Fundamentals - Wildland Fuel Fundamentals and Applications - page 24

quantity (loading), size, and shape of surface fuels in the following section by scale
and specific property. Arrangement, continuity, and pattern are discussed in detail in
Chap. 6. While some important fuel parameters listed in Table 2.2 refer to the prop-
erties of a fuel component, the component parameter is often computed as an aver-
age across particles within the component. Therefore, each fuel property is discussed
at the scale of measurement rather than the scale of model input.

2.3.1 Particle Properties

2.3.1.1 Particle Diameter ( d )

While fuel particle diameter ( d ) is a critical fuel parameter for fire behavior model-
ing, it is even more important in sampling for fuel loading ( W ). Thomas ( 1953 ),
for example, mentioned that the duration of burning is related to stick diameter
by approximately the 1.5 power. However, use of diameter in most fire and fuel
applications may be overgeneralized because nearly all fire behavior models as-
sume woody fuel particles are circular in cross section and use an assumption of
a cylinder to estimate volume for other fuel properties, such as SAVR (Eq. 2.13)
and density (Keane et al. 2012b ). Most woody fuel particles are not cylinders, but
rather, they are complicated volumes of highly variable cross sections and contorted
lengths. Moreover, particle diameters are not static; they change with weather con-
ditions, often becoming thicker when wet, and cracked when dry, making diameter
measurements difficult and further complicating the estimation of SAVR. Distri-
butions of diameters and lengths are also highly variable across woody particles.
The assumptions of circular cross sections and frustum volumes are necessary due
to current fire behavior modeling and fuel sampling limitations, but future efforts
should explore methods for estimating SAVR and particle volume by other means.
Diameter measurements are required for many fire modeling and fuel sampling
techniques (Chap. 8). Measuring particle diameter is relatively easy and is usually
often done with a ruler, caliper, or diameter tape. However, many have found that
these measurements are often too coarse for accurate fuel particle volume estima-
tion, especially for fine woody fuels, because of the large variation of diameters
across a fuel particle and the assumption that the particle is a cylinder or frustum
(Brown 1970a ). Using a single particle diameter often complicates efforts to evalu-
ate loading sampling method accuracy and precision because a major source of
uncontrolled error comes from the circular cross section assumption (Keane and
Gray 2013 ; Sikkink and Keane 2008 ).

2.3.1.2 SAVR

SAVR (m −1 ) is defined as the area of a particle surface (m −2 ) divided by the volume
of that particle (m −3 ), but it is often indirectly estimated from particle diameter ( d )
using Eq. 2.13. Particles that are thick, such as logs, have low SAVR values (less