Assessment of Long-term Groundwater Use Increase and Forest Growth Impact on Watershed Hydrology

Geum River basin is currently suffering from stream drying which is caused by various reasons. Among many reasons, the expansion of groundwater use and forest growth which are known to pose an influence on stream drying have significantly developed over the past 40 years in Geum River basin. Therefore, the periodic change of two factors were reflected to SWAT to figure out their influences on watershed hydrology and stream drying. The periodic change was considered by using 10-year period data from the 1980s (1976 ~ 1985) to the 2010s (2006 ~ 2015), and applying the condition to SWAT. The model was calibrated based on observed data of streamflow, evapotranspiration, at monitoring points including dam, weir, flux tower, and soil moisture sensor. The calibration result showed satisfactory result evaluated by coefficient of determination (R 2 ), Nash–Sutcliffe Efficiency (NSE), scatter index (SI), and percent bias (PBIAS). The impact of groundwater use and forest growth was evaluated by hydrologic responses obtained by differentiating and comparing their conditions by period while settling weather conditions. As a result, the increase of groundwater use lowered groundwater recharge and groundwater flow while forest growth led to the rise of evapotranspiration which lessened surface runoff and the infiltration to soil layer. These two series of processes reduced total runoff showing decreased value of 2.7%, 6.3%, and 8.9% in 1990s, 2000s, and 2010s compared to 1980s.