The undergoing changes in the global climate are expected to alter the hydrologic regime of many river basins worldwide. Understanding the potential vulnerabilities of water systems under these evolving conditions plays an important role in supporting strategic planning decisions. In this reserach, we contribute a multi-objective assessment of the impacts of hydrologic uncertainty on the operations of multipurpose water reservoirs systems in arid river basins. We focus our analysis on the Dez and Karoun river system, the largest catchment in Iran, under synthetically generated scenarios of streamflow and evaporation rates. The three modeled reservoirs, namely Dez, Karoun and Masjed Soleyman, are responsible for the production of 20% of total hydropower generation of the country. These dams control most of the water flowing to the downstream part of the system, where irrigation and domestic supply are strategic objectives, along with flood protection downstream of Masjed Soleyman dam. Our numerical results show that the performance of the multireservoir system is extremely sensitive to the alterations of both streamflow and evaporation rates, with evaporation rates’ uncertainty resulting to be as relevant as the one of streamflow. In addition, we show that long-term objectives are mainly vulnerable to inflows uncertainty, while evaporation rates’ uncertainty becomes more relevant on short-term objectives.