A risk assessment for hillslope erosion from wildfire and implications for water quality and watershed ecology

Abstract

In the western United States, wildfire is a natural disturbance and regulatory mechanism by which vegetated watersheds reset the natural succession regime. Wildfire management by humans protect property, people's lives, and values such as freshwater provision or recreational opportunity. However, erosion is a natural process that is directly exacerbated by wildfire. Erosion as debris flows, mass-wasting, or increased sedimentation is a continual phenomenon augmented by natural disturbance; fire plays an important role in stream channel development and floodplain connectivity. Hillslopes are the flanks of a valley and the margins of the eroding uplands and they vary in form and are indicative of the local geology and soils. However, ground observations to determine hillslope characteristics is expensive and negates the positive benefit of risk assessment as a "fast, cheap, good" method for targeting areas to manage a watershed for erosion risk. Therefore, my paper presents a risk assessment demonstrating the knowledge gap that limits economical risk management decisions to a pre-disturbance environment. I discuss hillslope erosion as the primary disturbance effect that can be managed pre-fire by the characterization of risk through an integrated ecological risk assessment. A beneficial outcome of accurate ecological risk assessment is knowledgeable resource triage in emergency situations, as well as the ability to proactively and economically manage for anticipated ecological effects. In addition, incorporating ecological risk metrics to a pre-disturbance management plan allows for inherently ecologically-sound operational objectives. I explore the risk of hillslope erosion from wildfire and how erosion from wildfire affects water quality and watershed ecology. I review the current understanding of erosion potential and how to minimize wildfire impact. I then demonstrate how to use the Total Risk Quotient to manage for total risk. I find that risk assessment for hillslope erosion requires interpreting layers of risk established by the topography, the fire, the storm, and the timing of these events. I conclude that whenever multiple natural phenomena affect resources and value, economic analysis to understand the total risk and risk mitigation options is required.