Surface Water-Groundwater Interactions Revealed by Coupling a Glacier-Enhanced SWAT Model With MODFLOW in Cryosphere Watershed

Climate warming has been altering cryospheric processes in cold regions, accelerating glacier retreat, reducing snow cover, leading to increased interannual variability in runoff and more frequent extreme hydrological events, thereby exacerbating hydrological instability. Consequently, exploring reliable approaches to investigate surface water-groundwater interactions in alpine cryospheric regions remains challenging and essential. Accordingly, we coupled the glacier-enhanced hydrological model (SWAT-GL) with the groundwater model (MODFLOW) to develop the integrated SWAT-GL-MODFLOW framework and to quantify such interactions in the cryospheric upper Shule River Basin on the northeastern Tibetan Plateau. The coupled model was calibrated and validated using observed streamflow and separated baseflow, demonstrating its capability to simulate basin-scale runoff dynamics and groundwater processes. For runoff, SWAT-GL-MODFLOW achieved Nash-Sutcliffe Efficiency values of 0.71 (calibration) and 0.65 (validation), improving by about 16.1% and 4.8% over SWAT and SWAT-MODFLOW, respectively. For baseflow, the simulated values showed good agreement with the baseflow separation values, with a FitNRMSE of 0.82. Our results reveal that: (1) the average contribution of glacier meltwater runoff to annual river discharge was 28.8%, highlighting its key role in runoff generation; (2) groundwater storage increased at an average annual rate of 4.2 mm/a, with 6.3 mm/a during the glacier melt season from June to September; (3) annual groundwater discharge to surface water accounted for 53.4% of the total exchange, and surface water leakage to groundwater accounted for 46.6%, with contributions of 17.1% from glacier meltwater, 7.3% from snow meltwater, and 22.2% from rainfall infiltration recharge during 2005–2020. This study provides a framework for evaluating surface water-groundwater interactions in cryospheric regions globally.