THE EVOLUTION AND DEPLETION OF SEDIMENT POOLS IN A STEEP HEADWATER CATCHMENT FOLLOWING WILDFIRE AS DOCUMENTED BY HIGH RESOLUTION CHANGE DETECTION

Wildfire is a highly effective short-term disturbance agent in semi-arid mountainous settings, where runoff and associated sediment yields may increase by orders of magnitude during the first few post-fire storm events. This brief period represents a critical time for downstream stakeholders, when understanding the threat to receiving water resources and infrastructure in the form sediment-laden flows during intense rainfall is crucial. A necessary aspect to effective hazard characterization is understanding the primary sources of material and processes that produce post-fire sediment-laden flows. In this study, we seek to understand how storm magnitude and sequencing affects sediment availability and transfers from hillslopes to channelized domains where material may be stored or exported downstream.

In order to achieve this, we monitored surface changes, precipitation, soil moisture, and runoff over the course of the first wet season following wildfire in a steep (>32 degrees) ~1 ha watershed burned by the 2018 Holy Fire in Southern California. Change detection was performed using terrestrial lidar scans to capture changes on the hillslopes and fringes of channelized features along with UAV-based structure-from-motion photogrammetry. The first effective events occurred four months following the burn and generated extensive erosion with sediment yields equivalent to ~390 tons/ha, with approximately half of this sediment derived from shallow hillslope erosion, and the remainder from gully formation and channel evacuation. The next few change detection sequences bracketed storms with greater rainfall intensities and runoff plot responses where hillslope sediment sources were elevated relative to more subdued channel sediment yields. Despite higher magnitude storms and hillslope runoff responses later in the wet season, storm-specific sediment yield declined as more easily mobilized sediment stocks became exhausted. These results indicate that following a burn the sequence of effective events exert a first-order control on sediment yields as supply-limited conditions are reached, particularly in channelized domains. Predictive models of post-fire sediment export would be improved by more explicitly incorporating the role of sediment availability.