Projections of Increasing Flood Frequency and Magnitude Across the Western United States

Abstract:

Climate disruption is changing the behavior of extreme hydroclimatic events. Increases in the frequency and intensity of extreme precipitation events in certain areas have been observed and are projected to accelerate through the 21st century, which affect hydrologic extremes. In addition, increased temperatures have effects on whether precipitation falls as rain or snow, as well as the rate of snow melt. These all have the potential to change the peak flood levels that produce large losses to society every year. This provides the motivation for this study, in which changes in flood frequency and intensity are explored. Focusing on 421 gauge stations across the western United States for 30-year time periods: 1971-2000, 2010-2039, 2040-2069, 2070-2099, flood levels are calculated using the standard flood frequency analysis method outlined in Bulletin 17B by the United States Interagency Committee on Water Data. Daily flow data was obtained, which had been produced by a hydrologic model forced with statistically downscaled climate projections produced by GCM runs from the 5th Coupled Model Intercomparison Project. Annual peak flow data is examined to determine the percent change in flow values at difference recurrence intervals and by inverting the process, the future recurrence intervals of historic flood levels are determined. We assess the ability to generalize the results, based on spatial coherence of the changes. Results indicate that floods with a historical recurrence interval of 100 years will be expected to occur more frequently by the end of the 21st century at 94% of the gauge sites.