Combining fire and erosion modelling to target forest management activities

Forests deliver a number of important ecosystem services including clean water. When forests are disturbed by wildfire, the timing, quantity and quality of runoff are altered. A modeling study was carried out in a forested watershed in California to determine the risk of wildfire, and the potential post-fire sediment delivery from approximately 6-ha hillslope polygons within a 1500-km2 basin following a wildfire event. Wildfire intensity was estimated with the FlamMap prediction tool and fire risk with the FSim tool, based mainly on topography, current vegetation conditions, and wind speed and direction. The estimation of soil burn severity was based on predicted flame length for each modeled 30-m pixel and the prefire vegetation for each hillslope polygon. Sediment delivery was estimated from each hillslope polygon using the Water Erosion Prediction Project (WEPP) Model in a GIS framework. Polygons that generated the greatest amount of sediment, impacted other values at risk in the basin, or were critical for reducing fire spread were treated by reducing the amount and type of fuel available for a wildfire. The fire and erosion models were run a second time for treated conditions to see if the treatment resulted in a reduced fire intensity and probability, and hence a reduced erosion rate. The estimated erosion rates the first year after the fire dropped from 46 Mg ha-1 before treatment to 26 Mg ha-1 for polygons that had received fuel treatments. If the reduction in the probability of wildfire occurrence and the effects of a quarter century of fuel treatments are considered together, then the treatments are predicted to significantly impact long-term #century scale# erosion rates by lowering average annual erosion rates by 19%