Analysis of water balance by surface–groundwater interaction using the SWAT model for the Han River basin, South Korea

The water balance and groundwater dynamics due to surface–groundwater interactions for watershed health assessment were investigated for the Han River basin (34,148 km2) of South Korea using the Soil and Water Assessment Tool (SWAT). The model was established considering 4 multipurpose dams and 3 multifunction weirs. The SWAT was spatially calibrated and validated using daily observed inflows for the dam (2005–2014) and weir (2012–2014) as well as evapotranspiration, soil moisture, and groundwater level data (2009–2013). The simulation results revealed the impact of surface–groundwater exchange fluxes on the water balance and baseflow by evaluating the vertical water budget and horizontal water transfer. Evapotranspiration in the surface and return flows from the shallow aquifer for the dry season was estimated to be 29 and 10% higher than for the wet season, respectively. Percolation’s role was also significant, providing approximately 24% of the annual groundwater recharge to shallow aquifers in the rainy season. On average, the February to August period (A) was characterized by a net flux of infiltration into the groundwater. For the September to January period (B), the proportion of groundwater flow into the river of the basin was nearly balanced by a slight increase in surface water infiltration. During period A of average surface water infiltration into the groundwater, the net groundwater recharge was positive and up to 20% of the infiltration during this period resulted from groundwater recharge. These results showed that groundwater recharge is strongly affected by the surface water and groundwater interactions.