Abstract:Fire-induced soil hydrophobicity is presumed to be a primary cause of the observed post-fire increases in runoff and erosion from forested watersheds in the Colorado Front Range, but the presence and persistence of hydrophobic conditions has not been rigorously evaluated. Hence the goals of this study were to: (1) assess natural and fire-induced soil hydrophobicity in the Colorado Front Range, and (2) determine the effect of burn severity, soil texture, vegetation type, soil moisture, and time since burning on soil hydrophobicity.Five wild and prescribed fires ranging in age from 0 to 22 months were studied. Each fire had four study sites in ponderosa pine forests that had been burned at high, moderate, or low severity, and three sites were in unburned areas. Additional sites were established in lodgepole pine stands and an area with unusually coarse-textured soils. At each site the soil hydrophobicity was assessed in two pits using the water drop penetration time (WDPT) and the critical surface tension (CST). Measurements were made at the mineral soil surface and depths of 3, 6, 9, 12, 15, and 18 cm.In sites burned at moderate or high severity the soils were often strongly hydrophobic at 0, 3, and 6 cm. Unburned sites or sites burned at low severity were typically hydrophobic only at the surface. Although soil hydrophobicity generally strengthened with increasing burn severity, statistically significant differences in soil hydrophobicity were difficult to detect because of the high variability within and between sites. Hydrophobicity also increased with increasing percent sand and was not present when soil moistures exceeded 12-25%. There were no significant differences in soil hydrophobicity between ponderosa and lodgepole pine stands, regardless of burn severity.Repeat measurements on one fire suggest a weakening of fire-induced soil hydrophobicity after 3 months. Comparisons between fires suggest that fire-induced soil hydrophobicity persists for at least 22 months. Overall, CST values were more consistent and more highly correlated with the independent variables than the WDPT, and the CST is recommended for assessing soil hydrophobicity rather than the more commonly used WDPT.
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.
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