The Keetch-Byram Drought Index (KBDI) is basically a mathematical system for relating current and recent weather conditions to potential or expected fire behavior. This system was originally developed for the southeastern United States and is based primarily on recent rainfall patterns.
The KBDI is the most widely used drought index system by fire managers in the south. It is also one of the only drought index systems specifically developed to equate the effects of drought with potential fire activities.
The result of this system is a drought index number ranging from 0 to 800 that accurately describes the amount of moisture that is missing. A rating of zero defines the point where there is no moisture deficiency and 800 is the maximum drought possible.
0 - 200 Soil and fuel moisture are high. Most fuels will not readily ignite or burn. However, with sufficient sunlight and wind, cured grasses and some light surface fuels will burn in spots and patches.
200 - 400 Fires more readily burn and will carry across an area with no gaps. Heavier fuels will still not readily ignite and burn. Also, expect smoldering and the resulting smoke to carry into and possibly through the night.
400 - 600 Fire intensity begins to significantly increase. Fires will readily burn in all directions exposing mineral soils in some locations. Larger fuels may burn or smolder for several days creating possible smoke and control problems.
600 - 800 Fires will burn to mineral soil. Stumps will burn to the end of underground roots and spotting will be a major problem. Fires will burn thorough the night and heavier fuels will actively burn and contribute to fire intensity.
Available Fuels - That portion of the total fuel that actually burns.
Burning Index - A numerical measurement of the difficulty of fire containment. Based on spread component and fire intensity.
Chain - This is a unit of measure used in land surveying. 1 chain equals 66’. This is a useful measurement for determining distance and area. 80 chains equals 1 mile and 10 square chains equals 1 acre.
Drought Index - A number representing the net effect of evaporation, transpiration, and precipitation in producing cumulative moisture depletion.
Firebrand - Any source of heat, natural or man made, capable of igniting wildland fuels. Flaming or glowing fuel particles that can be carried naturally by wind, convection currents, or by gravity into unburned fuels.
Fire Danger Rating - The process of evaluating fire danger that integrates the effects of selected factors into one or more qualitative or numerical indices to express current danger or protection needs.
Fire Intensity - The rate of heat release from an entire fire at a specific point in time.
Fuel Moisture - The amount of water in a fuel, expressed as a percent of the oven-dried weight of that fuel.
Fuel Type - An identifiable association of fuel elements of distinctive species, form, size, arrangement, or other characteristics. General fuel types are grass, brush, timber, and slash.
Ignition Component - A rating of the probability that a firebrand will cause a fire requiring suppression action. It is calculated using air temperature, shading, fuel moisture, fuel temperature, wind, slope, and fuel type.
Man Caused Risk - A numerical rating of the potential occurrence of a person-caused fire requiring suppression action per million acres.
Mineral soil - Soil with little organic matter. There are organic soils such as peat soils in Florida and the lake states. Also, there are organic soil layers often found above mineral soils such as humus layers of decayed leaves in forests ecotypes. Organic soils and organic soil layers can burn.
Rate of Spread - The relative activity of a fire in extending its horizontal dimensions. It can be expressed as a rate of increase for the total perimeter of the fire, as a rate of forward-spread for the fire front, or as a rate of increase in area. It is usually expressed as a rate in chains or acres per hour.
Spread Component - A rating of the forward spread of a fire front expressed in feet per minute.