Fundamentals - Wildland Fuel Fundamentals and Applications - page 30

components comprising a fuelbed have different scales of distribution (Chap. 6),
so large areas are rarely needed to visually estimate the loading of fine fuels. Many
studies now use SI units of kg m −2 for most fuel components (Keane and Dickinson
2007 ) because they are more easily visualized (it's easier to imagine a kilogram or
2.2 pounds of fuel over a square meter of ground than a ton of fuel over an acre).
However, these units may be inappropriate for CWD and canopy fuels. Surface and
canopy fuel loading measurement techniques are discussed in detail in Chap. 8.
Fuel loading is the primary fuel property discussed in this topic because it is
used extensively in wildland fire management for many purposes. The calculation
of fire intensity, for example, demands an estimate of fuel loading (Eq. 2.4), and fire
intensity is perhaps one of the most important fire behavior characteristics for esti-
mating fire effects (Reinhardt et al. 2001 ). Loading is also used to estimate smoke
emissions which directly affect human health and wellness. Loading often corre-
lates to both vertical and horizontal fuel connectivity; undisturbed fuelbeds with
high loadings are more likely to have greater canopy fuels and are more likely to
be connected to fuelbeds with high loadings. Loading is also important for issues
outside of fire science, such as habitat for small mammals, site productivity, carbon
dynamics, and soil erosion.

2.3.2.2 Bulk Density ( ρ b )

The bulk density of a fuel component is the mass of the fuel component material
divided by the volume of space within which it resides (Fig. 2.4a ). Bulk density is
different from wood or particle density (specific gravity) in that the volume includes
the empty space between fuel component particles. Bulk density is often used to
represent fuel arrangement in vertical dimensions; canopy profiles, for example,
display the vertical distribution of canopy bulk density for crown fire modeling
(Bebi et al. 2003 ). Past studies often used bulk density to represent fuel porosity
(Countryman 1969 ).
Bulk density has a number of uses in fire management. First, it is an input to
some important fire modeling programs (see Table 2.2 ); canopy bulk density is
used in FARSITE (Finney 1998 ) to simulate crown fire propagation (Chap. 4) and
fuelbed bulk density ( ρ b ) is used to simulate surface fire intensity (Eqs. 2.9, 2.13).
Bulk density can also be used to describe the rate at which heat can travel through
a surface fuel layer. Another common application is in calculating loading for those
fuel components that are difficult to sample. Duff, litter, shrub, herb, and tree re-
generation fuel component loadings, for example, are difficult to measure opera-
tionally, so many fire specialists use the volume method to approximate loading.
In this method, the depth of a fuel component is visually estimated or measured
as an integrated average across an area, and multiplying this depth by the area of
consideration gives the volume which the component occupies. The loading of that
component can then be estimated by multiplying volume by bulk density (details
are given in Chap. 8). The problem with calculating loading this way is deciding
the scale at which to measure loading. Should volume be calculated for the entire