The Keetch-Bryam Drought Index (KBDI) is one of the few daily indices used to monitor drought.  It is most prevalent in the wildfire community, as it provides a real-time indication of the drying potential for finer fuels, such as grasses and shrubs.  While most commonly used to monitor the risk for wildfire and wildfire spread, it offers the potential for many other applications as well.

The KBDI is built purely off of normal annual precipitation, daily precipitation amount, and daily maximum air temperature.  Algorithms are applied to derive the daily water balance and soil moisture (assumed to have a maximum storage capacity of 8 ”) (Texas Weather Connection).  Unlike most drought indices that are often standardized and reflect numbers ranging from below zero (typically indicating drier conditions) to above zero (wetter conditions), the KBDI ranges from 0 to 800, where the higher the number, the drier the conditions.  Often, after a minimum amount of threshold precipitation within a day, the KBDI will reset to 0.  With each dry day, the KBDI index increases.  The warmer the air temperatures, the greater the potential for moisture depletion; consequently, the KBDI value will increase faster.

When the algorithms are first run, it often takes several days for the index to stabilize.  Therefore, for these MRCC products, the algorithms are started 2 years before the data requested; output cannot be attained until 2 years into the period of record.  There are no “seasons ” for KBDI, since it is driven by liquid-equivalent precipitation and air temperature.

Handling of Missing Data
A station must have several variables available for KBDI to be calculated.  If a station does not have a 1991-2010 annual precipitation normal, KBDI will not be calculated for that station.  Maximum air temperature and daily precipitation are also necessary for calculating the KBDI values.  If fewer than three values are missing in a row of precipitation, they are assumed to be zero.  If a 3rd missing precipitation value occurs, KBDI is set to missing.  For missing maximum temperature values, the maximum temperature for the previous day is used in the KBDI calculation.  This is also done for two successive missing maximum temperatures in a row. If more than two consecutive days have missing temperatures, KBDI is set to missing.  Once KBDI is set to missing, depending on the length of time during which KBDI cannot be calculated, it may take some time for the value to stabilize again. 

References and Further Information
Alexander, M. E., 1990: Computer calculation of the Keetch-Byram drought index – Programmers Beware!  Fire Management Notes. 51(4), pp. 23-24.

Byram, G. and J. Keetch, 1988: A Drought Index for Forest Fire Control , US Dept of Agric. – Forest Service Report, SE-38, 33 pp.

Texas Weather Connection, “Keetch-Byram Drought Index (KBDI)” , https://twc.tamu.edu/kbdi , accessed May 2024.

Wildland Fire Assessment System, “Keetch-Byram Drought Index ” , https://www.wfas.net/index.php/keetch-byram-index-moisture--drought-49 , accessed June 2013.

Xanthopoulos, G., G. Maheras, V. Gouma, and M. Gouvas, 2006:  Is the Keetch-Byram drought index (KBDI) directly related to plant water stress? , Internatl Conf on Forest Fire Res., D. X. Viegas (Ed.), 9 pgs