SANTA CRUZ, Calif. — In a first-of-its-kind study, scientists compiled one year’s worth of soil and sediment erosion quantities occurring after large California wildfires between 1984 and 2021. Scientists found that postfire erosion has accelerated over time, particularly in northern California, likely reflecting both the increase in wildfire in the state and the frequency of wet water years. In addition, scientists found that 57% of postfire erosion by mass occurred upstream of reservoirs. This research helps planners understand the degree to which postfire erosion has impacted watersheds and can inform management actions to minimize the effects of runoff on clean water storage.

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Using the Water Erosion Prediction Project model developed by the USDA Forest Service, in combination with field-based debris-flow volume measurements and modeled debris-flow volumes, scientists from the USGS and California Geological Survey evaluated postfire erosion throughout California for 196 large wildfires, which includes fires over 100 km²,~25,000 acres, in California between 1984 to 2021.

Researchers estimated the magnitude of erosion contributed by postfire debris flows and hillslope erosion in the first water year following wildfire. Results show that postfire debris flows can mobilize vastly more sediment than hillslope erosion processes. As climate change exacerbates hillside erosion, it has become more common and could create more of a downstream hazard for water resources. Examples of this include the filling of storage space in reservoirs and damaging infrastructure as ‘nuisance’ sediment that blocks roads and culverts.

Results from the study show a tenfold increase in postfire hillslope erosion between 1984–1990 and 2011–2021 in northern California, with the majority of top sediment-producing fires occurring in the last decade.

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The team of researchers reviewed published studies of the Water Erosion Prediction Project model’s accuracy in application to burned areas and conducted their own tests comparing model output to field data. Results showed that the model often over-predicts erosion where little erosion has occurred and underpredicts erosion where significant erosion occurred, particularly in watersheds with greater burn severity.

Researchers acknowledged that targeting erosion in just the first water year following wildfire may underestimate postfire erosion, as those effects may last for several years.

Because fire extent and burn severity are projected to increase alongside extreme rain with ongoing climate change, postfire sediment mobilization will be a growing risk to water-resource security, as well as riparian, coastal, and marine ecosystems and communities.

The study “Postfire sediment mobilization and its downstream implications across California, 1984–2021” was published in the Journal of Geophysical Research: Earth Surface from the American Geophysical Union. 

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