MODELING THE EFFECTS OF CLIMATE CHANGE ON STREAMFLOW IN THE NOOKSACK RIVER BASIN, NW WASHINGTON

The Nooksack River watershed spans approximately 2300 km² in northwestern Washington State, with its headwaters in the North Cascades. The basin supports glaciers and an annual snowpack that is sensitive to the changes in temperature and precipitation forecasted by General Circulation Models (GCMs). Predicting the response of the Nooksack River to climate change is crucial for water resources planning purposes because regional municipalities, tribes, and industry depend on the river for water use and for fish habitat. We combine modeled climate forecasts and the Distributed-Hydrology-Soil-Vegetation Model (DHSVM) to assess changes to timing and magnitude of streamflow in the higher elevations of the Nooksack River basin due to climate change. The DHSVM is a physically based, spatially distributed hydrology model that simulates a water and energy balance at the pixel scale of a digital elevation model.

We use recent meteorological and landcover data to calibrate and validate the DHSVM to three watersheds that define the mountainous portion of the Nooksack River basin. Coarse-resolution GCM forecasts are being downscaled to the Nooksack basin for use as local-scale predictive input to the calibrated DHSVM. The downscaling procedure is consistent with the methodology used by previous studies examining the effects of climate change on water resources in the central Puget Sound (e.g., Palmer, 2007). The statistical downscaling process utilizes about 50 years of historical weather data from local stations, which results in meteorological forecasts that incorporate future climate trends and preserve local weather patterns and variability. We use multiple scenarios of possible future climate conditions to bracket the range of potential changes in the Nooksack River basin during three 30-year intervals in the future, centered on 2025, 2050, and 2075. Our preliminary modeling results are consistent with previous studies that predict decreases in summer flows, increases in winter flows, and a shift toward earlier spring snowmelt in western Washington drainages as regional climate warms.