Over the past few decades, increasing of greenhouse gases has led to climate change. Factors which increase greenhouse gases are divided into natural and human. Human factors are including fossil fuels, urban development, deforestation and desertification. Climate change is defined as changes in weather during a long period of time based on recorded information. The main goal of this research is to investigate the effect of climate change on surface runoff in the Gamasiab basin. Here we used HADCM3 model under B2, A2 scenarios for the near future (2021-2050) and far future (2070-2099). Results show that HADCM3 is a suitable model for the simulation of climate parameters.in this work we downscaled climate data using SDSM4.2 Model. According to SDSM4.2 calibration, among 26 climate parameters, 3 parameters have the strongest correlation with the average temperature and five parameters have the most correlation with the average precipitation in Gamasiab basin. Calibration of SDSM4.2 model in Gamasiab indicates that RS (coefficient of explaining) for average temperature and average precipitation is about 0.874 and 0.266 respectively. These results prove that for down scaling, SDSM 4.2 is an efficient model. In this research the periods 1971-1995 and 1996-2000 were used for calibration and validation respectively. Climate parameters were simulated for the near future (2021-2050) and far future (2070-2099) and were compared with observation period (1981-2010). Results demonstrate that the average temperature and average precipitation for both periods and both scenario have increased and decreased respectively. We simulated surface runoff by IHACRES model. Input data were daily temperature, precipitation and discharge. For calibration of IHACREC model aforementioned daily parameters (temperature, precipitation and discharge) from 1975 to 1990 were added to model and for validation again we used these daily parameters from 1991 to 2000. After evaluating the efficiency of model, discharge was simulated for near and far future. Based on the results it should be announced that runoff in the whole basin for both near and far future and both scenario was decreased. Key words: climate change, SDSM, IHACRES, surface runoff, Gamasiab Watershed
