Synthesis of soil‐hydraulic properties and infiltration timescales in wildfire‐affected soils

Abstract

We collected soil‐hydraulic property data from the literature for wildfire‐affected soils, ash, and unburned soils. These data were used to calculate metrics and timescales of hydrologic response related to infiltration and surface runoff generation. Sorptivity (S) and wetting front potential (Ψf) were significantly different (lower) in burned soils compared with unburned soils, whereas field‐saturated hydraulic conductivity (Kfs) was not significantly different. The magnitude and duration of the influence of capillarity during infiltration was greatly reduced in burned soils, causing faster ponding times in response to rainfall. Ash had large values of S and Kfs but moderate values of Ψf, compared with unburned and burned soils, indicating ash has long ponding times in response to rainfall. The ratio of S2/Kfs was nearly constant (~100 mm) for unburned soils but more variable in burned soils, suggesting that unburned soils have a balance between gravity and capillarity contributions to infiltration that may depend on soil organic matter, whereas in burned soils the gravity contribution to infiltration is greater. Changes in S and Kfs in burned soils act synergistically to reduce infiltration and accelerate and amplify surface runoff generation. Synthesis of these findings identifies three key areas for future research. First, short timescales of capillary influences on infiltration indicate the need for better measurements of infiltration at times less than 1 min to accurately characterize S in burned soils. Second, using parameter values, such as Ψf, from unburned areas could produce substantial errors in hydrologic modeling when used without adjustment for wildfire effects, causing parameter compensation and resulting underestimation of Kfs. Third, more thorough measurement campaigns that capture soil‐structural changes, organic matter impacts, quantitative water repellency trends, and soil‐water content along with soil‐hydraulic properties could drive the development of better techniques for numerically simulating infiltration in burned areas.

Postwildfire measurement of soil physical and hydraulic properties at selected sampling sites in the 2011 Las Conchas wildfire burn scar, Jemez Mountains, north-central New Mexico

“…The K fs magnitudes reported in this study are one to two orders of magnitude greater than the geometric mean burned K fs value from Ebel and Moody (2017) It is important to note that the timing of postwildfire measurements reported in this study (4 years after the wildfire) is atypical of the timing of most postwildfire measurements which usually occur within days to months of wildfire containment. In particular, and although speculative, it is possible there were substantial reductions in infiltration rates corresponding to the reductions in SHPs immediately following the wildfire, but that SHPs that control infiltration have returned to magnitudes that do not facilitate appreciable runoff generation during rainfall at these sites.…”

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

“…The tendency of fire to substantially reduce K fs is consistent with most individual studies (Table S1) and prior reviews on post-fire K fs (Neary, 2011) and infiltration (Cerdà and Robichaud, 2009). The arithmetic mean of the TI and SSRS K fs values from burned forest soils, which are themselves arithmetic means of the original datasets, gives 30.8 mm hr -1 with a standard deviation of these mean values of 19.6 mm hr -1 ; it is not surprising that, with significant decreases in sorptivity after fire (Ebel and Moody, 2017), that rainfall rates often exceed infiltration rates after fire. The means of the K fs datasets analyzed here were not lognormally distributed, agreeing with the work from Langhans et al (2016) that K fs may not be lognormally distributed after fire, although their work quantifies the distribution of values at a site whereas this work presents a distribution of mean values from many sites.…”

“…Surface runoff dominance after wildfire is primarily attributed to losses of canopy and litter-duff layer interception (e.g. Greatly reduced sorptivity, representing the contribution of capillarity to infiltration, after wildfire (Ebel and Moody, 2017) causes K fs to become the primary soil-hydraulic property determining infiltration rate and thus surface runoff generation. Robichaud, 2000;.…”

“…At the plot scale, multiple K fs measurement methods are commonly used. Furthermore, a recent synthesis (Ebel and Moody, 2017) of a small subset (number of datasets, N=9) of plot-scale K fs measurements from wildfire-affected watersheds suggested that K fs may not be significantly impacted by wildfire, which prompts further analysis with a much larger sample size. positive versus negative pressure head).…”

Sources of inherent infiltration variability in postwildfire soils

“…One SHP is saturated hydraulic conductivity, K s (mm hr −1 ), which is controlled by the force of gravity on the timescale of hours to days, and the second SHP is sorptivity, S (mm hr −0.5 ), which is controlled by capillary forces on the timescale of seconds to hours. At the present time, so little is known about postwildfire SHPs that many model predictions of postwildfire floods are empirical (Chen, Berli, & Chief, ; Foltz, Robichaud, & Rhee, ; Moody, ) or based on estimates of SHP of burned soils as inputs to traditional infiltration theories that assume a thick, single‐layer homogeneous media (Ebel & Moody, ; Flanagan & Nearing, ; Gochis, Yu, & Yates, ; Goodrich et al, ; Goodrich et al, ; Green & Ampt, ; Julien, Saghafian, & Ogden, ; Moody, Shakesby, Robichaud, Cannon, & Martin, ; Smith & Parlange, ; U.S. Army Corps of Engineers, ). Knowing both magnitude and variability of these SHPs is essential for improving probability density functions used in rainfall–run‐off modelling that can aid land and emergency managers.…”