The Keetch-Byram Drought Index (KBDI) is a continuous reference scale for estimating the dryness of soil and duff layers. It assesses fire risk by representing the net effect of evapotranspiration and precipitation in producing cumulative moisture deficiency in deep duff and upper soil layers.
The index ranges from 0 (no moisture deficiency, soil is at full field capacity) to 800 (maximum drought), representing a moisture regime of 0 to 8 inches of water through the soil layer. At any point on the scale, the index value indicates the amount of net rainfall (in hundredths of an inch) required to return the soil to saturation.
Inputs: Weather station latitude, mean annual precipitation, maximum dry bulb temperature, and the last 24 hours of rainfall. Rainfall must exceed 0.20 inches to count as "net rainfall" that reduces the index.
How to interpret KBDI values:
|
KBDI Range |
Conditions |
Fire Behavior Implications |
|---|---|---|
|
0–200 |
Soil and large-class fuel moistures are high |
Fuels do not contribute significantly to fire intensity. Typical of spring dormant season following winter precipitation. |
|
200–400 |
Lower litter and duff layers are drying |
Beginning to contribute to fire intensity. Typical of late spring / early growing season. |
|
400–600 |
Lower litter and duff layers actively contributing |
Active burning of duff and litter. Typical of late summer / early fall. |
|
600–800 |
Severe drought conditions |
Intense, deep-burning fires with significant downwind spotting. Live fuels may also burn actively. |
Source: U.S. Forest Service Wildland Fire Assessment System (WFAS), which provides current and forecast KBDI maps for the contiguous U.S.