Exactly what is drought and what does the Drought Index measure? Drought is a condition of dryness in the duff and upper soil layers, that progresses from total moisture saturation to an absence of available moisture. The Keetch-Byram Drought Index is loosely defined as "a number representing the net effect of evapotranspiration and precipitation in producing cumulative moisture deficiency in deep duff and upper soil layers". OK, so that is not so loose and it is rather formal. Generally speaking, it is a measure of moisture in the tested layers of soil. It is based on an arbitrary 8 inches of water in the litter/duff/soil column. When the full 8 inches of water are available, the Index value is 0. As water is removed from the soil column by evapotranspiration, the numerical value of the Index can increase to a maximum value of 800, which is the condition the 8 inches of water have been totally removed. Thus, an Index of 250 means there is a deficit of 2.5 inches of water out of the original 8 inches, leaving a content of 5.5 inches of water. The KBDI attempts to measure the amount of precipitation necessary to return the soil to full field capacity. It is a closed system ranging from 0 to 800 units and represents a moisture regime from 0 to 8 inches of water through the soil layer. At 8 inches of water, the KBDI assumes (by definition) saturation. Zero is the point of no moisture deficiency and 800 is the maximum drought that is possible. At any point along the scale, the index number indicates the amount of net rainfall that is required to reduce the index to zero, or saturation.
The official inputs for KBDI are weather station latitude, mean annual precipitation, maximum dry bulb temperature, and the last 24 hours of rainfall. Reduction in drought occurs only when rainfall exceeds 0.20 inch (called net rainfall). The computational steps involve reducing the drought index by the net rain amount and increasing the drought index by a drought factor. This information is from public domain.
You can see the general graphic picture for the USA in regard to the KBDI below the calculator. The general grouping is below...
KBDI = 0 - 200: Soil moisture and large class fuel moisture is high and do not contribute much to fire intensity. Typical of spring dormant season following winter precipitation.
KBDI = 200 - 400: Typical of late spring, early growing season. Lower litter and duff layers are drying and beginning to contribute to fire intensity.
KBDI = 400 - 600: Typical of late summer, early fall. Lower litter and duff layers actively contribute to fire intensity and will burn actively.
KBDI = 600 - 800: Often associated with more severe drought with increased wildfire occurrence. Intense, deep burning fires with significant downwind spotting can be expected. Live fuels can also be expected to burn actively at these levels.