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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B4. Istanbul 2004
out. The net radiation is estimated from the DSR flux, the
surface albedo, the DLR and the up-welling long-wave
radiation flux. The sensible heat flux is retrieved from the LST
and the air temperature. This method can only be applied for
clear sky conditions. As the LST is not estimated for cloudy
pixels, the sensible heat flux and, consequently, the latent heat
flux cannot be calculated in this way. In that case, it is assumed
that the Bowen ratio (sensible heat flux versus latent heat flux)
is constant during the period of cloud cover, so it is the same as
on the last cloudfree day.
E] Water
33
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55 Relative
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piration from
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4.10 Burnt Areas
Monthly global maps of burnt areas for the period 1998-2003
have been produced in the frame of the ESA-DUP2
GLOBCARBON project. The sensors for burn scar detection
are ERS-2 / ATSR-2, ENVISAT / AATSR, and SPOT /
VEGETATION. ENVISAT / MERIS is used to add confidence
to the preliminary results.
Figure 16. Burn scars detected in Mongolia on 11/5/2000, left:
ATSR image (RED, NIR, SWIR) and right: burn
scar map.
After a fire, two main physical variations can be detected in the
remote sensing data over a vegetation layer. The first one is the
strong top of atmosphere reflectance decrease: burnt areas have
a lower reflectance in the near infrared channel than healthy
vegetation. The second one is the LST increase that occurs over
a burnt surface during day time, due to strong solar irradiance
absorption and to the absence of evapotranspiration that
normally transfers energy to the atmosphere in form of latent
heat through water vapour. The presence of ash and carbon
constitutes a dry layer that does not allow this cooling process,
Increasing the surface temperature by 7K.
789
4.11 Water Bodies
A method to map and monitor small ponds in arid regions using
SPOT-VEGETATION 10-daily composites was developed in
the frame of the GEOSUCCESS project. A small pond can be
defined as a surface ranging between 1 km and few tens of km?
of either free water or water with vegetation. Monitoring small
ponds or water bodies is important for economic activities and
is of great environmental value. The method was applied for
arid and semi-arid regions in northern Africa within
GEOSUCCESS. The aim is to extend these calculations for
CSP to the total African continent.
S. SCIENCE CHALLENGES
We conclude this paper by reviewing the present scientific
limitations and the proposed approaches in the science field
covered by CSP.
The key challenge today for vegetation and albedo parameters
is to move from single sensor retrieval approaches to
multisensor approaches. This is an important step forward in
order to assure time continuity of retrieved variables, as well as
to improve parameter accuracy using the spectral, directional,
time and space synergies between sensors. This trend has been
accounted for here, since the developments in GEOLAND /
CSP largely rely on the progress undertaken in the FPS /
CYCLOPES and ESA / GLOBCARBON projects, for which
the multisensor aspect has been very important.
Global fields of downwelling radiation, a key input parameter
for vegetation functioning, are not yet available. Their retrieval
in the long wave regime can now be envisaged using sounder
radiances (i.e. TOVS) complemented with radiative transfer
models. A multisensor approach with both polar and
geostationary sensors has been undertaken in the short-wave.
This should constitute a significant progress.
The land surface temperature, which plays a crucial role in the
surface — atmosphere energy exchange, is difficult to derive due
to inhomogeneities and rapid change in time. Advanced
numerical methods in combination with growing storage
capacities allow progress in this area. The provision of adequate
ground based measurements for validation purpose remains
however a challenging task.
For precipitation the major goal is to improve the satellite
estimates with bias-corrected gauge data. The latter is needed,
because of the under-catch of operational rain gauges (on the
order of 5-30 % on average). Currently no operational global
daily precipitation product, based on bias-corrected gauge
analysis, is available.
The accuracy of global soil moisture products is currently well
known only on a few regions. The scientific orientation is here
to reinforce validation activities and to intercompare two
different retrieval methods, based respectively on active and
passive microwave sensing. The activities in the passive
microwave represent an extension of progress made in the FP5
project ELDAS to the global scale.
The foreseeable evolution of the science field relevant to CSP is
to move towards a generalization of multisensor approaches,
possibly combining optical and microwave techniques. Some
new parameters, such as vegetation biomass, chlorophyll
concentration, or vegetation height should become
