Historical and Future Trends in Preciptation Extremes and Their Impacts on the Stormwater Infrastructure of Washington State

Abstract

The design and performance of stormwater infrastructure is based on a probability distribution of precipitation extremes, together with an underlying assumption that this distribution is statistically stationary. This assumption is called into question by climate change. We therefore examined both historical precipitation records and simulations of future rainfall to evaluate changes in the probability distributions of precipitation extremes across Washington State. Precipitation data downscaled from two global climate models were also used as input to a hydrologic model to project future streamflow in two urban watersheds in central Puget Sound. Few statistically significant changes in extreme precipitation were observed in the historical records, but simulations generally indicate future increases in the magnitudes of extreme rainfall and streamflow. Although the range of these projections is too large to predicate engineering design, and actual changes may be difficult to distinguish from natural variability, the evidence suggests that drainage infrastructure designed using mid-20th century rainfall records may be subject to a future rainfall regime that differs from current design standards.