USE OF HYDROGEOMORPHIC AND SEDIMENTARY EVIDENCE TO REFINE CONVECTIVE RAINFALL ANALYSES AND FLASH-FLOOD CHARACTERISTICS

There is a critical need to improve rainfall analyses for extreme rainstorms and resultant streamflows. Such advancements will enhance emerging technologies designed to improve rainfall estimates, hydrologic modeling, and flash-flood forecasting. Convective rainfall amounts and intensities can vary substantially over temporal scales (minutes) and spatial scales (a few kilometers), particularly for watersheds located in semi-arid and arid regions in the west-central United States. In many areas, precipitation and streamflow gages are sparse to nonexistent, particularly in remote areas. Limited resources often preclude extensive data collection, and flood-producing rainstorms often do not occur in instrumented basins. Even in instrumented basins, observational networks may not work properly because of storm or flood damage.

NEXRAD weather radar (WSR-88D) is increasingly being used to quantify rainfall, however, validation is fundamentally necessary to determine a storm’s footprint and assess predicted rainfall estimates. NEXRAD data have limitations that can substantially affect the reliability of derived rainfall amounts. For example, hailstorms may result in substantially overestimated rainfall amounts and extreme winds may change the location of the storm’s footprint as reported by NEXRAD.

Hydrogeomorphic techniques have been developed to refine convective storm rainfall estimates in conjunction with flash-flood documentation efforts. The techniques use physical evidence remaining on hillslopes and in river channels, particularly in the documentation of sediment erosional and depositional features, to make inferences about the hydrometeorological conditions. These data are used to refine or validate NEXRAD rainfall estimates, the storm footprint, and rainfall-runoff footprint. This presentation includes discussions of several documented storms and floods with a focus on: 1) the approach, results, and benefits of documenting recent convective rainstorms and associated flood discharges; 2) coordination with the National Weather Service to determine rainfall thresholds that may produce hazardous flash floods for different land uses; 3) limitations and uncertainties of hydrometeorologic estimates, and; 4) transfer of the methodologies to other hydroclimatic regions.