result of the exploitation of groundwater for irrigation,when the groundwater level in the irrigated area decreases,the soil moisture content increases accordingly.(2)Based on the measured data,SWAT and MODFLOW models,an intergated distributed hydrological model(SWAT-MODFLOW)was constructed and calibrated for soil water-groundwater coupling simulation.By comparing the simulated results of MODFLOW and SWAT-MODFLOW,it is found that the simulated groundwater level of MODFLOW were obvisouly overestimated in the area with large thickness of vadose zone,while the simulated groundwater level of the SWAT-MODFLOW model is close to the measured groundwater level.For the SWAT-MODFLOW model,the determination coefficient(R~2)between the simulated and measured values is between 0.1-0.7,and the root mean square error(RMSE)is between 0.5-2.4 m.For the MODFLOW model,the R~2 of the simulated and measured values is between 0.1-0.56,and the RMSE is between0.5-3.3 m.(3)Based on the simulated results of the SWAT-MODFLOW model,the characteristics and influencing factors of water exchange between soil water and groundwater in the irrigated area were obtained,and suggestions on groundwater regulation were given based on the measured data and simulated results.Under the influence of groundwater extraction for irrigation,annual recharge of soil water to groundwater is 170-187 mm in the sandy loam region,eastern part of the irrigation area,and 150-170 mm in the silty loam region,central part of the irrigation area.The average annual groundwater evaporation in the irrigation area is 72 mm.Irrigation significantly increased the recharge of soil water to groundwater and evapotranspiration in the study site.The simulated results show that groundwater exploitation and irrigation are the main reasons for the opposite change trend of soil water and groundwater in the irrigated area.Groundwater and soil water in irrigated areas are greatly affected by precipitation and irrigation exploitation.In order to reduce the amount of groundwater exploitation and restore the groundwater level,water-saving irrigation and optimiztion irrigation schedule can be further carried out in irrigated areas.