The hydrology and hydrometeorology of extreme floods in the Great Plains of Eastern Nebraska

The Great Plains of eastern Nebraska occupy a distinctive hydroclimatological niche, characterized by a high frequency of organized thunderstorm systems. A consequence of the hydroclimatology of these systems is a sharp seasonal peak in the regional flood frequency in late June. Pebble Creek and Maple Creek are adjacent drainage basins in the Great Plains of Nebraska with drainage areas of 528 and 1165 km², respectively. The hydrometeorological and hydrologic controls of extreme floods are examined through analyses of a series of five major flood events that occurred in these catchments during the warm season of 1996. Particular attention is given to two storm systems. The 20-21 June flood event was produced by a series of tornadic supercell thunderstorms which tracked over Pebble Creek. The 4-5 August 1996 event, which resulted in record flood peaks in both Pebble Creek and Maple Creek, was produced by a system of multicellular thunderstorms. Analyses of the structure, motion and evolution of these two storm systems provide a conceptual framework for interpreting hydrometeorological controls of scale-dependent flood response. Hydrometeorological analyses are based on both volume scan WSR-88D reflectivity observations from the Omaha, Nebraska radar and composite reflectivity observations from the WSR-88D radar network. Analyses of composite reflectivity observations for the US east of the Rocky Mountains for the 4-year period from 1996 to 1999 are used to place the scale-dependent flood response of the Great Plains within a broader hydroclimatological context. Discharge data for Maple Creek and Pebble Creek, at 15 min time scale, serve as the basis for stream flow analyses. The striking contrasts in flood response between Maple Creek and Pebble Creek are related to contrasts in drainage network structure, infiltration properties and flood wave attenuation. The scale-dependent flood response of these catchments is analyzed in terms of the space-time variability of rainfall as viewed from the spatial perspective imposed by the drainage network of the basin.