Drought

Drought, days since the last precipitation, is an important prescribed burn fire behavior factor.  Drought is a combination of weather factors.  There are several drought indexes in use.  

          The Haines Index

          The Palmer Index

The Drought Monitor

          The Keetch-Bryam Index

The Keetch-Byram Drought Index (KBDI) is the most commonly used index for predicting fire behavior and effects in the Southeast.  All of the indices are reported periodically on the internet as maps.  These broad scale maps are based on numerous local weather stations. Some states have developed and refined drought indexes for their area.

The Keetch-Byram Drought Index (KBDI) is a continuous reference scale for estimating the dryness of the soil and duff layers. This system was originally developed for the southeastern United States and is based primarily on recent rainfall patterns.  KBDI is site specific.  It is only truly valid for a particular site if precipitation records are maintained on site.  However, data from near by sites may serve as a good indicator used with caution.

KBDI is measured in units from 0 800 with 0, representing soil saturation and 800 representing something close to oven dryness.  KBDI and its impact on fire may be broken down as follows:

0 - 200 Soil and fuel moisture are high. Most fuels will not readily ignite or burn. However, with sufficient sunlight and wind, cured grasses and some light surface fuels will burn in spots and patches.

200 - 400 Fires more readily burn and will carry across an area with no "gaps". Heavier fuels will still not readily ignite and burn. Also, expect smoldering and the resulting smoke to carry into and possibly through the night.

400 - 600 Fire intensity begins to significantly increase. Fires will readily burn in all directions exposing mineral soils in some locations. Larger fuels may burn or smolder for several days creating possible smoke and control problems.

600 - 800 Fires will burn to mineral soil. Stumps will burn to the end of underground roots and spotting will be a major problem. Fires will burn through the night and heavier fuels will actively burn and contribute to fire intensity.

 An example of a KBDI map for the US

Several states produce their own KBDI drought data.  The following is an example from Alabama:

The USFS in Alabama uses the following KBDI guidelines for prescribed burning:

Growing season MAX KBDI = 450

Dormant season MAX KBDI = 300

Site preparation MAX KBDI = 500

The method for tabulating KBDI locally can be found at  

http://www.wrh.noaa.gov/sew/fire/olm/KBDICOMP.htm