National threshold runoff estimation utilizing GIS in support of operational flash flood warning systems

Threshold runoff is the amount of excess rainfall accumulated during a given time period over a basin that is just enough to cause flooding at the outlet of the draining stream. Threshold runoff estimates are indicators of maximal sustainable surface runoff for a given catchment, and are thus an essential component of flash flood warning systems. Used in conjunction with soil moisture accounting models and areal rainfall data, they form the basis of the US National Weather Service (NWS) flash flood watch/warning program. As part of their modernization and enhancement effort, the NWS determined that improved flash flood guidance and thus improved threshold runoff estimation is needed across the United States, with spatial resolution commensurate to that afforded by the WSR-88D (NEXRAD) radars. In this work, Geographic Information Systems (GIS) and digital terrain elevation databases have been used to develop a national system for determining threshold runoff. Estimates of threshold runoff an presented for several locations in the United States, including large portions of the states of Iowa, Oklahoma, and California, and using several options in computing threshold runoff. Analysis of the results indicates the importance of channel geometry in flash flood applications. Larger threshold runoff estimates were computed in Oklahoma (average value of 34 mm) than in Iowa (14 mm) or California (9.5 mm). Comparisons of the threshold runoff estimates produced by the GIS procedure with those based on manually computed unit hydrographs for the selected catchments are shown as a preliminary measure of the accuracy of the procedure. Differences of up to about 15 mm for hourly rainfall durations were obtained for basins larger than 50 km(2). (C) 1999 Elsevier Science B.V. All rights reserved.