EFFECT OF RAINFALL-SAMPLING ERRORS ON SIMULATIONS OF DESERT FLASH FLOODS

The effect of rainfall-sampling errors on distributed hydrologic simul ations was evaluated in a study conducted with localized thunderstorms and a midsized (150 km2) semiarid watershed. Rainfall fields based on observations from a very dense rain gage network were compared to rai nfall fields based on observations from a subset of the original gages . The rain gage density of the ''sparse'' network (1 gage per 20 km2) was selected to represent the typical gage density of a local evaluati on in real time (ALERT) type flash flood warning system. Inadequate ra in gage densities in the case of the sparse network produced errors in simulated peaks that, on the average, represented 58% of the observed peak flow. Approximately half of the difference between observed and simulated peaks was due to rainfall-sampling errors. Simulations were also conducted with rainfall that is similar to the next generation we ather radar (NEXRAD) digital precipitation estimates in that it repres ents areal averages within 4 km x 4 km pixels. Spatial averaging of ra infall over 4 km x 4 km pixels led to consistent reductions in simulat ed peaks that, on the average, represented 50% of the observed peak fl ow. Hence it appears that the current spatial resolution of ALERT-type precipitation measurements and 4 km x 4 km radar precipitation estima tes may not be sufficient to produce reliable rainfall-runoff simulati ons/forecasts in midsized watersheds of the southwestern United States subject to localized thunderstorms and large infiltration losses.