The Keetch-Byram Drought Index (KBDI) is a drought index calculated by the Oklahoma Fire Danger Model. Ranging from 0 to 800, the index is used to increase the amount of dead fuel available to the fire. KBDI was included in the 1988 revisions to the National Fire Danger Rating System (NFDRS) on which the Oklahoma Fire Danger Model is based. KBDI values at Mesonet sites are updated daily at 4 p.m. CST. Drought, as defined by KBDI, is a condition of dryness in the litter, duff, and upper soil layers that progresses from saturation to an absence of available moisture. The KBDI is based on an arbitrary 8 inches of water in the litter/duff/soil column. When the column is completely saturated, KBDI = 0. As water is removed from the column by evapotranspiration, the KBDI increases in value. When KBDI reaches 800 (its max), all the plant available water has been removed. In the NFDRS and Oklahoma Fire Danger Model, as KBDI increases above a value of 100, increasing amounts of dead fuel are provided for burning. During combustion some of this fuel contributes directly to fireline intensity (BI), but most increases total heat release (ERC) and contributes to burn severity through smoldering combustion. In Oklahoma, the KBDI has shown itself to be more useful during the growing season than during the dormant season. Also, as it was developed mainly for forested landscapes, its usefulness for grassy landscapes is somewhat questionable. KBDI values in the 600-800 range represent the most severe drought conditions, and many states issue burn bans at these levels. In forested areas, prescribed fires should not be conducted at values over 700, as fires will be intense and deep burning.