Post‐fire, rainfall intensity–peak discharge relations for three mountainous watersheds in the western USA

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Abstract
Wildfire alters the hydrologic response of watersheds, including the peak discharges resulting from subsequent rainfall. Improving predictions of the magnitude of flooding that follows wildfire is needed because of the increase in human population at risk in the wildland–urban interface. Because this wildland–urban interface is typically in mountainous terrain, we investigated rainfall‐runoff relations by measuring the maximum 30 min rainfall intensity and the unit‐area peak discharge (peak discharge divided by the area burned) in three mountainous watersheds (17–26·8 km2) after a wildfire.We found rainfall‐runoff relations that relate the unit‐area peak discharges to the maximum 30 min rainfall intensities by a power law. These rainfall‐runoff relations appear to have a threshold value for the maximum 30 min rainfall intensity (around 10 mm h−1) such that, above this threshold, the magnitude of the flood peaks increases more rapidly with increases in intensity. This rainfall intensity could be used to set threshold limits in rain gauges that are part of an early‐warning flood system after wildfire. The maximum unit‐area peak discharges from these three burned watersheds ranged from 3·2 to 50 m3 s−1 km−2. These values could provide initial estimates of the upper limits of runoff that can be used to predict floods after wildfires in mountainous terrain. Published in 2001 John Wiley & Sons, Ltd.