Effects of the Three Gorges Dam Operation on the hydrological interaction between the Yangtze River and downstream aquifers

Abstract. The construction of the Three Gorges Dam (TGD) has profoundly altered the groundwater cycle downstream. The obscure spatiotemporal patterns of exchange fluxes between the Yangtze River and groundwater hinder the resolution of water resources and environmental issues in the watershed. In the Four-Lake Basin, the first river-lake wetland plain downstream of the TGD, this study investigated the spatial extent of the Yangtze River's influence on adjacent groundwater by leveraging multiple groups of monitoring wells installed along the river. A coupled SWAT-MODFLOW model was applied to quantify period-specific SW-GW exchanges. A counterfactual scenario without TGD operation-holding other conditions constant is also simulated for comparison. The results show: (1) The influence range of the Yangtze River on confined groundwater is larger in the ZJ-JLX2 section, whereas it is relatively minor on groundwater near HH1 profile and HH2 profile. The influence distance at the HH1 profile is the smallest, measuring as 1.94 km. (2) River and groundwater exchanges exhibit pronounced seasonal and spatial characteristics: river-to-aquifer recharge dominates during drawdown and flooding periods, while aquifer-to-river discharge dominates during impounding and dry periods. Using JLX2 as a divider, interaction rates are consistently higher in the upper section than in the lower one. (3) Relative to natural conditions, TGD operation dramatically dampens Yangtze River-groundwater interactions overall. The effect is most pronounced during the dry period in the upper section, when the interaction rate decreases by 40.6 %. These research outcomes serve as a vital theoretical foundation for assessing the effects of the Three Gorges Dam's regulation on the regional water cycle.