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