Impacts of Forecasted Climate Change on Snowpack, Glacier Recession, and Streamflow in the Nooksack River Basin

Abstract:

Like many watersheds in the North Cascades Mountain range, streamflow in the Nooksack River is strongly influenced by precipitation and snowmelt in the spring and glacial melt in the warmer summer months. With a maritime climate and a high relief basin with glacial ice (3400 hectares), the streamflow response in the Nooksack is sensitive to increases in temperature, thus forecasting the basins response to future climate is of vital importance for water resources planning purposes. The watershed (2000 km²) in the northwest of Washington, USA, is a valuable freshwater resource for regional municipalities, industry, and agriculture, and provides critical habitat for endangered salmon species. Due to a lack of spatially distributed long-term historical weather observations in the basin for downscaling purposes, we apply publically available statistically derived 1/16 degree gridded surface data along with the Distributed Hydrology Soil Vegetation Model (DHSVM; Wigmosta et al., 1992) with newly developed coupled dynamic glacier model (Clarke et al., 2015) to simulate hydrologic processes in the Nooksack River basin. We calibrate and validate the DHSVM to observed glacial mass balance and glacial ice extent as well as to observed daily streamflow and SNOTEL data in the Nooksack basin. For the historical period, we model using a gridded meteorological forcing data set (1950-2010; Livneh et al., 2013). We simulate forecasted climate change impacts, including glacial recession on streamflow, using gridded daily statically downscaled data from global climate models of the CMIP5 with RCP4.5 and RCP8.5 forcing scenarios developed using the multivariate adaptive constructed analogs method (Abatzoglou and Brown, 2011). Simulation results project an increase in winter streamflows due to more rainfall rather than snow, and a decrease in summer flows with a general shift in peak spring flows toward earlier in the spring. Glacier melt contribution to streamflow initially increases throughout the first half of the 21^st century and decreases in the latter half after glacier ice volume decreases substantially.