Changes in heavy rainfall in Midwestern United States

Characteristics of heavy rainfall events are important in the design of water-handling structures, agriculture, weather modification, and in monitoring climate change. Traditionally it is assumed that the extreme rainfall time series are stationary with no trends. This assumption may not be true for portions of the Midwestern United States. A newly digitized record of precipitation for 304 sites that extends back to 1901 was used to examine this assumption. Results for the entire Midwest show that stations are more likely to experience their heaviest rainfall events in more recent years. An analysis of the geographic distribution of changes in the annual maximum time series shows areas of increases across the Midwest. The impact of the changes in the annual maximum time series can be significant in determining rainfall frequency values and consequent runoff calculations. These results suggest that rainfall frequency studies should be updated on a regular basis for maximum usefulness.