U.S. Hydrologic Design Standards Insufficient Due to Large Increases in Frequency of Rainfall Extremes

Evidence for intensifying rainfall extremes has not translated into “actionable” information needed by engineers and risk analysts, who are often concerned with very rare events such as “100‐year storms.” Low signal‐to‐noise associated with such events makes trend detection nearly impossible using conventional methods. We use a regional aggregation approach to boost this signal‐to‐noise, showing that such storms have increased in frequency over much of the conterminous United States since 1950, a period characterized by widespread hydrologic infrastructure development. Most of these increases can be attributed to secular climate change rather than climate variability, and we demonstrate potentially serious implications for the reliability of existing and planned hydrologic infrastructure and analyses. Though trends in rainfall extremes have not yet translated into observable increases in flood risks, these results nonetheless point to the need for prompt updating of hydrologic design standards, taking into consideration recent changes in extreme rainfall properties.