Abstract: Changes in hydrologic extremes (floods and low flows) represents one of the most important impact pathways associated with climate change in the Pacific Northwest. Most human and natural systems function reasonably well under “normal” conditions, and substantial impacts are defined mostly by the extreme events. Water supply systems, for example, are typically limited by the worst drought in the historical record in formal planning exercises.
We report here on research conducted by the Climate Impacts Group over the last seven years or so focused on quantifying changes in hydrologic extremes in the 20th century, and for a suite of future climate change scenarios for the 21st century. The impacts of observed warming on flood risk in the 20th century have varied with mid-winter temperatures in each location. Relatively warm areas near the coast and at moderate elevation in more inland areas have seen strong increases in flood risk, due primarily to rising freezing levels that enlarge the effective basin area. Colder areas show decreases in flood risk related primarily to reduced antecedent snowpack (contributing either to rain on snow events, or spring melt) during flood events. Rain dominant basins show little response to temperature alone. Increases in precipitation variability in the late 20th century (increased variance since about 1975) have increased flood risk over much of the western U.S. Climate change scenarios in the PNW project warmer and wetter winters, springs, and falls, and warmer and drier summers. The combination of warmer and wetter winters creates strong increases in flood risk in near coastal watersheds west of the Cascades and in moderate elevation areas of the Rockies due to an enlarged effective basin area and increased storm intensity. Rain dominant and colder snowmelt dominant basins show modest increases in flood risk due primarily to increased precipitation intensity.
Ongoing work using high-resolution climate model simulations will estimate the effects of changing storm intensity combined with warming on PNW flood risk. Initial results suggest a much larger increase in flooding for the windward slopes of the Cascades and Rockies in comparison with studies based on statistically downscaled GCM data, because daily precipitation intensity is expected to increase dramatically in these areas. Leeward slope areas may show reduced flood risks in comparison with GCM based studies. Extreme low flows are generally predicted to decline across the region under natural conditions due to the combined effects of decreased snowpack and drier summers, although some areas buffered by glaciers or deep groundwater will need further study.
The current projections are an initial attempt to provide quantitative estimates of the magnitude and direction of changing hydrologic extremes across the PNW, but there are many missing pieces, including the effects of groundwater, glaciers, and water management on flooding and low flows, updated inundation maps for areas in the flood plain, and the combined effects of sea level rise and river flooding on the near coastal environment. Strategies for coping with non-stationary and uncertain design standards are also needed.
