The moisture content of the upper soils, as well as that of the covering layer or duff, has an important effect on fire suppression efforts in forest and wildland areas. In some areas of the United States, fire in deep duff fuels are of particular concern to fire control managers because when the fuels are dry fires burn deeply and damage is excessive. The cost of suppression can be excessive - even small fires can become costly and large fires can be disastrous.

This does not imply that difficult wildfires cannot occur without prior, long-term drought. For example, during early spring in north-central and northeastern states, snowmelt is often followed by a period of dry, windy weather. Since spring greenup has not occurred yet, the conditions can be ideal for fast-spreading surface fires. These type of wildfires can display crowning in conifer stands, but seldom burn fuels in the deeper duff material. Consequently, low moisture content in large fuels, deep duff, and the soil mantle are usually not a contributing factor in these types of fire situations.

The critical effects of drought are not confined to deep organic soils. Frequently, dried-out organic materials are embedded in the shallow upper layers of mineral soils. These fuel pockets can become a deciding factor in whether or not fire lines will hold. They can also cause further problems during the mopup stage. During extreme drought conditions, the moisture content of live fuels (forbs, grasses, brush) and tree crowns will be lowered, woody vegetation will die, and fires will crown more easily. Drought accelerates the curing process of herbaceous plants; it intensifies the behavior of wildfires, making control efforts more and more difficult as additional fuel becomes available for consumption.

Other factors, in addition to soil moisture, can and will influence the occurrence and behavior of both small and large wildland fires. However, experience over the years has established a close relationship between cumulative dryness, or drought, and extremely difficult fires.

Over the years, drought has been defined in many ways. For forest fire contol, a useful concept of drought is one that treats moisture difficiency as a continuous quantity that can be described in numerical terms. The values would range from zero, in which the soil and duff are saturated with water, up to a maximum value that corresponds to the absence of available moisture in the soil and duff. In other words, the upper portion of the scale corresponds to those situations when the moisture deficiency is "abnormal" or "unusual."

The Keetch-Byram Drought Index is a number that represents the net effect of evapotranspiration and precipitation in producing cumulative moisture deficiency in the deep duff or upper soil layers. It is a quantity that relates the flammability of organic materials in the ground. The material may be soil humus, in which case the upper soil may appear to burn if fire occurs when the index is high. It may also consist of buried wood, such as roots in varying degrees of decay, at different depths below the mineral soil surface. The relative dryness of these materials is directly related to drought and is of greater significance in fire suppression than in fire behavior, i.e., firelines are harder to hold.

The 1978 NFDR fuel models had no reservoir of additional fuel that could be used to simulate increased fuel availability as drought progresses. Therefore, in 1988, theNFDRS fuel models were modified to include a potential dead fuel load that can be added to the fuel model as a function of the Keetch-Byram Drought Index. The Keetch-Byram Drought Index was incorporated into the 1988 NFDR System as a standalone drought indicator. The Index is also used in an algorithm that estimates the amount of dead vegetation available for combustion. As drought increases, the amount of dead fuel in the fuel models is increased to reflect greater amounts of fuel available in the duff and litter layers. Studies have shown that the changes in the dead fuel load used in the 1988 NFDR System better reflectseasonal changes in fire danger. The chart below shows the relationship of fuel load addition due to drought. Note that this feature of the 1988 NFDRSystem can be disabled if the 1978 NFDRS fuel model set is selected when the station catalog is entered into WIMS.

As can be seen from the graph above, the Keetch-Byram Drought Index varies from 0 to 800, the assumption being that increasing values indicate increasingly severe drought. Keetch and Byram cautioned that it is necessary to begin the drought calculations from a time when it is reasonably certain that the upper soil layers are saturated. It is assumed that this condition exists right after snowmelt for areas receiving heavy snowfall, or right after substantial rains. However, fire weather observations are not necessarily started immediately after the spring snowmelt, but rather closer to the beginning of the fire season, at a time when soils are not certain to be saturated. Specifically, the amount of precipitation, in inches, required to zero the KBDI at any time is approximately the KBDI value divided by 100.

The Keetch-Byram Drought Index was designed to operate through a wide range of climatic conditions. The theory and framework are based on the following assumptions:

1. The rate of moisture loss in a forested area will depend on the density of the vegetation cover in that area. In turn, the density of the vegetation cover, and consequently, its transpiring capacity, is a function of the mean annual rainfall. Furthermore, the vegetation will eventually adjust itself to use most of the available moisture. 1.

2. The vegetation-rainfall relation is approximated by an exponential curve in which the rate of moisture removal is a function of the mean annual rainfall. Therefore, the rate decreases with decreasing density of vegetation, hence, with decreasing mean annual rainfall.

3. The rate of moisture loss from the soil is determined by evapotranspiration processes.

4. The depletion of soil moisture with time is approximated by an exponential curve in which the wilting point moisture is used as the lower moisture level.

5. The depth of the soil layer wherein the drought event occurs is such that the soil has a field capacity of 8 inches of available water.

From these assumptions, plus supporting data, a mathematical description of the overall process was developed. The final equations were incorporated into the 1988 NFDR System.

Manual Calculations of the KBDI

Initializing the KBDI

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