Abstract

Climate change is projected to result, on average, in earlier snowmelt and reduced summer flows, patterns not well represented in historical observations used in water planning. We extend efforts in the Puget Sound basin cities of Everett, Seattle, and Tacoma to characterize differences between historic and future streamflow and ability to meet future demands. We simulate streamflow for the 2020s, 2040s, and 2080s from the Distributed Hydrology-Soil-Vegetation Model, driven by downscaled ensembles of climate simulations archived by the 2007 IPCC Fourth Assessment Report. We use these streamflow predictions as inputs to reservoir system models for the three water supply systems. Over the next century, under average conditions all systems are projected to experience declines and eventual disappearance of the springtime snowmelt peak. How these shifts impact management depends on physical characteristics, operating objectives, and the adaptive capacity of each system. Without adaptations, average seasonal drawdown of reservoir storage is projected to increase in all three systems throughout the 21st century. Reliability of all systems in the absence of demand increases is robust through the 2020s, and remains above 98% for Seattle and Everett in the 2040s and 2080s. With demand increases, however, climate change effects progressively reduce system reliability.