Changes in the demand for water amongst the human and environmental systems in the coming decades may pose profound implications for water availability and management. With its ability to address uncertainty in global change, the MIT Integrated Global System Modeling (IGSM) framework has also enabled a Water Resource System (WRS) module to explicitly account for water management at large river basins as well as for more detailed, regional assessments over the United States.  We will present our latest results from a suite of projections that assess the impact of human and environmental changes on water availability for managed river basins in the coming decades. Case studies will be presented on developed and developing countries.