Fire-Water: A New Gis Framework To Assess Post-Wildfire Erosion, Watershed-Scale Sediment Dynamics, and Downstream Impacts In The Western Us

Wildfires create a cascading series of watershed effects that extend beyond the direct risks from burning ecosystems and infrastructure. Runoff and erosion after fire can deliver substantial sediment loads to river networks influencing river morphodynamics, aquatic habitat and populations, water quality and quantity, and downstream infrastructure. While models exist to predict the risks and impacts from individual hydro-geomorphic processes (e.g., debris flows, hillslope erosion), there is a need for efficient tools that incorporate the various models and are capable of predicting post-wildfire erosion, sediment delivery, and transport through large watersheds and river networks. Our group has developed a new modeling framework to better predict the integrated and interacting impacts of post-fire processes, constructed as an easy-to-use Python-based ESRI ArcGIS toolkit, that we are calling the Fire-Watershed Assessment Toolkit for Erosion and Routing (Fire-WATER).

The Fire-WATER framework links three toolkits that can all be operated individually for other applications: 1) the Utah State University AppLied (USUAL) Watershed Tools, 2) the Wildfire Erosion and Sedimentation Toolkit (WEST), and 3) the Network Sediment Transporter (NST). First, the USUAL Watershed Tools provide users with a streamlined, efficient, and easy-to-use set of ArcGIS toolboxes to delineate watersheds, sub-catchments, river-adjacent interfluves, and discretized river networks for 1-D routing applications. Second, WEST combines the current USGS post-fire debris flow model (Staley et al., 2017) with regional debris flow volume models (Gartner et al., 2008; 2014; Wall et al., 2022), an updated debris flow sediment delivery model (based on Murphy et al., 2019), and a post-fire version of RUSLE that predicts soil erosion and river delivery from hillslope sheetwash and rilling (Gannon et al., 2019). Collectively, the WEST components can all be run using publicly available datasets (at least for the western US), including but not limited to topography, rainfall intensity, soil erodibility, land cover, and wildfire severity. Structured around the delineations from USUAL, WEST will output a set of spatially explicit locations of sediment inputs to the river network from debris flow catchments and interfluves that include estimates of both the volume and grain sizes. Finally, the NST 1-D sediment routing model uses the attributed and discretized river network from USUAL, sediment inputs from WEST, and additional streamflow inputs to route sediment through the river network based on mixed grain size sediment transport equations (Pfeiffer et al., 2020; Czuba et al., 2018). Collectively, the new Fire-WATER framework provides a streamlined and user-friendly approach to predicting source-to-sink sedimentation impacts in large watersheds (> 10 km2) after wildfire.

Notably, Fire-WATER can be run for either pre-fire or rapid post-fire assessments. For pre-fire assessment, users can input simulated fire perimeters and fire severity layers. For post-fire assessment, standard BAER or MTBS layers can be used. And while the individual components of Fire-WATER have potential applications far beyond just wildfire analysis, this new framework will allow users to more easily and comprehensively assess potential post-wildfire erosion and sedimentation risks, including downstream impacts to aquatic habitat, water resources, and infrastructure.