Swat, developed by the Institute of water resources, Texas University of agriculture and engineering, is a long-term Distributed Watershed Model Based on the SWRRB model (Williams et al., 1985; Arnold et al., 1990). SWAT is a physical based model, which can simulate continuous time series. The watershed hydrological process simulated by SWAT is divided into land surface part of water cycle (i.e. runoff generation and slope confluence part) and water surface part of water cycle (i.e. River confluence part). The former controls the input of water, sand, nutrients and chemicals in the main channel of each sub basin; the latter determines the transport of water, sand and other substances from the river network to the outlet of the basin. The whole water circulation system follows the law of water balance. SWAT model involves precipitation, runoff, soil water, groundwater, evapotranspiration and river confluence.