Coronado National Forest
U.S. Forest Service
300 W. Congress St.
Tucson, AZ 85701
Burned Area Emergency Response (BAER) specialists recently completed their data gathering and analysis for the Bighorn Fire burn area to produce a preliminary burned severity map. This map and data display categories of unburned/low, moderate, and high. Ecosystems mapped in the Catalina Mountains contain approximately 30% rock outcrop which are considered unburned. Approximately 64% of the 119,236 acres is either unburned and/or low burn severity, while 32% sustained a moderate burn severity, and 4% identified as high burn severity.
The low category of soil burn severity indicate that there was only partial consumption of fine fuels and litter coverage still remains, to some extent, on the soil surface. Residence time at the soil surface in low areas was short, leaving root systems and structure intact. Recovery time in the low category will vary based on ecological community but is expected to be relatively short.
A moderate category of soil burn severity indicates consumption of litter and fine fuels at the soil surface. In forested communities, the heat from moderate severity fire will result in water repellant conditions at the mineral soil surface. The canopy in the moderate forested system is browning and it is expected that trees in this area will drop needles and leaves that can provide some litter cover at the soil surface. In these systems, recovery can take longer for tree species to reestablish.
The moderate soil burn severity category in lower elevation communities indicates complete consumption of shrub cover that can but does not necessarily result in water repellant conditions at the soil surface. Several shrub species in the lower elevation communities do root crown after fire and recovery time will be variable. Moderate soil burn severity category in the lower elevation shrub communities that did not express water repellant behavior can still result in a runoff potential category of high as a result of the soils inherent qualities and the removal of effective vegetative cover.
A high soil burn severity category is the result of higher intensity fire behavior or longer residence time at the soil surface. This category is found in forested or dense Woodland communities and the litter and fuels, including an overstory canopy, was consumed. The soil structure is weakened, roots are charred, and water repellant soil conditions persist through the upper horizon of mineral soil. Recovery time in the conifer systems can be significant.
The Bighorn BAER assessment team used initial remote sensing imagery with their initial soil burn severity data, along with other soils data, plus Aspen Fire BAER data to develop and produce a map showing runoff potential and a preliminary burn severity map. This runoff data and map were released on July 1, 2020. The BAER team and the US Geological Survey (USGS) both utilized the runoff potential map as an analysis tool to estimate post fire flows and debris flow probability.
The BAER team relied on their refined burn severity map to produce an updated runoff potential map and data which they used in their subsequent modeling and determination of post fire runoff and sedimentation. This further analysis was deemed necessary to capture likely magnitude of runoff events given that the area of re-burn within this fire was substantial and soil burn severity alone is likely to underestimate runoff events.
BAER Team Leader Greg Kuyumjian said, “The BAER team is projecting increased erosion and runoff within the Bighorn Fire area as a result of the increased area of high runoff potential, where 36% of the burned area experienced moderate or high soil burn severity. In specific areas that experienced moderate to high burn severity with much higher runoff potential, there could be concern for runoff from steep hillslopes and resultant increases in post-fire soil erosion and potential debris flows. When you add these post-fire changed conditions to the pre-fire conditions that have a high proportion of high runoff due to the amount of rock outcrop, the cumulative effect may double the output of potential runoff and debris flows. The BAER Team favored using the potential runoff product as it provides a more realistic depiction of post-fire conditions relating to runoff and sediment delivery.”
The Bighorn Fire updated runoff potential map along with the preliminary burn severity map can be downloaded at the interagency “2020 Coronado BAER” InciWeb site (https://inciweb.nwcg.gov/incident/6796/) as a JPEG or PDF version under the “maps” tab.
SPECIAL NOTE: Everyone near and downstream from the burned areas should remain alert and stay updated on weather conditions that may result in heavy rains over the burn scars. Flash flooding may occur quickly during heavy rain events-be prepared to take action. Current weather and emergency notifications can be found at the National Weather Service website: www.weather.gov/twc/.