1 Spatial Modelling of the Terrestrial Environment - Georeferencial

Fireline Intensity and Biomass Consumption in Wildland Fires 177
burnt area and the biomass actually combusted in the affected region is dependent upon
highly variable factors such as the combustion efficiency and biomass density, which are
very difficult to measure remotely to the required accuracy. Thus, Andrea and Merlet (2001)
demonstrate that whilst average ‘emission factors’ for many important pyrogenic species
are known to uncertainties of perhaps 20–30%, much larger uncertainties persist for regional
and global fire emissions because of the difficulty inherent in estimating the amount
of biomass combusted with the current EO approaches. For this reason it can be difficult
to settle disagreements where significant differences exist between EO-based emissions
estimates and data from non-EO sources (Barbosa et al., 1999; Andrea and Merlet, 2001).
In part to tackle this limitation, a recent development has been the suggestion that a
new generation of satellite-based infra-red (IR) imagers can accurately measure the rate
of emission of energy radiated by large fires, and that this may be well related to the rate
of combustion of the vegetation, which is itself directly proportional to the rate of release of
pollutant species (Kaufman et al., 1996; 1998a). Integrating the measurements of emitted
energy over time for individual fires should then provide information on the total amounts of
vegetation combusted and pollutants emitted. If correct, then this more direct remote sensing
method could be a candidate for the independent EO-based emissions–estimation route
called for by Andreae and Merlet (2001), which would greatly assist in helping solve the
disagreements that often arise between biomass consumption estimates derived via current
approaches. The purpose of this chapter is to review this new methodology, which Kaufman
et al. (1996) termed fire radiative energy (FRE), to indicate the theoretical framework in
which it has been derived and show how it relates to the understanding of the energy
liberated in free-burning vegetation fires as modelled by fire scientists. Finally, we present
some examples of FRE derivation from several new EO instruments and fire scenarios, and
indicate some of the characteristics of this new EO measure that will likely influence its
ability to contribute to fire science, including the estimation of biomass consumption and
the related pyrogenic pollutants and the modelling of fire propagation.
9.2 Pyrogenic Energy Emissions: Combustion and the Energy Flux
9.2.1 Combustion: The Physico-Chemical Reaction
When observing an active fire, the thermal energy measured by a remote sensing instrument
is a direct result of energy stored in the biomass being released as heat when the fuel
combines with oxygen. This process also liberates carbon dioxide, water vapour and small
amounts of other substances, the very emissions that scientists wish to quantify at global and
regional scales. The chemical equation for combustion can be illustrated in the complete
combustion of a simple sugar (e.g. D-glucose):
C 6 H 12 O 6 + 6O 2 −→ 6CO 2 + 6H 2 O + 1.28 × 10 6 kJ
The fuels burned in wildland fires are much more complex than the simple glucose described
above. Vegetative material is made up of cellulose, hemicelluloses (together 50–75% of
most dry-plant matter), lignin (15–25%), proteins, nucleic acids, amino acids and volatile
extractives (Lobert and Warnatz, 1993). Furthermore, the different components of the fuel
(e.g. dead leaf litter, dead wood and live foliage) contain energy stores in a variety of forms
(Chandler et al., 1983; Rothermel, 1972). However, in a wild fire combustion is never