The Water Supply Stress Index Model (WaSSI) is a web-based tool that can be used to project the effects of land use change, climate change, and water withdrawals on river flows, water supply stress, and ecosystem productivity (i.e. carbon sequestration dynamics) across the conterminous United States and Mexico. As water yield and carbon sequestration are tightly coupled, WaSSI is useful for evaluating trade-offs among management strategies for these ecosystem services.