沙漠研究 19-1, 339- 342 (2009 )Journal of Arid Land Studies“Desert Technology IX ”
Refereed Paper

The Use of Remote Sensing and GIS to Map Unstable Sand dunes-A Case Study in the Oudia Area of TunisiaMongi BEN ZAIED1, Mohamed Osman ARNOUS2, Mohamed Ahmed SIDI CHEIKH3, Riadh ALIOUT4,Rabah HADJ ALI4, Yassine GAROUNI4, Saleh SID4, Francisco MUNTONI5
Abstract: Desertification constitutes a major concern of countries in the Sahelian region ofNorth Africa. This phenomenon is responsible for the degradation of the natural habitat and forthe arable land disappearance. With the development of remote sensing techniques, it became
possible to study this phenomenon at spatial and temporal scales and to analyze the interaction
between the various elements of the environment in relation to soil dynamics and humanactivity. This study was an attempt to use high-resolution satellite data, specifically Quick Bird,
to thematically map the geomorphology of the study area, particularly active dunes. Visualinterpretation combined with supervised classification using training site tests allowed us toperform a precise identification and led us to an accurate mapping desertification sensibility. Asa result, it appears that Oudia may be threatened by the encroachment of sand dunes andsignificant wind activity. In conclusion this study strongly recommends the use of these newtechniques of remote sensing in assessment and mapping of desertification.
Keywords: Desertification, Quick Bird, Remote sensing, Tozeur

1 Introduction
Mapping of desertification serves to define the current limitsof zones affected by the desertification and as source data to thedecision-makers for the implementation of means to effectively
fight against desertification.
The south of Tunisia, particularly the Chott El-Garssadepression, is strongly threatened by desertification amplified bypopulation activities and overgrazing. Presently, the area of thezones affected by the desertification is in progression. In fact,many factors of desertification (Wind, Rain, overgrazing, types ofsols, etc.) are implicated in the increase of the infected area. The
study area, Oudia, in spite of the apparent state of stable sand dunes,
is threatened by the formation of barkhanic dunes that constitute themost mobile shape of dunes (Ben Zaied et al., 2006). Our approachwas to use Quick Bird satellite data to map the landscapes’
geomorphology, plant formations, and use GIS to predictsusceptibility to desertification.
2. Materials and Methods
2.1. Study area
The study area represented in Figure 1 is located in theSouthwest of Tunisia at Latitude and Longitude. It is about 400 km
from the capital Tunis. It is a depression that is surrounded byimportant geological formations (Quaternary formations). The
annual precipitation ranges between 50 mm to 100 mm (Talbi et al.,2006). The mean annual temperature is about 22 ˚C (Talbi et al.,2006). The region has high evapotranspiration that is combined
1Fig.1. Location of study area
(Ben Zaied, 2007).

Institut des Régions Arides de Medenine, Tunisie. E-mail: benzaied_m@yahoo.fr
Suez Canal University, Egypt. E-mail: m.arnous@scuegypt.edu.eg
3
Parc National du Banc d’Arguin, Mouritanie. E-mail: ouldsidicheikh@yahoo.fr
4
Ministère de Agriculture Algerie Ministre. E-mail: Aliouat_riadh@yahoo.fr
5
Laboratory TELEGIS,University Cagliari, Italy. E-mail: fdessi@unica.it
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with low precipitation causing a strong annual deficit in the waterbalance. The zone of study is subjected to important flows ofwind drained by a vast network of wind corridors through abordering west-east axis mountain chain. Need the soil type,vegetation, and geology. The zone of study is characterized byvery extended and localized soils Types (Sand, silt and limon).
The scarcity of soils in this region finds its explanation in therevealed aridity, the very scattered plant cover, moreover to theinfluence on the quality of the soils by the depressions closed
during the Quaternary (Talbi et al., 2006). The study area isdistinguished by sparse low vegetation, mainly tamarix. (BenZaied et al., 2006). The study area is used mainly for camelgrazing.
2.2. Methods of Data Collection
2.2.1 Remote sensing data resources
The data used for this study are collected from:- A scene of Quickbird with panchromatic and multispectral
layers acquired in February 2006 (panchromatic with 60cm
resolution and multispectral of 4 m resolution containing 4bands)A.
- A Landsat image ETM+ with 8 bands (panchromatic with15m of resolution)B.
2.2.2. Field Work
Prior to field work we produced a survey map of the area
using a Landsat image where we added the main localities (Fig. 2).
Also produced a via interpretation land cover map that allowed
comparison between the reality of ground and the satellite images.
During a period of three days, fieldwork consisted of using car
4×4, a Global Positioning System (GPS) and a digital camera to
identify16 representative training sites of Sabkha, sand dune,
barkhan and salt crust on the satellite imagery based on texture,pattern and tone (Fig. 3).
2.2.3. Data processing
We used ERDAS Imagine (Version 8.6) for thematic land
classification (ERDAS Imagine Manuel). We carried out texture
analysis/radiometric enhancement of contrast and unsupervisedand supervised classification (the maximum likelihood method
and visual interpretation of different classes of imagery withinlandscape). The validity of the training data was evaluated bothfrom visual examination and from quantitative characterization.
The spectral signatures of the main features such as sand dune,
sabkha and barkhan were evaluated. The dynamic vision of thedesertification process is important for the identification of thezones already affected as those which in the future risk being it.

Fig. 2. Survey map of the study area.

Fig. 3. Profile path and stations.

3. Results and discussion
3.1. Geomorphology map
The multiple projects that were established to fight against
Fig. 4. Geomorphology map.
the sand encroachment also contributed to landscape change (BenZaied et al., 2006).
El-Oudia region corresponds to the eastern part of chott El-Gharssa altitude between -5 m and -25 m.

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These areas made decisions-makers about a channel as Suezfrom Gabès to the Algerian Sahara. In this area we have a lotof salt, so this means that the sea was be there. It is located in
the catchment area of Wadi El-Maleh, this unit is characterized
by a wind action.
Therefore many deposits come from the deposit ofphosphate factory of Gafsa in upstream. The Figure 4
represent the different morphology forms, such us: Sand dunes,
Barkhans, Sabkhas Gypsum and Phostphates, Highly Saline
Sabkhas, Moderately Thickness Sabkhas, Silty Sand sabkha,
Dry Sabkha and wet Moderately thikness sabkha
3.2. Land Cover map
Land cover map is one of key parameters inenvironmental studies.
The study area is distinguished by the weak depth andFig. 5. The land cover map.
weak content in organic matter. It is dominated by a pseudo
steppe with psamophiles and gypsophiles species,
characterized by three states (50 to 200cm, 15 to 50cm and 0 to 20cm). This steppe assures a soil coveringof about 20% in the best situations. But we can generally distinguish three plant groupings, in relation withthe type of soil:• Grouping on the salty formations: Haloconemum Strobiloceum, Arthrocnemum indicum, Salsola Siberico,
Salsola tetrandra, Sueda mollis;• Grouping on the sandy formations: (Stipagrostis pungens), Retama raetam, Calligomum spp., Aristidapungens is indicating of mobile sand. The presence of this species indicates that sand moves.
• Grouping on the gypseous formations: Astragalus armatus, Zygophyllum album, Erodium glaucophyllum.
The offered pasture is in poor because of the over grazing and constitutes only a summer supplementin particular for the dromedary. In Figure 5, we can see the different classes of soils and their use forgrazing. Land use types mapped in this study are: Active sand dunes, Hazardous sabkhas, Moderatelysabkhas, Low sabkhas, and Sand dunes.
3.3. Vulnerability map to the desertification
The conservation of natural resources andthe protection of the public infrastructures
constitute the best means of fighting against thedesertification. The objective to establish thedesertification sensitivity map is to provide
support for the local decision-makers for better
understanding the processes in progress andplanning the interventions of attenuation of thedesertification and direct the follow-up of theirefficiency. The establishment of this map was animportant step in this work. By visual photointerpretation,desertificationunitswererecognized. The personal knowledge of the sameland use, similar physical conditions (climate, soil,vegetation) and common characteristics of landdegradation (types and class of desertification)
are very important to achieve this work.
Adopted methodologies are based on theanalysis of the dynamics of the landscape. The
Figure 6 which represent the result of images

Fig. 6. The sand cover map.

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processings and the field work shows the vulnerabily map of desertification. This map represents the resultof superposition of many information layers (Land use, Geeomorphology, climat data).
The wind represents the primary mechanical agent of desertification. It was the cause of sand deposits
were made in dunes. In the process of elaboration of desertification sensitivity map, the sand chart
constitutes an important element. This map elaborated from Quick Bird data shows the big types of dunes
which are discriminated in step with their reflectance: The final result was an image map of vulnerability ofdesertification, which gives good indication of areas affected by desertification and those under risk ofdesertification. The areas were divided into seven classes including high, Moderate and low hazardous:
Active sand dunes, moderate sand dunes, low sand dunes, highly hazardous sabkhas, moderately hazardous
sabkhas, low hazardous sabkha and flash flood. On the ground, we saw fences, and lineaments which areused for the fight against the desertification. But the barriers were built in perpendicular with the dominantwind direction. In this zone, there are two dominant sectors of the wind: the West during the winter andthe east during summer but 65 % of wind come from the east. It appears that the fences amplify thetrapping of a lot of sand. The question is if these fences embarrass the existing plantation? We have tostudy the zone between two fences and verify if the sand is going to become a plateau of sand. In ouropinion, it is necessary to define the situation before to know if the intervention was justified or not? To
look for the alternative, it is necessary to make a comparison between the natural sector and theanthropological sector.
4. Conclusion
This paper shows the application of remote sensing to map land cover (vegetation) andgeomorphology in an arid region. Application of the remote sensing and geographic information systems
constitute good tools for investigation to help decision-makers fight against desertification and provide
reasoned management of natural resources. Reliable and constantly updated information constitutes a needand a guarantee of sustainable development of natural resources. These techniques allow us to measure theimpact of the phenomenon on the people as well as on the natural environment. As regards the elaborationof the Vulnerability map to the desertification we insist on the need of harmonizing and to standardize theapproaches and the methods used everywhere in Africa.
It is necessary to revise the current models concerning the physical, ecological and socioeconomic
aspects of the degradation of lands, for their integration in a framework of dynamic modelling andinstruments of support for the decision. The visual interpretation combined with the digital processing of
the satellite data with high resolution Quick bird and Landsat permitted to classify the territory studied in
its various components on the geomorphological level.
This study shows that the use of the satellite data with high resolution allows the elaboration of a
cartography which reports the state of degradation of the ground under the influence of the desertification .
These documents are essential for the rehabilitation of the sector Oudia, today strongly threatened by thedesertification.
AnnotationsA) Laboratorio TELEGIS, Dipartimento Scienze della Terra, Università di CagliariB) Global Land Cover Facility: ftp://ftp.glcf.umiacs.umd.edu/glcf/Mosaic_Landsat/N-32/N-32-30.ETMEarthSat-MrSID/
References
Ben Zaied M. (2007): Carte de la Tunisie Laboratoire d’Erémologie et lutte contre la desertification, unite de Télédétection et de SIGde l’IRA.
Ben Zaied M., Bedoui M., Ferchichi A. (2006): Plan d’Action Régional de la Lutte Contre la Désertification (PARLCD) dugouvernorat de Tozeur, Aout 2006, 100p.
Talbi M., Gasmi N., Marini A. (2006): Présentation générale de la région de Tozeur (Le Jerid), Cartographie des paysages arides
pour la lutte contre la désertification, Tozeur, Tunisie. 9-19 décembre 2006, p. 28