Soil erosion from California wildfires has increased since 1984

Santa Cruz, California — In a first-of-its-kind study, scientists compiled soil and sediment erosion one year after wildfires in California between 1984 and 2021. The scientists found that post-fire erosion accelerated over time, especially in Northern California, likely reflecting the increase in wildfires and the frequency of wet years in the state. Additionally, the scientists found that 57% of the large-scale post-fire erosion occurred upstream of reservoirs. The research helps planners understand the extent of post-fire erosion impacts on watersheds and can inform management actions to minimize the impact of runoff on clean water storage.

Using a model developed by the USDA Forest Service’s Water Erosion Forecasting Program, combined with field debris flow volume measurements and simulated debris flow volumes, scientists from the U.S. Geological Survey and the California Geological Survey assessed post-fire erosion from 196 large fires in California, including fires with an area of ​​more than 100 kilometers.²Between 1984 and 2021, California had approximately 25,000 acres of farmland.

The researchers estimated the extent of erosion caused by post-fire debris flows and hillslope erosion in the first water year after a fire. The results show that post-fire debris flows can move more sediment than hillslope erosion processes. As climate change intensifies hillslope erosion, this is becoming more common and could cause greater harm to downstream water resources. Examples of this include filling up storage space in reservoirs and damaging infrastructure by clogging roads and culverts as “harmful” sediment.

The findings show that the area of ​​hillside erosion following fires increased tenfold in Northern California between 1984-1990 and 2011-2021, with most of the fires that produced top sediments occurring in the past decade.

The team reviewed published research on the accuracy of the Water Erosion Prediction Project model when applied to fire areas and conducted their own tests comparing model output with field data. The results showed that the model tended to overestimate erosion where little erosion was occurring and underestimate it where erosion was severe, especially in watersheds with high fire severity.

The researchers acknowledge that focusing only on erosion in the first water year after a wildfire may underestimate post-fire erosion because the effects can last for years.

As fire extent and burn severity are projected to increase with climate change and extreme rainfall, post-fire sediment flows will pose an increasing risk to water security and riparian, coastal, and marine ecosystems and communities.

Research”Sediment flows and downstream impacts following fires in California from 1984 to 2021” was published in the American Geophysical Union’s Journal of Geophysical Research: Earth Surface.