Effects of Climate Change on Extreme Low-Flows in Small Lowland Tributaries in the Skagit River Basin
Matt Stumbaugh and Alan F. Hamlet
Management of extreme low-flows requires fundamental trade-offs between water extraction for human use (e.g. for irrigation and municipal water supply) and in-stream flows to protect aquatic ecosystems. In the context of protecting endangered salmon and other cold-water fish species in small streams, extreme low-flows are one of the most important aspects of the flow regime. We quantify projected changes in low-flow magnitude and timing for several lowland tributaries of the Skagit River basin in response to regional climate change. Ten hydrologic model simulations of mid-21st century (2030–2059) streamflows are compared against a historical period (1917–2006). Each of the hydrologic simulations are forced by atmospheric variables developed from respective CMIP3 global climate model (GCM) output downscaled to 1/16th degree resolution using the Hybrid-Delta downscaling method. Baseline historical simulations are forced by historical gridded meteorological data sets of temperature and precipitation, and additional meteorological variables reconstructed using the MTCLIM weather preprocessor. Hydrologic simulations were performed using the Distributed Hydrology Soil Vegetation Model (DHSVM) implemented at 30–m resolution. Analysis of the DHSVM streamflow simulations projects future low-flows in Skagit lowland tributaries will decrease by 5–20% and low-flow conditions will persist on the order of a week longer into early fall. For the Samish and Nookachamps basins, the projected changes in future low-flow regimes are larger than for the smaller basins included in the study. Projected changes in near-average low-flows are larger and more consistent between different climate change scenarios than are projected changes for the most extreme low-flow events.