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

Abstract

This study used Soil and Water Assessment Tool (SWAT) to investigate the impacts of groundwater use increase and forest growth on the watershed hydrology of Geum River basin (9,645.5 km 2 ), South Korea. Groundwater use increase and forest growth data from 1976 to 2015 were prepared in 10-year interval and were reflected to SWAT corresponding to each decade. SWAT was calibrated in the aspect of evapotranspiration, soil moisture, and streamflow using the observation data. The model performance for streamflow was evaluated by coefficient of determination (R 2 ), Nash-Sutcliffe efficiency (NSE), root mean square error (RMSE), and percent bias (PBIAS). The calibration achieved the average R 2 value of 0.73 ~ 0.82, NSE value of 0.75 ~ 0.81, RMSE value of 0.53 ~ 2.35 mm/day, and PBIAS value of -2.51 ~ +11.74%, respectively. The model performance for evapotranspiration and soil moisture was evaluated by R 2 . The calibration result of evapotranspiration and soil moisture achieved average R 2 value of 0.45 and 0.44, respectively. The calibrated model evaluated the impact of two factors on watershed hydrology. Decadal increase of groundwater use has decreased groundwater flow and increased groundwater recharge while decadal forest growth has mainly increased evapotranspiration that led to the decrease of other hydrological components. Resultingly, the change of two factors have imposed temporal decrease of total runoff on the watershed while the influence of two factors on annual streamflow loss was bigger in lower flow rate.