A burn scar is the area a wildfire has burned, where the fire removed or killed vegetation and changed the soil surface. The term covers both the visible mark on the landscape and the altered ground, which absorbs rain less well and can take years to recover.
What the fire changes
After a fire, vegetation is removed and soil properties change, reducing the ground's ability to absorb rainfall. Soil burn severity describes those ground effects, including char depth, organic matter loss, altered color and structure, and reduced infiltration.
Heat can also make soil water repellent. A U.S. Forest Service field guide explains that compounds from burning organic matter vaporize and condense in the soil, typically 0.25 to 2 inches below the surface.
Why it matters in rain
With cover gone, the exposed soil erodes, runoff may increase and cause flooding, and sediment can move downstream into homes and reservoirs. The National Weather Service (NWS) office in Spokane notes that locations downhill and downstream of burn scars are very susceptible to flash flooding and debris flows, especially near steep terrain.
That hazard is covered in detail under burn scar flooding. The NWS Spokane office says the susceptibility is greatest during the first two years after the fire, and that vegetation can take many years to re-establish.
How burn severity is mapped
Under the Burned Area Emergency Response (BAER) program, the BAER Imagery Support Program compares pre-fire and post-fire Landsat and Sentinel images to map the fire's effect on the landscape. The resulting Burned Area Reflectance Classification (BARC) has four classes and is a satellite layer of post-fire vegetation condition.
The Monitoring Trends in Burn Severity (MTBS) program, which maps large fires back to 1984, defines a key change measure, the differenced Normalized Burn Ratio (dNBR), as the post-fire Normalized Burn Ratio subtracted from the pre-fire value.
What a severity map does not tell you
A BARC map is not considered a soil burn severity map until BAER teams verify it in the field and refine it where needed. Vegetation burn severity, which describes scorch and plant mortality, is a separate measure from what happened to the soil beneath.