Abstarct: Climate change is one of the most important challenges of the current century. Because of the importance that climate change effects have on the future of the Earth and its inhabitants, it should be further investigated to understand the characteristics of future climatic events. In this study, HEC-HMS hydrologic model was used to study the potential impacts of climate change on surface water resource of the Latyan Dam watershed. For this purpose, climate data from HadCM3 and Canesm2 models under two emission scenarios, namely, A2 and B2 for HadCM3 and Rcp8.5, Rcp2.6 for Canesm2 for two future periods (near future 2010-2039, far future 2070-2099) was utilized. To downscale climate data, SDSM model was employed. The HEC-HMS model calibration and validation showed reasonable performance of the model with respect to selected performance criteria. Climate change impacts on river runoff were then investigated. Unlike temperature that would increases in both models, annual average runoff would increase in the Canesm2 and decreases in the HadCM3. The maximum increase in the average maximum temperature is related to the Canesm2 equal to 3.8 degrees Celsius by the year 2100 in October. The pessimistic scenario A2 would cause 21 million cubic meters decrease in runoff volume in the far future compared to the baxseline period. In this research, in order to simulate the operating conditions of Latyan Reservoir affected by climate change, WEAP model was used to simulate reservoir operation. Assuming the change in supply resources as well as changes in demand suggest reducing the average percentage of annual supply with increase in time horizon in a way that in the pessimistic scenario A2, the most severe changes would happen. Percent of the average monthly water in Latyan Reservoir in January, February and March is almost zero in all scenarios and future periods while in May, it has the highest level in the near future period (about 60%). It clearly indicates the effect of different time horizons on reservoir storage and subsequently on supply the demand.