CHARACTERISTICS OF DEBRIS-FLOW TRIGGERING RAINSTORMS FOLLOWING WILDFIRE IN WESTERN NEW MEXICO

Moderate and high severity fires promote increases in runoff and erosion, leading to a greater likelihood of extreme responses such as debris flows. Post-fire debris flows often initiate when runoff rapidly entrains a substantial amount of sediment. The high sediment concentration in debris flows can produce flows with peak discharges that greatly exceed those of water-dominated flood flows. Therefore, it is important to characterize the watershed properties and rainfall characteristics that promote debris-flow responses versus those that promote water-dominated flood flows. Constraining debris-flow timing within rainstorms is critical to this effort since the rainfall intensities that initiate debris flows may be substantially less than the peak rainfall intensities during a debris-flow producing rainstorm. In this study, we monitored a series of watersheds following the 2020 Tadpole Fire in western New Mexico, USA, and documented 16 debris flows during the first 3 months following the fire. Debris flows were generated by runoff and were associated with high intensity rainfall over short durations (< 30 minutes). We were able to constrain the timing of debris flows at watershed outlets to within approximately one minute in seven of the 16 cases, while we were only able to constrain the date of debris flow occurrence in the remaining nine cases. Based on the seven debris flows where the triggering rainfall intensities could be more accurately estimated, the recurrence interval of 15-minute rainfall intensities generating debris flows ranged from approximately 1-1.3 years. Despite debris-flow activity being concentrated in watersheds burned primarily at moderate and high soil burn severity, in-situ measurements of soil hydraulic properties following the fire indicated that areas burned at moderate and high severity had increased infiltration capacity relative to soils burned at low severity. Results provide constraints on the rainfall characteristics associated with post-fire debris flows in New Mexico and highlight the importance of factors other than reductions in soil infiltration capacity for increasing debris-flow potential in recently burned areas, including increases in percent bare ground and decreases in surface roughness and interception.