UNDERSTANDING AND QUANTIFYING THE INFLUENCE OF BURN SEVERITY, LANDFORM AND ASPECT on POST-WILDFIRE HILLSLOPE EROSION RATES IN MOUNTAINOUS TERRAIN

Wildfires can profoundly affect soil erosion rates by physically and chemically altering soils and removing stabilizing vegetation. Increased soil erosion in turn can augment the sediment supply to streams, which will alter channel morphology and aquatic habitat structure and could increase downstream dam siltation rates. In the summer of 2007, a severe fire burned over a half million acres of forest in Central Idaho; the frequency of severe fires such as this is expected to increase with current climate change. Our goal is to quantify post-fire hillslope erosion rates, and to understand the influence of burn severity, landform (convex or concave hillslope form), aspect and slope on erosion rates. In mountainous terrain, aspect controls the amount of solar radiation received which in turn strongly influences local moisture, soil, and vegetation conditions and therefore affects soil production and erosion rates. Hillslope form may influence the dominant erosion process occurring (i.e. diffusive processes or rilling), and slope impacts erosion rates. Thus, in the high relief terrain of our study area, we expect aspect, landform, and slope (in addition to burn severity) to strongly influence post-fire erosion rates. To address the impact of each of these variables on post-fire erosion, we measure hillslope erosion rates using radionuclide (Lead-210 excess, Cesium-137) inventories in several small catchments with a wide spectrum of burn severities. Erosion rates are measured and compared for severely, moderately and unburned hillslopes; north vs. south facing aspects; concave vs. convex landforms; and slopes ranging from 20 to over 45 degrees. Despite predictions of increased drought severity and subsequent forest fires with climate change in the intermountain West, controls on post-wildfire hillslope erosion rates and processes remain poorly understood. It is therefore critical to gain a quantitative understanding of the influence of variables such as burn severity, landform, aspect and slope on erosion rates. A better understanding of the factors that influence hillslope erosion would enable us to predict and potentially mitigate the adverse impacts of severe fires on land and water resources by identifying priority areas for post-fire rehabilitation.