7th Workshop on Forest Fire Management - EARSeL, European ...

3 - Results
Fuel moisture content estimation based on hyperspectral data for fire risk assessment 93
FMC was found to be lower in May than July for all the plots, with an average
of 75 % and 116% respectively. The largest temporal variation in FMC
values was for regenerated Calluna, where FMC increased from 66% to 136%
between May and July. The smallest variation in FMC was for building
Calluna, where values changed from 84% in May to 107% in July.
Statistically significant negative correlations were found between FMC and
spectral reflectance at 685nm, 689nm and 704nm when all plots were considered.
When considering only Calluna plots, there were significant negative
correlations with visible wavelengths and positive correlations in the
NIR between 728nm and 915nm. Over the SWIR there were significant negative
correlations between 1428nm and 1782nm and between 2034nm and
2337nm. The strongest correlation was in the visible region and the highest
value was in the blue region, where r = 0.8. When the first derivative
was considered, a greater number of wavelengths were significantly correlated
with FMC. The main areas of significant positive correlation were
between 678nm and 764nm and between 1782nm and 1807nm, and additionally
between 506nm and 538nm, and at 1416nm, 1668nm and 2375nm
for Calluna-only plots. The main areas of significant negative correlations
were at 594nm and 934nm and between 1081nm and 1182nm, and additionally
between 1258nm and 1315nm, and at 1454nm for the Calluna-only
plots. In terms of VIs, the strongest correlation was with the moisture
stress index (MSI= TM5/TM4) where (r= -0.98).
4 - Discussion and conclusions
FMC showed important variation with vegetation type and it exhibited
important temporal variations, where FMC values increased from May to
July. This may be due to the larger portion of green material in the canopy
in July compared to May. There was no significant correlation between FMC
and the spectral reflectance in the visible region with the exception of a
few wavelengths which had significant negative correlation with FMC. This
corresponds to the findings of Bowyer and Danson (2004), who showed that
FMC had no direct impact on spectral reflectance in the visible domain. This
is expected as the main parameter that controls the leaf spectral variation
for visible wavelengths is leaf chlorophyll content (Zhang et al., 2008).
However, when considering Calluna plots separately, FMC was correlated
with a large number of visible wavelengths, which may be due to the cocorrelation
between FMC and canopy chlorophyll content. This correlation
became stronger and positive for wavelengths in the NIR region, which may
be due to the co-correlation between FMC and leaf area index (LAI). Higher
LAI and biomass produces more scattering within and between leaves,
increasing the reflectance due to the secondary effects that FMC has on the
internal cell structure of the leaves (Liu et al., 2004). In the SWIR, the