KBDI is a reference scale used for estimating the dryness of the soil and duff layers.
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.
For different soil types, the depth of soil required to hold 8 inches of moisture varies (30 inches of loam is required, 25 inches of clay is required, and 80 inches of sand is required). A prolonged drought (high KBDI) influences fire intensity largely because more fuel is available for combustion (i.e., fuels have a lower moisture content). In addition, the drying of organic material in the soil can lead to increased difficulty in fire suppression.
High values of the KBDI are an indication that conditions are favorable for the occurrence and spread of wildfires, but drought is not by itself a prerequisite for wildfires. Other weather factors, such as wind, temperature, relative humidity and atmospheric stability, play a major role in determining the actual fire danger.
The moisture content of the upper soil and the duff layers is an important variable in evaluating the potential danger of wildfires. The Keetch-Byram drought index (KBDI) is used to track the moisture content of the upper soil and duff layers. To calculate the KBDI, 24 hour rainfall totals and the maximum temperature need to be measured on a daily basis. With this information it is a simple matter to calculate the KBDI for a location; however, problems arise when one wishes to determine the drought index for a region such as the state of Florida.
Temperature is routinely measured at a number of sites across Florida and since temperature is continuous, interpolation to regions that lack measurements is straightforward. Rainfall on the other hand is not continuous, limiting the usefulness of interpolation. With Florida's rainfall being largely tied to local thunderstorms, precipitation can vary dramatically over short distances. Due to the hit or miss nature of these storms a particular rain gauge may receive substantially more precipitation than surrounding areas and may cause the KBDI to underestimate the severity of the drought in that region.
The Florida Forest Service combines traditional rainfall observations with data derived from the National Weather Service's WSR88D (NEXRAD) radar network to provide a detailed view of rainfall across the state for use in the KBDI. The radar data is provided to the Florida Forest Service by the National Weather Service in the form of a composite rainfall estimate that combines precipitation information from all of the NEXRAD radars and is adjusted by rain gauge data to improve quality.