Investigating the effects of small dams and land use changes on hydrological processes using SWAT in Kordan watershed

This study aimed at investigating the effects of watershed management operations including check dams and vegetation restorations on the amount of runoff and hydrological processes of the catchment. For this purpose, the Kordan watershed located in Alborz province, with an area of 357 square kilometers was selected, where the mentioned operations have been performed in parts and predicted for most parts. In addition, the SWAT model was used to simulate monthly hydrological components. The model was calibrated for a 14-year period (1998-2012) and validated for a 4-year period from early 2013 to late 2016. Statistical coefficients of calibration period NS = 0.88 and R2 = 0.91 and validation period NS = 80 and R2 = 0.85 confirmed the model calibrated well. The simulation results of the model for the period 1998 to 2016 showed that the total surface runoff and lateral flow were 21 and 18% of the average annual rainfall and subsurface flow was about 20%, respectively. Applying different scenarios showed that the combination of biological operations (e.g. land cover rehabilitation) along with mechanical structure operations (e.g. check dams) made maximum effects on runoff reduction, so that almost all of the high drainage and steep surface runoff in the basin will be gentle. In these conditions, the direct runoff of the basin decreases from 39% to 16%. Therefore, combinational operations have significant effects on flood control, runoff reduction and consequently increase subsurface flow components.