The large-scale Bobcat Fire that hit Los Angeles in 2020 led scientists to explore the after-effects on soil and water. They made a startling discovery, contrary to previous understanding, that the waxy coating formed on burnt soil from burnt plants actually has the ability to absorb water. This absorption contributes to an increase in water and debris flow in streams, escalating the risks of floods and landslides.
Previously, researchers believed that the waxy layer on scorched soil caused water to run off on the surface. However, recent findings reveal that burnt land does, in fact, soak up water. These new insights can enhance the accuracy of predictions for flooding and mudslides after a fire event.
In 2020, Los Angeles County’s San Gabriel Mountains were severely affected by one of the worst wildfires, scorching over 115,000 acres and damaging or destroying over 150 structures. This disaster imposed further stress on residents already grappling with pandemic-related challenges.
Though firefighters eventually subdued the enormous Bobcat Fire, its aftermath created additional dangers. Such “mega-fires,” becoming more common due to climate change, set the stage for post-fire hazards like flooding, mudslides, and debris flows, worsening the destruction.
The ability to comprehend how water accumulates and track runoff and streamflow in burned areas equips authorities to forecast these post-wildfire events, enabling timely warnings for residents.
An Unexpected Observation
Long-standing general belief has maintained that the absence of vegetation during a fire renders the soil prone to erosion as plant roots holding the soil die. Scientists had a different notion, arguing that the waxy coating from burning leaves forms an organic, oily substance on the soil. They thought this layer repelled water, causing quick runoff like a Slip ‘N Slide, carrying mud and debris.
But new research published in Nature Communications has challenged this scientific assumption.
A Pivotal Discovery
A team of scientists from USC Dornsife College of Letters, Arts, and Science, the University of Michigan, the U.S. Geological Survey, and Rutgers University monitored the aftermath of the Bobcat Fire for two wet seasons. They discovered that the burnt ground with the waxy coating was actually absorbing water.
The team observed three watersheds in Southern California’s San Gabriel Mountains, two affected by the fire and one largely untouched. They determined that a considerable part of the water flow in all watersheds came from the ground where the water had been absorbed.
This observation contradicts previous scientific thought that little water would be absorbed in the burnt areas due to waxy soils. The unexpected result was the increase in water in rivers coming from the burnt areas, as burned trees couldn’t retain water, but not from the soil’s inability to soak up water.
This finding reinforced the team’s hypothesis that water in streams originates from both rain and groundwater, leading to more flooding in burnt areas compared to unburnt ones.
The Lingering Danger of Water Accumulation
Understanding how water infiltrates soil and contributes to stream flow is crucial for pinpointing high-risk areas for debris flow and mudslides and predicting debris flow post-rainfall in burn areas.
Water flow dynamics and sub-surface accumulation can notably influence landscape recovery post-wildfire, affecting hill slope stability and forest resilience against drought. Conversely, this accumulation can instigate landslides for up to four years after a fire due to increasing pressure in the soil.
Joshua West, who led the study, warned that the possibility of landslides extends well beyond two years after a fire, constituting a long-lasting issue. For instance, the water stored in the Bobcat Fire-affected areas could foreshadow future flooding problems.
The research’s findings carry valuable data that the USGS can employ to enhance burn area monitoring and predictions for flooding and mudslides post-wildfire.
Reference: “Importance of subsurface water for hydrological response during storms in a post-wildfire bedrock landscape” by Abra Atwood, Madeline Hille, Marin Kristen Clark, Francis Rengers, Dimitrios Ntarlagiannis, Kirk Townsend and A. Joshua West, 29 June 2023, Nature Communications.
DOI: 10.1038/s41467-023-39095-z
Funding for this study was provided by USC Dornsife’s Department of Earth Sciences and the National Science Foundation.
