Abstract

Climate change is projected to result, on average, in earlier snowmelt and reduced summer flows, patterns that are not well represented in the historical observations used for planning and reliability analyses by water utilities. We extend ongoing efforts in the Puget Sound basin cities of Everett, Seattle, and Tacoma to characterize differences between historic and future streamflow and the ability of the region’s water supply systems to meet future demands. We use future streamflow simulations for the 2020s, 2040s, and 2080s from the Distributed Hydrology-Soil-Vegetation Model (DHSVM), driven by climate simulations archived by the 2007 Fourth Assessment Report (AR4) of the Intergovernmental Panel on Climate Change (IPCC). We use ensembles of streamflow predictions produced by DHSVM forced with multiple downscaled ensembles from the IPCC climate models as inputs to reservoir system models for the Everett, Seattle, and Tacoma water supply systems.

Over the next century, under average conditions, all three systems are projected to experience a decline and eventual disappearance of the springtime snowmelt peak in their inflows. How these shifts impact water management depends on the specifics of the reservoir system and their operating objectives, site-specific variations in the influence that reductions in snowmelt have on reservoir inflows, and the adaptive capacity of each system. Without adaptations, average seasonal drawdown of reservoir storage is projected to increase in all of the systems throughout the 21st century. The reliability of the three water supply systems in the absence of demand increases is, however, generally robust to climate changes through the 2020s, and in the 2040s and 2080s reliability remains above 98% for the Seattle and Everett systems. With demand increases, however, system reliability is progressively reduced by climate change impacts.