Fundamentals - Wildland Fuel Fundamentals and Applications - page 23

of the physical process of combustion are commonly empirical. This means that
most fire behavior modelers had to make broad assumptions of the combustion
process, specifically with respect to the physical description of fuels, and these as-
sumptions may be inappropriate for a given physical process or scale of application.
The fuel property surface-area-to-volume ratio (SAVR) is a good example. Finney
et al. ( 2013 ) show that SAVR may not be the principal factor governing boundary
layer thermal dynamics and vertical surface flow length may be more important,
yet SAVR is an important fuel property used to simulate fuel effects on thermal dy-
namics (Table 2.2 ). Therefore, many fuel properties and components were selected
because they best correlated to fire processes using limited empirical relationships
and it was assumed that they are representative of the causal mechanisms govern-
ing fire behavior everywhere. This results in an imperfect fit between the ecology
of fuels and the prediction of fire behavior, and it is the primary reason why the
study of fuels is so difficult. Fuel description and management will continue to be
difficult when fuels are described in the context of fire behavior without a theory of
fire behavior and without being fully integrated with ecology.
The above description of the representation of fuel in fire behavior modeling
is mostly limited to the US fire behavior prediction systems and is meant only to
generally describe those fuel properties that are commonly used in fire behavior
and effects simulation. The list of eleven variables (Table 2.1 ) is by no means
exclusive; there are other fire behavior models in the world that use additional
fuel-related variables in their structure (Sullivan 2009a , b ; Linn 1997 ; Parsons
et al. 2010 ). Moreover, there are many other fuel particle and fuelbed properties
that are important to the field of wildland fuel ecology, such as degree of rot,
particle length, and fuelbed cover, that are not discussed here. However, this list
(1) probably represents those fuel properties used across most of the world's fire
behavior modeling systems, (2) is perhaps the most important for the merging of
fire behavior with ecology, and (3) contains properties that can be measured by fire
behavior practitioners and wildland fuel managers. These properties are discussed
below fuelbed scale.

2.3 Surface Fuel Properties

Past fuel studies have identified the fundamental properties of fuels as quantity, size,
shape, arrangement, continuity, and pattern (Bebi et al. 2003 ; Ottmar et al. 2007 ),
but this classical list has many limitations. First, there are scale inconsistencies, in
that some properties refer to individual particles, while others refer to all particles
in fuel components, layers, and fuelbeds. Second, missing are some physical prop-
erties that describe the role of the fuel in the combustion process, especially in the
context of fire behavior (see Sect. 2.2). There is also a missing linkage between
many of these fundamental properties and how they are used to simulate fire or how
they are employed in fire and fuel management. For example, arrangement remains
unaddressed in point-scale fire models. And last, this list is missing critical metrics
and variables that can be used to quantify the properties. This chapter discusses the