Agriculture Reference
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in the bound and free water of live fuels may influence ignition (Jolly et al. 2012 ;
McAllister et al. 2012 ). Bulk densities and packing ratios of canopy live fuels may
facilitate rapid ignition processes to generate great heat outputs that overwhelm
moisture effects. Hopefully, future research will provide answers to these important
live fuel moisture questions.
5.4
Moisture of Extinction ( M x )
The moisture of extinction (  M x ) is the dead or live fuel moisture at which combus-
tion cannot continue (Chap. 2, 3). Trabaud ( 1976 ) defined M x as the maximum FMC
above which a fire cannot be sustained and found 45 % as threshold value; combus-
tion did not occur above that value or it was delayed more than 15 min. In general,
studies have found that fire spread usually stops when dead fuels have an FMC
greater than 30 %, yet live fuels with FMCs above 100 % can support a spreading
fire (Scott et al. 2014 ). Rothermel ( 1972 ) first formulated the effect of moisture
content on the burning rate by defining a threshold M x , above which fire cannot
be sustained. The concept of extinction moisture is difficult to define in field and
laboratory experiments because it varies by diverse fuel (size, shape, density) and
environmental (temperature, humidity, wind) factors. As mentioned, FMC increases
specific heat and thermal conductivity of fuel so that more heat is absorbed by the
fuel particles surface layer to drive out moisture, delaying preheating and ignition of
fuel until it reaches ignition temperature. If there is too much water, ignition will not
occur. The most comprehensive M x model was provided by Wilson ( 1985 ) based on
laboratory data using milled wood sticks and shaved excelsior. Ideally, M x should
be an emergent property of fire models that simulate combustion using mechanistic
physical process; simulated combustion should cease when moistures are too great.
However, most fire models do not have the resolution and detail to accurately simu-
late the extinction of combustion due to high moistures. Instead, MEs are consid-
ered static parameters in combustion models and are assigned as properties of a fuel
or particle (Chap. 2). Rothermel ( 1972 ), for example, used the ratio of FMC to M x
to calculate a dampening coefficient that reduced fire intensity (Table 2.2).
5.5
Measuring Moisture Content
The most common method of measuring FMC is called gravimetric sampling where
fuel is dried in an oven. Fuel moisture is calculated from the difference in fuel
weight before and after drying. In general, this method involves collecting fuels
in the field and weighing them as soon as possible or placing them in a waterproof
container and storing the container in a cold or cool place for transport to the labo-
ratory. Once in the laboratory, fuels are first weighed and then dried in an oven set
at temperatures that range from 50 to 105 °C until their weight is stable, typically
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