U.S. hydrologic design standards insufficient due to large increases in frequency of rainfall extremes

Numerous studies have shown that heavy rainfall in the United States and elsewhere is becoming more common and more severe in a warming climate. These studies have not shown, however, how the most extreme rainfall events are changing, since these storms are so rare that they are difficult to assess using conventional techniques, which generally focus on changes at individual geographic locations.

This report's authors instead use a simple aggregation technique to “pool” multiple observations within a region. This “pooling” allows the authors to show that rainfall events that exceed common engineering design criteria, including 100‐year storms, have increased in frequency in most parts of the United States since 1950—a period of widespread infrastructure construction. The findings show that in most locations, these increases are likely due to climate warming. The authors also show that much of the existing and planned hydrologic infrastructure in the United States based on published rainfall design standards is and will continue to underperform its intended reliability due to these rainfall changes.