CHALLENGES OF MODELING POST-FIRE RUNOFF AND SEDIMENT TRANSPORT IN EPHEMERAL FORESTED WATERSHEDS IN THE ARID SOUTHWEST

Severe wildfires in the forested, arid Southwestern U.S. make watersheds highly susceptible to post-fire flooding, sediment transport, and debris flows. Wildfires have increased in size and severity due to land-use practices including fire suppression throughout the twentieth century, and climate change that has increased the occurrence of drought. Forest restoration treatments are being planned and implemented to reduce risks of severe wildfire and subsequent flooding and erosion. The Four Forest Restoration Initiative (4FRI) is a large scale forest thinning project being planned by the United States Forest Service and collaborators on ~1.5 million acres of federal land in the Southwest. Due to excessively steep terrain and non-market supported timber costs, two high risk watersheds on federal land near the City of Flagstaff, AZ, will not be restored by 4FRI. The city is planning the Flagstaff Watershed Protection Project (FWPP) to restore forested watersheds feeding a vital city reservoir (Lake Mary) and upslope of city infrastructure and residential areas at risk of fire and post-fire flooding (Dry Lake Hills).

Schultz Creek is a 17 km² drainage network within the Dry Lake Hills area that flows into densely populated residential areas and the cultural center of Flagstaff. Previous studies and local catastrophic wildfires provide evidence to suggest that runoff from post-fire monsoonal precipitation could result in flooding 2.2 to 6 times greater than the pre-fire 100 year flood, with costly damages caused by hyperconcentrated flows and erosion.

Hydrologic and hydraulic models are being constructed using Hydrologic Engineering Center (HEC) software to predict how peak discharges will be affected by FWPP treatments in the Schultz Creek watershed. The sediment record is being analyzed to determine the individual watershed’s sediment transport dynamics, using radiocarbon dating of macroscopic charcoal for age constraints on deposition. Peak discharges have been successfully modeled using HEC software, but modeling transported sediment and erosion, particularly in ephemeral channels, poses unique challenges. Existing sediment mobilization models have limited applications. Without an appropriate sediment model, predictions of the full impact of post-fire runoff and sediment yield are limited.