Insights into temporal changes in debris flow susceptibility following fire in the Southwest USA from monitoring and repeat estimates of soil hydraulic and physical properties

Abstract

Wildfire influences geomorphic process rates, increasing the potential for runoff-generated debris flows in steep watersheds. Runoff-generated postfire debris flows (PFDFs) often initiate when overland flow rapidly mobilizes sediment from steep hillslopes and channels. Fire effects on soil hydraulic properties, including their magnitude and temporal persistence, can therefore play an influential role in determining the degree to which fire increases debris-flow potential and the time period for heightened debris-flow hazards following fire. There is a paucity of measurements that quantify the timing of changes in soil hydraulic properties throughout the first 1–2 years after fire. Here, we monitored rainfall and debris-flow activity in two watersheds burned by the 2022 Contreras Fire in Arizona, USA, over the first 1.5 years following fire. We quantified changes in soil hydraulic properties during 11 site visits using in-situ measurements with a tension infiltrometer to provide insight into the temporal persistence of heightened debris-flow hazards. Specifically, we estimated field-saturated hydraulic conductivity (K_fs), wetting front potential (h_f) and sorptivity (S). We further tracked changes in soil water repellency, ground cover and soil physical and chemical properties, including bulk density, carbon and organic matter content to help explain temporal trends in soil hydraulic properties. Seasonal variations in K_fs, h_f and S were substantial, leading to non-monotonic relationships between these properties and time since fire. Rainfall-runoff modelling demonstrates that the magnitude of these seasonal changes are sufficient to influence runoff ratios and suggest postfire debris-flow susceptibility could change over timescales as short as several months. A comparison of K_fs, h_f and S at similar times during the first and second postfire years indicates that K_fs h_f and S decreased immediately following the fire. We observed two debris flows, which occurred during the first three months after the fire. The relatively short time associated with notable fire effects on soil hydraulic properties, combined with substantial increases in ground cover during the first postfire year, help explain observations that PFDFs primarily initiate in the first rainy season following fire in the Southwest USA.