Chapter 6 Fire Behavior, Fuels and Topography
FIRE BEHAVIOR, FUELS AND
TOPOGRAPHY
I. INTRODUCTION
Most courses and material concerning fire behavior typically deal with wildfires
in the mountainous western United States. This course, however, deals with
fire behavior in a different setting: prescribed fire in the 11 ecoregions of
Texas. The influence of Texas’s climate, vegetation, topography, coastline and
other unique factors known to people familiar with the Lone Star State, can
cause both wildfires and prescribed fires to vary radically from fires in the west.
Fires are like snowflakes. Collectively they look alike, but when fires are
closely examined it is apparent that no two fires burn in the exact same way.
The overall objective of this course is to give the trainee a basic understanding
of fire behavior in Texas and allow him/her to make some basic predictions on
how a prescribed fire will act under the weather, fuel and topographic
conditions present on the prescription area.
II. FIRE TERMINOLOGY
Before we begin discussing fire behavior and the fire environment, let’s review
some fire terms and nomenclature, so that we all speak the same language:
1. Head - the forward, wind-driven edge of a fire usually the hottest and
fastest-moving area with the highest flames
2. Flanks – the parts of the fire burning perpendicular to the wind
3. Rear – the part of the fire burning into the wind (also known as
backing)
4. Islands – unburned patches of fuel inside a burn
5. Fingers – projections of the fire perimeter separate from the head
6. Pockets – areas of unburned fuel outside the fire perimeter that occur
between fingers and the head.
7. Spotting – the process by which a fire spreads by lifting or
transporting burning or glowing ember and ignition fuels in advance
of the main fire.
8. Torching – the process by which a fire races upward from the
ground to treetop to treetop.
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Chapter 6 Fire Behavior, Fuels and Topography
9. Surface Fire – a fire burning above ground level but below the tree
canopy
10. Crown Fire – a fire burning from treetop to treetop either in
conjunction with or independently of a surface fire
11. Ground Fire – a fire that smolders underground in organic soils
12. Flare-up – a sudden increase in a fire’s rate of spread and intensity
13. Ground Fuels – material located in the duff and soil (organic
material)
14. Surface Fuels – grasses, shrubs, and small trees
15. Ladder Fuels – trees and vines located between the surface and the crown
16. Aerial Fuels – the crowns or canopy of the overstory
III. PHASES OF COMBUSTION
The process of combustion is divided into four distinct phases. Understanding
these phases is important to comprehending the overall effect a fire will have
towards accomplishing the objectives of the burn. These phases are:
1. Pre-ignition – the products are water and organic gases. This phase
is important to preparing the fuel just prior to the next phase so that
actual combustion can take place. Some of the fuels, particularly
those in the crown may reach this phase, but due to the fire’s
velocity, not actually ignite, as a result they will “brown” but not
actually burn.
2. Flaming – the major products are water, carbon dioxide, and visible
smoke. The importance of this phase is obvious, the visible flame
that consumes some of the available fuel. The amount of fuel
consumed depends on the speed of the fire.
3. Smoldering – the product is visible smoke. This phase is important
due to the continued consumption of available fuels and heat
transferred to both the soil and the surrounding vegetation.
4. Glowing – the products are carbon monoxide and carbon dioxide.
This phase continues to transfer heat to the area as well as consuming
available fuel.
The last three phases are the most critical to the prescribed burn manager. The
total amount of fuel consumed and the total amount of heat generated determine
the overall effect of the fire applied to the area. It is important to be able to
determine, and regulate the total effect of the fire in all four areas with
particular emphasis on the last three.
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Chapter 6 Fire Behavior, Fuels and Topography
IV. FIRE ENVIRONMENT
There are three factors that make up the fire environment. They are:
1. Fuels
2. Weather
3. Topography
V. PRINCIPLES OF COMBUSTION
Three main elements of the fire triangle must be present in order for a fire to
burn. These elements are:
1. Heat
2. Fuel
3. Oxygen
Without one of these elements, fire cannot exist. In the planning phases of
prescribed burning, all three elements should be considered. Once the burner
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Chapter 6 Fire Behavior, Fuels and Topography
understands the role of each side of the triangle and their relationship to time,
they will have come a long way towards consistently achieving the objectives
of their burns. Heat can be regulated by choosing favorable weather conditions
and appropriate firing techniques. Fuel can be regulated by maintaining a
particular fire regime and fire breaks. Oxygen can be regulated by choosing
weather conditions that will contribute or restrict air movement.
VI. HEAT
A fire spreads by transferring heat from one piece of fuel to another. There are
three ways this occurs.
1. Convection
2. Radiation
3. Conduction
The three methods above are listed in the order of their importance.
Convection – is the movement of heat through a liquid or a gas (like air). An
example is heat rising in a column above a fire. Convection pre-heats fuel
quickly, raising them to their ignition temperature. Convection heat usually
rises, but may be pushed laterally by wind or slope.
Radiation – is emitted from a burning fuel. The simplest example is the heat
we feel from the sun. Radiation spreads in all direction, preheating fuels in the
immediate vicinity of a fire.
Conduction – is movement of heat through solids. An example would be the
transfer of heat through a copper pipe. In woodland fires, conduction usually
takes place in larger fuels such as fallen logs. However, wood is a poor
conductor of heat, which makes the method of heat transfer the least important
to the fires spread but very important to the fires effect on stems, root systems
and soil.
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Chapter 6 Fire Behavior, Fuels and Topography
At this time, the only measurements that we have of the amount of heat released
during a fire is the heat within the flaming front, regardless of location, head,
flank, or back. The three measurers of heat are:
1. Reaction Intensity – which is the heat released per minute within a
square foot of the flaming front.
2. Fireline Intensity – which is the heat released per second per linear
foot of flaming front.
3. Heat per Unit Area – the total heat released within the flaming front
in a square foot dependent of time.
The important point to remember is that the total heat released and the fuel
consumed is not measured by any three of the above. The smoldering and
glowing phases of combustion are not taken into consideration by any of the
three, however, the Heat per Unit Area does allow for the most complete picture
during the flaming phase of combustion.
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Chapter 6 Fire Behavior, Fuels and Topography
EXERCISE 1: Heat Transfer Methods.
Fill in the figure below with the appropriate methods of heat transfer.
Remember the preheating of fuels may be occurring by all of these methods at
the same time, depending on the arrangement or loading of the fuels. All three
heat transfer methods are important factors in determining the spread of a
wildland fire.
VII. OXYGEN
Oxygen is necessary for any type of combustion and the surrounding
environment contains sufficient oxygen to sustain the fire. However, the
environment is able to “feed” the fire to a greater or lesser degree depending on
wind velocity. If there is no wind the fire stands straight up and the heat from
the fire also goes straight up. If the wind bends the fire over in the direction
towards which it is blowing, the flame tip is closer to the ground, and more of
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Chapter 6 Fire Behavior, Fuels and Topography
the heat is dissipated laterally, than vertically. This can be very important
during the spring and summer if crown scorch has to be kept to a minimum.
The bending or leaning of the flame is where the two terms flame height and
flame length come into play. Flame height is the distance from the tip of the
flame straight down to the mid-point of the fuel bed, with no wind this could
equal flame length because the flame is standing straight up. Flame length, is
the distance from the tip to the mid-point of the fuel bed along the flame, so
with any kind of wind, flame length will be longer than flame height.
The reason we us the mid-point of the fuel bed, rather than the ground is
because the flame may not begin on the ground, particularly where crown fires
are concerned.
Another term applied to wind with respect to flame is “Mid Flame Wind Speed”
which is the speed of the flame at the mid-flame of the total height. Most often
the speed taken at eye level in the field is accepted as mid flame wind speed.
Flame moving up slope will be closer to the ground and therefore closer to the
fuels. This allows for faster preheating and preparing of the fuels for when the
fire arrives to ignite them. Of course this is why fire tends to move much faster
up hill than down or on flat ground.
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Chapter 6 Fire Behavior, Fuels and Topography
FUELS
I. INTRODUCTION
Fuel is any organic material that is living or dead, that can ignite and burn.
Fuels are found in almost infinite combinations of kind, amount, size, shape,
position and arrangement. The fuel on a given acre may vary from a few
hundred pounds of sparse grass to 100 or more tons of logging slash. It may
consist of dense conifer crown, deep litter and duff, moss layers and
underground peat, or a mixture of any of these forming a fuel complex.
We can estimate potential fire behavior by analyzing the physical properties and
characteristics of fuels. Topographic and weather factors must also be
considered before rate of spread and general fire behavior of fires can be
determined.
A. Fuel Levels and Components
A systematic approach to looking at the fuel complex is to divide it into
three broad groups or levels—ground, surface and aerial fuels.
Aerial Fuels: All green and dead materials located in the upper forest
canopy including tree branches and crowns, snags, moss, and high shrub.
When aerial fuels are present we are concerned with crown or canopy
closure. Timber stands with open canopies usually have a faster
spreading surface fire than closed canopy stands, and torching of
individual trees with possible spotting. Unless very strong winds are
present, crowning is unlikely without a closed canopy. Closed canopy
stands that are greater than six feet in height, whether timber or tall
shrubs, offer the best opportunity for a running crown fire. It is
important to note that few fires become running crown fires.
Surface Fuels: All materials lying on or immediately above the ground
including needles or leaves, duff, grass, small dead wood, downed logs,
stumps, large limbs, low shrubs, and reproduction. Surface fuels are less
compact than ground fuels and have other characteristics more favorable
for faster rates of spread. Surface fuels include litter, grass and shrubs to
about six feet in height. If no aerial fuels are present, surface fuels have
an open environment subject to stronger winds and more heating and
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drying by solar radiation. Thus, fires run through this fuel level with low
to high rates of spread. Surface fuels are what generally carry a
prescribed fire and are therefore considered the most important the most
important fuel type and will receive the most emphasis in this section.
Ground fuels: All combustible materials lying beneath the surface
including deep duff, roots, rotten buried logs and other woody fuels.
Ground fuels are important in relation to line construction and mop-up
operation, but contribute very little to the actual fire intensity and rate of
spread of the fire front. Do to the compactness of ground fuels they
produce very slow burning or smoldering fires.
II. FUEL CHARACTERISTICS
A. Fuel Characteristics which affect Fire Behavior
1. Loading
2. Size and shape
3. Compactness
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Chapter 6 Fire Behavior, Fuels and Topography
4.
5.
6.
7.
Horizontal arrangement
Vertical arrangement
Chemical content
Moisture content
Fuel
Characteristic
Fire
Behavior
Fuel
Characteristic
1. Fuel Loading:
Fuel loading is the oven-dry weight of fuels in a given area usually
expressed in tons/acre or pounds/acre. Fuel loadings vary greatly by
fuel groups. For example grass fuel types can vary from <1 to 5
tons/acre, shrub fuel types from 2-80 tons/acre. Logging slash from 10
to 200 tons/acre and timber litter from 4 to 12 tons/acre.
When interpreting and predicting fire behavior, we are more concerned
with the surface fuel loading; in particular those dead fuels that are less
than 3 inches in diameter and live fuels of less than 1.4 inch diameter.
The total fuel loading on a site can be much more than what we have
shown here. Much of the vegetation on a site may not be available to
carry fire due to its height above the ground or high moisture levels.
Fuel loadings are generally separated by different sizes of live and
dead fuel particles. Dead fuels are broken into four size classes
according to their diameter. They are:
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Chapter 6 Fire Behavior, Fuels and Topography
1. Grasses, litter and duff; < ¼ inch in diameter (1 hour
fuels).
2. Twigs and small stems; ¼ to 1
inch in diameter (10 hour fuels).
3. Branches; 1 to 3 inches in
diameter (100 hour fuels).
4. Large stems and branches; > 3
inches in diameter (1000 hour
fuels).
Fuel loading is usually measured in tons/acre (T/A) or pounds/acre
(lb/A) for lighter fuels such as grass.
2. Size and Shape:
Surface-area-to volume ratio is the ratio of the surface area of a fuel to
its volume using the same unit of measurement. The higher the ratio
(1:3,000) the finer the fuel (grass). The lower the ratio (1:6) the larger
the fuel (logs). We know from our experience in starting campfires,
wood stoves, or fireplaces that small fuels ignite and sustain combustion
easier than large pieces of fuel. Less heat is required to remove fuel
moisture and raise a small fuel particle to ignition temperature.
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Chapter 6 Fire Behavior, Fuels and Topography
The size and shape of firebrands affect the amount and distance of
spotting. Small embers ordinarily produce short-range spotting only,
because they cannot sustain combustion for the period of time required
for long-distant transport. Live cedar
often produces short-range
are
spotting due to the smallness of the firebrands. Oak leaves
larger and more aerodynamic (especially Live Oak) than live cedar.
Therefore, firebrands from oak trees can often cause spotting over 300
ft. Cones, cedar fronds, bark plates and pine needles are examples of
some firebrands which have been lifted into convection columns and
then deposited 10 miles or more down wind from the fire. In these
cases, their flatness and greater surface-area-to-volume ratios have
increased the aerodynamic qualities of the particles, thus making it
easier for convection columns to lift them to greater altitudes. The
shape of fuels is also important to spotting down slope by rolling
firebrands. Pine cones, round logs and round yucca plants are
particularly troublesome in their respective area.
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Chapter 6 Fire Behavior, Fuels and Topography
3. Compactness
Compactness can be simply defined as the spacing between fuel
particles. The closeness and physical arrangement of fuel particles
affects both ignition and combustion. Rates of spread in closely
compacted fuels (forest litter) are usually slower than rates of spread in
loosely compacted fuels (grasses).
Fuel bed depth is the average height of surface fuel that is contained
in the combustion zone of a spreading fire front. Orientation of the
fuel refers to the horizontal or vertical orientation o the fuel
arrangement that carries the fire. Vertically oriented fuels are found in
the grass and shrub groups, which rapidly increase in depth with an
increase in fuel load. Horizontally oriented fuels are found in the
timber litter and logging slash fuel groups and slowly increase in depth
as the load is increased.
Observation of the location and orientation of fuels in the field help
one decide which fuel groups are represented. For more information
and assistance on determining fuel categories and groups refer to Aids
to Determining Fuel Models for Estimating Fire Behavior in this
notebook.
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Chapter 6 Fire Behavior, Fuels and Topography
4. Horizontal Continuity
Horizontal continuity is the horizontal distribution of fuels at various
levels or planes. Horizontal continuity applies to all levels of the fuel
complex but the continuity of fine fuels is especially important to the
spread of surface fires, since prescribed fires burn most often in this
fuel level. This characteristic influences where a fire will spread, how
fast it will spread and whether the fire travels through surface fuels,
aerial fuels or both.
Discontinuous or patchy fuels are difficult for a fire to travel through
and usually require strong winds with spotting for good fire coverage.
Continuous fuels, however provide available fuels at one or more
levels giving allowing the fire to spread uniformly for great distances.
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Chapter 6 Fire Behavior, Fuels and Topography
Horizontal continuity in aerial fuels or the effects of a closed versus
open timber canopy plays a major role in fire behavior. A forest
canopy not only shades surface fuels and prolongs moisture retention
but also greatly reduces wind speeds from levels above the canopy to
levels near the surface. Generally, the greater the crown closure, the
greater the wind speed reduction. This certainly does have an effect on
surface fires burning in these closed environments. If torching out of
individual trees occurs then we have an entirely new fire environment
with which to be concerned.
5. Vertical Arrangement
Vertical arrangement is the relative height of fuels above the ground as
well as their vertical continuity, both of which influence fire reaching
various fuel strata.
When fuels are mostly vertically continuous, we call this a fuel ladder,
or ladder to transport fire into the forest canopy. The intensity of the
surface fire and the live fuel moisture usually determine whether a fire
will travel up through the green ladder fuels.
A dangerous condition exists when a fire has only burned through the
surface fuel level, drying the aerial fuels. A slight change in the
environment and the fire can cause a reburn of the canopy—a very
dangerous situation.
6. Chemical Content
All fuels, living and dead, contain fiber that is known as cellulose.
Fuels also contain chemicals and minerals than can enhance or retard
combustion. Chemical contents include the presence of volatile
substances such as oils, resigns, wax and pitch. There are certain fuels
having rather high amounts of these volatile substances that can
contribute to rapid rate of spreads and high fire intensities. On the
other hand, certain fuels may be high mineral content, which can
reduce fire spread and intensity.
A burn manager is primarily concerned with volatile substances that
make the job more difficult. A few fuels such as duff and “cow pies”
are excellent receptors of fire brands that hold over fire primarily due
to their high mineral contents that enhance smoldering at much lower
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Chapter 6 Fire Behavior, Fuels and Topography
ignition temperatures. Other moderate to highly volatile fuels in Texas
include Juniper, Oak, and Pine.
7. Fuel Moisture
Fuel moisture content is the amount of water in a fuel expressed as a
percent of the oven-dry weight of that fuel. Fuel moisture can exceed
100% and ranges from 0-30% for dead fuels and 30-300% for live
fuels depending on species and their growth stage (Table 1). The
formula for obtaining fuel moisture is:
[(Wet Weight – Dry weight)/Dry Weight] x 100 = % Fuel Moisture
Table 1. General estimates for live fuel (foliage) moisture content.
Stage of vegetative development
Fresh foliage, annuals developing, early in
growing cycle.
Maturing foliage, still developing with full vigor.
Mature foliage, new growth complete and
comparable to older perennial foliage.
Entering dormancy coloration starting, some
leaves may have dropped from stem.
Completely cured.
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Moisture content
(Percent)
300
200
100
50
Less than 30, treat as dead
fuel
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Chapter 6 Fire Behavior, Fuels and Topography
A technique used for obtaining live fuel moisture in Juniper and other species is
described in the appendices.
Fine dead fuels less than ¼ inch, such as grass
and needle/leaf litter, are most responsible for
the spread of fire. In fact, the fine fuels are
considered the primary carrier of a surface
fire. The live-to-dead ratio becomes critically
important when evaluating the potential for a
fuel to burn. The greater the amount of dead
fuel compared to live fuel, the more
flammable the fuel.
The dead component of the fuel is extremely important since it is the dead
material that carries the fire and heats the live component t ignition. With
insufficient dead fuels present, a live stand may not burn even under good
burning conditions. Normally, at least one-third of the fine fuel complex must
be dead or cured in order to have an adequate ratio of live-to-dead to carry a
fire.
In Review: Fuel Characteristics Influencing Fire Behavior:
Of the seven fuel characteristics discussed above compactness, loading,
chemical content, size and shape and moisture content are the characteristics
that influence fire ignition. While Compactness, loading, horizontal continuity,
chemical content, size and shape and moisture content influence rate of spread
and fire intensity. It is important to note that not all fuels burn during a
prescribed fire, depending on factors of fuel availability. Consumption tends to
vary by fuel category with nearly 100% consumption n grass fuels and as little
as 5-25% in timber litter fuel types.
Specialists that work with fire and try to develop methods of predicting fire
behavior have broken down all ecosystems into physical models that represent
how a fire will burn. Many different inputs and fuel characteristics are involved
with these models, but the basic one is fuel loading as it relates to fuel size.
How much dead fuel, how much live fuel and how much dead fuel in the size
classes of 0 – 0.25 inches (1 hour fuels), 0.26 – 1 inch (10 hour fuels). 1 to 3
inches (100 hour fuels) and 3 to 7 inches (1000 hour fuels).
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Chapter 6 Fire Behavior, Fuels and Topography
All of the above has contributed to the development of 13 basic fuel
models for predicting fire behavior. These 13 fuel models have been
classified into 4 categories. Models 1-3 are the grass models, models 4-7
are the brush or shrub models, models 8-10 are the timber models and
models 11-13 are the slash models (Table 2). For a detailed explanation
of these models and their characteristics refer to the publication Aids to
Determining Fuel Models for Estimating Fire Behavior provided in this
notebook.
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Chapter 6 Fire Behavior, Fuels and Topography
Table 2. Description of the 13 fuel models in fire behavior as documented by
Albini (1976).
Moisture of
extinction
Fuel loading
Fuel
Typical fuel complex
Model
Fuel Bed
Dead Fuels
Depth
Percent
--------------------Tons/acre------------------- Feet
1 hour 10 hours 100 hours
Live
1
2
3
Grass and grassShort grass (1 foot)
Timber (grass and
Tall grass (2.4 feet)
0.74
2
3.01
0
1
0
0
0.5
0
0
0.5
0
1
1
2.5
12
15
25
4
5
6
7
Chaparral and shrub
Chaparral (6 feet)
Brush (2 feet)
Dormant brush,
Southern rough
5.01
1
1.5
1.13
4.01
0.5
2.5
1.87
2
0
2
1.5
5.01
2
0
0.37
6
2
2.5
2.5
20
20
25
40
8
9
10
Timber litter
Closed timber litter
Hardwood litter
Timber (litter and
1.5
2.92
3.01
1
0.41
2
2.5
0.15
5.01
0
0
2
0.2
0.2
1
30
25
25
11
12
13
Slash
Light logging slash
Medium logging slash
Heavy logging slash
1.5
4.01
7.01
4.51
14.03
23.04
5.51
16.53
28.05
0
0
0
1
2.3
3
15
20
25
Fuel Model 1
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Chapter 6 Fire Behavior, Fuels and Topography
III. FUEL MOISTURE
Fuel moisture is one of the most important characteristics of fuels in Texas.
Burn managers are primarily concerned with two kinds of fuel moisture. There
is live fuel moisture and dead fuel moisture, which should be self explanatory.
Live fuel moisture is affected by live plant’s metabolism and growing season.
Dead fuel moisture is affected by the humidity and soil moisture to a large
degree. As mentioned earlier in the unit fuel moisture is measured by obtaining
a fuel sample, weighing the sample immediately after clipping and then drying
the sample to a constant weight and then using the following formula:
[(Wet Weight – Dry weight)/Dry Weight] x 100 = % Fuel Moisture
Fuels are constantly exchanging moisture with the surrounding air. Factors that
contribute to a change in dead fuel moisture include precipitation duration,
precipitation amount, temperature, relative humidity and wind. Wind can both
dry and wet fuels depending on the relative humidity. During periods of high
humidity and precipitation there is a net gain in fuel moisture. However, when
the air is dry, with low humidity, fuels are giving up more moisture to the air
than they receive.
The fuel itself can regulate the amount of moisture it can absorb or lose, for
example waxy coatings can prevent moisture from getting in, while rotten
material tends to prevent loss of moisture because it holds it like a sponge.
Several factors influence the rate of moisture exchange between fuels and the
air. These include water vapor pressure difference, presence or absence of
wind, size of fuels, compactness of fuels and proximity of fuels to damp soil.
A. Equilibrium Moisture Content
Equilibrium Moisture Content is the point at which the fuel has
absorbed 70% of the total amount of water it could hold at that
particular temperature and relative humidity. This is the point at which
the vapor pressure in the air is equal to the vapor pressure in the fuel
and the exchange stops.
Equilibrium moisture content can occur in small, fine fuels, but never
occurs in larger fuels, as the time required to reach equilibrium in
larger fuels is much longer.
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Chapter 6 Fire Behavior, Fuels and Topography
B.
lag
Time
Fuels respond to changes in moisture at different rates. Time lag is a
measure of the rate at which a specified size of dead fuel gains or loses
moisture. Time lag can also be defined as the time necessary for a fuel
particle to gain or lose approximately 2/3 of the difference between its
initial moisture content and its equilibrium moisture content. The
smaller the fuel the greater the surface-area-to-volume ratio, therefore
the quicker it responds to environmental changes such as rain. The
dead fuel time lag categories are as follows:
1.
2.
3.
4.
1 hour: fuels that are 0 inch to 1.4 inch diameter.
10 hour: fuels that are ¼ inch to 1 inch diameter.
100 hour: fuels that are 1 inch to 3 inches diameter.
1000 hour: fuels that are 3 inches to 8 inches diameter.
The gain and loss of moisture does not occur at a constant rate When
conditions change, fuels respond quickly at first. The change in
moisture content becomes slower as the fuel moisture gets closer to the
equilibrium moisture content.
C. Environmental Factors Influencing Fuel Moisture
Fuel moisture is directly influenced by temperature, relative humidity,
precipitation and shading. Wind alters the exchange of moisture
between the fuels and the air. Other site factors of weather and
topography influence atmospheric temperature and relative humidity.
Since these factors indirectly affect fuel moisture they must be
considered when estimating fuel moisture content. These factors are
shown below.
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Chapter 6 Fire Behavior, Fuels and Topography
D. Moisture of Extinction
Moisture of extinction is defined as the fuel moisture content at which
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Chapter 6 Fire Behavior, Fuels and Topography
predictable manner. Moisture of extinction varies by fuel situation and
is dependent on various fuel characteristics such as fuel loading, fuel
size, arrangement and chemical content. Moisture of extinction is
lowest (around 12%) for light porous grasses such as buffalograss and
tends to be higher (around 30%) for more compacted fuels such as
needle litter. Southern rough fuels located in the Southeastern US have
a moisture extinction as high as 40 percent. The moisture extinction
ranges for different fire behavior fuel models are found in Table 3.
Table 3. Moisture of Extinction for each Fuel Model
Fuel Model
Presence of Fuel Class
Moisture of
1-H 10-HR 100-HR LIVE
Extinction (%)
1 Short Grass
X
2 Timber and Grass
X
3 Tall Grass
X
4 Chaparral (6ft)
X
X
5 Brush (2ft)
X
X
6 Intermediate Brush
X
X
X
7 Southern Rough
X
X
X
8 Closed Timber Litter
X
X
X
30
9 Hardwood Litter
X
X
X
25
10 Timber with Litter
X
X
X
11 Light Logging Slash
X
X
X
15
12 Medium Logging Slash
X
X
X
20
13 Heavy Logging Slash
X
X
X
25
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X
X
X
15
25
X
X
20
X
20
25
X
X
40
25
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Chapter 6 Fire Behavior, Fuels and Topography
E. Fine Dead Fuel Moisture Tables
Now you will be introduced to tables which can give you acceptable
estimates of 1-hour timelag fine dead fuel moistures, during daylight
hours, under a variety of conditions. We want to discuss briefly how
the tables are to be used. First of all, you will determine a reference
fuel moisture (RFM) by entering dry bulb temperature and relative
humidity into the table below.
REFERENCE FUEL MOISTURE (DAYTIME 0800-1959)
Relative Humidity (%)
Dry
0
5
10 15
20
25 30
35
40 45
50
55 60
65
70 75
80
85 90
Bulb
95
…..100
Temp (F) 4
9
10 - 29 1
2
2
3
4
5
30 - 49 1
2
2
3
4
50 - 69 1
2
2
3
70 - 89 1
1
2
90 - 109 1
1
1
109+
1
14 19
24
29 34
39
44 49
54
59 64
69
74 79
84
89 94
99
5
6
7 8
8
8
9
9
10 11
12
12 13
13
14
5
5
6
7 7
7
8
9
9
10 10
11
12 13
13
13
4
5
5
6
6 7
7
8
8
9
9 10
11
12 12
12
13
2
3
4
5
5
6 7
7
8
8
8
9 10
10
11 12
12
13
2
2
3
4
4
5
6 7
7
8
8
8
9 10
10
11 12
12
13
2
2
3
4
4
5
6 7
7
8
8
8
9 10
10
11 12
12
12
Exercise 4
Determine the reference fuel moisture given the following conditions:
Temperature 75° and relative humidity 34%:________________RFM
Temperature 95° and relative humidity 22%:________________RFM
Next you will determine a fuel moisture correction (FMC) value from
the tables by considering the month, cloud/canopy cover shading, time
of day, site location elevation difference, aspect and slope percent.
The correction value is then added to the reference fuel moisture to get
the adjusted fine dead fuel moisture (FDFM).
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Chapter 6 Fire Behavior, Fuels and Topography
Fuel Moisture Correction Values
Correction Values For Daytime 0800 - 1959 (MAY, JUNE, JULY)
Time
Apect
North
East
South
West
North
East
South
West
0800
1000
1200
1400
1600
1800
3
2
3
4
Clear and/or Canopy (Less than 50% shaded)
1
0
0
0
0
0
1
0
0
2
0
0
1
2
1
0
3
4
3
2
5
4
4
5
Cloudy and/or Canopy (More then 50% Shaded)
4
3
3
4
3
4
4
3
3
4
3
3
4
4
4
4
5
5
5
4
Correction Values For Daytime 0800-1959 (FEB.,MARCH, APRIL, AUG., SEPT., OCT.)
Time 0800
1000
1200
1400
1600
1800
Apect
Clear and/or Canopy (Less than 50% shaded)
North
4
2
2
2
2
4
East
3
1
1
1
3
4
South
4
2
1
1
2
4
West
4
3
1
1
1
3
Cloudy and/or Canopy (More then 50% Shaded)
North
5
5
4
4
5
5
East
5
4
4
4
5
5
South
5
4
4
4
4
5
West
5
5
4
4
4
5
Correction Values For Daytime 0800 - 1959 (NOVEMBER, DECEMBER, JANUARY)
Time 0800
1000
1200
1400
1600
1800
Apect
Clear and/or Canopy (Less than 50% shaded)
North
5
4
4
4
4
East
5
3
2
3
4
South
5
3
2
1
3
West
5
4
3
2
3
Cloudy and/or Canopy (More then 50% Shaded)
North
5
5
5
5
5
East
5
5
5
5
5
South
5
5
5
5
5
West
5
5
5
5
5
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5
5
5
5
5
5
5
5
25
Chapter 6 Fire Behavior, Fuels and Topography
Dry Bulb Temperature
Relative Humidity
Reference Fuel
Moisture
+
Month
Shaded or Unshaded
Time of Day
Aspect
Fuel Moisture
Correction Value
(FMC)
=
Adjusted Fine Dead
Fuel Moisture (FDFM)
Exercise 4
Determine the reference fuel moisture (RFM), Fuel moisture correction value (FMC) and
the fine dead fuel moisture (FDFM) in the following examples:
On June 10 at noon, the temperature was 93° and the relative humidity was 34%:
RFM_________ + FMC_________ = __________FDFM
On a cloudy March 23 at 2:00 pm (1400), the temperature was 56° and the relative humidity was 45%:
RFM_________ + FMC_________ = __________FDFM
IV. Combining Influences Affecting Fire Behavior
All actions on a prescribed burn should be based on current and expected fire
behavior. In this section we will describe the response of fire to important
elements of the fire environment.
A. Fire Behavior Characteristics
As the rate of spread (ROS) of a fire increases, there is a correlated
increase in fireline intensity. ROS is often difficult to see and calculate.
However, ROS is interrelated to flame length, which is often easier to
discern in the field. Therefore, fire suppression limitation can be based
on flame length.
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Chapter 6 Fire Behavior, Fuels and Topography
B. Effects of Fuels on Fire Behavior
1. Fuel loading
Higher fuel loadings create greater flame lengths (intensities) for a
given rate of spread. Increased loading of larger fuels will decrease
rate of spread because of the additional time required t for larger
fuels to become preheated.
2. Live-to-dead fuel ratio
The higher the ratio of dead fuel to live fuel, increases the
flammability of fuel complex. The dead fuel is the material that
carries the fire and preheats the live component to ignition.
3. Surface-area-to-volume ratio
The greater the ratio, the finer the fuel, and the faster a fuel bed will
burn.
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Chapter 6 Fire Behavior, Fuels and Topography
4. Compactness
Closely compacted fuels restrict oxygen and inhibit
convective/radiant heat transfer and consequently burn slowly (e.g.
pine needles). Fuels that are too open and sparse limit heat transfer,
allow too much cooling, and will burn poorly.
5. Orientation
Vertically oriented fuels (grass and shrub) will burn faster and more
efficiently than those lying horizontally (timber litter and logging
slash).
6. Examples of Fire Behavior for Fuel Groups
In the following example the environmental conditions assumed are
a midflame wind of 4 mi/h and a 30% slope. The fuel moisture
contents are representative of low and high fuel moisture
conditions.
Low
Fuel Moisture
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High
Fuel Moisture
28