Watersheds are the fundamental unit for managing water and natural resources. Watershed analysis using science-based tools such as numerical models allows PNNL researchers to assess the impacts of climate change, hydropower, and irrigation on watershed health. PNNL staff developed a state-of-the-art modeling system (DHSVM) that describes the complex interactions of the atmosphere, biosphere and hydrosphere. We use this modeling system to assist our clients in assessing the impact of land-use changes on stream flow and water quality. The system is also used to study possible regional impacts of climate change on the magnitude and timing of runoff and the resultant effect on the timing and availability of water for fisheries and irrigation. Our research in watershed hydrology has been recognized by the National Academy of Sciences. According to the Academy, this work "represents the state of the art in spatial modeling and emphasizes the need for more complete and informative spatial data for model inputs and environmental decision making."