Model Description

The Oklahoma Fire Danger Model, operational since 1996, is a prototype next-generation model of the U.S. Forest Service's National Fire Danger Rating System (NFDRS). Developed in conjunction with the Missoula Fire Sciences Laboratory of the U.S. Forest Service in Missoula, Montana, this NFDRS model is the first to utilize a real-time automated weather station network (the Oklahoma Mesonet) rather than once-per-day manual observations. The model also utilizes weekly 1-km resolution AVHRR satellite data for NDVI (Normalized Difference Vegetative Index) to estimate the "relative greenness" of the earth's surface, which is directly related to live fuel moisture. One kilometer resolution colored maps of both "relative" and "visual" greenness are available.

The Oklahoma Fire Danger (OKFD) Model produces 1-km resolution colored maps of four NFDRS fire danger indices (dimensionless): Spread Component (SC), Energy Release Component (ERC), Burning Index (BI), and Ignition Component (IC). Colored maps of 1-hr dead fuel moisture and the Keetch-Byram Drought Index (KBDI) are also produced, the latter map once per day. In addition, for each model run, a table of fire danger indices, fuel moistures, KBDI, and selected weather variables is produced for each Mesonet site.

It is important to understand some of the limitations of the Oklahoma Fire Danger Model:

1. Every 1-km "grid" square of Oklahoma has been assigned one of five surface fuel models (so many tons/acre of 1-hr, 10-hr, 100-hr dead fuels, herbaceous and woody live fuels, etc.). If the particular fuel model of concern (e.g, an open grassy area) within a given grid square is not the same as the assigned fuel model (e.g., a deciduous forest), then the OKFD Model results for that 1-km square can be expected to be different from what they would be for your fuel model of concern.

2. The OKFD Model, like the NFDRS, is only a surface-fuel based model and does not apply to crown fires.

3. The Oklahoma Fire Danger Model assumes an NFDRS Slope Class of 1 (terrain of 0-25% slope), so actual upslope conditions over steep terrain will be amplified over OKFD predictions.

4. As can be inferred from the above, the OKFD Model is not designed for specific fire behavior predictions for a given field, fuel type, slope, etc., but rather for the predominant vegetative fuel type over a 1-km square, mainly flat region.

5. The fuel models utilized are for native vegetation. Accordingly, for 1-km grid squares of primarily agricultural cropland, the OKFD predictions will not be accurate (e.g., fields of bare soil, wheat fields, etc.). Nevertheless, live fuel moisture over such grid squares will be a function of what the satellite "sees" (the relative greenness maps). So in the sense that live fuel moisture values strongly influence fire behavior, the OKFD predictions for such squares will reflect reality, even though based on a totally different fuel model. Thus, 1-km squares of green wheat fields will show low fire danger, as they should. Bare soil areas, on the other hand, will show high fire danger - in such cases, OKFD predictions are erroneous, as there are no surface fuels to burn.

6. Because the current Mesonet sensors are unable to detect the presence of a snow cover, the OKFD output will be unreliable for locations having a snow cover and should be ignored. After the snow cover melts in such locations, model output will become valid once again.

Fire Behavior and the Weather

In Oklahoma, as mentioned earlier, the OKFD Model uses one of five surface fuel models (NFDRS Model A for shortgrass prairie; L for mixed prairie and western cropland; T for tallgrass prairie and eastern/central cropland; R for deciduous forests; and P for pine forests). These models consist of so many tons/acre of 1-hr, 10-hr, and 100-hr dead fuels and live herbaceous and woody fuels. Grassy fuel models, such as predominate in the panhandle and western Oklahoma, contain only "fine" 1-hr dead (and live) fuels, while forest fuel models such as are in eastern Oklahoma have equal amounts of 1- and 10-hr fuels and a lesser amount of 100-hr fuels. In addition, the 1988 revisions to the NFDRS include a "drought dead fuel load" in the subsurface - increasing amounts of this fuel become available (in the same proportions as the "non-drought" dead fuels) to burn as the KBDI value increases above 100.

It is important to realize that fire initiation and behavior are primarily determined by the current weather conditions, of which solar radiation, temperature, relative humidity, and wind speed are most important. Fine fuels (1-hr dead class) respond quickly to changing weather conditions - the subsurface can be completely saturated from a recent rain, but the surface fine fuels can have very low fuel moisture and carry a fire. The fire danger indices (especially SC and BI) are primarily a function of the current weather conditions, even though fuel types with 10-hr and 100-hr components are taken into account.

On the other hand, the Keetch-Byram Drought Index (KBDI) relates to the moisture levels in the subsurface (litter, duff, and upper soil layers). Increasing KBDI values add extra amounts of fuel to the fuel load, but the moisture content of the 1-hr fuels is still determined by current weather conditions. Thus, one can have extremely high KBDI values, but if wind speeds are low and relative humidities are high, there will be low fire danger. Conversely, one can have very low KBDI values, but if wind speeds are high and relative humidities low, there will be high fire danger. KBDI, because it is based on the subsoil moisture profile, is also a fairly reliable indicator of live fuel moisture. High KBDI values means the understory vegetation is low in water content and susceptible to ignition with a minimum of preheating.

Descriptions of SC, ERC, BI, IC, and KBDI

With these thoughts in mind, we now present some descriptions and interpretations of the four fire danger indices (SC, ERC, BI, IC) and of KBDI:

Spread Component

For fire control planning, the first consideration is the rate of spread. The Spread Component (SC) is numerically equal to the predicted rate of spread
of the headfire in feet/minute. It is the most variable of the indices, with daily variations being caused by changes in wind speed and in the dead and live fuel moisture contents.

SC = (1 min/ft)*(Rate of Spread in ft/min)

[MPH=0.0114*SC]

Energy Release Component

ERC is a measure of the heat released per unit area in the flaming zone of the fire. It is the least variable index on a day-to-day basis. Variations are caused by varying fuel moistures in the fuel bed.

ERC = (0.04 ft**2/Btu) * (Heat Release in Btu/ft**2)

Burning Index

The fireline intensity, given by I, in Btu/ft-min is given by:

I = SC * ERC * (25 Btu/ft-min)

The larger this number, which is based on both SC and ERC, the greater the difficulty of containment of the fire.

From this variable, a flame length (FL) relationship has been developed:

FL (ft) = j(I/60)**0.46

Finally, the burning index (BI) is related to the flame length as follows:

BI = k (FL), where k = 10/ft.

Or,

Flame Length (ft) = BI/10

Thus, the burning index contains information related to both the fireline intensity and flame length. It is the most important single fire danger index (besides fireline intensity I).

Ignition Component

The ignition component is the probability (0-100%) that a reportable fire requiring suppression action will result from a firebrand. It says nothing about the intensity of the fire.

Keetch-Byram Drought Index

Drought, as defined by the KBDI, is a condition of dryness in the litter, duff, and upper soil layers that progresses from saturation to an absence of available moisture. The KBDI is based on an arbitrary 8 inches of water in the litter/duff/soil column. When the full 8" of water are available, KBDI = 0. As water is removed from the column by evapotranspiration, the KBDI increases in value. When KBDI = 800, all the water has been removed. As a drought proceeds, the upper soil layers dry and the amount of dead fuel available for consumption increases. During combustion some of this fuel contributes directly to fireline intensity (BI), but most increases total heat release (ERC) and contributes to burn severity through smoldering combustion with its resulting smoke. The interpretations below are based on experience within forested areas in the southeastern United States.

KBDI Value	Interpretations
0 - 200	Nearly all soil organic matter, duff, and litter are left intact after a burn. Once the fire passes, remaining embers extinguish quickly and, within a few minutes, the area is completely extinguished and smoke free.
200 - 400	At these levels, litter and duff layers begin to contribute to fire intensity. Heavier fuel classes can become involved in the burn. Soil exposure is minimal. Smoke management can become a real hazard, especially if there are larger fuel classes available. Smoldering with resulting smoke can carry into the night.
400 - 600	These levels represent the upper range at which most understory type burning should be conducted. Most of the duff and organic layers will ignite and actively burn. The intensity can be expected to increase almost exponentially from the lower to upper ends of this range. Considerable soil exposure occurs. Complete consumption of all but the largest dead fuels can be expected, and larger fuels not consumed may smolder for several days, leading to smoke and possible fire control problems.
600 - 800	These levels represent the most severe drought conditions, and many states issue burning bans at these levels. Prescribed fires should not even be attempted at levels over 700. Fires that do occur will be intense and deep-burning. Live understory vegetation (2-3" range) should be considered part of the fuel complex due to its low fuel moisture. Most subsurface soil organic material will be consumed; great soil exposure will occur with great future erosion potential. Smoldering may occur for many days, with smoke and fire control problems.

 

Fire Suppression Interpretations