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Burn scars less water-repellant than thought

August 2, 2023

Burnt watershed soils more absorbent than believed

New research from the University of Southern California (USC) reveals that burnt soils or organic substances are less hydrophobic, or less water-repellant, than commonly thought. Water is still absorbed. Rainfall and soil moisture are responsible for increased flood and mudslide risk after wildfires.

While loss of vegetation during a fire leaves soil vulnerable to erosion, the waxy coating or organic, oily substance on the soil’s surface does not absolutely prevent the soil from absorbing water. Research suggests that “burnt watersheds may hold more water in their soil due to lack of vegetation.”

Joshua West, professor of Earth Sciences who led the study at USC, was not surprised the water flow and debris in the burned area’s stream was four to 10 times greater than the flow in the unburnt area’s stream.

However, stormwater had unexpectedly permeated the ground in both of the burnt watersheds.

Scientists at USC collaborated with researchers from the University of Michigan, the U.S. Geological Survey (USGS) and Rutgers University to monitor two wet seasons following the Bobcat Fire. Of the three watersheds in the San Gabriel Mountains of California, two were burned during the 2020 Bobcat Fire and the third watershed was mainly untouched.

The team concluded from data collected over December 2020 to March 2022 that the burnt ground that contained this waxy coating was absorbing water. Post-wildfire, a significant portion of the water flow in all three watersheds came from water that had been absorbed in the ground. In the unburnt watershed, researchers found that trees absorbed the water as anticipated, preventing it from reaching streams.

Other factors include hillslope steepness and seismic activity; University of Kansas researchers found slope to be a more significant factor to stormwater runoff and floods than precipitation and aridity. “If we can understand how hillslopes respond to rainfall inputs, we will be better able to predict potential future responses,” noted Admin Husic, the Phelps Chair’s Council Assistant Professor. He added, “Looking forward, an important consideration is the increased intensity of (storm) events … (and) the amount of rainfall absorbed by hillslopes versus (that) transmitted to rivers.”

To help prevent or reduce the risk of flooding, we support the safe monitoring of stream discharge and precipitation as well as comprehensive analysis of environmental data, from rainfall to soil moisture to water quality. In particular, noncontact discharge radar sensors can be mounted on bridges or cableways to keep field staff on solid ground while they record water flow and water level measurements. Within the sensors, machine learning accelerates the building of a rating curve, saving months of time to return to the site and take new measurements.