2.1.1 Groundwater Abstraction
Consumptive water use is a management tool in SWAT that removes water from a basin. Water removed for consumptive use is designed to be lost from the system. SWAT allows water to be removed from a shallow aquifer, deep aquifer, reach, or pond within any subbasin in the watershed selectively. Consumptive water use varies from month to month. For each month in the year, an average daily volume of water is removed from the source and is specified (Neitsch, Arnold, Kiniry & Williams, 2005). A shallow aquifer composed of sand, gravel, and clay is distributed throughout South Korea. The aquifer has good porosity with adequate nutrients for cultivation so that it is mainly used as a major source of groundwater (Park, Lee, Koo & Kim, 2016). Thus, monthly groundwater use was applied to be removed from a shallow aquifer using consumptive water use tool in SWAT.
The National Groundwater Information Center updates annual groundwater use report which provides groundwater abstraction (GA) information with three types: agricultural, industrial, and domestic. Based on the report, the annual use data in study area from 1976 to 2015 were collected and they were monthly averaged in 10-year periods (the 1980s, the 1990s, the 2000s, the 2010s) so that SWAT could simulate GA scenario periodically. Figure 2 illustrated the monthly and annual groundwater use of the Geum River basin corresponding to the four decades. The annual groundwater use has gradually increased since the 1980s, appeared to have 107.6% increase rate in the 2010s compared with the 1980s. The monthly use was highly concentrated in June, July, and August. This pattern coincided with the precipitation trend of South Korea, in which most rainfall occurs in Summer. The groundwater use database has been constructed in subbasin unit for every decade. Figure 3 shows the spatial increment of groundwater use in every decade compared with the 1980s. Middle and Northern part of the target catchment showed relatively higher increase of groundwater use.
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