Seasonal change is greatest in the area of blistered 
thin crust with polygonal ridging. In winter much 
of the area of covered by shallow standing water which 
by late spring exhibits a relatively featureless thin 
crust. As evaporation increases through the summer 
polygonal ridging and blistering develop. The areas 
of thick salt crusts show far less variation and the 
main differences relate to water on the surface in 
winter and the thrusting of the thick salt crusts 
during the summer. The area of aioun shows little 
variation as well and the circular patterns of ground- 
water associated with it (Jones & Millington, 1986) 
can be seen in both the wet and dry seasons. All 
three of the above areas however show one major 
change between the wet and dry seasons. In the 
winter wet season many of the areas exhibit standing 
water and upon drying out a relatively clean salt 
crust develops. However, in the summer, dry season 
much silty and sandy material is blown onto the 
salt crusts. This gives them a thin veneer of brown 
material and affects the amount of reflectance in 
all bands. The wind sculpted and hummocky facies 
show the least variation between seasons of all of 
the zones. The only noticeable difference which will 
affect reflectance patterns is the slight increase 
in surface moisture during the winter. 
Longer term changes can be identified from 
anniversary change detection images which compare 
images taken at similar times but in different years. 
Satellite orbits and problems of image acquisition 
means that it is almost impossible for imagery of 
the same day and week to be acquired. Imagery 
acquired during the same month is feasible especially 
in the dry season at the desert margin but it is more 
difficult to obtain in the wet season because of 
increased cloud cover. 
Changes detected on anniversary images should 
ideally indicate longer-term changes as well as areas 
of little change indicating areas of long-term 
stability. However there are two problems that need 
to be considered in interpretating geomorphological 
change in anniversary images in desert margin 
environments. Firstly, the erratic nature of wet 
season rainfall and runoff changes in fluvial and 
lacustrine geomorphological phenomena may be 
attributable to rainfall and runoff fluctuations 
within a season. Consequently the changes may be 
seasonal rather than long-term. Secondly, intrinsic 
geomorphological changes may be detected which 
although representing a long-term change in the 
system do not indicate long-term instability as would 
be the case with extrinsically-controlled changes. 
Examples of longer term changes on the Chotts el 
Djerid and el Fedjadj can be divided into two groups. 
Firstly, processes which operate each year creating 
an annual incremental adjustment of the system. 
These can be subdivided into chott marginal processes, 
strongly influenced by geomorphological and 
hydrological processes in the areas adjacent to the 
chotts, and processes operating in the centre of the 
chotts, which are mainly artefacts of internal 
adjustment of the water-salt-sediment balance. The 
second group of processes occur less frequently and 
fall into the high magnitude - low frequency category. 
These are most likely to be linked to catastrophic 
events such as storms with very long return periods 
and tectonic activity. Erosion and sedimentation 
are responsible for long-term geomorphological changes 
on the chott margin. Chott margin erosion occurs in 
two ways. Firstly, by cliff retreat at the junction 
of the playa and fan sediments (eg. the fan delta and 
Chott el Guettar) or at the junction of old remants 
or terraces of playa sediments and the contemporaneous 
playa surface. Secondly, between the bare playa 
surface and the vegetated chott margins where runoff 
from adjacent slopes erodes into the vegetated margins 
forming a series of runnels which enlarge into 
embayments. 
Other marginal facies are dominated by sedimentation 
related to either the influx of sheetwashed material 
from actively eroding adjacent areas or debris 
transported out of alluvial fans. The first 
situation is commonly found on the Chott el Fedjadj, 
and in places occurs in association with marginal 
erosion. Deposition of alluvial fan material is 
more commonly found in the southern Chott el Guettar 
and the northern Chott el Djerid (Fig. 4). 
Marginal processes operating each wet season, 
result in small rates of annual geomorphological 
change. Consequently, detection of the retreating 
vegetated-bare playa boundary and chott marginal 
cliffs is unlikely to be seen on remotely sensed 
imagery unless the interval between image acquisition 
is relatively large. 
Sedimentation from alluvial fans onto the chott is 
dependent on discharge events with greater return 
periods than sheetwash. Consequently, they fall into 
the high magnitude - low frequency event category 
and the resultant sedimentation can easily be seen 
on remotely sensed imagery (Fig. 4) 
In the more central facies of the larger chotts in 
Tunisia geomorphological redistribution of material 
on the chotts related to surface water movement and 
groundwater seepage can be identified on remotely 
sensed imagery. Examples can be seen in the Chotts 
el Fedjadj and el Djerid. (Fig 5& 6,) 
Winter runoff gathers on the Sebkhet el Hamma each 
wet season and then flows westwards into the Chott 
el Fedjadj along very low gradients. Well developed 
geomorphological features related to these flow 
patterns are found in areas where the playa is 
narrow. In Sebkhet el Hamma an inland delta to the 
north and a large, mobile sand body to the south 
constrict the playa (Fig. 5). Here it is about 2.5km 
wide and, for the most part, sparsely vegetated. 
The central Chott el Fedjadj is constricted by a 2m 
high cliff to the north and the gypsiferous Djebel 
Klikr, to the south. Here the playa is less 
than 2km wide and mainly unvegetated. Field 
observations made in May 1986 found evidence of 
recent water flow such as eroded vegetation hummocks, 
scoured channels and ripple marks. These flow 
features can be seen on the imagery (Fig 5) and 
represent zones of relatively fast water flow for a 
playa environment and are areas of actively eroding 
playa sediments. 
On the Chott el Djerid, flow- and splay-like 
features are evident in thin salt crust areas. The 
trends evident in the salt crusts suggest they were 
formed by salt-rich water flowing into a topographic 
depression or an area with a lower water table. In 
some of the flow lines, which are up to 0.5km wide, 
darker channels can be seen suggesting more than one 
flow event. Further evidence to support a multiple 
flow hypothesis can be seen at the splayed ends of 
the features where there is evidence of the uppermost 
crusts overlapping other crusts, or salt-rich zones, 
beneath them. 
These flow features start to the south of the 
embanked road (Fig. 6) and measurements of the water 
table carried out in 1984 showed that this embank 
ment had a marked affect on the water table up to 
2km away. It is likely that these features represent 
flow of surface water which has dammed up to the 
north of the road and then reappears after flowing 
under the embankment. It then continues to flow 
southwards to the depression or low water table area. 
5. CONCLUSIONS 
Evidence from southern Tunisia has shown that 
seasonal and long-term changes in playa geomorpho 
logical phenomena can be detected using digitally 
processed Landsat and SPOT imagery. However, when 
examining such changes it is important to establish 
a framework of the temporal and spatial dimensions 
of change for comparison with changes detected on 
different types of imagery. 
Three classes of geomorphological changes were 
identified with reference to satellite data - sub 
sampling unit changes, seasonal changes and long-term 
Table 1. G 
Sub-sampli 
Seasonal c 
surface 
surface 
surface 
vegetat 
aeolian 
Longer-ten 
in marg: 
- erosii 
- sedimi 
in cent: 
- flow : 
Figure 1. C 
produced bj 
1983 with £ 
indicated ( 
moisture ch 
areas of wi 
and from Gu 
moisture le 
discharge f 
from the sc 
changes. T 
on both MSS 
changes are 
coarser spa 
spectral re 
enable chan 
moisture le 
Whereas on i 
be seen are 
reflectance 
The impor 
ing sedimen 
in this stu< 
equally if i 
Furthermore, 
634