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Quantification of wildfire-induced perturbations to soil-water retention curves and their impact on rainfall runoff
AdvisorMcCoy, Scott W
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Increased frequency and magnitude of wildfires can cause elevated post-fire rainfall-runoff and debris-flow hazards in and around steeplands. Improved prediction of the severity and spatial extent of post-fire increases to these hazards requires understanding which soil properties are most strongly affected by wildfire and what role these changes play in runoff generation and debris-flow initiation. Previous studies have shown significant post-fire perturbations to saturated hydraulic conductivity (Ks) and sorptivity, and in some cases these perturbations appear to exert a strong control on the observed increase in post-fire runoff and debris flow. However, a variety of other soil properties also influence rainfall infiltration rates into unsaturated soils. The soil-water retention curve (SWRC), also called the soil-water characteristic curve, is the relationship between matric suction and soil-water content and determines many critical unsaturated soil properties such as unsaturated hydraulic conductivity. The SWRC is commonly described using the Van Genuchten equation, in which key parameters are the residual water content, θr, saturated water content, θs, the pore-size distribution index, n, and the inverse of the air-entry pressure head, α. To date, the degree to which SWRCs are perturbed by wildfire is less clear, and as a result, they are commonly assumed to be unaffected by wildfire and obtained based on soil texture from databases composed of unburned agricultural soils. To quantify potential wildfire perturbations to SWRCs, we collected a suite of 43 unburned and 70 burned soil cores from seven recent wildfires in Nevada and California. We measured burned and unburned SWRCs and found best-fit Van Genuchten parameters θr, θs, n, α, using ASTM standard methods and the Meter HYPROP system. We found high quality fits to SWRCs from both unburned and burned samples using the Van Genuchten equation. While the magnitude of fire perturbation varies across fire sites, in most cases we found post-wildfire reductions in soil water retention at most values of matric suction, as quantified by increases in n and α, and decreases in θs and θr relative to the unburned state. To generalize the importance of the observed perturbations to SWRC parameters for rainfall runoff generation, we conducted a suite of numerical rainfall-runoff simulations in which we solved Richards equation with HYDRUS 1-D and with parameters constrained from burned and unburned SWRCs. These simulations suggest that the most commonly observed perturbations to SWRCs decrease time to ponding and strongly increase runoff generation at the sub-hourly timescales relevant to flash flooding and debris-flow initiation by rainfall runoff. These results highlight that field constraints on soil-hydraulic properties beyond just the saturated hydraulic conductivity are critical to more accurate prediction of rapid rainfall runoff following fire. The large database of such measurements presented here provides an important step towards increasing our understanding of the magnitude of expected post-fire perturbations to soil properties.