Impacts of 21st‐Century Climate Change on Hydrologic Extremes in the Pacific Northwest Region of North America

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Abstract
Climate change projections for the Pacific Northwest (PNW) region of North America include warmer temperatures (T), reduced precipitation (P) in summer months, and increasedPduring all other seasons. Using a physically based hydrologic model and an ensemble of statistically downscaled global climate model scenarios produced by the Columbia Basin Climate Change Scenarios Project, we examine the nature of changing hydrologic extremes (floods and low flows) under natural conditions for about 300 river locations in thePNW. The combination of warming, and shifts in seasonalPregimes, results in increased flooding and more intense low flows for most of the basins in thePNW. Flood responses depend on average midwinterTand basin type. Mixed rain and snow basins, with average winter temperatures near freezing, typically show the largest increases in flood risk because of the combined effects of warming (increasing contributing basin area) and more winterP. Decreases in low flows are driven by loss of snowpack, drier summers, and increasing evapotranspiration in the simulations. Energy‐limited basins on the west side of the Cascades show the strongest declines in low flows, whereas more arid, water‐limited basins on the east side of the Cascades show smaller reductions in low flows. A fine‐scale analysis of hydrologic extremes over the Olympic Peninsula echoes the results for the larger rivers discussed above, but provides additional detail about topographic gradients.
Funding Information
  • Washington State Department of Ecology
  • Bonneville Power Administration
  • Northwest Power and Conservation Council
  • British Columbia Ministry of the Environment
  • Oregon Water Resources Department
  • U.S. Forest Service

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