Using WEPP Technology to Assess the Distribution of Erosion Risk, Mitigation Benefit, and Peak Flow Following Wildfire

A number of interfaces have been developed for the Water Erosion Prediction Project (WEPP) model. These include a GIS interface (GeoWEPP), a Windows interface, and an online interface to assist in analyzing erosion risks, variability, and mitigation effectiveness on forest and rangeland hillslopes that have been burned (the Erosion Risk Management Tool ERMiT). This presentation describes how each of these tools was used to assist in analyzing the erosion and peak runoff risk, and mitigation benefits following a large wildfire. In August, 2005, a wildfire burned 20,000 ha of forest south of Pomeroy, WA. Ten watersheds of greatest interest were identified for analysis and were modeled with GeoWEPP. The watersheds covered 7650 ha, or about 38 percent of the total burned area. Hillslopes were specified high, moderate or low burn severity by examining the BAER fire severity map. The results of this analysis highlighted areas of greatest erosion risk. Most of these areas were on steep south or west facing slopes. Following the GeoWEPP analysis identifying hillslopes of greatest erosion risk, the ERMiT interface was run to evaluate potential erosion risks on these hillslopes, and benefits from seeding or mulching during the first two years following the wildfire. An average of about 12 randomly selected hillslopes, or 5 percent of the hillslopes on larger watersheds, were run with ERMiT for each watershed. The results showed that untreated, there was a ten percent chance that erosion would exceed 13 to 29 t/ha, averaging about 25 t/ha in the first year. Seeding had no affect on first year erosion rates, but reduced erosion on the average about 40 percent in the second year. Mulching at 2 t/ha reduced erosion about 86 percent the year following the fire and 60 percent the second year. For the third analysis, the WEPP windows interface was run for each watershed to estimate return periods for peak runoff rates. From the peak runoff estimates for watersheds of varying area, the ten year peak runoff from any watershed following this fire can be estimated by: Peak runoff (cubic meters per second per square km) = 3.1512 × (Watershed area (sq km))^{-0.5446} (R2 = 0.90). We concluded that these three WEPP tools can be of great benefit to watershed managers when determining the spatial distribution of soil erosion, the risks of a given amount of erosion occurring, and the return period of peak flows the year following the wildfire.