Post‐fire Soil Water Repellency

Abstract

Fire‐induced soil water repellency is a key control on post‐fire runoff and erosion rates, but there are few data on the persistence of soil water repellency and the soil moisture threshold at which water repellent soils become hydrophilic. This study used repeated sampling to quantify changes in soil water repellency over time and identify soil moisture thresholds for the loss of soil water repellency. The study area was a wildfire in the northern Colorado Front Range that burned 43 km2 of ponderosa and lodgepole pine forests in June 2000. Soil water repellency and soil moisture were measured periodically from June 2000 through June 2001 at 36 sites stratified by burn severity and nine unburned sites. Water repellency was assessed in the field at depths of 0 to 18 cm using the critical surface tension (CST) test. Soil water repellency was strongest in sites burned at high and moderate severity, decreased with increasing depth, and was spatially highly variable. The fire‐induced soil water repellency progressively weakened and became statistically nondetectable by 1 yr after burning. The effect of time since burning on soil water repellency was increasingly significant with increasing burn severity and progressively less important with increasing soil depth. The soil moisture thresholds at which water repellent soils become hydrophilic apparently increase with increasing burn severity. The data suggest soil moisture thresholds of approximately 10% for unburned sites, 13% for sites burned at low severity, and no less than 26% for sites burned at moderate and high severity.