Fuel's Effect on Fire Behavior

The various weather elements that affect fire behavior, are interrelated and affect each other. This is also true of the fuel elements and the elements of topography.

Not only does fuel affect fire behavior but we must have fuel to complete the fire triangle and thus have combustion.  Unlike weather, fuel does not change rapidly but most elements remain rather constant. (People, tornados etc., can make a drastic change.) The elements of wildland fules are:

FUEL LOADING

The amount of fuel has a decided effect on fire behavior. Very low volumes of fuel can result in a low intensity, creeping fire. On the other hand, large volumes of fuel could result in a blow-up fire that is difficult to control. The more fuel burning, the more heat produced. Generally, the greater the volume of fuel, the more intense the fire will be. To measure the amount of fuel, weight is used--not mass. The definition of “fuel loading” is the oven dry weight of all fuel in an area. Loading is usually expressed in tons/acre (T/A).

FUEL LOADING -- THE OVEN DRY WEIGHT OF ALL EXISTING FUELS IN A GIVEN AREA.

Similarly, the amount of fuel affects the amount of smoke produced by a prescribed burn.

TONS of FULE = TONS of SMOKE

Available Fuels

The fuel loading is the total amount of fuel on an area (acre) that could burn under the most extreme conditions.  Available fuel is the amount of fuel that will burn at a given time under the situation that exists at that time.  Fuel loading remains rather constant changing more on a yearly basis.  On the other hand, the amount of fuel that is available to enter the burning process can change rapidly--even on an hourly basis.  Many factors affect the percent of the fuel loading that is available, the major one being the amount of moisture.  Many of the weather elements will have an impact on the amount of moisture as well as many of the other elements we will discuss under “Fuels” and “Topography.”

AVALABLE FUELS - Those fuels which will burn during passage of a flaming front under specific burning conditions.

On most days, roughly half of understory fuel loading will be available and will be consumed.  During long periods of drought however, most of the fuel can be consumed resulting in a potentially more damaging fire with high intensity and rapid spread.  Such fires will also burn deeper into the duff and are more difficult to control.

Site preparation burns where no overstory is present and large-size fuel is present may be done under more severe drought conditions- IF:

  1. Escape is no problem

  2. No soil will be exposed

SIZE OF FUEL 

Anyone who has started a fire is well aware that small fuels ignite more readily than larger fuels.  In building a campfire smaller size fuel (called kindling) is used (There are also other properties of “kindling” that we will discuss later).  The reason lies in the rapidity with which small fuels can be heated and ignited.  All of the heat that enters a piece of fuel must go through the surface.  So the time required to heat fuel enough to ignite it depends greatly on the amount of surface area compared to its volume.  And--this ratio increases rapidly as the fuel size decreases.  A cylindrical piece of wood two inches in diameter has about one square inch of surface area to each cubic inch of volume.  However a piece of wood one-half inch in diameter will have approximately eight square inches of surface area per cubic inch.  Small fuel in the fuel bed greatly increases the chance of a fire starting even from small firebrands. Small fuels also burn more rapidly than large fuels and heat is released in a shorter period of time.  The more rapid burning is due largely to the greater ratio of surface to volume and less moisture.  Small sized fuel is usually necessary throughout the area to carry the fire; the fuel bed.  It is needed to ignite and in turn produce enough heat to ignite the larger fuel that has a longer burnout period. 

Fuels are classified by diameter as follows:

       (less than 0.25”)     1-hour fuel

       (0.25”-1”)               10-hour fuel

       (1-3”)                      100-hour fuel

       (3-8”)                      1000 hour fuel

The time required for the specific size fuel to reach moisture equilibrium with the atmosphere is referred to as LAG TIME.

Small fuels (called light, flashy or fast-burning) generally consist of dry grass, dead leaves, pine needles, twigs and brush.  Large fuels (called heavy or slow-burning) generally consist of limbs, large stems of brush, logs and stumps.  Small fuels and flat fuels are called “flashy” fuels because of their property of igniting and burning very quickly.  Some fuels such as pine needles, palmetto, gallberry and ti-ti also have rosins or waxes that have lower ignition temperature and consequently, tend to react the same way.

ARRANGEMENT 

Fuel can be arranged in different ways that will affect the behavior of the fire.  Loosely arranged fuel will ignite quicker and burn more intensely because drying is quicker and also more oxygen is available among the fuel for the combustion process.  As fuels are compacted, less oxygen will be available and less wind can penetrate the mass to drive off the moisture.  They take longer to dry out and as a result will be wetter.  As the fuel is compacted further, it will begin to act like large fuel--takes more time and more heat to dry it out so that it will burn.  A floor of pine needles compacted by the rain is a good example. 

When there are fuels up off the ground such as brush, vines, and draped pine needles; they are very loosely arranged and will dry out rapidly.  If the volume is enough to carry fire and they are close enough for the convective heat as well as the radiant heat to pre-heat them to their ignition level, the fire can spread vertically into high brush and even the crown of the overstory; a FUEL LADDER.

If the horizontal arrangement becomes patchy, the fire may not continue to burn except on the most extreme days when the radiant heat is intense enough to pre-heat the fuel across the open areas.  A good example is scattered logging debris.

FUEL MOISTURE CONTENT 

The amount of moisture in fuel is the major element that will determine how much of the fuel will burn (available fuel).  According to how much moisture is in the fuel, all will burn, only part will burn or, if wet enough, none will burn. 

Of the fuels that do burn (the available fuel), the moisture that is in them affects the behavior of the fire.  When the available fuels are moist, combustion is slow because much of the heat is expended in converting moisture in the fuel to steam.  Fuel will not enter the combustion process and burn until the moisture is driven out of that particular particle of fuel and it has reached ignition temperature.  As fuels become drier, the rate of combustion is increased because less heat is required to drive off the moisture that is left. 

Dead forest fuels pick up and give off moisture continuously according to dryness of the air (relative humidity), wind, temperature and precipitation.  As with temperature in the atmosphere which is continuously trying to equalize (causing winds and unstable atmosphere), the moisture in dead fuel is continuously trying to equalize with the amount of moisture in the adjacent air.  Fine fuels, such as small twigs and grass, lose moisture rapidly and will have the greatest day-to-day variation.  Heavy fuels such as logs and large limbs fluctuate slowly in the exchange of moisture.  Only the surface of large limbs is exposed to the air and therefore drying or wetting must take place progressively from the surface to the center.  For this reason heavy fuels do not become very dry until long periods of dry weather have occurred.  For the same reason, rains of short duration will wet only the outer layer.

Fine fuels (1-hour fuel) are those fuels whose moisture content reaches equilibrium with the surrounding atmosphere within one hour. Fuels may be referred to as 1 hour, 10 hour, etc. fuels depending on their tendency to dry. Fuels 0" to 1/4" are "one-hour fuels. Fuels 1/4" to 1" are known as 10-hour fuels.

A large log, wet from winter precipitation dries through the summer from the outside in. In the fall, as rains begin and temperatures and humidities moderate, the process is reversed and the log begins to take on moisture from the outside in.

Fuel moisture will increase during periods of rain or snow and high humidity.  Sunlight will lower the relative humidity and increase the temperature resulting in the fuel losing moisture.  The longer fuel is exposed to dryer conditions, the dryer it will get. Wind will also help to dry fuel. It will blow away the moisture laden air next to the fuel and replace it with drier air.

At night, surface temperature drops fairly rapidly, atmospheric moisture (R.H.) will increase, winds will decrease or become calm and the atmosphere will become more stable. As a result, fuels will not continue to lose moisture to the atmosphere. In most cases, it will begin to draw moisture from the now damp air adjacent to them. 

Even though pine needles are fine fuels and can exchange moisture rapidly, they give off moisture very slowly when on the ground and compacted, because they are in a more moist environment.  The moisture in the adjacent needles and the space in between stays wetter because the sunlight and air with lower relative humidity is exposed only to the top layer.  Consequently they will react more like larger size fuel.  They also absorb more moisture from precipitation because the excess does not all run off but is held by the ground.  As these fuels lose moisture, they in turn will absorb more moisture from the ground.

Draped fuels that are caught on limbs and brush will dry out rapidly because they are exposed to the sunlight and wind. 

Only dead fuels interact with the adjacent air in the exchange of moisture.  Green fuels gain and lose moisture on a seasonal basis (or severe droughts).  They are usually the highest in moisture content in the spring and are the lowest in the winter when in the cured stage. 

FUEL MOISTURE CONTENT - The amount of moisture in fuel expressed as a percent of the oven dried weight. It is necessary to weigh a sample of fuel then dry it in an oven to determine the moisture content.  The process requires a substantial amount of time and is not readily available on the fire line though new devices for measuring fuel moisture are becoming available.  Reliable estimations of fuel moisture may be made based on sound science using the Fireline Handbook Appendix B: Fire Behavior, NFES 2165, available from the National Interagency Fire Center .  Prescribed burn managers and planners should be able to estimate the MOISTURE of EXTINCTION and the PROABILITY of IGNITION as part of the prescribed burn prescription process.  Monitoring fine fuel moisture through the day, diurnal cycle, is an important fire line task to insure that the prescribed burn is within prescription parameters.  In most southern fuel beds fine fuel moisture between 6 and 10% is effective.  A fine fuel, 1 hour fuel, moisture of 5% or less may result in a burn that is difficult to control and too intense, outside of prescriptive parameters.  The maximum fuel moisture, moisture of extinction, that will support continuous ignition is published in Aids to Determining Fuel Models for Estimating Fire Behavior, NFES 1574.  (See the segment on fuel models)

SHAPE OF FUEL 

Shape affects the ignition and behavior of fire much the same as size.  Flat shaped fuel is similar to small size fuel in that it has a larger surface-to-volume ratio.  It dries out faster and there is also more surface area for the heat to enter, thus it will ignite more readily because it takes less heat to dry it out.  It will also burn more rapidly contributing its heat energy to the fire quicker resulting in a more intense fire.

Fuels shapes:

         Needles

         Leaves

         Limbs

         Logs

         Palmetto

         Peat