Evaluation of Long-Term Increased Groundwater Abstraction Impact on Watershed Hydrology in Han River Basin, South Korea

Distinguishing the hydrological impacts of anthropogenic groundwater withdrawal from natural climate variability is a critical yet complex challenge in sustainable water resource management. This study quantitatively evaluated the watershed-scale hydrological response to the increased groundwater abstraction in the Han River basin (35,770 km2) of South Korea using the Soil and Water Assessment Tool (SWAT). Groundwater use datasets for the 1970s and 2010s were constructed using groundwater statistical yearbooks. By applying the groundwater use datasets under 2010s weather conditions, we effectively isolated the specific effects of human usage. The results indicated that a rise in the annual groundwater abstraction from 9.6 to 22.3 million m3 reduced the average streamflow by 6.59%. The baseflow and groundwater recharge were identified as the most sensitive components, decreasing by 20.7% and 20.8%, respectively. Notably, intensive summer withdrawal (53% of the annual total) depleted aquifer storage, directly exacerbating streamflow reductions during the autumn and winter seasons. A flow duration analysis further confirmed that the duration of the dry season—defined by the flow exceeded for 275 days (Q275)—extended by 13 days, as the exceedance duration for the specific flow duration shifted from Q275 to Q263. These findings highlighted that excessive groundwater withdrawal compromises seasonal hydrological stability, necessitating integrated management strategies to secure the streamflow during critical dry periods.