OVERVIEW OF THE MAGNITUDE AND SCALE OF POST-WILDFIRE EROSION (Invited Presentation)

Some climate change scenarios suggest that there may be an increase in the magnitude and scale of wildfires across the western United States and perhaps in other countries around the world. This portends an increase in post-wildfire floods and landscape erosion. Some effects of wildfires are: (1) the loss of the upper vegetation canopy that intercepts rainfall and protects the soil from rain-splash erosion, (2) the combustion of the surface litter and vegetative ground cover that are obstructions to overland flow, and (3) changes in the soil properties that affect infiltration. If these effects are followed by sufficient rainfall, then the potential consequences can be extreme post-wildfire runoff, flooding, and erosion of hillslopes and channel networks in burned basins.

Post-wildfire erosion is not uniform across the landscape. Erosion ranges from the scale of millimeters near the ridges on some hillslopes, to the scale of centimeters or several meters in previously un-channelized hillslope swales where erosion can create gullies, to the scale of several meters to tens of meters in depth and width in pre-wildfire channels. Post-wildfire erosion has been documented by numerous methods such as erosion pins at the point scale, silt fences at the hillslope scale, debris dams at the basin scale, and more recently by terrestrial LiDAR, which has permitted better spatial resolution of erosion at the hillslope scale and airborne LiDAR at the basin scale. Early pre-LiDAR results, from precipitation-fire regimes in the Colorado Rocky Mountains characterized by short-duration, high-intensity rainfall and wildfire recurrence intervals of 15-40 years, have shown that about 20% of the erosion comes from hillslopes and about 80% from channels. By contrast, in a different precipitation-fire regime in Southern California Mountains characterized by long-duration, low-intensity rainfall and wildfire recurrence intervals <10 year, these percentages shifted to about 50% from the hillslope and 50% from channels. These differences suggest that the magnitude and scale of post-wildfire erosion varies between precipitation-fire regimes. Thus, the post-wildfire erosion response is controlled by different dominant geomorphic processes in these two regimes, and probably in other precipitation-fire regimes as well.