USU Soil Scientists Examine Link Between Fires and Floods

It’s becoming more common to see fires and floods in the same news cycle. This spring both disasters devastated Ruidoso, New Mexico, within a month of each other. Soil scientists at Utah State University are beginning to see how they are related.

The simplest description is severe fires seem to lead to severe flooding. Low-severity fires can add organic matter to the soil and reduce the chance of runoff, while high-severity fires can destroy nearly all the organic matter, increasing the chances of erosion.

“We are trying to measure erodibility and how it increases or decreases after fires,” says Professor Grant Cardon, Utah State University Extension soils specialist. He and graduate student Justin Allred are studying several burn scar sites such as the one in Spanish Fork Canyon near Price, Utah. The four components that determine erodibility are soil texture, soil organic matter, soil permeability and soil structure.

Allred and Cardon are using drone mapping and on-the-ground analysis to measure erodibility and help predict the risk of flooding after a fire.

“Low-severity fires are generally thought to be good for the soil because they add organic matter,” Allred says. “There are several factors that can impact how long it takes organic matter to be restored back into soil. The largest factor is the amount of plant biomass, which over time becomes soil organic matter. This is another factor we can measure”

After a high-severity fire, nearly all the plants are burned, which leads to slow recovery times, Allred explains. Soil scientists measure organic carbon levels as an indicator of organic matter. Extremely hot fires can consume most or all of the organic matter that is primarily responsible for soil aggregate stability, permeability and resistance to erosion. According to some studies, it can take 40 years or more for forest soils to recover to their pre-fire levels.

Many fires, such as those in New Mexico, are caused by lightning strikes or other unexpected natural causes. Some result from accidents or criminal activity. Others are planned, or “prescribed” burns.

“Prescribed fires have been shown to reduce the severity of wildfires,” Allred says. “In areas where erosion may be of concern, prescribed fires may be a tool used to reduce those soils’ erodibility. That may seem counterintuitive, since plant cover would be removed for the short term. However, in the long run, prescribed burns can benefit the soil.”

Allred and Cardon’s research could be used to ensure erodibility is considered when making prescribed burning decisions.

“That’s one of the ways our research should be useful in measuring how much erodibility is impacted by wildfire,” Allred says. “This could help various agencies make decisions about prescribed burns. Other researchers have also concluded that lower-severity fires have potential benefits to soil properties, which can positively impact a soil’s erodibility. The economic and community safety impacts also need to be considered.”