Dixie Fire Severity – 8/12/2021

We use the term ‘fire severity’ to describe a fire’s level of impact on the vegetation, soil, and overall ecology of a place. High severity isn’t always bad – some ecosystems like chaparral are well-suited to it. You can really go down a rabbit hole on this topic, and every place is different in what kind of fires it has evolved with and what sort of fire it needs to stay in any particular sort of condition. When we humans talk about severity, there is usually a major value given to things we want from the land: like timber, or water, wildlife habitat, or forest health. And then in the background, the land is kind of on autopilot, going into recovery mode as soon as the fire hits, as it has for millions of years.

We use satellite imagery to rapidly assess fire severity across large swaths of landscape. Satellites are kind of like a fancy digital camera, except that instead of capturing the visible light, they capture all of the wavelengths of light from heat or infrared all the way up to ultraviolet light. All of this information gets saved in the same basic way a camera works, with pixels for each wavelength of light. We can slice and dice this imagery, trying out different combinations of the different wavelengths until we get a ‘false color’ image that gives us a graphic that is useful for our particular question.

The satellite we use to do this goes over every 5 days, and we got an orbit yesterday about noon. These images are a snapshot of the fire right before it blew up. You can see the thunderheads that caused the big outflow winds starting to build up over Caribou Wilderness. These images also cut through the smoke, and you can see where the heat was concentrated at the time.

I colored these images with two different band combinations. The blue and brown images are for use in looking thru the smoke and seeing the severity of the burn. Brown areas have lost most of their living tree canopy. Many of these areas had widely-spaced trees, and there may be many tress surviving in brown areas, but in general, they burned hot. Shades of blue still have living tree canopy. Many of these areas are outside of the burn perimeter.

The second set of images use tones of green and red to do the same thing. These images are better at capturing the subtlety of the burn, but these images don’t see through smoke as well. Overall, there is a lot to unpack in these images, but the amount of high-severity burn is kind of dumbfounding. I am not used to seeing such a high-proportion of the landscape burned with such high severity.

Images from European Space Agency’s Sentinel 2 Satellite, captured 12:03 yesterday, 8/12/2021.
Click to zoom in.

We’ll do more with this imagery later. I don’t want too jump to deep into interpreting them until I have had a chance to go out in the field.

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