Temporal changes in soil water retention in burned and unburned areas after a wildfire and implications for slope stability

Temporal changes in the hydrologic and mechanical behavior of burned slopes result in changes in post-wildfire slope stability. The lack of long-term monitoring data on landscape recovery hinders our understanding of the interdependencies between wildfire-induced alterations in soil and vegetation properties, near-surface processes, and landslides. This study presents results from a 4-year field monitoring project of soil water retention and the corresponding long-term changes in landslide susceptibility after the 2019 Williams Flats Wildfire on the Colville Indian Reservation, WA, USA. Water retention was measured near both a burned and an unburned tree at multiple depths. The measured data were integrated into a suction stress-based infinite slope stability analysis to determine the corresponding temporal changes in shallow landslide susceptibility. The results showed that the lowest factor of safety (FS) near the burned tree was in June of the first year, which was attributed to macropores acting as preferential flow paths. The FS near the burned tree increased in the following years, which was attributed to vegetation regrowth. Vegetation recovery was quantified using normalized difference vegetation index mean values and soil temperature data. Field observations supported contributions of the macropore clogging on temporal changes in slope stability. The results suggest a dynamic interaction of vegetation recovery and soil physical changes and discuss the challenges of quantifying short- and long-term post-wildfire slope stability.