POSTWILDFIRE HYDROLOGIC AND GEOMORPHIC RESPONSE IN A SEVERELY BURNED, SEMI-ARID WATERSHED: IMPLICATIONS FOR EARLY FLOOD-WARNING SYSTEM INSTALLATIONS

Postwildfire damage to vegetation and soils in the severely burned Rito de los Frijoles watershed following the 2011 Las Conchas wildfire resulted in a period of dramatic hydrologic and geomorphic adjustment, followed by a period of increasing hydrologic and geomorphic stabilization, as quantified by Flood Travel Time and Stream Efficiency (a method for normalizing flood magnitude). The initial period of adjustment was characterized by floods of historically unprecedented magnitude associated with increased Runoff Efficiency, decreased Flood Travel Time, and changes to channel morphology from large-scale erosion and sediment deposition. The subsequent period of stabilization exhibited a trend towards pre-fire conditions including decreased magnitude of floods, increased Flood Travel Time, and re-establishment of inset channel morphology with vegetated overbanks. The transition between the initial period of hydrologic adjustment and subsequent period of hydrologic stabilization is identified as the “Recovery Inflection Point”. The Recovery Inflection Point is that period in the hydrologic recovery continuum when the channel is the most hazardous to downstream values at risk because Runoff Efficiency is at a maximum, meaning floods are larger for given rainfall events, and Flood Travel Time is at a minimum, meaning downstream, flood-response time is shorter. We show that the Recovery Inflection Point in the Rito de los Frijoles Watershed occurred in 2013, two years after the fire. While mechanisms are similar for other burned watersheds, timing of recovery will vary, based on timing and intensity of postwildfire rainfall in addition to watershed physical and ecological characteristics. Because postwildfire flood peaks reach their maximum and arrive sooner during the recovery inflection point, early-warning systems should remain in place and hyper-vigilance should persist until the recovery inflection point -- as constrained by Flood Travel Times and Runoff Efficiency -- after which early-warning systems may incorporate the steady recovery of the system.