Simulation of runoff and sediment yield using SWAT in Gawshan dam watershed

Soil erosion by water is a global threat to the environment which negatively affects soil and water quality. Soil erosion modeling is an efficient method to simulate soil erosion, to identify sediment source areas, and to evaluate soil conservation measures. In this study the Soil and Water Assessment Tool (SWAT) was used to model runoff and sediment in Gawshan dam watershed in west of Iran. For this purpose, climatic data obtained from a period of 11 years, from January 1, 2005 to the end of December 2015, from the stations of Bavale, Ravansar, Sanghor, Ghorve, Sanandaj, Kermanshah, Kangavar and Kamiyaran were used. We used two different approaches for estimating of model parameters. In the forward deterministic approach model parameters were independently derived from digital elevation model (DEM), soil and land use maps and climate data, whereas in the inverse stochastic approach model calibration were performed by sensitive model parameters using Sequential Uncertainty Fitting (SUFI-2), which is one of the programs interfaced with SWAT, in the package SWAT-CUP (SWAT Calibration Uncertainty Programs). Model performance was evaluated using the coefficient of determination (R2) and Nash-Sutcliff efficiency (NS) for forward approach and P factor and R factor for inverse approach. Results showed that in the forward approach, based on monthly data (2008-2013), the model performance was satisfactory for runoff (R2=0.66; NS=0.63) and fair for sediment (R2=0.42; NS=0.3). However, model prediction improved for both runoff (R2=0.94; NS=0.83) and sediment (R2=0.61; NS=0.53) when they were averaged on monthly basis. Calibration of the model parameters improved its performance for runoff (P factor>0.6; R factor<1) and sediment (P factor>0.4; R factor <1). These results demonstrated that the SWAT has the potential to be used as a decision making tool for Gawshan dam watershed management.