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 rainfall-runoff partitioning and geomorphic process rates, increasing the potential for runoff-generated debris flows in steep watersheds. Runoff-generated postfire debris flows (PFDFs) often initiate when infiltration excess 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 scale 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 during which time debris flow susceptibility may change rapidly. Here, we monitored rainfall and debris-flow activity in two recently burned watersheds in southern Arizona, USA, over the first 1.5 years following fire. We quantified changes in soil hydraulic properties during 11 site visits throughout the monitoring period, using repeat 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 (Kfs), wetting front potential (hf), and sorptivity (S) and used a point-scale Green-Ampt infiltration model to quantify the combined effects of changes in Kfs and hf on runoff generation. 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 observed temporal trends in soil hydraulic properties. Seasonal variations in Kfs, hf, and S were substantial, leading to non-monotonic relationships between these properties and time since fire. A comparison of Kfs, hf, and S at similar times during the first and second postfire years indicate that Kfs hf, and S decreased immediately following the fire. We observed two debris flows, which occurred during the first three months following 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 to explain prior observations that PFDFs primarily initiate in the first rainy season following fire in Arizona.