Postfire Sediment Mobilization and Its Downstream Implications Across California, 1984–2021

• Published in: Journal of geophysical research. Earth surface 2024-08, Vol.129 (8), p.n/a
• Main Authors: Dow, H. W., East, A. E., Sankey, J. B., Warrick, J. A., Kostelnik, J., Lindsay, D. N., Kean, J. W.
• Format: Article
• Language: English
• Subjects: Climate change; Climate models; Debris flow; debris flows; Detritus; disturbance; Downstream; erosion; Erosion models; Extreme values; Extreme weather; fire; Fires; geomorphology; Heavy rainfall; Modelling; Precipitation; Rain; Rainfall; Reservoir storage; Reservoirs; Sediment; Sediment yield; Sediments; Soil erosion; Soil investigations; Soil water movement; Soil water storage; Storage capacity; Storage conditions; Time measurement; Upstream; Vegetation; Water resources; Water storage; Wildfires
• ISSN: 2169-9003; 2169-9011
• DOI: 10.1029/2024JF007725

Fire facilitates erosion through changes in vegetation and soil, with major postfire erosion commonly occurring even with moderate rainfall. As climate warms, the western United States (U.S.) is experiencing an intensifying fire regime and increasing frequency of extreme rain. We evaluated whether these hydroclimatic changes are evident in patterns of postfire erosion by modeling hillslope erosion following all wildfires larger than 100 km2 in California from 1984 to 2021. Our results show that annual statewide postfire hillslope erosion has increased significantly over time. To supplement the hillslope erosion modeling, we compiled modeled and measured postfire debris‐flow volumes. We find that, in northern California, more than 50% of fires triggering the top 20 values of sediment mass and sediment yield occurred in the most recent decade (between 2011 and 2021). In southern California, the postfire sediment budget was dominated by debris flows, which showed no temporal trend. Our analysis reveals that 57% of postfire sediment erosion statewide occurred upstream of reservoirs, indicating potential impacts to reservoir storage capacity and thus increased risk to water‐resource security with ongoing climate change. Plain Language Summary Large amounts of soil erode following wildfire, particularly if heavy rainfall occurs on the burned area within the first year after the fire. Climate change is increasing wildfires as well as extreme rain in many regions, including California. We investigated whether documented changes in fire and climate in California in the years 1984–2021 resulted in measurable changes in the amount or patterns of sediment produced from soil erosion following wildfire. Using a combination of models and measurements, we estimated annual postfire sediment erosion statewide, and separately for northern and southern California. We found that postfire erosion increased statewide over the time frame of interest. In southern California, postfire sediment erosion is dominated by debris flows, which are a liquefied mix of rock, mud, and soil, and no clear temporal patterns are apparent. In northern California, most cases of major postfire erosion occurred recently between 2011 and 2021. Most of the sediment mass eroded postfire statewide in the years 1984–2021 occurred upstream of reservoirs, rather than in places where sediment would move downstream directly to the ocean. These results indicate increasing pressure on water resources from postf