The Keetch-Byram Drought Index was added to the 1988 NFDRS because it was specifically designed for use in a forest environment, it was familiar and meaningful to fire mangers in the eastern Untied States (Melton 1989), and it could also be applied to the western United States. However, an initial KBDI value must be defined to begin KBDI calculations at the start of the fire season. This is particularly troublesome for fire weather stations since observations are not taken throughout the year.

Byram cautioned that it is necessary to begin drought calculations from a time when it is reasonably certain that the upper soil layers are saturated. This condition is assumed to be met right after the spring snowmelt for those areas receiving heavy winter snowfall, or right after substantial rains. Unfortunately, fire weather observations are not necessarily started at the end of the spring snowmelt. Fire weather observations usually begin later in the spring, at a time when it is no longer certain the soils are saturated with water.

Fujioka (1991) reported that while the effect of an incorrect starting value of the KBDI diminishes over time, it can take a long time to become negligible. If sufficient rainfall occurs to drive the index to zero, the error resulting from an incorrect initialization will immediately vanish. However, if a lesser amount of rainfall occurs during the course of the fire season, convergence to the "correct" value will take a much longer period of time.

A Method of Initializing the Keetch-Byram Drought Index

The proposed method for initializing the KBDI uses the percentage field capacity (PFC) from the Palmer Drought Index. Various parameters associated with the Palmer Drought Index are calculated weekly by the Climate Prediction Center (CPC) for each state by climate division. The percentage field capacity (PFC) is the ratio of soil moisture in the upper and lower soil layers to the available water capacity, expressed as a percentage. The upper soil layer is loosely defined as the plow depth and is assumed to contain 1 inch of available moisture at field capacity. The lower soil layer encompasses the effective root zone and is a function of the average soil type within the climate division. In the Palmer Drought Index, the water-holding capacity of the lower soil layer ranges from 3 to 10 inches.

The Keetch-Byram Index assumes that all soils have an 8 inch field capacity. While this is not true for all soils, it should be noted that Keetch and Byram presented their work as an index, not as an estimator of actual soil moisture. The assumption of an 8 inch field capacity scales the KBDI from 0 to 800 and is likely to cause serious error only in those locations having such low annual precipitation that the index does not periodically get reset to zero. If fire weather observations are begun at a time when the soil is not obviously saturated from snowmelt or recent rains, a nonzero starting valaue must be estimated. The proposed method for calculating an initial value of the KBDI is shown below.

Thus, the model for initialization satisfies the extreme conditions of drought and saturated soils. It also matches the assumption of an 8 inch field capacity in the Keeth-Byrma Drought Index calculations.

This method of initialization relies on the assumption that the percentage field capacity values provided by the Climate Prediction Cener (CPC) reflect whatever drying has occurred at the fire weather station. To use this method for initializing the KBDI at the start of the fire season, fire managers need to obtain the most recent PFC value for the climate division in which their fire weather station is located. Some climate divisions can be rather large, and this method provides only a single starting KBDI estimate for each fire weather station located within the climate division. Precipitation variability within any given climate division is obviously not accounted for with this method. Thus, fire managers, particularly in mountainous areas of the west, may need to adjust the estimated KBDI starting value, if their fire weather station is typically wetter or drier than the climate stations within their climate division.
The sources of information listed below will aid in the initialization of the Keetch-Byram Dought Index.

State Climate Division Maps for the Pacific Northwest

Other State Climate Division Maps

Weekly Palmer Drought and Crop Moisture Data

Weekly Palmer Drought MAP

NOTE: Percentage Field Capacity (PFC) can be found in column 7 (titled "PCT FLD CPC END WEEK") of the Weeklly Palmer Drought and Crop Moisture Data for the selected state (col. 1) and climate division (col. 2).

REFERENCES:

BURGAN, ROBERT E. 1993. A method to initialize the Keetch-Byram Dourgh Index. Western Journal of Applied Forestry. 8(4):109-115.

FUJIOKA, F.M. 1991. Starting up the Keetch-Byram Drought Index. P. 74-80 in Proc. 11th conference on fire and forest meteorology, Andrews, P.A., and D.F.Potts (eds.).

MELTON, M. 1989. The Keetch/Byram Drought Index: A guide to fire conditions and suppression problems. Fire Management Notes 50(4):30-34.

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