1 Spatial Modelling of the Terrestrial Environment - Georeferencial

180 Spatial Modelling of the Terrestrial Environment
but through various empirical relations they can be related indirectly to observable fire
phenomena which themselves serve as indirect measures of intensity.”
Fire intensity was first defined by Byram (1959) as being the effective radiative temperature
of a fire front. Although Tangren (1976) and certain other physicists studying fire
disagree with the use of the term fire intensity, preferring the term prefer ‘fire power’, we
use the term intensity here since it is the most widely used in the literature. The most
commonly used measure of fire intensity is Byram’s (1959) fireline intensity:
I = HWR (1)
where, I is the fireline intensity (kW m −1 ), H is the heat yield of the fuel (kJ kg −1 ), W is
the mass of fuel consumed in the active flaming zone per unit area (kg m −2 ) and R is the
forward rate of spread of the fire (m s −1 ).
In quantitative terms, fireline intensity can be summarized as the heat release per unit
length at the fire front. In practical terms it requires estimation of the total fuel load (typically
partitioned into different particle size classes based on the time for each particle to reach
its equilibrium moisture content (Pyne, 1984) and the rate of (forward) spread of the
fire using model-based and field observations). It is very difficult (if not impossible) to
determine with any accuracy the amount of fuel consumed in the active flaming part of
the fire front under field conditions. Alexander (1982, p. 351) noted that ‘the amount of
fuel consumed by secondary combustion and residual burning after passage of the main
fire front will increase W and consequently result in an overestimation of fire intensity.’
Hence for accurate estimation of fire-line intensity, reductions in the measured value of W
are necessary if significant quantities of fuel are consumed subsequent to the passage of the
flaming front. Put simply, it may not be sufficient to simply compare pre-fire and post-fire
fuel loads.
As mentioned earlier, the heat yield parameter (H) is remarkably constant and in many
cases it is assumed static at 18 608 kJ kg −1 (Burgan and Rothermel, 1984). The value
is actually derived by measuring oxygen consumption under complete combustion of the
fuel in a cone or bomb calorimeter and published tables of H exist for many common fuel
types. However, these values are usually measured under conditions (oven-dried material
and complete combustion) seldom achieved in a wild-fire situation and thus adjustments
may be necessary. Two reductions are generally required, one for the presence of moisture
in the fuel, and another for incomplete combustion (Alexander, 1982). Reduction for latent
heat absorbed when the water of reaction is vaporized is 1263 kJ kg −1 (Byram, 1959),
with another reduction for FMC of 24 kJ kg −1 per FMC percentage point. This moisturerelated
correction to the theoretical heat yield results in the so-called low heat yield of
the fuel or the low heat of combustion parameter. Unfortunately the second correction,
for the (in)completeness of combustion, is extremely difficult to make due to the extreme
variable and difficulty in measuring this parameter.
The length scale for determining the local fireline intensity is provided by the size of the
coherent flame structure, measured normal to the fire edge (Cheney, 1990). The length of
the flame at the fire front has been correlated to a fractional power of the fireline intensity
by a number of authors (e.g. Byram, 1959; Nelson and Adkins, 1986; Cheney, 1990;
Fuller, 1991). Fireline intensities in excess of 20 000 kW m −1 have been estimated for
high-intensity, fast-burning fires in eucalyptus fuels (Bradstock et al., 1998); and also in