Abstract

This report, prepared by the Climate Change Technical Subcommittee of the central Puget Sound (WA) Regional Water Supply Planning Process, presents the results of using the climate impacted meteorology in hydrology models for the five major water supply drainage basins (the Sultan, South Fork Tolt, Cedar, Green and White rivers) in the study area.

Climate impacts to streamflow were found to be substantial in each of the five basins, with the magnitude differing across basins. In all basins, earlier snowmelt caused the spring peak in the hydrograph to occur earlier. This leads to lower early summer flows at each location investigated. The least amount of change occurs in the GISS-B1 model, where, despite being drier than the other models, warming is substantially lower than in other GCMs. Relatively large changes in early summer flows are seen in the IPSL-A2 and ECHAM-A2 models as median summer flows become increasingly reduced in the projected future. Basins where precipitation falls predominantly as rain (Green River) were less affected in spring shifts than those in which snow has been more dominant in the past. For the primary sites used to characterize inflows into the reservoirs , by 2075 the ensemble averages across all five basins compared to historic flows decrease by 37% during the summer and increase by 48% during the winter.