Measurement Method Has a Larger Impact Than Spatial Scale For Plot‐Scale Field‐Saturated Hydraulic Conductivity (K_fs) After Wildfire and Prescribed Fire in Forests

Abstract

Wildfires raise risks of floods, debris flows, major geomorphologic and sedimentologic change, and water quality and quantity shifts. A principal control on the magnitude of these changes is field‐saturated hydraulic conductivity (Kfs), which dictates surface runoff generation and is a key input into numerical models. This work synthesizes 73 Kfs datasets from the literature in the first year following fire at the plot scale (≤ 10 m2). A meta‐analysis using a random effects analysis showed significant differences between burned and unburned Kfs. The reductions in Kfs after fire, expressed by the ratio of Kfs Burned/Kfs Unburned, were 0.46 (95% confidence interval of 0.31‐0.70) combining wildfire and prescribed fire and 0.3 (95% confidence interval of 0.13‐0.71) for wildfire. No significant differences for Kfs were observed between wildfire and prescribed fire or moderate and high fire severity. Both Kfs magnitude and variability depended more on measurement method than measurement support area at the plot scale, with methods applying head ≥0.5 cm producing larger estimates of Kfs. It is recommended that post‐fire efforts to characterize Kfs for modeling or process‐based interpretations use methods that reflect the dominant infiltration processes: tension infiltrometers and simulated rainfall methods when soil matrix flow dominates and ponded head methods when macropore flow is critical. Published 2019. This article is a U.S. Government work and is in the public domain in the USA.

Parameter estimation for multiple post‐wildfire hydrologic models

“…The magnitude of K fs reduction expressed by a ratio of burned over unburned geometric means (Table 2) was 0.37. This reduction is consistent with a global compilation of K fs ratios of 0.3 that had a 95% confidence interval of 0.13-0.71 (Ebel, 2019). Increases in ρ b from 0-1 cm depth, and the lack of change in LOI, point to the two most likely causes of fire-related K fs reduction being the loss of soil structure and soil seal formation.…”

“…K fs is typically reduced following wildfire (Benavides-Solorio & MacDonald, 2001;Blake et al, 2010;Ebel & Martin, 2017;Ebel, Moody, & Martin, 2012;Martin & Moody, 2001;Neary, 2011;Parks & Cundy, 1989). A recent global review of wildfire and prescribed fire impacts on K fs at the point to small plot scale (≤10 m 2 ) by Ebel (2019) showed a post-wildfire reduction, expressed by the ratio of K fs Burned /K fs Unburned , equal to 0.3 (95% confidence interval of 0.13-0.71). However, the wide spread in the confidence intervals from Ebel (2019) indicates substantial site-to-site variability of the impact of wildfire on K fs .…”

“…A recent global review of wildfire and prescribed fire impacts on K fs at the point to small plot scale (≤10 m 2 ) by Ebel (2019) showed a post-wildfire reduction, expressed by the ratio of K fs Burned /K fs Unburned , equal to 0.3 (95% confidence interval of 0.13-0.71). However, the wide spread in the confidence intervals from Ebel (2019) indicates substantial site-to-site variability of the impact of wildfire on K fs . Different causal factors have been cited for observed changes in soil-hydraulic properties.…”

“…Measuring soil-hydraulic properties after wildfire for model parameterization is challenging. For example, the soil-hydraulic properties change with time since the fire (Ebel & Martin, 2017), fire enhanced soil-water repellency can complicate analysis (Ebel & Moody, 2013;Moody et al, 2009;Nyman et al, 2010), and issues of measurement scale bear consideration (Ebel, 2019;Langhans et al, 2016). Despite these obstacles, K fs and S have been measured directly in the field (Moody et al, 2009;Nyman et al, 2010) or on soil cores collected in the field (Ebel, Romero, & Martin, 2018;Moody et al, 2016) after wildfire to provide parameters for physically-based infiltration models (Ebel et al, 2016;McGuire et al, 2018;Rengers, McGuire, Kean, Staley, & Youberg, 2019).…”

Soil Physical, Hydraulic, and Thermal Properties in Interior Alaska, USA: Implications for Hydrologic Response to Thawing Permafrost Conditions

“…By contrast, burned and unburned conditions at sampling sites had little impact on physical, hydraulic, and thermal properties of mineral soil at the depths of this investigation. Wildfires in interior Alaska do not typically have the major impacts on physical and hydraulic properties of mineral soil (e.g., Brown et al, 1983;Viereck & Dyrness, 1979) that can be present in the continental United States, for example, see reviews in Certini (2005), Ebel and Moody (2017), and Ebel (2012), Ebel (2019b). The direct heat impulse into mineral soil during interior Alaska wildfire is minimal to absent owing to the thick, often moist, organic layer at the surface (Brown et al, 1983;Dyrness & Norum, 1983;Viereck & Dyrness, 1979), which reduces or eliminates direct wildfire impacts on mineral soil properties.…”