Canopy Fuels - Wildland Fuel Fundamentals and Applications - page 65

specifically designed to estimate canopy fuel characteristics, computes both CBH
and CH by multiplying the maximum CBD by 0.1 up to the CBD threshold of
0.012 kg m −3 . Since CBH and CH can be computed from so many diverse methods,
it is important that those who use canopy fuel data to simulate fire behavior know
how CBH and CH were calculated.
There are many aspects of the two canopy height estimates that may compro-
mise their use in fire behavior analyses. First, regardless of what method is used to
estimate CBH and CH from the modeled CBD canopy profile, the resolution of that
estimate is governed by the width of the horizontal canopy layer used to summarize
CBH vertical distribution. This is somewhat of a double-edged sword in that thin
layer widths (high resolution) rarely match the resolution of the tree input data used
to calculate the CBD estimates, but, the thicker layer widths that better match the
scale of the data (> 1 m) may be too coarse to recognize subtle differences created
by fuel treatments. Input tree data often contain only heights of the tree and crown
base, and rarely have finer crown measurement resolutions. Second, surface fuels
may have more importance in crown fire initiation than low CBH values. A small
fire can ignite a canopy if there are sufficient ladder fuels at a tree-level, and a large
fire can ignite a canopy no matter the CBH . And lastly and most importantly, the
scale of CBH and CH measurements do not match the scale of the point-scale simu-
lation models that simulate these processes, similar to CBD . The coarse resolutions
of point-level, crown fire simulation in most fire behavior prediction systems is
often incompatible with the fine-scale spatial distribution of CBH and CH, and the
fine scales at which fire spreads. As a result, one can view CBH and CH as indices
rather than actual measurements, and it is often a common practice to adjust the
calculated or measured values to values that represent more realistic fire behavior.

4.3.4 Canopy Fuel Load

CFL (kg m −2 ) is used to estimate the amount of canopy material consumed in a
crown fire ( CFB in Eq. 4.7), and it is defined as the amount of burnable canopy fuel
per unit area. CFL is also quite useful in fire effects simulations, such as the esti-
mation of smoke emissions (Ward 1995 ), because it provides a somewhat accurate
representation of actual biomass consumed in a crown fire.
CFL isn't really related to any of the other three canopy characteristics
(Table 4.1 ). While an estimate of CFL could be made by multiplying CBD by the
difference between CH and CBH, this would assume an even distribution of CBH
across the canopy fuel profile similar to van Wagner ( 1977 ) assumptions, but this is
rarely observed as is evidenced by the profiles in Fig. 4.3 only a few methods cal-
culate CBD as an average across the canopy profile (Cruz et al. 2003 ). FuelCalc, for
example, estimates CBD as a maximum running average (Reinhardt et al. 2006a ).
Therefore, CFL needs to be estimated and mapped directly to ensure accurate and
consistent fire applications.