(upbeat rock music)
- [Speaker] In this video you will learn
about dead fuel moisture and its importance
in wildland fire.
You will learn, one, what is dead fuel moisture?
Two, why is it important information for fire managers?
Three, what is moisture of extinction?
Four, what is equilibrium moisture content?
Five, what is the time lag concept?
Six, how do other factors influence
dead fuel moisture content?
One, what is dead fuel moisture?
Fuel moisture content of dead vegetation is a measure
of the amount of water in a piece of fuel
and is expressed as a percentage ranging
from about 2% to 30% during the fire season
for typical wildland fuels.
Dead fuels include all dead plant parts,
such as litter, duff, fallen logs,
as well as cured plants like annual grasses
that dry out and lose moisture.
This also includes dead vegetation that is still attached
to living vegetation and standing dead trees.
Two, why is it important information for fire managers?
Dead vegetation is critical in determining fire potential.
Unlike live fuel, dead fuel responds solely
to ambient environmental conditions,
and is highly susceptible to daily changes
in moisture content levels.
Dead fuel cells can absorb water vapor from the air
and absorb liquid water in the form of dew or rain.
Dead fuel loses water through evaporation
when the atmosphere is drier than the fuel.
Fine fuels such as grasses, needles and twigs
can react daily, yet larger fuels such as branches and logs
will take longer to absorb and lose moisture,
especially in the center of the fuel.
For fire managers, using dead fuel moisture levels
to determine the potential for fire to ignite and spread
is critical on wildland and prescribed fires.
Fine dead fuel moisture such as grasses and pine needles
is an important factor in determining fire behavior,
including rate of spread, flame lengths and fire intensity.
Generally, the drier the fuels the faster
and harder the fire will spread.
Fire behavior is discussed in another video.
Fire and forest managers monitor fuel moisture levels
frequently during the fire season to determine the potential
for a wildfire to ignite and spread.
The forest fire danger rating system is based partly
on dead fuel moisture levels.
Prescribed fire managers use fuel moisture levels
as a deciding factor of how best to meet burn objectives.
This information helps to determine the potential fire
behavior and fire effects.
If the moisture content is low, the fire can
become intense and erratic, and fire effects
may be severe.
If the moisture content is high,
little fuel may be consumed.
Three, what is moisture of extinction?
At some moisture level fuel is too wet for fire to spread.
This is called the moisture of extinction,
and in dead fuels can be between 12 and 30%.
This percentage will vary with fuel characteristics
such as the size, amount, and arrangement
of the fuels on the landscape.
In general, fine fuels such as grasses will have
a lower moisture of extinction, while in larger size fuels
the percentage will be higher.
Four, what is the equilibrium moisture content?
In a wildfire situation, the lower the EMC
the more fuel is readily available to burn
because the fuel is dry and ready to ignite.
Dead fuel reaches its equilibrium moisture content,
or EMC, when the fuel has reached equilibrium
with its environment.
However, the equilibrium is dynamic
and changes with relative humidity and temperature.
For example, with high relative humidity,
the water content at which fuel will stop
exchanging moisture with the atmosphere,
the EMC, is fairly high.
In a drier atmosphere, there will be higher
evaporative demand for water vapor from the fuel.
Thus, the EMC is lower.
Temperature also has an effect on EMC.
Thus, EMC will decrease as temperature increases.
The material, or the size of the fuel,
is also a determining factor of the EMC of that fuel.
Next, we will discuss these different fuel size classes
with the time lag concept.
Five, what is the time lag concept?
The time lag principle states that when the environment
changes fuel requires time to wet or dry to approach EMC.
A time lag is the amount of time that it takes
for a fuel to approach its EMC due
to changes in the environment.
Moisture gain or loss does not occur at a constant rate,
and when conditions change,
the fuels will respond quickly at first.
The time lag of fine fuels is short
because they reach their EMC quickly.
To explain further, smaller fuels have a greater
surface area to volume ratio than larger fuels,
which means it takes less energy and time
for smaller fuels to lose moisture.
Water near the surface of any fuel can
be evaporated fairly easily, but it requires
more energy to evaporate water from the center of the fuel,
especially in larger size class fuels.
Heavy fuels have a much longer time lag,
and generally will never reach their EMC
due to constantly changing conditions.
Time lag is related to fuel size.
For example, fuels zero to 1/4 inch in diameter
generally approach equilibrium moisture content
at a much faster rate than larger fuels.
This is why they are called one hour fuels.
Larger fuels such as 10 hour, 100 hour,
and 1,000 hour fuels approach the equilibrium
moisture content at progressively slower rates.
As you can see, the 1/2 inch sticks gain
and lose moisture more quickly than the 12 inch log.
The smaller the fuel size, the less time it takes
for that fuel to gain or lose moisture.
Wildland fuels are heterogeneous across a landscape.
The wide variety of fuel sizes and shapes
plus weather changes make it impossible
for an entire fuel complex to be at EMC at the same time.
Remember, when fuels get a day of precipitation,
they will both gain and lose that moisture
at different rates depending on their sizes.
This is important information for predicting fire behavior
and for assessing the potential effects of a fire.
Six, how do other factors influence
dead fuel moisture content?
Dead fuel moisture is affected by all three legs
of the fire environment triangle,
with weather as the primary influence.
Weather influences atmospheric temperatures
and relative humidity, which in turn affect fuel moisture.
Shade from clouds or overstory canopy cover
can raise fuel moisture levels slightly
in one and 10 hour fuels.
Fuel moisture in those same size fuels can
be influenced by wind.
Typically, winds during fire season
speed evaporation rates and decrease fuel moisture.
Topography can affect fuel moisture
of all size classes of fuels.
South facing aspects tend to be hotter and drier.
Fuels at high elevations have a shorter growing season
and generally greater fuel moisture content
due to the lower temperatures and higher humidity.
Narrow canyons can provide more shade
and are cooler than more exposed areas.
From this graph we can see how aspect
affects fuel moisture levels.
Except for the early morning hours,
fuel moistures are lower throughout the day
on south slopes than on north slopes
in the Northern Hemisphere.
Southwest slopes usually have the lowest
afternoon fuel moistures.
The properties of the fuel directly influence
the fuel moisture content.
Compacted fuels such as a slash pile or dense pine needles
will have slower evaporation rates than fuels
that are more loosely arranged.
And now, let's review.
One, dead fuel moisture is the amount of water
in a piece of fuel, which typically ranges
up to 30% during the fire season
and can fluctuate on a daily basis according
to the weather and the time of day.
Two, fire managers use this information to help determine
the potential for a fire to ignite and spread,
as well as potential fire behavior and affects
of wildfires and prescribed fires.
Three, moisture of extinction occurs
when the moisture level of a fuel is too high
for a fire to spread.
Four, dead fuels approach equilibrium moisture content
when they can neither lose nor gain moisture,
and this can change many times in a given day.
Five, a time lag is an indication of the rate
a fuel gains or loses moisture
due to changes in its environment.
Time lag is related to fuel size and is expressed in hours,
one, 10, 100 and 1,000 hour fuels.
When the environment is hot and dry,
the EMC of all size classes of fuels will be lower,
which means that the potential for a fire
to ignite and spread is high.
Six, topography, atmospheric conditions
and fuel properties all affect fuel moisture levels.
(upbeat rock music)