The present study provides a preliminary analysis of the impact of climate change on a water resources system of Alentejo region in the South of Portugal. Regional climate model HadRM3P forced by the Global Circulation Model HadAM3P A2 of the Hadley Centre, is used to derive temperature and precipitation data, which in turn is used as input to hydrological model (SHETRAN) for simulation of future streamflow. Dynamic programming based models are used for operation of reservoir system in order to examine the impact of climate change. Results are obtained for future streamflows and observed historical series, in order to analyze the performance of system operation in climate change scenario.