Fuel Moisture - Wildland Fuel Fundamentals and Applications - page 72

Fig. 5.1 Drying curve of a
5 cm-deep litter layer under
temperature and humidity
conditions for which the
equilibrium moisture content
(EMC) is 5.5 %. (Schroeder
and Buck 1970 )
rainfall, not the amount, is often used in fuel moisture modeling (Viney 1991 ). Dew
can also condense on a fuel particle provided that the particle's temperature is at
dew point and this condensed free water can be directly absorbed by the particle.
Moisture from the ground can also be conducted to a fuel particle if the ground is
wet, and water evaporated from the ground surface can condense on the cooler fuel
particle (Pyne et al. 1996 ).
Any discussion of dead FMC dynamics requires the introduction of another term
to illustrate how fuel dries over time. The equilibrium moisture content (EMC) is
the final FMC of a dead fuel particle when it is exposed to constant temperature
and humidity conditions for a period of time. When the difference in vapor pres-
sure between air and the fuel particle is zero, the moisture of a dead fuel particle
is in equilibrium with the constant environment (Fig. 5.1 ). EMC curves differ for
each type and size of particle as well as for each set of temperature and humidity
conditions. Most fuel particles rarely reach EMC because the surrounding environ-
ment changes rapidly and desorption rates are slow. The process of adsorption and
desorption can also be illustrated using the relationship of EMC to relative humidity
(Fig. 5.2 ). Low humidities allow desorption to decrease EMCs resulting in drying
fuels (lower FMCs), while higher humidities increase adsorption to increase EMCs
and ultimately FMCs. One application of EMC is in the construction of drying
curves (Fig. 5.2 ) that show how the relative humidity affects the EMC at a constant
temperature, in this case, of 27 °C (80 °F) of a fuel particle (needle) during both the
adsorption (when the air is moister than the particle) and desorption processes.
Fire scientists have condensed the complex process of dead woody fuel moisture
dynamics into the time-lag concept discussed briefly in Chap. 3. A time lag is the
time it takes for a fuel particle to lose 63 % of the difference between its initial mois-
ture content and its EMC under constant conditions (80 °F, 20 % relative humidity)