2. DATA AND METHODS
2.1 Study Area
The Geum River is one of the four national and major rivers in South Korea, located in the Midwest part of the country with a basin area of 9,912 km2 and a river length of 397.79 km. The rivers in South Korea are generally classified into three groups: rivers of state, local level 1, and local level 2. Currently, there are 11 rivers of state with a total of 397.79 km, 20 rivers of local level 1 with a total of 358.70 km, and 460 rivers of local level 2 with a total of 3000.87 km in the Geum River basin. However, among 460 rivers of local revel 2, 39 rivers are experiencing stream drying phenomena and their total length is 25km (Korea Water Resources Corporation, 2009).
The Geum River originates from southwestern South Korea, where Yongdam Dam (YDD) is located. It passes through Daecheong Dam (DCD), Sejong Weir (SJW), Gongju Weir (GJW) and Baekjae Weir (BJW) and discharges into the West Sea. The five hydraulic structures support five million people living in the region and are crucial since most hydrologic inflows arriving during the flood season are stored in the facilities and used throughout the dry season.
The dominant soil textures of the study are sandy loam and loam which occupies 58% and 24%. The predominant land use in the basin is forest that accounts for 62% (5,980 km2). Cultivated cropland, composed of paddy fields and upland crops, is mainly distributed in lowland fertile areas covering 26% (2,508 km2). The remaining areas are filled with urban, grassland, and bare fields that each cover 3-5% of the whole catchment.
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2.1 Soil and Water Assessment Tool (SWAT)
SWAT is a watershed hydrology model developed to quantify the impact of land management practices. The model operates by dividing the watershed into subbasins, with each subbasins being connected to stream channels. Subbasins are further divided into hydrological response units (HRUs), which are portions of a subbasin that possess unique land use, management, and soil attributes. The simulation of the hydrological cycle is based on the water balance equation: