Potential climate chance impacts on mountain watersheds in the Pacific Northwest

Global climate change due to the buildup of greenhouse gases in the atmosph ere has serious potential impacts on water resources in the Pacific Northwe st. Climate scenarios produced by general circulation models (GCMs) do not provide enough spatial specificity for studying water resources in mountain watersheds. This study uses dynamical downscaling with a regional climate model (RCM) driven by a GCM to simulate climate change scenarios. The RCM u ses a subgrid parameterization of orographic precipitation and land surface cover to simulate surface climate at the spatial scale suitable for the re presentation of topographic effects over mountainous regions. Numerical exp eriments have been performed to simulate the present-day climatology and th e climate conditions corresponding to a doubling of atmospheric CO2 concent ration. The RCM results indicate an average warming of about 2.5 degrees C, and precipitation generally increases over the Pacific Northwest and decre ases over California. These simulations were used to drive a distributed hy drology model of two snow dominated watersheds, the American River and Midd le Fork Flathead, in the Pacific Northwest to obtain more detailed estimate s of the sensitivity of water resources to climate change. Results show tha t as more precipitation falls as rain rather than snow in the warmer climat e, there is a 60 percent reduction in snowpack and a significant shift in t he seasonal pattern of streamflow in the American River. Much less drastic changes are found in the Middle Fork Flathead where snowpack is only reduce d by 18 percent and the seasonal pattern of streamflow remains intact. This study shows that the impacts of climate change on water resources are high ly region specific. Furthermore, under the specific climate change scenario , the impacts are largely driven by the warming trend rather than the preci pitation trend, which is small.