What drives spatial variability in rainfall intensity-duration thresholds for post-wildfire debris flows? Insights from the 2018 Buzzard Fire, NM, USA

Abstract

Wildfire alters the hydrologic and geomorphic responses of burned areas relative to nearby unburned areas, making them more prone to runoff, erosion, and debris flow. In post-wildfire settings, debris flows often initiate when runoff concentrates on steep slopes and rapidly mobilizes sediment. Rainfall intensity-duration (ID) thresholds have been proven useful for assessing post-fire debris-flow potential but can vary substantially from one location to another as a result of hydrologic factors that control rainfall-runoff partitioning. Debris-flow initiation thresholds based on a slope-dependent dimensionless discharge criterion, which have the theoretical benefit of being consistent from site to site, have also been proposed but not extensively tested. We monitored debris-flow activity in 12 small (< 1 km2) watersheds burned by the 2018 Buzzard Fire in New Mexico, USA, documenting 24 debris flows during the first several months following the wildfire. We use a recently proposed dimensionless-discharge threshold in combination with rainfall-runoff modeling to estimate basin-specific rainfall ID thresholds for debris-flow initiation. These model-derived thresholds compare well with observations. Areas burned at low severity are characterized by higher infiltration capacity, rainfall interception, and hydraulic roughness relative to areas burned at moderate or high severity, but differences in rainfall ID thresholds between these two areas can be predominantly attributed to wildfire-induced changes in hydraulic roughness. Results highlight the utility of thresholds based on dimensionless discharge relative to those based on rainfall intensity and also provide additional data that will help constrain general models for the prediction of rainfall ID thresholds.