Variation in Post-Wildfire Perturbations to Soil Hydraulic Properties Across a Climatic Gradient

Abstract:

Elevated debris-flow hazards following wildfire are associated with increased efficiency of runoff generation and sediment entrainment. However, the degree to which wildfire increases debris-flow hazard does not appear to be uniform in different climatic zones, which makes prediction of post-fire debris-flow hazard difficult across climatically diverse areas such as the western U.S. Accurate prediction of post-wildfire debris-flows requires a better understanding of how post-fire perturbations to infiltration rates, runoff characteristics, and sediment flux vary with climate zone. Here we present preliminary results from a field and laboratory-based study of post-fire perturbations to soil hydraulic properties in recent fires that have occurred across a steep climatic gradient between the wet northern California Coast Ranges and the deserts of Nevada. Post-fire perturbation to saturated hydraulic conductivity and sorptivity was not uniform across our sites and these parameters did not always decrease after wildfire. Across all sites we find significant recovery towards pre-burn values one or two years after the fire. Mean values of post-fire saturated hydraulic conductivity across sites show a positive correlation with the maximum 15-minute rainfall intensity of storms with a one-year recurrence interval. Laboratory measurements of soil water characteristic curves (SWCC) show that burned soils generally retain water over a narrower range of suction as compared to unburned soils. If SWCC parameters are pulled from a database of unburned soils, as is often done when interpreting in situ infiltration measurements using disk infiltrometers, some of the effects of fire on soil infiltration properties might be missed. These results suggest that a growing collection of measurements quantifying post-fire perturbations to soil properties in different climatic zones will be useful for rapid post-fire debris-flow hazard assessments across climatic gradients.