Abstract
This study used Soil and Water Assessment Tool (SWAT) to investigate the
impacts of groundwater abstraction and forest growth on the watershed
hydrology of Geum River basin (9,645.5 km2), South
Korea. Groundwater abstraction (GA) and forest growth (FG) data from
1976 to 2015 (1980s;1976~1985, 1990s;
1986~1995, 2000s; 1996~2005,
2010s;2006~2015) were prepared with 10-year interval as
SWAT input data, respectively. SWAT was calibrated
(2006~2015) using daily observation data from two
multipurpose dams and three multifunction weirs. The dam and weir
calibration result showed coefficient of determination
(R2) of 0.78, 0.81, Nash-Sutcliffe efficiency (NSE) of
0.79, 0.76, root mean square error (RMSE) of 1.96 mm/day, 0.55 mm/day,
and PBIAS of -5.48%, 8.56%, respectively. The SWAT ran at each decade
using corresponding GA and FG information under the same weather
condition of the 2010s to evaluate the impact of GA and FG on hydrologic
cycle. Influenced by both GA and FG, the streamflow at the watershed
outlet showed the decrease of 1.3% (10.1 mm/year), 4.4% (34.2
mm/year), and 7.8% (60.3 mm/year) in the 1990s, the 2000s, and the
2010s, respectively. The hydrologic response of surface runoff, lateral
flow, groundwater flow, and soil moisture showed decreasing trend while
evapotranspiration and groundwater recharge showed increasing trend. GA
imposed bigger influence on the spatial and temporal loss of streamflow
than FG. Especially, it was discovered that the agricultural water use
from groundwater was the most influential factor that has decreased
total runoff in the target watershed for the last four decades.
Keywords: Stream drying phenomena, Groundwater abstraction,
Forest growth, SWAT.