Water Supply Vulnerability To Post-Wildfire Reservoir Sedimentation: A New Modeling Framework For The Western Us and Applications To Salt Lake City, Utah

Following the widespread damming of rivers and the establishment of the US Forest Service, the American West was largely settled under a perception that water was plentiful and that wildfire was not a serious risk. However, a century of fire suppression combined with the increasing effects of anthropogenic climate change has resulted in rapidly increasing wildfire activity and decreasing water supplies relative to the boom of the mid-20th century. Today, more than 70 million people live in this region (with over a third residing in the Wildland-Urban Interface) and they all depend on a reliable and sustainable water supply. Snowpack is decreasing in many regions and delivering less water to reservoirs, the reservoirs are aging and filling with sediment, and more extreme wildfire activity is increasing the risk of catastrophic losses in water supply. This has already occurred in western cities like Flagstaff, Arizona and Fort Collins, CO; yet few reservoir management plans contain detailed models or evaluations of potential post-fire sedimentation. This is changing rapidly though, and recognizing the risks, many municipalities are seeking to assess their post-fire vulnerabilities and establish plans to address them. However, one limitation has been a lack of tools to comprehensively model and predict post-wildfire erosion, watershed sediment dynamics and sediment routing through the large watersheds that drain to reservoirs.

Our group has created a new modeling framework, known as the Fire-Watershed Assessment Toolkit for Erosion and Routing (or Fire-WATER), that was developed as an easy-to-use, ArcGIS toolbox (see presentation by David et al. for more details). The Fire-WATER framework links three modeling tools developed within our group: 1) the Utah State University AppLied (USUAL) Watershed Tools, 2) the Wildfire Erosion and Sedimentation Toolkit (WEST), and 3) the Network Sediment Transporter (NST). Further, we have collected new field data and developed additional predictive models that have made it possible to use Fire-WATER as a pre-fire assessment tool. Namely, this includes a new machine-leaning model to predict wildfire severity (Klimas et al., in prep), and a suite of empirical models to predict the volumes and grain sizes of post-fire debris flows trained exclusively with data from the Intermountain West (Wall et al., 2022).

In our first application of this framework, we evaluated post-wildfire reservoir vulnerability (measured as the potential sedimentation losses relative to storage capacity) for reservoirs serving the municipalities of Salt Lake City and Park City, Utah, USA under a range of potential wildfire and post-fire environmental conditions. Specifically, wildfire severity was modeled under varying fuel moisture conditions, erosion was modeled with a range of post-fire rainfall intensities, and sediment routing was modeled under various discharge responses. Our efficient and automated Fire-WATER framework enables modeling many scenarios, which provides a range in estimates of potential sedimentation. This is critical given the inherent uncertainties in modeling and stochasticity of both wildfire and post-wildfire landscape response. We anticipate this methodology, along with our new Fire-WATER tools, will enable more natural resource agencies and public utilities to assess the vulnerability of reservoirs to post-fire sedimentation.