The disparate temporal and local distribution of freshwater and the rapid growth of the population in recent decades have led to problems in providing water resources for various applications. Hence, the supply of water in many countries such as Iran has become one of the most important challenges of the present century. Modern mathematical models have been developed for studying the complex hydrological processes of a watershed. The application of conceptual hydrological models is an important issue in watersheds for researchers, especially in arid and semiarid regions. The hydrological behaviors are complicated in such watersheds, and their calibration is more difficult. In this research, the conceptual and semidistributed SWAT model is used for Samalqan watershed, Iran, with 1148 km2 area. Streamflow simulation is considered for 13 years. Samalqan watershed modeling led to 22 subbasins and 413 hydrologic response units. Water balance components have been computed, the results have been calibrated with the SUfI2 approach in the SWAT-CUP program, and finally, the performance of the model is evaluated. The sensitivity analysis was conducted using 26 SWAT parameters. The most sensitive parameters were CN2 (moisture condition II curve number), GWQMN (threshold water level in shallow aquifer for base flow), GW_Delay (delay time for aquifer recharge), GW_REVAP (Revap coefficient), and ESCO (Soil evaporation compensation coefficient). The model was calibrated from 2004 to 2012 and validated from 2012 to 2014. The coefficient of determination (R2) and Nash and Sutcliffe efficiency (NSE), On monthly basis, were between 0.60–0.80 and 0.80–0.95 respectively, for calibration, and 0.70–0.90 and 0.70–0.80, respectively, for validation periods, which indicates that the model results are satisfactory. Results show that the SWAT model can be used efficiently in semiarid regions to support water management policies and development of sustainable water management strategies. Due to the arid and semiarid climate of Iran and water resources constraints, the need for modeling is clear. It can be concluded that determination of the existing water potential is necessary for water planning and management of water resources in the watershed.

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