1 Spatial Modelling of the Terrestrial Environment - Georeferencial

Fireline Intensity and Biomass Consumption in Wildland Fires 181
tropical savanna environments, where average intensities of c. 8000 kW m −1 have been
noted (Williams et al., 1999). However, these may be best regarded as maximum potential
estimates as they appear to (tacitly) assume complete combustion of the entire fuel load
and are dependent on the accuracy with which the rate of fire spread is either modelled
or measured. Byram (1959) believes that fireline intensities could realistically be expected
to vary from 15 to 100 000 (!) kW m −1 ; the primary uncertainties in their estimation
being the rate of spread of the flaming front, followed by the fuel consumption and finally
the estimation of the low heat of combustion. These huge extremes of fireline intensity
described by Byram (1959) have been documented in, for example, wildfires in the Canadian
forests (Alexander, 1982) and provide clear evidence as to the widely varying nature of this
parameter.
Given the potential importance of accurately estimating fire intensity for a range of
applications, and the inherent difficulties and uncertainties involved with estimating it in
the field, there is a pressing need for remote sensing methods for calculating this parameter.
In terms of the remotely sensed measurement of fire-related energy emission and fire
intensity, only the fraction of the liberated energy (I ) that is radiated away from the fire
is available to be detected by a remote sensing device. However, if this fraction is known,
because equation (1) relates the rate of heat release (kW m −1 ) to the rate at which biomass is
combusted by the fire (kg m −2 × m sec −1 ), there exists the possibility of estimating the rate
at which vegetation is consumed in the fire via observations of the radiant energy produced
(both measures being per unit length of the fire front in this case). Furthermore, since it is
expected that such biomass combustion estimates are likely to be strongly correlated to the
rate at which emissions of trace gases and aerosols are produced (Kaufman et al., 1996),
if sufficient radiant energy observations are available and can be integrated over the fire’s
lifetime, then in theory the total amount of radiative energy released can be calculated and
used to estimate the total amount of biomass consumed.
Perhaps surprisingly, estimation of biomass consumption (a first-order fire effect) has not
been a prime concern of researchers involved with modelling fire behaviour and dynamics
(Reinhardt et al., 2001), with much more effort placed on developing physical models of
fire spread and intensity (reviewed by Perry, 1998, and see Perry et al., 1999, for an example).
Initial attempts at measuring fireline intensity using remote sensing were made
via airborne sensors. For example, Budd et al. (1997) modelled head-fire intensity over a
series of experimental bushfires in Australian eucalyptus forests using airborne IR imagery
to measure the speed of fire propagation. They estimated that the head-fire intensity (averaged
over 6 minutes) exceeded 1000 kW per metre of fire front (kW m −1 ) for most of the
fires when they were at their most intense, and ranged as high as 3280 kW m −1 . Kaufman
et al. (1998a) used the Moderate Resolution Imaging Spectroradiometer (MODIS) Airborne
Simulator to measure the rate of release of thermal energy from spreading cerrado
fires in Brazil, and went on to successfully relate the integral of this parameter to the rate
of generation of the burn scar, which was believed proportional to the rate of biomass
consumption. The strong relationship between these parameters (r 2 = 0.97, n = 21) supported
the idea of using such radiative energy measures to directly estimate the amount of
biomass combusted and Kaufman et al. (1998a) found that the relationship between FRE
and the change in burn scar size was four times stronger than that between simple fire size
and the rate of change of the burn scar. This clearly illustrated the improvement provided
when the actual energy emission from the fires was considered.