Fundamentals - Wildland Fuel Fundamentals and Applications - page 26

Sect. 2.3.1.1). However, there are great measurement errors when fuel particles
are small because of highly variable fuel dimensions and inaccurate measurement
procedures (Keane et al. 2012b ). The other technique involves dipping the particle
in liquid and calculating the displacement in volume or mass as mentioned above
for SAVR. This technique is more accurate, but there are several problems that
must be addressed to get more precise measurements. First, care must be taken to
ensure the particle does not absorb the liquid, and this is done in a number of ways,
including dipping the particle in wax or some other substance that prevents absorp-
tion or using a liquid that will not be readily absorbed by the particle. Displacement
by weight can be estimated if the specific gravity of the liquid is known, while
displacement of volume is somewhat problematic in that it is difficult to accurately
estimate displaced volume for small and large particles.
Particle density is another property that is difficult to measure because of its high
variability within a particle, across fuel types, and among fuelbeds. The density of
some particles, especially woody fuels, can vary substantially along the length of
the particle. Logs, for example, can be in various stages of decay along their lengths
because of their contact with the ground resulting in a wide variety of densities
within one particle. Most material in the litter fuel component often exists as foliar
material in various states of decomposition because of their position in the vertical
litter profile. And, similar to SAVR, particles are constantly changing in volume
in response to environmental conditions resulting in changes in density. And each
fuelbed results from a unique combination of disturbance history, vegetation devel-
opment, and moisture regime, all influencing particle densities.

2.3.1.4 FMC and Moisture of Extinction ( M x )

FMC is one of the most important and dynamic fuel properties, so it is discussed in
a separate chapter (Chap. 5) and will not be detailed here. Some refer to the mois-
ture level in wildland fuel as the fuel state or condition (DeBano et al. 1998 ), and
some refer to those fuels that can burn because they are dry enough as available fu-
els (Brown and Davis 1973 ). Fuel moisture provides the important link to estimate
fire danger (Deeming et al. 1977 ), and is perhaps one of the most critical inputs in
fire behavior prediction models (Andrews 1986 ; Table 2.2 ). Fuel moisture also is
important to many other ecological processes, such as decomposition, evapotrans-
piration, and nutrient cycling.
The moisture of extinction ( M x ) is the moisture content at which combustion can-
not be sustained (moisture above which fire does not burn; Rothermel 1972 ), and
greatly depends on the type, quantity, and arrangement of fuels and their interaction
with weather, mainly wind. Dead woody fuels are often assigned M x of 30 % while
M x s for live fuels are much harder to quantify. This property is actually a static pa-
rameter used in fire behavior modeling algorithms at the fuel component level (see
Table 2.2 ) to drive combustion to zero at high moisture contents (Rothermel 1972 ).
It would be difficult to estimate M x under field conditions because it would change
with ambient weather (e.g., temperature, humidity, incident radiation) and particle
qualities (e.g., rot, density, shape, size), and live fuel plant condition (e.g., phenology,