CLIMATE-RELATED FLUCTUATIONS IN MIDWESTERN FLOODS DURING 1921-1985

Historical records of warm- and cold-season floods and associated heavy-precipitation events during 1921-1985 in the Midwest were examined for temporal fluctuations and trends. Floods in basins in the northern Midwest exhibited upward trends in both seasons but no statistically significant temporal changes existed elsewhere. The incidence of heavy-precipitation events also increased in this same area, a region where thunderstorm incidences and cyclone frequencies have also been on the increase since 1920. Pentads of high flood incidences all occurred in major wet periods (generally the 1970s and 1980s), and pentads of lowest flood incidences occurred in notable droughts (1930s and 1951-1965). The times of these droughts (early) and wet periods (late) further help explain the tendency for floods to increase during the 1921-1985 study period. When precipitation over 5-year periods decreased 8% or more below average, or increased by 7% above average, the number of floods was greatly reduced or increased. This suggests that future significantly drier climate conditions in the Midwest could have few floods, and significantly wetter conditions could have increased flood activity.