What drives spatial variability in rainfall intensity-duration thresholds for post-wildfire debris flows? Insights from
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Luke A. McGuire I Ann M. Youberg
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 intensityduration (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 dimensionlessdischarge 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. Keywords Debris flow . Runoff . Erosion . Discharge Introduction Wildfire alters the hydrologic and geomorphic response to rainfall (Shakesby and Doerr 2006; Moody et al. 2013). Decreased canopy and ground cover in burned areas promotes reduced interception (Stoof et al. 2012) and lower hydraulic roughness (Stoof et al. 2015), which lead to greater amounts of runoff and more efficient erosion via overland flow and raindrop-driven processes. Infiltration rates are generally lower in burned areas (Robichaud 2000; Ebel and Moody 2017) relative to nearby unburned areas due to fireinduced soil water repellency (DeBano 2000; MacDonald and Huffman 2004), surface soil sealing (Larsen et al. 2009), and hyper-dry conditions (Moody and Ebel 2012). Debris flows are one consequence of the increased runoff and erosion that characterize recently burned, steep terrain (Cannon 2001; Gabet and Bookter 2008; Kean et al. 2011; Nyman et al. 2011). Debris flows in the first year after fire often initiate from runoff (Kean et al. 2011), as opposed to debris flows that mobilize from shallow landsl
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