Predicting Streamflow, Snowpack, and Stream Temperature Sensitivities to Climate Change in the Pacific Northwest’s Green River Basin

Climate change will have significant impacts on Pacific Northwest hydrology. Rising temperatures and shifts in precipitation will lead to changes in snowpack, runoff, and streamflow timing. For water and environmental resource managers, an improved understanding of climate impacts at the watershed scale is critical for regional mitigation and adaptation. The Pacific Northwest’s Green River Basin is a valuable water supply, major flood risk, and provides habitat to cold-water aquatic species including the threatened Puget Sound Chinook salmon. Streamflow in the basin is seasonally regulated for flood prevention and flow modulation for ecosystem health, and changes in the annual hydrologic cycle could have consequences for flood/drought risk and ecosystem habitat. This study investigates the implications of climate change on streamflow, snowpack, and stream temperatures in the Green River Basin. Climate sensitivity analysis and future climate change impacts are simulated using two models with varying spatial and process complexity: 1) the conceptual Snow17/Sacramento Soil Moisture Accounting model (Snow17/Sac) implemented with two elevation zones and 2) the process-oriented Structure for Unifying Multiple Modeling Alternatives (SUMMA) model implemented using twelve USGS HUC12 subareas. Future climate change impacts on basin hydrology and stream temperatures are assessed using an ensemble of statistically downscaled climate projections from Global Climate Models (GCMs) run as part of the Intergovernmental Program on Climate Change 5th Assessment Report. The application of future warming scenarios lead to moderate changes in streamflow volume, timing, and risk of extremes (flood and drought) which differ depending on the watershed model. Presented are key results and findings from the study, and comment on potential impacts to related variables such as stream temperature.