Journal of Watershed Management Research

     Simulation of stream flow, prediction of the hydrological behavior of watersheds and understanding of various components of the hydrological cycle is important in order to plan for protection of water resources. On the other hand, lack of proper estimation of runoff in the watersheds, may cause serious problems in the optimum management of water resources and particularly the utilization of dams and water transfer systems. The main objectives of present study were application and test the performance of Soil and Water Assessment Tool (SWAT) as a simulator of monthly stream flow in the Ghareh Su watershed with an area of 4062 km². The simulation was carried out using flow data from the Samian, Pol Almas and Yamchi hydrometric stations for 8-hydrological years (from January 2003 to December 2010), where data for the six years (from January 2003 to December 2008) were used as a calibration period and validation period was done for two years (from January 2009 to December 2010). The performance of the model was evaluated by coefficient of determination ( ) and Nash–Sutcliffe efficiency ( ). The  values in Yamchi, Samian and Pol Almas hydrometric stations during the calibration and validation periods were 0.70, 0.68, 0.34 and 0.82, 0.53, 0.63, respectively. The  values were 0.51, 0.58, 0.09 and 0.51, -0.33, 0.60, respectively. The statistical analysis showed a satis factory agreement between observed and simulated monthly discharge values, in Yamchi hydrometric station during the calibration and validation periods. The weakness of the model to simulate flow for some months was probably due to a poor characterization of snowmelt processes, the discordance between model assumptions with flow transmission in frozen and saturated layers, lack of sufficient discharge data, and lack of input data for simulation of groundwater recharge and groundwater-river interaction.