249
/
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i border
slope
r very
change
change
rate
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. May >
sry
, May <
iry
ctance for
surfaces
3r several
iand 7 (low
) a small
.n May. The
iootslopes,
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nge may be
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is in the
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, In other
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ifluence of
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opment and
:es between
very small
present in
istern part
of the playa border zone is somewhat more important
than more to the west, due to spatial differences
in rainfall or runoff. Within the footslope area
small differences in change of reflectance exist.
It is not clear wether these last mentioned
differences are a function of sealing, displacement
of sand sheets or grains, exposition or roughness.
A study to better atmospheric and reflectance
models will be a good way to reveal small
differences. The eventual effect of hygroscopic
salts is overruled by the effect of salt
efflorescence.
6. CONCLUSIONS AND DISCUSSION
With the aid of Thematic Mapper data of two
different days dynamic features were studied for an
area in Tunisia. An understanding of the spectral
reflectance is necessary for a good interpretation
of the changes. A number of main groups are
presented. For the multitemporal comparison the
data of reflectance have been made comparable. The
use of atmospheric models and models describing the
inflence of different solar zenith angles on
reflectance are described. These atmospheric models
were not used due to the limited knowledge of the
atmosphere and the wavelength dependancy of path
radiance and transmittance. General ways to
describe the influence of sun elevation on
reflectance are not used. A wavelength dependancy
may exist, and the solar zenith angle dependent
reflectance is probably also a function of the
roughness of the surface. Therefore with the aid of
a reference surface all spectral reflectance data
of May have been converted to values comparable
with January. In general the May reflectance for
the playa and playa border zone is above that of
January due to the effect of salt cover. This will
give a slightly higher reflectance, as is also
supported by field measurements. In most parts of
the footslope area only small changes in
reflectance occur. Parts show however a clear
inlfuence of healthy vegetation cover. Also the
influence of exposition can be read. Since
reference objects are used in stead of a physical
model, no exposition model was developed.
From the above it is clear that, comparing
Thematic Mapper data of two days, increase or
decrease in reflectance can be observed for a
change in salt cover, condition and coverage of
vegetation and relief. Other changes, for instance
due to hygroscopic salts, small displacements of
dunes or sealing of the surface remain within the
limits of uncertainty. If more detatiled
information is wanted, the research has to be
directed in several complementary ways. More
research has to be directed to atmospheric models
and especially to the wavelength dependancy of it,
while also measurements of the atmosphere in this
area have to be performed. Measurements of
reflectance in different parts of the reflective
part of the spectrum with different solar zenith
and observation angles will be useful. An other
important approach will be a range of field
experiments. An example is the application of
artificial rain to monitor changes in reflectance
in relation with changes of the surface. The
measurements have to be supported by laboratory
measurements. The selection of locations has not
only to be based on Thematic Mapper data. It is
important that the location is also a function of
the features of the surface and subsurface itself.
A physiographic and spatial statistical approach
are both important. The experiments and model
developments will lead to a better use of Thematic
Mapper for multitemporal analysis. But it is clear
that also with a simple approach, as described in
this paper, valuable information is gathered with
Thematic Mapper over dynamic processes and the
distribution of these processes over the area.
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