Thresholds and relations for soil‐hydraulic and soil‐physical properties as a function of burn severity 4 years after the 2011 Las Conchas Fire, New Mexico, USA

Abstract

Wildfire effects on soil-physical and -hydraulic properties as a function of burn severity are poorly characterized, especially several years after wildfire. A stratified random sampling approach was used in 2015 to sample seven sites representing a spectrum of remotely sensed burn severity in the area impacted by the 2011 Las Conchas Fire in New Mexico, USA. Replicate samples from each site were analysed in the laboratory.Linear and linear indicator regression were used to assess thresholds in soil-physical and -hydraulic properties and functional relations with remotely sensed burn severity.Significant thresholds were present for initial soil-water content (θ i ) at 0-6 cm depth between the change in the Normalized Burn Ratio (dNBR) equal to 618-802, for bulk density (ρ b ) at 3-6 cm between dNBR equal to 416-533, for gravel fraction at 0-1 cm between dNBR equal to 416-533, for fines (the silt + clay fraction) at 0-1 cm for dNBR equal to 416-533, and for fines at 3-6 cm for dNBR equal to 293-416. Significant linear relations with dNBR were present between ρ b at 0-1 cm, loss on ignition (LOI) at 0-1 cm, gravel fraction at 0-1 cm, and the large organic fraction at 1-3 cm.No thresholds or effects on soil-hydraulic properties of field-saturated hydraulic conductivity or sorptivity were observed. These results suggest that ρ b and LOI at 0-1 cm have residual direct impacts from the wildfire heat impulse. The θ i threshold is most likely from delayed groundcover/vegetation recovery that increases evaporation at the highest burn severity sites. Gravel and silt + clay thresholds at 0-1 cm at the transition to high burn severity suggest surface gravel lag development from hydraulic erosion. Thresholds in ρ b from 3 to 6 cm and in silt + clay fraction from 3 to 6 cm appear to be the result of soil variability between sites rather than wildfire impacts.This work suggests that gravel-rich soils may have increased resilience to sustained surface runoff generation and erosion following wildfire, with implications for assessments of postwildfire hydrologic and erosion recovery potential.

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

“…Moody et al (2016) showed that K fs did not depend on severity until a threshold was reached and then decreased exponentially with increasing remotelysensed burn severity following wildfire. Ebel et al (2018) did not observe wildfire severity impacts on K fs , however that work was 4 years after fire. In a meta-analysis of simulated rainfall experiments, Vieira et al (2015) did not observe significant fire severity effects on runoff generation and did not separate wildfire from prescribed fire results, suggesting no effect of fire type.…”

Parameter estimation for multiple post‐wildfire hydrologic models

“…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-water retention has also been measured on intact soil cores (Ebel, 2012) and used for model parameterization after wildfire (Ebel, 2013;Ebel et al, 2016).…”

Sources of inherent infiltration variability in postwildfire soils

“…Parameter β ranges from 0 to 1 (Fuentes, Haverkamp, & Parlange, ), and we used β = .6 as suggested by Haverkamp et al (Haverkamp, Ross, Smettem, & Parlange, ) and to be consistent with previous SHP measurements (Ebel et al, ; Moody et al, ; Romero et al, ; Wieting et al, ). Bubble filtered cumulative infiltration, I , from field cores measured in the laboratory was plotted as a function of t 0.5 , and K fs and S f were determined by a non‐linear least squares fit of Equation (Figure ).…”

“…The effects of water repellency (natural and that induced by wildfire) are also a source of infiltration variability (DeBano, ; Doerr, Shakesby, & Walsh, ; Robichaud, Lewis, & Ashmun, ). More recently, burn severity (Ebel, Romero, & Martin, ; Moody et al, ) and hyper‐dry conditions (Moody & Ebel, ) have been investigated as sources, and sources related to soil sealing, preferential flow, and multiple layers have been reviewed by (Moody et al, ).…”