Baking dirt to predict erosion after a fire

During a forest fire, the temperature at ground level can range from 50°C to more than 1,500°C. Earth denuded of plant cover by fire becomes vulnerable to erosion, but the severity of that erosion depends on the particular high-temperature conditions that the soil experienced, says John A. Moody of the U.S. Geological Survey in Boulder, Colo.

To quantify this relationship, Moody and his colleagues subjected lab-baked forest soils to flowing water. The researchers used soils derived from a variety of bedrock materials, including granite, sandstone, and volcanic ash.

In soils that had been heated to temperatures less than 175°C for an hour, erosion resistance was variable, depending on such particulars as the presence of tiny roots and rotted leaves, says Moody.

When samples were baked at temperatures ranging from 175°C to 275°C, however, chemical bonds formed between clay particles and hardened the soil to an adobelike consistency, rendering its surface extremely resistant to erosion.

Surprisingly, when roasted at temperatures above 275°C, samples became more prone to erosion. The higher temperatures apparently destroyed the chemical bonds that had formed at intermediate temperatures, says Moody.

These results could be combined with readings of soil temperatures during or after a fire to forecast an area’s vulnerability to erosion and even the mudslides that often follow fires, the researchers suggest. Moody and his colleagues report their findings in the January Journal of Geophysical Research (Earth Surface).