Spatial and temporal changes in runoff caused by climate change in a complex large river basin in Oregon

SummaryWe estimated potential changes in annual, seasonal, and high and low runoff and associated uncertainty in the 218 sub-basins of the Willamette River basin of Oregon for the 2040s and the 2080s. The US Geological Survey’s Precipitation–Runoff Modeling System (PRMS) was calibrated and validated for representative river basins between 1973 and 2006. A regionalization method and GIS analysis determined the PRMS model parameters for ungauged basins. We used a combination of eight general circulation models (GCMs) and two emission scenarios downscaled to 1/16° resolution to estimate spatial and temporal changes in future runoff at a sub-basin scale. The seasonal variability of runoff is projected to increase consistently with increases in winter flow and decreases in summer flow. These trends are amplified under the A1B emission scenario by the end of the 21st century with increases in top 5% flow and decreases in 7-day low flow. The ratio of snow water equivalent to precipitation declined consistently throughout the basins extending into the Cascade Range. The center timing of runoff, the day when half of the water-year flow has passed, is projected to occur earlier in the water year. Snowmelt-dominated basins exhibit large reductions in summer flow in response to increased temperature, while rainfall-dominated basins show large increases in winter flow in response to precipitation change. The spatial and temporal variability of runoff may increase in the future, but the direction and magnitude of these changes depend on sub-basin characteristics such as elevation and geology. Streams flowing from High Cascade basins that contain a large component of groundwater are projected to sustain summer flows, although the uncertainty associated with future projections is high. The main source of uncertainty stems from GCM structure rather than emission scenarios or hydrologic model parameters, but the hydrologic model parameter uncertainty for projecting summer runoff and 7-day low flow is relatively high for Western Cascade basins.