Soil Burn Severity

Soil Burn Severity (SBS) maps are critical components to post-fire analysis.

SBS maps are needed to calculate debris flow hazard probabilities, perform runoff analysis, and identify treatment options. The US Forest Service makes SBS maps of nearly all recent fires on USFS lands available for download on its BAER Imagery Support Data page.

SBS maps are derived from a combination satellite imagery and field verification.

More details on creation and use of SBS maps can be found here.

The following text is courtesy of the USFS’s Central Washington Fire Recovery website, which is a valuable resource for recent WA wildfire information.

Soil Burn Severity (SBS) refers to effects of fire on soil characteristics. These include the depth of char (a downward heat pulse into the soil), loss of organic matter through consumption of duff, litter, and fine roots, altered surface color, damage to soil structure, and reduced infiltration. Burn severity can vary across the fire area depending on topography, weather conditions, fuel types, and rate of fire spread. When organic materials on the ground and within the soil structure burn hot and slow, they form a layer on the surface and within the soil, reducing the ability of water to move downward into the soil and resulting in an increase in surface runoff.

Burn severity levels:
Low severity- These zones were subjected to predominantly an understory burn that consumed approximately 10-30% surface vegetative ground cover. Black charring of litter intermixed with grey ash is present. Hydrophobicity is generally absent but there are areas that are extremely dry that experienced weak hydrophobicity.

Moderate severity– Tree canopy cover of 50-80% usually has been consumed. The majority of the ground cover has been consumed with approximately 30 – 75% vegetative ground cover being retained. Grey ash is predominantly present, larger diameter organic matter material of .25-.5 inches is consumed, and soils are strongly hydrophobic. This indicates that high levels of heat were introduced to the soil. These sites are expected to be a source of ash, infiltrations rates will decline, runoff will increase, and soil loss will be accelerated.

High severity– Usually tree canopy cover has been consumed to 90% or greater. Ground cover has been fully consumed with approximately 5-10% coarse woody debris material remaining. Finer organic material and woody material .5-1 inch has been fully consumed. Ash is deep and generally grey, and soils are strongly hydrophobic. Infiltration will be reduced drastically and accelerated soil loss is predicted to occur. High levels of ash will be immobilized from these sites.