WILDFIRE AND EROSION: WHEN TO EXPECT THE UNEXPECTED

Wildfire is a major ecological process and management issue in the western U.S. The 2000, 2001 and 2002 fire seasons were some of the biggest in history with over 2 million ha burned annually. What happens when the rains come? Most wildfires create a patchwork of low, moderate, and high severity burn areas, often causing spatially varied hydrologic surface conditions. Severely burned areas often have increased erosion due to loss of the protective forest floor layer and loss of water storage as well as creation of water repellent soil conditions. Erosion is a natural process occurring at varying rates and scales depending on soil type, topography, vegetation, climate and type of disturbance. Erosion rates in a forest environment are generally small except when the ground surface is disturbed by human or natural causes. First year surface erosion rates after the Bitterroot Valley, Montana wildfires indicate that short duration high intensity thunderstorms (10-min max. intensity 75 mm/hr) caused the highest erosion rates (2 to 40 t/ha). Whereas the long duration low intensity rain events produced little erosion (0.01 t/ha).

To control erosion and flooding potential, post-fire rehabilitation treatments are commonly used. Some mitigation treatments may help reduce erosion for some rain events but not all events. Using low-intensity rainfall simulation techniques we compared three common post-fire emergency rehabilitation treatments: contour-felled logs, straw wattles, and hand trenches at reducing erosion as compared to non-treated areas. The straw wattles and contour-felled logs reduce erosion by 70 percent for these small rain events. But during intense summer thunderstorms (40 mm/hr, 10-min max intensity) there were smaller differences between treatments with the straw wattles showing the most reduction in erosion. Other laboratory studies suggest that brown needles that commonly fall to the ground after low and moderate severity burns may reduce rill erosion by 30 to 40 percent and interrill erosion by 50 to 70 percent.