Changing temporal and spatial characteristics of midwestern hydrologic droughts

Hydroclimatic parameters were examined to identify spatial variability among 19 Midwestern basins during the 1950-1990 period. The magnitude of annual streamflow variability values increased westward across the Midwest and appeared to be associated with a similar pattern in annual precipitation variability. Basin seasonal streamflow values were analyzed to determine the occurrence of hydrologic drought. A criterion that combined duration and severity of low-flow characteristics was used to determine hydrologic droughts. Sixteen of the 19 basins experienced between one and four hydrologic droughts, whereas 3 experienced none. The temporal distribution and spatial extent of the 45 hydrologic droughts indicated that there were two classifications of Midwestern hydrologic drought during this period. The first type-''long-term'' hydrologic drought-occurred over: (1) a period of 12 or more consecutive seasons and (2) a broad latitudinal belt across the Midwest. Long-term hydrologic droughts occurred during the period from the early 1950s through the late 1960s. Since 1970, hydrologic droughts can be described as ''short-term'' and are characterized as having occurred over: (1) periods generally less than two years and (2) a smaller region. This information could assist hydrologists and water managers in understanding the more precise nature of temporal and spatial differences that exist in Midwestern hydrologic drought.