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ARS Home » Pacific West Area » Boise, Idaho » Northwest Watershed Research Center » Research » Publications at this Location » Publication #125724
Title: INFILTRATION RATES AFTER WILDFIRE IN THE BITTERROOT VALLEY

Author
item BRADY, J - UNIV CALIFORNIA DAVIS
item ROBICHAUD, P - FOREST SERVICE
item Pierson Jr, Frederick

Submitted to: American Society of Agricultural Engineers
Publication Type: Trade Journal
Publication Acceptance Date: 7/30/2001
Publication Date: N/A
Citation: N/A

Interpretive Summary: Recent fires have renewed interest in the effect of fire on 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, the effect of fire on infiltration rates was studied in high severity burned areas as well as undisturbed areas on the Sula Ranger District, Bitterroot National Forest in western Montana after the 2000 fire season. Results indicate that soils in these ecosystems are naturally water repellent at the soil surface, while burned areas have a water repellent layer centimeters beneath the soil surface. Therefore, infiltration into these soils is quite low on both burned and unburned areas. However, the unburned areas have large amounts of little and duff on the soil surface that protects the soil surface from erosion and stores water allowing time for the water to infiltrate.

Technical 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 as well as 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 sq.m plots for 60 min at 100 mm/hr. 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 have a water repellent layer centimeters beneath the soil surface. This allows for some water storage in the surface layer while also maintained low infiltration rates even below what was expected, especially towards the end of the simulation runs. These results are most like due to extremely dry conditions and natural water repellency conditions common in volcanic ash-cap soils.