FIRE EFFECTS GUIDE - National Wildfire Coordinating Group

actively through the night.
(6) 74 percent and below. Fires will have ADVANCED FIRE BEHAVIOR with
high potential to control their environment. Large acreage will be consumed in
very short time periods. Backfiring from indirect line such as roads must be
considered. Aircraft will need to be cautious of hazardous turbulence around the
fire.
b. Eastern Oregon. Fire behavior and its relationship to moisture content in
sagebrush has been monitored on Oregon rangelands east of the Cascades
(Clark 1989). The following moisture levels indicate how readily a fire can
propagate, given that adequate fine fuels are present between sagebrush plants
to carry the fire, or that sagebrush density is high enough for flames to reach
between plants where herbaceous fuels are sparse.
(1) Above 90 percent. Fire behavior is docile. The fire may or may not spread
and is easy to control.
(2) 60 to 90 percent. Fire is much more difficult to control. Fire is likely to burn
actively throughout the night, especially if wind is present.
(3) Less than 60 percent. The fire displays extreme fire behavior and rates of
spread, and is essentially uncontrollable by normal suppression methods.
3. Effect of Fuel Type Changes on Fire Behavior. The dominating factor
regulating fire behavior is wind in some fuel types and moisture content in others.
The behavior of fires in fuel types with a large component of fine materials, such
as the grass models, is most influenced by wind. Fuel moisture is much more
important than wind in regulating the activity of fires in fuel types with a lot of
larger diameter, dead woody fuels. Wind, while influential, is not so dramatically
important in heavy fuels as it is in grass or shrub type fuels.
Fire behavior can drastically change when a fire moves into a different fuel type. If
a fire moves from an area of logging residue to one dominated by cured grassy
fuels, flame length and fireline intensity will probably decrease, but the rate of
spread is likely to increase significantly. An optimal prescription for burning the
logged unit to reduce hazard fuels would include low moisture content in smaller
size classes of fuels and low windspeeds. Under these conditions, the desired
amount of consumption in the harvested area would be achieved, and any escape
into the grass fuels outside of the unit could easily be caught.
4. Effect of Long-Term Drying on Heat Release. Long periods of limited
precipitation result in deep drying of the surface organic layers. Deeper drying of
the entire fuel complex leads to an increase in fire behavior because of greater
involvement of larger fuels and surface organic fuels in the fire front. Because
more fuels burn in the initial stages of flaming combustion, fireline intensities and