This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5). Fires in steep chaparral landscapes are often followed by catastrophic debris flows. Because of this threat, another potentially significant post-fire erosional process, wind erosion, has been under-appreciated. Observations from a burned area in Southern California suggest that wind erosion may be as important as runoff erosion in burned semi-arid landscapes. Although the removal of vegetation, which protects the soil surface from the wind, undoubtedly plays a role in higher rates of wind erosion from burned areas, fire-induced changes within the soil itself may be more important in increasing the vulnerability of the soil to eolian transport. Undisturbed chaparral soils are covered by a biotic mat, a dense network of fine chaparral roots overlain by algae, lichen, and moss, which acts to armor the soil underneath. High severity fires, however, appear to combust the biotic mat, leaving the soil exposed to the wind. In Southern California, large fires are often driven by Santa Ana winds that can reach hurricane speeds. Because the conditions for these winds generally occur from September to March, the areas scorched by fire are then subjected to high winds for many months. Field observations and satellite images testify to extreme rates of wind erosion under these conditions. This project will investigate the relationship between fire, the biotic mat, and wind erosion through a program of wind-tunnel experiments. Undisturbed soil surface samples, collected from chaparral hillslopes, will be subjected to controlled wildfire conditions in the laboratory to simulate fires of varying temperatures and durations. The samples will then be placed in a wind tunnel to determine their threshold friction velocity, a measure of the soil's erodibility by wind.
The main impact of this project will be to provide information on a source of airborne dust that can affect human health in Southern California. It should have implications for many similar environments elsewhere. It has been estimated that airborne dust pollution is responsible for thousands of premature deaths annually in California; satellite images indicate that burned areas are important sources of dust blown westward to the Los Angeles area during Santa Ana conditions. One of the goals of this project is to develop relationships between easily measured soil parameters and wind erosion potential for use by air pollution control districts. This project will also provide an educational opportunity for an undergraduate student from a minority-serving institution.