Impacts of successive wildfire on soil hydraulic properties: Implications for debris flow hazards and system resilience

Abstract

Climate and land use changes have led to recent increases in fire size, severity, and/or frequency in many different geographic regions and ecozones. Most post-wildfire geomorphology studies focus on the impact of a single wildfire but changing wildfire regimes underscore the need to quantify the effects of repeated disturbance by wildfire and the subsequent impacts on system resilience. Here, we examine the impact of two successive wildfires on soil hydraulic properties and debris flow hazards. The 2004 Nuttall-Gibson Complex and the 2017 Frye Fire affected large portions of the Pinaleño Mountains in southern Arizona, creating a mosaic of burn severity patterns that allowed us to quantify differences in wildfire-induced hydrologic changes as a function of burn severity and recent fire history (i.e. burned in only the Frye Fire or burned in both fires). Field observations after the 2017 Frye Fire indicated debris flow activity in areas burned predominantly at low severity. Many of these areas, however, were also affected by the 2004 Nuttall-Gibson Complex, suggesting that the relatively short recovery time between the two wildfires may have played a role in the geomorphic response to the most recent wildfire. Field measurements of soil hydraulic properties suggest that soils burned at moderate severity in 2004 and low severity in 2017 have a lower infiltration capacity relative to those that remained unburned in 2004 and burned at low severity in 2017. Simulations of runoff demonstrate that measured differences in infiltration capacity between once-and twice-burned soils are sufficient in some cases to influence the rainfall intensities needed to initiate runoff generated debris flows. Results quantify the impact of wildfire history and burn severity on runoff and debris flow activity in a landscape affected by successive wildfires and provide insight into how the resilience of geomorphic systems may be affected by successive wildfires.

Controls on the Spatial Distribution of Near‐Surface Pyrogenic Carbon on Hillslopes 1 Year Following Wildfire

“…Soil burn severity, also referred to here more generally as burn severity, is likely to play a role in the detachment and transport of PyC because the magnitude of wildfire‐induced effects on hydrologic and geomorphic systems is known to be dependent on soil burn severity (Moody et al., 2013). Soil hydraulic properties, including saturated hydraulic conductivity, play a strong role in determining the duration and intensity of rainfall required to generate runoff and have been shown to depend on burn severity (McGuire & Youberg, 2019; Moody et al., 2016). Infiltration capacity is generally lower in areas burned at moderate or high severity relative to nearby areas that are unburned or burned at low severity (Ebel & Martin, 2017; McGuire & Youberg, 2020; Moody et al., 2016; Robichaud, 2000; Robichaud et al., 2016), though soils burned at moderate and high severity are occasionally characterized by increased saturated hydraulic conductivity (Raymond et al., 2019) and decreased soil water repellency (Pierson, Robichaud, Moffet, Spaeth, Williams, et al., 2008; Tessler et al., 2013).…”

Wildfire and earth surface processes

“…One of the largest threats to life and infrastructure after a wildfire in steep mountainous terrain is post‐fire debris flows. Several works in the special issue focus on debris flow hazards (McGuire & Youberg, 2019; Raymond et al 2020; Tillery & Rengers, 2020). Raymond et al (2020) monitored debris flow activity in five burned watersheds and demonstrated that rainfall intensity‐duration thresholds for debris flow initiation are consistent with the rainfall intensity required to exceed a critical water discharge.…”

“…In particular, they found that debris flows on unburned slopes were typically initiated from shallow landslides while debris flows on burned slopes initiated from dispersed runoff. McGuire and Youberg (2019) focused on exploring the impacts of successive fires on debris flow processes. They used a hydrologic model, with soil hydraulic properties constrained from field measurements, to illustrate how areas that burned at moderate severity in 2004 and low severity in 2017 were likely to be more vulnerable to debris flows relative to areas that were unburned in 2004 and burned at low severity in 2017.…”

“…used field observations to contrast different debris flow initiation mechanisms in burned and unburned areas during a single, extreme rainstorm. In particular, they found that debris flows on unburned slopes were typically initiated from shallow landslides while debris flows on burned slopes initiated from dispersed runoff McGuire and Youberg (2019). focused on exploring the impacts of successive fires on debris flow processes.…”

Extreme Precipitation Across Adjacent Burned and Unburned Watersheds Reveals Impacts of Low Severity Wildfire on Debris‐Flow Processes

“…Wildfire is a disturbance that is becoming more impactful as warmer temperatures, longer fire seasons, and invasive species help promote increases in fire size and frequency (Abatzoglou & Williams, 2016; Fusco et al., 2019; Miller & Safford, 2012; Pausas & Fernández‐Muñoz, 2012; Westerling et al., 2006). Wildfire can influence soil characteristics, including soil hydraulic properties (McGuire & Youberg, 2019; Moody et al., 2016) and erodibility (Moody et al., 2005; Wagenbrenner et al., 2010) as well as ground and vegetation cover (Stoof et al., 2015). Disturbance caused by wildfire typically results in increased runoff and erosion (Moody et al., 2013; Shakesby & Doerr, 2006), with debris flows being one of the more extreme geomorphic responses observed in recently burned landscapes (Cannon et al., 2001; Gabet & Bookter, 2008; Kean et al., 2011; Meyer & Wells, 1997; Nyman et al., 2011).…”