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Table 1. Geomorphological change detection categories on Tunisian chotts using remotely sensed imagery 
Sub-sampling unit changes (cannot be detected on imagery as less than spatial and/or temporal resolution) 
Seasonal changes (can be detected on multidate imagery corresponding to different sensors) 
surface moisture 
surface texture 
surface chemistry 
vegetation cover 
aeolian activity 
Longer-term changes (can be detected on anniversary imagery) 
in marginal facies 
- erosion 
- sedimentation 
in central facies 
- flow features 
Figure 1. Change detection image of Chott el Guettar 
produced by ratioing TM Band 7 imagery of January 
1983 with September 1985. El Guettar oasis is 
indicated (EG). The areas of highest surface 
moisture change can be readily identified - the 
areas of winter surface runoff on the fan delta (a) 
and from Guettaria (b) and the area of high summer 
moisture levels on the chott related to groundwater 
discharge from the Gafsa Fan (c). The gullied areas 
from the southwest can also be seen (d). 
changes. The latter two types of changes can be seen 
on both MSS and TM imagery. This suggests that the 
changes are large enough to be detected even with 
coarser spatial resolution MSS imagery. However the 
spectral resolving power provided by TM imagery does 
enable changes in salt and sediment patterns and 
moisture levels to be more readily recognised. 
Whereas on MSS imagery many of the changes that can 
be seen are related to large variations in overall 
reflectance and vegetation. 
The important role of surface water in redistribut 
ing sediment and salts on Tunisian playas is evident 
in this study. In many facies it appears to be 
equally if not more important than groundwater. 
Furthermore,interactions with adjacent landforms and 
Figure 2. A third principal component image of Chott 
El Guettar MSS data from the wet and dry seasons of 
1981. Areas with high levels of moisture change can 
be seen in one of the fans to the south of the chott 
and south-central chott. This area of change is 
related to a runoff event on the fans which also 
eroded sediments from the fans and deposited them on 
the chott. 
the role of aeolian activity are also locally 
important. The extent and magnitude of the 
contribution of these factors in the geomorphological 
evolution of playas needs to be monitored using 
remotely-sensed data backed up by ground observations 
wherever possible. 
ACKNOWLEDGEMENTS 
This work has been carried out as part of a NASA 
Principal Investigatorship under the Scientific 
Applications of TM Program to two of the authors 
(ACM & JRGT). Part of the work was also carried 
out as part of a NERC Studentship (GT4/83/GS/87) to 
another of the authors (ARJ). 
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