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

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IV - BURNED LAND MAPPING, FIRE SEVERITY DETERMINATION, AND VEGETATION RECOVERY ASSESSMENT
2 - Study area and datasets
Zuera study area is characterized by a mediterranean climate with continental
characteristics. The topography is moderate to steep, elevation ranging
from 500 m to 700 m above sea level on slopes up to 25°. The dominant
land cover is forest of Pinus halepensis L. interspersed with agricultural
fields. The fire was ignited on 6 th of June 2008 after a traffic accident
and burned around 2200 ha. The SAR dataset consisted of single (VV) and
dual polarization (HH&HV) images acquired by the Environmental Satellite
(ENVISAT) Advanced SAR (ASAR) and TerraSAR-X (TSX) sensors.
3 - Methods
Pre- and post-fire Landsat TM images were used to estimate burn severity
by means of dNBR. SAR data were absolutely calibrated, multi-looked and
geocoded to 25 m pixel using UTM projection. To correct the radiometric
distortions still present in the geocoded images topographic normalization
for the varying incidence angle from near to far range and the effective
pixel area was applied (Ulander, 1996).
Remotely sensed dNBR index was used as indicator of burn severity levels.
To minimize the effects of topography on the backscatter the analyses were
carried out by 5º groups of local incidence angle which provided the optimum
grouping interval (Tanase et al., 2009).
The study was carried out using a set of pseudo-plots generated based on
optical data. The similar trends and the high correlation between dNBR and
field estimates of burn severity suggested that the use of pseudo-plots
would result in consistent relations (Tanase et al., 2009). Pseudo-plots were
generated by averaging pixels of similar dNBRs and identical local incidence
angles (rounded to unity), by reclassifying the continuous dNBR interval (-
150 to 1050) into twenty-four classes. To avoid uncertainty related to size
only pseudo-plots containing nine pixels were allowed. From all generated
pseudo-plots 900 were randomly selected for the analyses.
4 - Results
4.1 - Backscatter properties of burned areas
Scatter plot of average backscatter with respect to burn severity (dNBR) is
presented in Figure 1 for pseudo-plots located on flat terrain. Figure 2 presents
the average backscatter levels for X-band (HH- and HV-polarization).
To provide a reference, the average backscatter of bare soils outside the fire
perimeter was computed for each polarization (horizontal lines).