Modeling the effects of climate change on streamflow and stream temperature in the South Fork of the Stillaguamish River

The South Fork of the Stillaguamish River in Northwest Washington State is an important regional water resource and habitat for several threatened salmonid species. Snowpack is the main contributor to spring and summer streamflow and currently serves as a stream temperature buffer. Previous modeling studies of mountainous Puget Sound river basins predict that forecasted climate warming will result in less precipitation falling as snow in the winter, resulting in a reduced snowpack and a lower spring runoff. Forecasted lower spring and summer flows have been shown to increase stream temperatures (e.g., Cao et al., 2015). To predict changes in streamflow and stream temperature in the South Fork Stillaguamish River, we used projected, gridded meteorological data in a physically-based streamflow model, the Distributed Hydrology Soil Vegetation Model (DHSVM), and a spatially-distributed stream temperature model, the River Basin Model (RBM). We established digital spatial characteristics of the South Fork basin at a 50-meter grid resolution, including soils, landcover, and elevation. We calibrated the DHSVM to measured stream discharge from a Washington State Department of Ecology (Ecology) stream gauge using historical gridded meteorological data processed by the Climate Impacts Group at the University of Washington. Field work was conducted in the summer and fall of 2018 to determine stream morphology, discharge, and temperature at several sites throughout the basin. Data collected from field work were used to calibrate the RBM to stream temperature data from the Ecology stream gauge. We will use the calibrated models to simulate the effects of climate warming into the 21st century at 3-hour time steps using gridded downscaled data from ten global climate models of the Coupled Model Intercomparison Project Phase 5 with representative concentration pathways scenarios 4.5 and 8.5. Simulations will be processed at 30-year intervals surrounding the years 2025, 2050, and 2075.