Water Balance of the 1993 Midwest Flood

Abstract

Throughout the spring and summer months of 1993, extended rainfall throughout much of the Midwestern United States caused record flooding that inundated much of the Upper Mississippi River Basin (UMRB). Precipitation in May was more then twice the normal over an area that extended from southeastern South Dakota across Iowa to eastern Kansas. From early June to the end of July, high amounts of precipitation persisted over the upper Midwest (Wahl, et al., 1993). USGS records indicated that at 45 streamflow gauging stations, the peak discharge recorded during 1993 had recurrence intervals of greater than 100 years. However, because of the natural and man-made changes in the flood region, some sites had less-than-record peak discharges (Parret, et al., 1993). The storage of large volumes of water in reservoirs significantly reduced the peak flow and flood damages downstream from the dams (Southard, 1993). Following the 1993 Midwest flood, President Clinton established the Scientific Assessment and Strategy Team (SAST) on November 24, 1993, to study the effects of the flood and to make recommendations about future flood preparedness. The SAST joined the Interagency Floodplain Management Review Committee (FMRC) on January 10, 1994 (FMRC, 1994). As part of this effort, the SAST project identified a need for a daily water balance of the flooded area to determine how much water fell and how quickly it moved through the landscape. There were two significant policy issues resulting from the flood: (1) how did the flood volume and velocity of flow increase by land use changes associated with agricultural development in the Midwest, including extensive drainage of wetlands; and (2) what plan should be adopted for restoration of failed levee systems. The first of these questions is hydrologic, the second, hydraulic. The hydraulic issues were addressed by the SAST project and related efforts by modeling the motion of water through the main tributaries of the Mississippi and Missouri Rivers where the major levee failures occurred. The hydrologic questions were not so readily addressed because of the huge region affected by the flood, some 700,000 km2 in area. Flood hydrology models are normally applied to regions 100 to 1,000 times smaller than this area. Thus, the need for the present study arose – to model the movement of water through the landscape of the SAST study area by constructing a daily water balance in a series of subwatersheds in the flooded area. A USGS WEB site designated for SAST is located at: http://edcwww2.cr.usgs.gov/sast-home.html . Figure 1.1 shows the location and the extent of the SAST study area. This region covers all of the UMRB above St. Louis and that portion of the Missouri Basin whose drainage enters the Missouri River by watershed (Missouri, Platte, Kansas, Osage, and Gasconade Rivers). The contribution of the remainder of the Missouri Basin was accounted for by using gauged data from tributary flows at the border of the study region. The goal of this project was to calculate the daily water balance for the SAST region for 1993. A Geographic Information System (GIS) was used to determine the balance. GIS offers a technology to formulate more objective and consistent methods to synthesize collected data and to assess water quality and quantity over large areas (Maidment, 1996). The spatial resolution of the SAST region was defined by the location of discharge gauging stations as well as the completeness and quality of the discharge record. The preliminary analysis was performed using daily discharge values recorded at 261 USGS stations from 01/01/1993 to 09/30/1993. The final water balance was estimated for 132 watersheds defined by the stations that have a complete discharge record for all days of 1993. The cumulative storage values were then spatially averaged over 4, 6, and 8 digit hydrologic units (HUC) to provide an alternative way of viewing the water balance. Missouri River at Yankton, SD Platte River at Luisville, NE Kansas River at Desoto, KS Osage River near St. Thomas, MO Gasconade River near Rich Fountain, MO Mississippi River at Inflow Thebes, IL Outflow Figure 1.1 Location and extent of the SAST region.