5. CONCLUSIONS
The long-term groundwater abstraction and forest growth impact on the watershed hydrology was evaluated using SWAT. Using groundwater use and forest growth database from 1976 to 2015 that are prepared in 10-year interval (1980s;1976~1985, 1990s; 1986~1995, 2000s; 1996~2005, 2010s;2006~2015) as SWAT input data, SWAT was calibrated based on the daily observation data at two multipurpose dams and three multifunction weirs in the study area. Calibrated SWAT estimated the watershed hydrology of three scenarios in terms of hydrological components. The hydrologic response to GA and FG was measured by comparing estimated components.
The long-term change of GA and FG in the target watershed affected the watershed hydrology and caused the spatial and temporal decrease of TR. Temporally, the decrease of TR influenced by GA and FG showed 3.6%, 10.3%, and 14.9% in the 1990s, the 2000s, and the 2010s, respectively. Spatially, the subbasins with small stream were appeared to be more vulnerable to the loss of streamflow. Furthermore, the analysis on flow-duration curve showed that the decadal change of GA and FG has consistently worsened the annual streamflow condition and their impact on the streamflow loss became stronger in lower flow rate.
The monthly analysis on the change of hydrological components explained that the decadal change of GA and FG mainly influenced the hydrological components in Summer due to the development of cultivation under structure and the vegetation growth while GF showed the biggest decrease. And decreasing GF in Winter came from water curtain cultivation expanded for the last four decades. By the way, GA showed bigger influence on the spatial vulnerability of TR loss than FG. Among three types of GA, agricultural use from groundwater showed the highest increase temporally and accordingly the fluctuation of GF in the monthly hydrologic response was biggest in Summer, farming season in South Korea. Therefore, we can infer that the agricultural water use pumped from groundwater use imposes the biggest impact on the stream drying in the Geum River basin.
In this study, groundwater abstraction and forest growth were selected as the stream drying factors that possibly influence the watershed hydrology. However, there are still many factors that are expected to affect the hydrology including land use change, road network development, and soil erosion. These factors have temporally developed like groundwater abstraction and forest growth. Thus, further studies on them are necessary to figure out comprehensive impact of stream drying factors on the watershed hydrology and to determine the priority of water resource management.