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Identification of zones of mobile dunes in desert and sahelian areas, Draa Valley (Morocco) and Damagaram (Niger).

Marc Salmon(1) and André Ozer(1)

(1) Université de Liège, Allée du 6 Août, 2 - B11, B-4000 Liège, Belgium

Abstract

Remote sensing constitutes a foreground tool in the study of dynamic phenomena. As such, radar images have a specific interest because of their capability to highlight and measure even tiny changes between various situations. Thus, the displacement of sand grains making up dunes can be observed too. The present study consists in underscoring moving sandy zones located near agricultural and settlement sites in order to allow the installation of effective protections. Two sites are jointly studied: the palm groves in the Draa valley (Southern Morocco) at the border of Saharan desert and the basins of Damagaram (South Niger) within the sahelian area. The methodology consists in creating phase coherent images - i.e. image pairs, each pixel of which presents a phase difference proportional to the ground displacement between two radars acquisitions - and in cross-checking those with other data sources: satellite images and field observations. For this study, the Centre Spatial de Liege (Belgium) carried out all InSAR processings and the geocoding of the results. These are still partial; the study launched in the frame of the first ENVISAT call for proposals, has been spread out in two phases, an ERS-1/2 data analysis followed by a similar processing of ENVISAT images, when acquired. The Draa valley takes shape, downstream from the city of Ouarzazate, at the junction of the Dades and Ouarzazate wadis. In its middle portion, i.e. from Agdz to M’Hamid, the presence of ordovicians schists allowed the development of wide depressions surrounded by sandstone ridges. These last protect six palm groves separated by transverse valleys, named “cluses” or “foums”: Mezguita, Tinzouline, Ternata, Fezouata, Ktaoua and M’Hamid. Rainfall decreases from 300 down to 40 mm per annum from upstream to downstream and presents very strong variations from year to year. Besides, in these areas, irrigation is essential and required the building of the Manssour-Edhabhi dam in order to provide more regularly water. However, the last oasis only seldom receives water. Moreover, next to this water diet, the vegetation is threatened by anthropic degradations, such as trampling and grazing. Palm trees are also weakened by the bayoud - a mushroom which attacks their roots. These phenomena lead to an invasion by sand dunes, in particular around the palm grove of M’Hamid. The radar data, i.e. a tandem pair of ERS-1/2 images dating from 18 and 19 May 1996, allowed the localization of the dunes showing a shift. The image processing consisted in generating intensity images and one coherence image. To facilitate their analysis and interpretation, a coloured composition has been created with the results. This composition is obtained by adding the coherence image in red, the average of both images phase intensities in green and the difference between these phase intensities in blue. As such, a high coherence level (value close to 1) appears in red and indicates a very good phase correlation between the two images – a consequence of the various mechanisms of radar signal scattering and of the lack of change which could affect the target. On the contrary, a weak coherence level (value tending to 0) appears in black and indicates a movement in these zones during the 24 hours interval which separated the two image shots. Lastly, the white spots correspond to areas with a strong radar signal backscattering i.e. with zones of hills or vegetation. The confrontation of the results with optical data, in particular with an IKONOS image dated 29 October 2001, confirmed and refined the observations. In the vicinity of the oasis, the moving sandy zones correspond to mobile dunes. In conclusion, by this methodology, the moving sandy zones could be highlighted and correspond each time to active dunes This methodology was also applied in the sahelian area of Damagaram (Niger). This zone is characterized by precipitations of 300 mm per annum and is primarily covered by dunes fixed by the vegetation. However this vegetation gradually degraded and the dunes started moving again under the combined effects of the periods of drought (since the Sixties) and the anthropic action. These processes lead to a remobilisation of sand and the silting-up of the multiple, more or less circular, basins the aera is dotted with. These lasts are evidence of an ancient network of temporary rivers active during wet periods from Quaternary and constitute the main source of production and agricultural diversification in these areas with reduced potentialities. The procedure tested in Morocco has also been applied on a larger scale within this second area in order to highlight moving dunes. Therefore, four tandem pairs of ERS-1/2 images have been processed: two pairs dating from April 9 and 10, 1996 and two from April 25 and 26, 1996. However, as the environmental conditions, in particular the moisture and vegetation cover conditions, are not identical; the analysis of the results is less obvious. The moving dunes have in particular more reduced extensions.

 

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry