Fuel Moisture - Wildland Fuel Fundamentals and Applications - page 71

high moisture to areas of lower moisture to achieve equilibrium. Water vapor can
either condense on cell walls or continue to diffuse through cell voids to any area
of lower moisture. The steeper the particle water vapor gradient, the faster the dif-
fusion process. Diffusion is governed by cell structure (i.e., pore space, cell wall
composition), which varies by plant life-form and species of the dead particle, and
particle age (stage of decomposition). In summary, water and water vapor move
through a fuel particle in a progression of processes consisting of evaporation of
water from cell walls, diffusion of the water vapor across cell voids and through cell
walls, condensation on another cell wall, and bound water transport across a cell
wall, all aided by capillary forces and infiltration. Eventually, water is evaporated
from the particle surface into the atmosphere and lost from the fuel particle.
Two physical properties of the fuel represent these processes in modeling dead
fuel moisture dynamics within a particle. Particle permeability is the ability of water
to flow through cell cavities or how fast water can be transported across the particle.
Cell structure, chemical composition, size, shape, and degree of decomposition all
play a role in influencing the permeability of water through a dead fuel particle.
Moisture diffusivity is the potential for the flow of water molecules across cell walls
and is mostly governed by the degree of hygroscopy of the cell walls within the
dead fuel particle.
To simplify complex physical FMC processes, dead fuel moisture dynamics is
mostly governed by the water vapor pressure difference (dry to wet) between the
atmosphere and the fuel particle. This difference is primarily driven by tempera-
ture, relative humidity, and the presence of water on the particle surface (Matthews
2013 ). If the atmosphere-particle vapor pressure difference is positive, such as
when the temperature is high, relative humidity is low, and fuel is wet, then water is
lost from the particle into the atmosphere through a process known as desorption .
The free water in the intercellular space is easily evaporated to the atmosphere, but
the vapor pressure deficit (difference in humidity of cell spaces and the air) must be
great to drive bound water from the cell walls. Adsorption occurs when atmospheric
humidity increases and dry fuel particles gain moisture as water molecules adhere
to the particle surface resulting in increasing FMC (Simard 1968 ). The processes of
desorption and adsorption introduce an important aspect of dead FMC dynamics—
the FMC for any dead fuel particle is greatly dependent on its past conditions. This
is called fuel moisture hysteresis in which today's fuel particle's FMC is dependent
on past FMCs. As a result, most empirical equations that simulate fuel moisture
include the previous days' FMC values (Viney 1991 ).
If there is rain, then free water on the particle surface can be absorbed by the
particle until either the fuel particle has reached the fiber saturation point (fully
saturated with water) or the precipitation stops and free water on the particle sur-
face evaporates. Water absorption into fuel particles is a slow process, especially
for large woody fuels, that depends on the many factors including initial moisture
content, particle condition (percent rot, amount fragmentation), and type of particle
(wood, grass, needle litter). As a result, the amount of rainfall may be unimport-
ant because, during heavy rainfall events, most of the precipitation may run off
or evaporate from the particle before being absorbed. Therefore, the duration of