Firewise/Firesafe - Homeowner's information on wildfire safety
http://www.floridadisaster.org/bpr/EMTOOLS/wildfire/wildfire.htm

Keetch-Byram Drought Index (KBDI)
 

The Keetch-Byram drought index (KBDI) is a continuous reference scale for estimating the dryness of the soil and duff layers. The index increases for
each day without rain (the amount of increase depends on the daily high temperature) and decreases when it rains. The scale ranges from 0 (no moisture deficit) to 800. The range of the index is determined by assuming that there is 8 inches of moisture in a  saturated soil that is readily available to the vegetation.