Climate change, insect defoliation, and management practices alter species composition and hence plant water use and catchment water yield. Tree based sap flow data measures water yield on a tree basis allowing us to gather more accurate and detailed data regarding evapotranspiration for individual tress and thus models for individual species.  Scaling up these species-specific sap flow models to the watershed level across a range of climatic conditions, sizes, elevation gradiaents and compositions allow us to determine the impact of changes in climate and/or species composition to catchement water yield.