Guilinger, James J., Foufoula‐Georgiou, Efi, Gray, Andrew B., Randerson, James T., Smyth, Padhraic, Barth, Nicolas C., and Goulden, Michael L.
Predicting sediment yield from recently burned areas remains a challenge but is important for hazard and resource management as wildfire impacts increase. Here we use lidar‐based monitoring of two fires in southern California, USA to study the movement of sediment during pre‐rainfall periods and postfire periods of flooding and debris flows over multiple storm events. Using a data‐driven approach, we examine the relative importance of terrain, vegetation, burn severity, and rainfall amounts through time on sediment yield. We show that incipient fire‐activated dry sediment loading and pre‐fire colluvium were rapidly flushed out by debris flows and floods but continued erosion occurred later in the season from soil erosion and, in ∼9% of catchments, from shallow landslides. Based on these observations, we develop random forest regression models to predict dry ravel and incipient runoff‐driven sediment yield applicable to small steep headwater catchments in southern California. Wildfire makes watersheds more susceptible to hazardous flash flooding and debris flows, yet characterization and prediction of these hazards remains limited. In this study, we used repeat airborne laser mapping to quantify the movement of sediment in steep burn areas during initial dry periods and subsequent erosion from runoff events. Based on these observations, we developed two predictive models: one to predict the filling of channels with sediment prior to rainfall and a second one to predict erosion by debris flows and floods during initial storm events, which showed improvement over another commonly used model. After initial runoff events, much of the available sediment in channels was transported downstream, however small landslides and extensive erosion of soils across the landscape continued to supply sediment to floods and debris flows, in line with studies elsewhere showing continued debris flow activity despite reduced sediment in channels. Our study demonstrates that airborne laser mapping together with data‐driven modeling offer opportunities to increase predictive ability of post‐fire erosion and such approaches should be further explored in regions such as northern California, where fire is expanding and models of post‐fire erosion need to be tested and refined. Postfire dry and wet sediment transport were quantified with lidar and dominant controls were identified using random forest regressionSlope and sediment supply, including dry ravel, were the strongest controls on initial sediment yield by debris flows and floodsContinued sediment bulking occurred from soil erosion and patchy mass wasting later in the wet season as channels became supply limited Postfire dry and wet sediment transport were quantified with lidar and dominant controls were identified using random forest regression Slope and sediment supply, including dry ravel, were the strongest controls on initial sediment yield by debris flows and floods Continued sediment bulking occurred from soil erosion and patchy mass wasting later in the wet season as channels became supply limited