Coupling Simulation Of Surface Water And Groundwater In Beishan Reservoir Watershed Based On SWAT-MODFLOW

The distributed hydrological model SWAT has been widely used in the study of basin water resources,but the groundwater module of SWAT model is relatively simple.As an important part of regional water resources,the interaction between groundwater and surface water in space and time is of great significance to the joint management of regional surface and groundwater resources.Therefore,based on QGIS,this paper constructed a coupled SWAT-MODFLOW model for the Beishan Reservoir Basin in Jurong City to illustrate the interaction relationship between surface water and groundwater systems in this region,which was of great significance to improve the simulation and management of water resources in middle and small basins.On the basis of measured hydrological data from 2016 to 2020,as well as the elevation data,land use data,soil type data,the SWAT model of Beishan Reservoir watershed was constructed firstly.The three coefficients of determination(R~2),Nash-Shutcliffe efficiency(NSE)and root-mean-square error(RMSE)were selected as the model evaluation indexes and SWAT-CUP software was used to calibrate the parameters and analyze the sensitivity of them.The results indicated that the simulated value of Beishan Reservoir’s monthly surface runoff were in good agreement with the measured value.The model evaluation indexes,including R~2,NSE and RMSE,were 0.91,0.90,and 0.33m~3/s in calibration period,and 0.95,0.87 and 0.26m~3/s in validation period,respectively.This suggested that the calibrated model could approximately reflect the characteristics of runoff in the study area,indicating the applicability of SWAT model in this watershed.Moreover,the sensitivity analysis of the main parameters based on the validated hydrological model indicated that GWQMN(threshold of shallow groundwater regression flow)and CN2(number of SCS runoff curves)were the most important parameters be of significant effects on the model results.Secondly,based on the hydrogeological data and observation data of the study area,a MODFLOW model of Beishan Reservoir Basin was constructed,and the coupling of SWAT and MODFLOW was realized on the QGIS platform.The three coefficients of determination(R~2),Nash-Shutcliffe efficiency(NSE)and root-mean-square error(RMSE)were selected as the model evaluation indexes.The results showed that the simulated values of monthly runoff and groundwater level were in good agreement with the measured results.The surface runoff evaluation indexes R~2,NSE and RMSE of the coupling model were 0.90,0.90 and 0.35(m~3/s)in the whole simulation period,and groundwater level evaluation indexes R~2 and RMSE were 0.99and 0.64m,respectively,indicating the simulation accuracy of the coupling model was good.Based on the verified SWAT-Modflow coupling model,the calculation results were analyzed and the following conclusions were drawn:(1)In terms of time,The amount of groundwater infiltration and recharge showed a trend of decreasing year by year,the main reason of which was the decrease of precipitation.Soil type and land use type had significant effects on the spatial recharge.The combination of Haplic Luvisols,Dystric Regosols with forest land showed higher groundwater infiltration rate.(2)In terms of time,the surface water-groundwater replenishment and discharge relationship showed that groundwater replenished surface water all year round.In space,the relationship between them was complex and the main rule was that the East and West tributaries of the reservoir supplied surface water with groundwater,and the middle tributaries supplied groundwater with surface water.(3)In the water balance,the proportion of the total water entering the river from low to high was groundwater recharge to the river,lateral recharge to the river,and surface runoff recharge to the river.It showed that although the contribution rate of groundwater to the streamflow was the lowest,it still reached 8.72%.In the period of less precipitation,groundwater was the main source of the river.It was an important part of water resources in this region.The above results showed that there was a strong interaction between surface water and groundwater in the study area,and it showed a complex spatio-temporal interaction law,which was affected by many factors such as precipitation,terrain,land use type,soil type and so on.Besides,groundwater was an important part of regional water resources.It supplied surface water all the year round,especially in the dry season.The SWAT-MODFLOW coupling model well described the surface water-groundwater interaction process in the study area,which fully reflected the necessity of considering the interaction between them in numerical simulation,and provided a scientific basis for the joint operation of water resources in Beishan reservoir watershed.At the same time,it also provided a reference for hydrological process simulation of valley type small watershed similar to Beishan reservoir.