4.1. Relationships between RBR indices and fire severity — local empirical
models
The relationship between RBRs and field estimated fire severity was
assessed for each fire separately. For all fires a linear relationship
was observed between fire severity and RBR. Fig. 3 reports the trend
in RBRHV and RBRRFDI with fire severity under dry environmental conditions.
For the Kinglake fire individual trends for each forest type are
plotted. For unchanged areas, values around 1 (0 dB change) were
observed, as expected. With increasing fire severity RBRHV decreased
since vegetation scattering reduces due to combustion of scattering elements
(mostly leaves and branches and occasionally stems). For RBRHH
(not shown) a similar trendwas observed, while an opposite trendwas
observed for RBRRFDI. Increasing RBRRFDI with fire severitywas the result
of higher post-fire RFDI values. Higher post-fire RFDI values were explained
by a differentiated effect of fire severity on HH and respectively
HV polarizations, with HH backscatter decrease due to removal of scattering
elements being compensated by increased surface scattering.
Such an effect is not present for the volume dominated HV polarization,
which explains the increasing post-fire RFDI with fire severity. The RBR
dynamic range fromunburnt (unchanged) to high severity levels varied
across fires. For the RBRHV polarization the signal decreased by about
8 dB for the Zuera fire (dB values were obtained by multiplying the