Introduction - Wildland Fuel Fundamentals and Applications - page 8

and is usually described using different metrics, such as reaction intensity, fireline
intensity, and radiant energy (Keeley 2009 ). Fuel types that foster rapid fire spread
(fine, flashy fuels) are quite different from those that create hot, intense fires (logs,
deep duff, canopy foliage). Fire effects are the physical, biological, and ecological
impacts of fire on the environment; fire ecology is the study of those impacts on
living organisms and their environment; and the term fire severity is often used to
describe the magnitude of fire impacts on the ecosystem (Morgan et al. 2014 ). Fire
danger is a term used to describe the combination of both constant and variable
factors on fire behavior, such as fuels, weather, and topography, which affect the
initiation, spread, and difficulty of control of wildfires.
Wildland fires are usually divided into three types based on the fuel layer in
which they are burning. A crown fire is the combustion of the canopy fuels above
the surface fuel layer, and similarly, a surface fire is a fire burning the surface fu-
els (Chap. 2). A ground fire slowly burns the duff and soil organic matter through
smoldering combustion. Most wildland fires have all three of these fire types at the
same time.
Fires are also described in terms of their effects (Morgan et al. 2014 ). A nonlethal
surface fire burns in the surface fuel layer and usually doesn't kill the majority of
plants (< 20 % mortality), while a lethal surface fire results in high plant mortality. A
stand-replacement fire kills most plants, especially trees in the burned area (> 70 %
tree mortality; Agee 1993 ; Morgan et al. 2001 ). In mixed severity fires that contain
evidence of the gradient between the other two types of fires distributed across
space (Arno et al. 2000 ).

1.2.4 Modeling

Most fuelbed characteristics are described in terms of input requirements of the
fire models that predict fire behavior and effects, so it is important to know the
differences between model designs and approaches. This terminology is more de-
scriptive than categorical so it is possible that models can be described by com-
binations of these terms. An empirical model is one that is based on observation
and experiment and not on theory. Empiricism forms the basis for much of current
fire and fuels research and generally provides the reference against which theory
is tested (Sullivan 2009b ). Empirical models are often composed of statistical cor-
relations using data measured in the field or derived from laboratory experiments.
Some use the term phenomenological models when taking a statistical modeling
approach because they use information about how a system has typically behaved
in the past to develop predictive equations and algorithms; the outcome of the pro-
cess is predicted using surrogates for the causal mechanisms. Others use the term
statistical models to indicate that statistics were used to develop the empirical
model.
Theoretical models are generated from physical laws, such as those that govern
fluid mechanics, combustion and heat transfer. Validation of these kinds of models