SEASONAL DISTRIBUTION OF HEAVY RAINFALL EVENTS IN MIDWEST

The seasonal distribution of extreme rainfall is important in hydrology and agriculture, especially in problems related to water control. The amounts at selected return periods vary by season and geographic location, as do the surface conditions of soil moisture and vegetation. Therefore, a particular size storm may have a different impact if it occurs during summer as opposed to winter (over frozen ground). In terms of total precipitation and the top-ranked storms, summer is the dominant season for extreme rainfall in the northern states, while the other seasons increasingly contribute more in the central and southern portions of the Midwest. This effect is maximized in Kentucky, as well as in southern Illinois and Indiana, where winter precipitation approaches that of summer in magnitude. This has important consequences, since soil moisture is generally quite high in the Midwest at this time of year. The combination of large winter rainfall events and near-saturated soil conditions can lead to high runoff. An examination of seasonal rainfall frequencies, along with seasonal soil conditions, may lead to the optimal solution to a water control design problem.