Parameter estimation for multiple post‐wildfire hydrologic models

Abstract

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 (Kfs) 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 (θS). The van Genuchten parameters α, 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 in Kfs, θs, 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 between Kfs, S, and the Green–Ampt wetting front potential (ψf) with measurements at other southern CA burned sites. We suggest that ratios of burned to unburned Kfs (0.37), S (0.36), and ψf (0.66) could be used to scale unburned values for model parameterization. Alternatively, typical burned values (Kfs = 20 mm hr−1; S = 6 mm hr−0.5; ψf = 1.6 mm) could be used for model parameterization.

The timing and magnitude of changes to Hortonian overland flow at the watershed scale during the post‐fire recovery process

“…Langhans et al (2016) demonstrated that plot‐scale infiltration rates were consistently higher than those in small watershed scales. Therefore, it is not surprising that the watershed scale K sh inferred here is smaller than that suggested by point‐scale measurements made shortly after other fires in the Transverse Ranges of southern California (Ebel, 2020; Kean et al, 2019; McGuire et al, 2018, 2019). However, our results in comparison with those of Rengers, McGuire, et al (2016) and Rengers et al (2019) suggest that the changes in effective saturated hydraulic conductivity are minimal when moving from small watersheds (<1.5 km 2 ) to large watersheds (~41 km 2 ).…”

Impacts of wildfire and post‐fire land management on hydrological and sediment processes in a humid Mediterranean headwater catchment

“…These hydrological patterns did not appear to be affected by the fire. It is possible that fire led to an increase of surface runoff at the slope scale, as suggested by the occurrence of rill erosion in the burnt area ( Figure 3d) and as observed after a previous fire in the catchment (Ferreira, 1997); this has often been observed in burnt areas, both due to the presence of soil water repellency (Malvar et al, 2016) and the degradation of soil physical structure, decreasing hydraulic conductivity (Ebel & Moody, 2020;Varela, Benito, & Keizer, 2015). However, rip-ploughing might have broken the topsoil repellent layer, and limited hydrological connectivity might have prevented the effects of repellency from manifesting at the catchment scale, as observed elsewhere (Ferreira et al, 2008;Stoof et al, 2012).…”

Impacts of wildfire and post-fire land management on hydrological and sediment processes in a humid Mediterranean headwater catchment

“…These hydrological patterns did not appear to be affected by the fire. It is possible that fire led to an increase of surface runoff at the slope scale, as suggested by the occurrence of rill erosion in the burnt area (Figure 3d) and as observed after a previous fire in the catchment (Ferreira, 1997); this has often been observed in burnt areas, both due to the presence of soil water repellency (Malvar et al, 2016) and the degradation of soil physical structure, decreasing hydraulic conductivity (Varela et al, 2015;Ebel and Moody, 2020). However, rip-plowing might have broken the topsoil repellent layer, and limited hydrological connectivity might have prevented the effects of repellency from manifesting at the catchment scale, as observed elsewhere (Ferreira et al, 2008;Stoof et al, 2012).…”