249 
/ 
i and 
i border 
slope 
r very 
change 
change 
rate 
je 
ig change 
. May > 
sry 
, May < 
iry 
ctance for 
surfaces 
3r several 
iand 7 (low 
) a small 
.n May. The 
iootslopes, 
e units is 
nge may be 
rences and 
>ther parts 
it. However 
is in the 
re used in 
phenomenon 
.ncrease in 
.ope. This 
comparison 
n which is 
etation is 
>and 4 and 
, In other 
vegetation 
ce. It has 
permanent 
% of the 
ion is a 
ardly less 
Dt be read 
reflectance 
a whole. 
i the playa 
limits of 
reference 
ifluence of 
reference 
ice are due 
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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