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IANGE DETECTION 
playas. 
tudied using 
tions in this 
The Chotts el Djerid and el Fedjadj occur to the 
south of the Atlas Mountains in a zone of subsidence 
and they form part of a series of playas stretching 
from southern Tunisia to central Algeria . Chott el 
Djerid is the largest of these playas (5360km 2 ), it 
has an elongated north eastern arm, the Chott el 
Fedjadj, which continues eastwards into the Sebkhet el 
Hamma (770km 2 ). Their geomorphology and hydrology 
have been the subject of a number of investigations 
(Coque, 1962; Coque & Jauzien, 1967). 
Chott el Guettar (75km 2 in area) is situated in an 
enclosed basin about 20km SE of Gafsa. It is 
bounded to the north by the Djebel Orbata and the 
south by the Djebel Berba; both of these mountain 
ranges have active fans encroaching on the chott. 
Low ground and smaller mountains and hills are found 
to the east and west of the chott. 
4.2. Geomorphological changes on playas 
Geomorphological changes have been detected on these 
playas from satellite imagery and ground survey, 
and they can be attributed to the seasonality of 
the hydrological regimes, surface salt concentrations, 
interactions with adjacent landforms and aeolian 
activity. These factors support the existance of 
seasonal patterns of climatically-controlled 
geomorphological change. Other geomorphological 
phenomena respond to fluctuations with either 
shorter or longer periodicities than the seasonal 
patterns and are not necessarily related to climatic 
fluctuations. If the temporal and spatial parameters 
of the geomorphological changes on playas are combined 
they can then be classified into 3 groups (Table 1). 
This subdivision is of fundamental importance because 
it recognises that specific types of change can be 
observed with different types of satellite data and 
different intervals between image acquisition as 
distinct from those that can only be observed by other 
techniques. 
Seasonal changes are readily detected by change 
detection algorithms applied to satellite imagery 
from the different seasons. Imagery corresponding to 
wet and dry conditions have been compared for all 
three playas in this study and four types of 
geomorphological changes in parameters affecting the 
geomorphology of the playas have been identified - 
1. - surface moisture (standing water and subsurface 
moisture) 
2. - surface texture and composition 
3. - vegetation cover 
4. - aeolian activity 
Seasonal patterns related to surface moisture and 
vegetation are best illustrated by examining change 
detection images between winter 1983 and summer 1985 
TM imagery of the Chott el Guettar (Fig. 1). The 
greatest changes are those related to the fan delta 
at the end of the eastern fork of the Gafsa Fan and 
the adjacent chott. The fan delta is relatively 
well vegetated with Crassula spp, Limoniastium 
gyonianum and Limonium spp, and slopes very gently 
towards the chott without any obvious distal channel. 
Subdued sparsely vegetated topographic depressions 
act as surface water collecting areas which drain 
into rills, these feed into channels of about 2m 
depth and 50-100m width which drain into the chott. 
The individual channels and their restriction to the 
distal end of the fan delta can be clearly seen 
(Fig. 1) . The adjacent chott surface is moist throu^i- 
out the year and has a similar, but less dense, 
vegetation than fan delta. 
The change detection imagery (Fig. 1) indicates 
higher middle IR absorption on the fan in winter than 
summer. This can be explained by a combination of 
higher levels of soil and foliar moisture. The 
channels and adjacent chott areas are characterised 
by higher reflectance in the summer than the winter . 
The high summer middle IR absorption is related to 
higher soil moisture levels found in these areas, 
a response to the relatively high groundwater seepage 
through the Gafsa Fan due to irrigated agriculture. 
The higher winter reflectance values appear 
paradoxical considering the increased surface flow 
but are probably related to the fact that as flow is 
intermittent, water evaporates between flow events 
leaving surface salt effloresences. Combined field 
and satellite data suggests that surface water 
gathers on the fan delta and flows into the eastern 
part of the chott in winter and that in the summer 
the area is dominated by high levels of groundwater 
seepage. The area influenced by winter surface water 
and high summer moisture forms a roughly semi 
circular area adjacent to the fan with a lobe to the 
south. Similar patterns in winter surface water 
contributions have been seen on MSS change detection 
imagery. 
The other areas contributing surface water appear 
to be far less important. Increased winter surface 
or subsurface flow along the Oued el Rahr and Oued 
es Sedd systems to the east and northeast of the 
chott respectively is visible (Fig. 1). However, 
the chott area affected by water from the east is 
far less extensive than the west and according to 
the imagery is more variable in extent. Water 
flowing onto, or seeping through, the alluvial fans 
to the north shows little seasonal variation on the 
imagery. This is probably due to a lack of surface 
flow in the years examined, the dominance of a 
relatively constant groundwater seepage or the 
'filtering' effect of the El Guettar oasis. Most 
of the fans to the south of the chott also show a 
similar effect despite the lack of a similar 
vegetation 'filter'. However substantial changes in 
moisture levels in the south-east chott were seen on 
1981 change detection imagery (Fig. 2) which can 
clearly be linked to surface water flow in fan- 
channels . 
As there is little evidence of any kind of aeolian 
activity groundwater seepage and surface water control 
the transport of salts and sediments onto chott and 
their redistribution within the chott. The main 
seasonal changes in surface water and near-surface 
moisture are related to winter runoff from the eastern 
fan delta, and to a lesser, extent the channels 
draining Guettaria. A less frequent source of 
surface water is from the southern alluvial fans. 
Surface runoff acts as the main transport mechanism 
for sediment movement into the chott. This is 
particularly important in the areas adjacent to the 
southern fans. In addition there is an area of 
active gullying on fine-grained old chott sediments 
to the south-west of the chott. These gullys drain 
into the chott and act as a further sediment source 
area. They can be distinguished on the imagery as a 
cluster of small patches of high winter middle IR 
reflectance (Fig. 1). The sediment moving into the chott 
from the south forms a south-central depositional 
wedge which is recognisable in all change detection 
images as an area of lower summer Band 7 reflectance 
(Figs. 1 & 2). This is due to the increased water 
holding capacity of the sediments resulting in high 
soil and foliar moisture levels in winter. Salt 
transport, unlike sediment transport, is related to 
both surface water and groundwater fluxes and it is 
likely to mirror the hydrological regimes more 
strongly than the sedimentation patterns. 
Seasonal changes in surface texture and composition 
are most noticeable on imagery from the Chott el 
Djerid. These are particularly well developed in 
the north-central Chott el Djerid where the Fatnassa- 
Deajache road traverses it (Fig 3). Mitchell (1982) 
has divided the Chott el Djerid into 7 surface detail 
classes and 4 of these occur on the area of the chott 
traversed by the road:- 
1. Wind sculpted, hummocky terrain 
2. Thick salt crusts 
3. Aioun (standing water and salt efflorescences) 
4. Blistered thin crust with polygonal ridging