Modeling 21st Century Peak Flows in the Nooksack River Basin in Northwestern Washington State Using Dynamically-Downscaled Global Climate Model Projections

The Nooksack River in northwestern Washington State provides freshwater for agriculture, municipal, and industrial use and serves as a vital habitat for endangered salmon, a resource that is of cultural and economic importance to the Nooksack Indian Tribe and the surrounding region. As more landscape becomes exposed to rain rather than snow and heavy winter precipitation events intensify (i.e., atmospheric rivers), peak flows and sediment delivery to streams will increase due to rapid runoff, resulting in salmon habitat degradation and increased flood risk. Thus, anticipating the effect of climate change on peak flows is crucial for salmon habitat restoration efforts and flood mitigation planning. To quantify the timing and magnitude of future peak flows, I use a calibrated Distributed Hydrology Soil Vegetation Model (DHSVM) and meteorological forcings from an ensemble of high-emission GCMs dynamically-downscaled using the Weather Research and Forecasting (WRF) model. By the end of the 21st century, results indicate a decrease in annual peak snow-water equivalent (-72% to -82%), an increase in winter flows (+31% to +56%), and the disappearance of the snowmelt derived spring peak in the hydrograph as the basin transitions from transient to rain-dominant. Future peak flows are projected to increase in magnitude by 34-60% across all flow durations (3 to 168 hours) and return periods (2 to 100 years) that were analyzed, with the largest changes occurring in the high relief subbasins. The frequency of high magnitude, flood-inducing peak flows will also increase into the future, lengthening the flood season by approximately three months