Changes in Seasonal and Extreme Hydrologic Conditions of the Georgia Basin/Puget Sound in an Ensemble Regional Climate Simulation for the Mid-Century

Abstract

In this study we examine an ensemble of climate change projections simulated by a global climate model (GCM) and downscaled with a regional climate model (RCM) to a 40-km spatial resolution for western North America. One control and an ensemble of three future climate simulations were produced by the GCM. The ensemble members followed a ’business as usual’ scenario for greenhouse gases and aerosols emissions from 1995 to 2100. The RCM was used to downscale the GCM control simulation (1995–2015) and each ensemble future GCM climate (2040–2060) simulation. Analyses of the regional climate simulations for the Georgia Basin/Puget Sound showed a warming of 1.5–2 °C and statistically insignificant changes in precipitation by mid-century. The temperature change has large impacts on snowpack (about 50% reduction) but relatively smaller impacts on the total runoff for the basin as a whole. The changes can strongly affect small watersheds such as those located in the transient snow zone, causing both a higher likelihood of winter flooding, as a higher percentage of precipitation falls in the form of rain rather than snow, and a reduced streamflow in early summer. In addition, there are large and different changes in the monthly total runoff above the one-percentile from October through May, and the December flood volume of the future climate is 60% above the maximum monthly flood volume of the control climate. Uncertainty of the climate change projections, as characterized by the spread among the ensemble future climate simulations, is relatively small for the basin mean snowpack and runoff, but increases in smaller watersheds, especially in the transient snow zone, and the associated extreme events. This emphasizes the importance of characterizing uncertainty through ensemble simulations.