A Climatic Perspective on the 1993 Flooding Rains in the Upper Mississippi River Basin

ABSTRACT

The major hydroclimatic cause of the 1993 Upper Mississippi River Basin flood was the persistence of heavy rains over a period of several months. As a result, precipitation totals for April-September 1993 were unprecedented, exceeding by a large margin any previous six-month precipitation total since 1895. Using standard hydrometeorological methods, the frequency of occurrence of these precipitation totals is expected to be less than once every 200 years. The core of the heavy rain area was located in Iowa and Missouri near the main stems of the Mississippi and Missouri Rivers. Thus, runof from many of the heavy rain events was deposited directly into these main stems. The persistence of the heavy rains maintained high soil moisture levels throughout the summer period. Thus, the heavy rains during the summer were accompanied by very high runoff rates. Also, the frequent rains were accompanied by many cloudy days. Primarily for this reason (and to a lesser extent because of lower temperatures), potential evapotranspiration was considerably below the long-term average. This was a contributing factor to maintaining the high soil moisture levels and high runoff.