In recent years, the climate change impacts on the water supply and the water resources availability have become an important issue. Reservoirs planning and management, as one of the most important water resources, are crucial components in the basin management. Therefore, studying the climate change impacts on operation and the reservoirs optimal water allocation are the major issues that should be considered in the basin management. This study has investigated Zayanderud basin, which is considered as one of the prominent basins in the country that has been suffered from lack of integrated management, climate change and increasing water consumption. To tackle the future challenges, a hydrological model of Zayanderud basin has been constructed using soil and water assessment tool (SWAT) model. The simulation results have been calibrated and validated using Sequential Uncertainty Fitting Algorithm (SUFI-2). To study the impact of climate change on reservoir inflow, precipitation and temperature data of five general circulation models (GCM) for scenarios RCP2.6, RCP4.5, RCP6.0, and RCP8.5 have been downscaled. The results have been used in the calibrated SWAT model as inputs to predict the future reservoir inflow. The optimal water allocation of the Zayanderud dam has been estimated considering climate change impacts using the Lingo software and the reservoir performance has been investigated. Based on the calibration period results (1991-2009), the Nash-Sutcliff coefficient and the coefficient of determination in Qaleh Shahrokh and Eskandari stations have been 0.75, 0.79, 0.6 and 0.65 respectively. For the Validation Period (2009-2016), the coefficients have been 0.82, 0.83, 0.70 and 0.75 respectively. The weighting results of GCMs indicate a high uncertainty in the GCMs prediction for the precipitation variable compared with temperature. The simulation results indicate that reservoir inflow reduces 40 to 50 percent in different climate change scenarios compared with the base period. According to the optimal water allocation results of Zayanderud dam, the maximum monthly water deficit occurs in spring for different scenarios. The maximum volume of annual water deficit equals to 841 MCM which occurs in RCP8.5 scenario. Also, the minimum sustainability index is estimated regarding to the RCP8.5 scenario based on the calculation of reservoir performance indices for the future. Keywords : optimal water allocation of the reservoir, SWAT model, SUFI-2 algorithm, climate change, Lingo software
