Infiltration rates after wildfires in the Bitterroot Valley

Brady, J.; Robichaud, P.R.; Pierson, F.B. 2001. Infiltration rates after wildfires in the Bitterroot Valley. ASAE paper number 01-8003, presented at the 2001 ASAE Annual International Meeting sponsored by American Society of Agricultural Engineers, Sacramento Convention Center, Sacramento, California, USA, July 30--August 1, 2001. ASAE--2001: An Engineering Odyssey. Technical Session 21: Forest soil erosion and water quality. St. Joseph, MI: ASAE.

Keywords: infiltration, Montana, post-fire assessment

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Abstract: Recent fires have renewed interest in fire's effect on different components of the ecosystem, in particular fire's effects on infiltration and runoff. Forests subjected to high severity burns often develop water repellent soil conditions. Under this condition, the infiltration of water into the soil is lowered and consequently additional runoff occurs. Increased runoff can contribute to surface erosion and lead to sediment deposition in downstream areas. In this study, a rainfall simulator was used to compare infiltration rates from plots located in high severity burned areas to plots in undisturbed areas on the Sula Ranger District, Bitterroot National Forest in western Montana after the 2000 fire season. Simulated rainfall was applied to 102 0.5 m2 plots for 60 min at 100 mm hr-1. Bottles collected runoff at 1 and 2-min intervals. There were 15 plots for each of the four burned sites, and 14 plots for the three unburned sites, half of which had their rootmat removed. Results indicate naturally occurring water repellent soil conditions of the surface on the unburned, rootmat-removed treatment, which had the lowest infiltration rates. The burned plots on the other hand had a water repellent layer centimeters beneath the soil surface. This allowed for some water storage in the surface layer while also maintaining low infiltration rates even below what was expected, especially towards the end of the simulation runs. These results are most likely due to the extremely dry state of the soil and the natural water repellent condition that is common in volcanic ash-cap soils.

Moscow FSL publication no. 2001i