First-year post-fire erosion rates in Bitterroot National Forest, Montana

Spigel, K.M.; Robichaud, P.R. 2007. First-year post-fire erosion rates in Bitterroot National Forest, Montana Hydrological Processes 21: 998-1005

Keywords: hillslope erosion; forest fire; silt fences; WEPP; rainfall intensity; water repellency; site productivity

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Abstract: Accelerated runoff and erosion commonly occur following forest fires due to combustion of protective forest floor material, which results in bare soil being exposed to overland flow and raindrop impact, as well as water repellent soil conditions. After the 2000 Valley Complex Fires in the Bitterroot National Forest of west-central Montana, four sets of six hillslope plots were established to measure first-year post-wildfire erosion rates on steep slopes (greater than 50%) that had burned with high severity. Silt fences were installed at the base of each plot to trap eroded sediment from a contributing area of 100 m². Rain gauges were installed to correlate rain event characteristics to the event sediment yield. After each sedimentproducing rain event, the collected sediment was removed from the silt fence and weighed on site, and a sub-sample taken to determine dry weight, particle size distribution, organic matter content, and nutrient content of the eroded material. Rainfall intensity was the only significant factor in determining post-fire erosion rates from individual storm events. Short duration, high intensity thunderstorms with a maximum 10-min rainfall intensity of 75 mm h^-1 caused the highest erosion rates (greater than 20 t ha^-1). Long duration, low intensity rains produced little erosion (less than 0.01 t ha^-1). Total C and N in the collected sediment varied directly with the organic matter; because the collected sediment was mostly mineral soil, the C and N content was small. Minimal amounts of Mg, Ca, and K were detected in the eroded sediments. The mean annual erosion rate predicted by Disturbed WEPP (Water Erosion Prediction Project) was 15% less than the mean annual erosion rate measured, which is within the accuracy range of the model.

Moscow FSL publication no. 2007n