Bowing
Source: Journal of Geophysical Research: Earth Surface
Like many states, California is facing a growing number of climate-related extremes: The annual acreage scorched by wildfires in the state increased fivefold between 1972 and 2018, and burns are also growing more intense. In addition, excessive rain is increasing flooding, landslides, and erosion, which can devastate terrain already reeling from fire damage. Large amounts of soil are prone to eroding after a wildfire, especially if heavy rainfall occurs within a year of the burn.
Dow et al. studied 196 fires that occurred between 1984 and 2021 and found that postfire sediment erosion increased statewide during this period. They used a combination of postfire hillslope erosion modeling and measurements of debris flow volume from both real and modeled events.
Both Northern and Southern California have been affected by large fires, but the two regions experience differences in climate and resulting fire patterns and postfire erosion patterns. In Northern California, postfire damage most often occurs as typical hillslope erosion, or the wearing away of the top layer of soil. There, the researchers found that the mean amount of erosion occurring in the year following a fire increased tenfold from 1984–1990 to 2011–2021. The highest sediment load (about 7 metric megatons) occurred in 2018, when the destructive Carr and Camp fires burned and a particularly rainy year followed.
In Southern California, postfire erosion more frequently occurs as debris flows, or fast-moving, liquefied blends of mud, rock, and soil. The years 2003, 2007, and 2017, during which major fires occurred, were the largest sediment-producing years (at 6.9, 3.9, and 2.7 metric megatons of sediment load, respectively). All were followed by low water periods. The researchers didn’t find an obvious pattern in how erosion magnitudes changed over time in Southern California, though they say that may be because of the episodic nature of major postfire debris flow events. The frequency of these events is forecast to increase with climate change.
Statewide, 57% of erosion occurring in the year after a fire occurred upstream from reservoirs, meaning the resulting sediment also poses a threat to water supply in a state already contending with water concerns. The database provided by this study could help water resources managers plan to mitigate these effects. As the likelihood of large fires and historically damaging rains in California increases with climate change, postfire erosion will present a growing risk to waterways and surrounding communities, the researchers say. (Journal of Geophysical Research: Earth Surface, https://doi.org/10.1029/2024JF007725, 2024)
—Rebecca Owen (@beccapox), Science Writer
