Parameter estimation for multiple post-wildfire hydrologic models

Predictions of post-wildfire flooding and debris flows are needed, typically with short lead times. Measurements of soil-hydraulic properties necessary for model parameterization are, however, seldom available. This study quantified soil-hydraulic properties, soil-water retention, and selected soil physical properties within the perimeter of the 2017 Thomas Fire in California. The Thomas Fire burn scar produced catastrophic debris flows in January 2018, highlighting the need for improved prediction capability. Soil-hydraulic properties were also indirectly estimated using relations tied to soil-water retention. These measurements and estimates are examined in the context of parameterizing post-wildfire hydrologic models. Tension infiltrometer measurements showed significant decreases (p < .05) in field-saturated hydraulic conductivity (K-fs) and sorptivity (S) in burned areas relative to unburned areas. Wildfire effects on soil water-retention were dominated by significant decreases in saturated soil-water content (theta(S)). The van Genuchten parameters alpha,N, and residual water content did not show significant wildfire effects. The impacts of the wildfire on hydraulic and physical soil properties were greatest in the top 1 cm, emphasizing that measurements of post-fire soil properties should focus on the near-surface. Reductions inK(fs),theta(s), and soil-water retention in burned soils were attributed to fire-induced decreases in soil structure evidenced by increases in dry bulk density. Sorptivity reductions in burned soils were attributed to increases in soil-water repellency. Rapid post-fire assessments of flash flood and debris flow hazards using physically-based hydrologic models are facilitated by similarities betweenK(fs),S, and the Green-Ampt wetting front potential (psi(f)) with measurements at other southern CA burned sites. We suggest that ratios of burned to unburnedK(fs)(0.37),S(0.36), and psi(f)(0.66) could be used to scale unburned values for model parameterization. Alternatively, typical burned values (K-fs= 20 mm hr(-1);S= 6 mm hr(-0.5);psi(f)= 1.6 mm) could be used for model parameterization.