Numericalsimulationof Groundwater-Surface Water Exchange Inxilinriver Duringfloodseasonbasedon MODFLOW

Groundwater and surface water are usually closely related,understanding the exchange between the two water bodies help the maintainess of river ecosystems functions and the sustainable management of water resources.In this study,two typical river sections in the upper and middle reaches of Xilin River in Inner Mongolia Autonomous Region were selected as the research objects.It aims to understand the evolution trend of groundwater in river corridors during spring and summer flood seasons and the dynamics of interaction between river and groundwater.A groundwater-surface water interaction model was established at monthly and daily scales in flood season based on groundwater model MODFLOW.Based on the established model,the influences of precipitation,slope of riverbed and hydraulic conductivity on the direction of groundwater-surface water interaction and exchange volume were discussed.The main conclusions are as follows:(1)A groundwater-surface water transformation model was established based on MODFLOW.The model was calibrated and verified by using the measured groundwater level.The average residual between measured and simulated values is 0.002 and the standard deviation is 0.018.It shows that the model is accurate,and can be used to analyze and predict the dynamics of interaction between groundwater and surface water.(2)Both the measured and simulated results of monthly scale(2018.4-2018.10)and daily scale(2018.4.9-4.23,7.6-8.5)in flood season show the same trend of groundwater level changes in the monitoring holes at two sampling river sections.From April to May,the groundwater level declined slowly,and then rise gradually until July.The water level showed an increasing trend again after the decrease in August.During the flood season,the variation of rainfall affected the direction of groundwater-surface water interaction and the exchange capacity.Surface water level rose rapidly with rainfall increased while groundwater increased after several days because of hysteresis effect.The gap between groundwater and surface water decreased gradually,and the exchange capacity decreased.(3)The results of scenario analysis based on the sensitive factors of groundwater and surface water interaction including precipitation,slope of riverbed and hydraulic conductivity showed that the groundwater level increased with the increase of rainfall,and the exchange capacity also increased.When the rainfall is 0.5 times as the precipitation in 2018(baseline),the exchange direction does not change.When the rainfall is 0.25,1.25,1.5 and 2 times as the baseline respectively,the exchange direction changes,with groundwater recharging surface water.Because it is impossible to predict the change of surface water level with rainfall,it is impossible to accurately calculate the change of groundwater-surface water exchange.The change of slope does not affect the direction of exchange,but the slope of riverbed and exchange quality is correlated.The exchange volume increases with increasing slope of riverbed.The higher the hydraulic conductivity is,the higher the groundwater level is and the larger the exchange capacity is.Study of the interaction between groundwater and surface water is conducive to the joint dispatch of groundwater and surface water and the sustainable utilization of water resources in the study area.