HINDCASTING SPATIAL AND TEMPORAL PATTERNS IN POST-WILDFIRE EROSION ACROSS CALIFORNIA

Global climate change is already impacting California’s hydroclimate via compression of the rainy season and an increased frequency of hydrologic extremes. The western US has also seen a twofold increase in the number of fires and a fourfold increase in median annual area burned in recent decades. Post-wildfire studies reveal that fire greatly facilitates erosion via changes to vegetation and soil properties, with extreme erosion observed when extreme rainfall follows wildfire. This suggests that the spatial and temporal patterns of post-wildfire erosion across the state may carry signatures of global climate change, with potential impacts to water resources, aquatic and riparian ecosystems, and near-shore environments. To quantify the potential impacts of post-wildfire erosion across California, we used the process-based model, Water Erosion Prediction Project (WEPP), to simulate post-fire erosion in watersheds impacted by wildfires greater than 100 km² in the time period 1984-2021 for a total of 201 fires and ~21,500 watersheds.

We find that the mass of sediment predicted to erode by the WEPP model correlates strongly with area burned. Inclusion of the standard precipitation index, a metric used for comparing hydrology across climatic regions, strengthens the correlations. From all years modeled (1984-2021), WEPP predicts 9.5 and 4.5 Mt of sediment to have eroded from basins draining into reservoirs from northern and southern CA, respectively, constituting 51% and 68% of the total mass of eroded sediment from each region and reflecting ongoing challenges for water resource security. The distribution of sediment yields (mass/area) is also significantly different between northern and southern California with northern California producing higher yields, with particularly high yields (>2000 t/km²) emerging as outliers in recent years. To account for post-fire debris flows, which are not included in WEPP, we compile measured and estimated debris flow volumes and convert these to mass. Where post-fire debris flows have been documented but no volume measurements exist, we use the USGS Landslide Hazards Program Gartner (2014) model to quantify debris flow volume. Our results provide the first estimates of the magnitude of sediment mass and yields generated statewide by post-fire erosion.