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Researchers Study Fire's Effects On Soil Moisture And Runoff
December 14, 2018

Massive and devastating wildfires across the country’s Southwest and West have unfortunately become an almost continuous topic during the daily news cycle. Beyond the obvious destruction of forests and the loss of habitat for wildlife large and small, scientists and researchers are now studying the impact these fires have on the soil’s ability to retain water and the impact of runoff from precipitation events in those burned areas.

In 2011, a massive wildfire in New Mexico burned more than 150,000 acres and threatened the Los Alamos National Laboratory. Photo credit: Brian Klieson.

In 2011, a massive wildfire in New Mexico burned more than 150,000 acres and threatened the Los Alamos National Laboratory. Now, using data from the Las Conchas mega-fire, researchers at Los Alamos have created an experimental model that will enable researchers to better understand the interactions of fire and water in the soil.

With funding provided by Los Alamos National Laboratories Laboratory Directed Research and Development (LDRD), Adam L. Atchley, a researcher at Los Alamos, and his team set off with a goal: to evaluate how the soil’s water balance changes before and after a fire, depending on the burn severity.

According to information presented in their research paper, “Simulating Surface and Subsurface Water Balance Changes Due to Burn Severity,” published in the Vadose Zone Journal, “Forest fires have a significant impact on hydrology, such as reduced infiltration rates leading to increased flooding. However, post-fire water balance changes and the competing hydrologic response of increased runoff and evapotranspiration as a function of burn severity are not well understood. Comparing pre- and post-fire water balance changes is challenging because measurements of fire-disturbed landscapes with the previously undisturbed character are impractical due to non-repetitive observational conditions. We used a physically based modeling experiment to incorporate burn severity data from the Las Conchas fire to approximate model parameterization to evaluate continuous water balance progression for pre- and post-fire simulations using the same forcing conditions.”

Using measurements taken several years before the fire by the Bandelier Fire Ecology Field Team, the researchers discovered that low- to moderate-severity wildfires result in wetter soil. However, “this modeling experiment also identified a tipping point where increased overland flow from high burn severity sites eclipses the effect of reduced evapotranspiration on the water balance, causing comparatively drier post-fire soils,” Dr. Atchley noted.

“It is well known that fire disturbances can have a strong effect on how water interacts with land,” noted Dr. Atchley. “Fire often dramatically increases flashy runoff responses to storms following the fire disturbance. But it also reduces evapotranspiration. What is not well understood, and is hard to measure, is how these two competing processes change the site water balance, or how wet or dry a burn site will be after the fire. Increasing the runoff would make the site dryer overall while decreasing the evapotranspiration will keep water on the site and make it wetter,” Dr. Atchley said.

“What we found,” Dr. Atchley explained, “is that burn sites will generally become wetter because the change in evapotranspiration is bigger than the change in runoff. However, in the case of high-burn severity, the site could become drier because the change in runoff shortly after the fire becomes bigger than the change in evapotranspiration.”

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