Abstarct: The hydrological cycle has been substantially influenced by climate change and human activities. In this paper, the Water and Energy Transfer between Soil, Plants and Atmosphere (WetSpa) hydrologic model was used to investigate the potential impacts of climate change on hydrology of the Jajrood watershed, North Iran. For this purpose, climate data from HadCM3 model was used under three emission scenarios, namely A1B, A2 and B1 for three future periods (2010-2039, 2040-2069, 2070-2099). To downscale climate data, LARS-WG model was employed. Climate change impacts on river runoff were then investigated. The WetSpa model calibration and validation showed reasonable performance of the model with respect to selected performance criteria. Finally, results showed that the mean annual runoff will decrease in all scenarios and future periods: the most reduction in annual runoff is related to the pessimistic scenario A2 in the far future (2070s) while B1 scenario will have the minimum runoff reduction during the near future period (2010s).For the study watershed, climate change will increase the mean monthly runoff to increase with respect to the future periods for all scenarios in the cold months of the year e.g. Jan, Feb, Mar and Dec, while it will decrease for the rest months of the year. In this research the WEAP model was used to simulate the allocation in terms of climate change, how to encounter the needs and water storage changes of Latyan Reservoir was assessed in terms of constant resources. Comparing the status of the reservoir storage to provide percentages for each of the scenarios and time steps due to changes in supply, it will be discovered that when the reservoir is empty, supply percent is less than when, the reservoir storage is higher.