Satellite Observations of the Interplay between Wind and Water Processes in the Great Sahara Farouk ECBaz Abstract Although the Sahara is now hyperarid and is subjected to Images of the Great Sahara of North Africa obtained by the the action of strong winds from the north, geographical, geo- Advanced Very High Resolution Radiometer (AVHRR), manned logical, and archaeological evidence indicate that it hosted wet- missions, and Landsat show regional linear patterns of sand ter climates in the past (e.g., Said, 1997).Pre-historic archaeo- dunes resulting from wind action. These patterns trend logical sites are mainly concentrated along shores of palaeo- southward in the northern part and southwestward in the lakes. Surface water during wet climates must have been center of the desert. Radar data from the Spaceborne Imaging responsible for the erosion, transportation, and deposition of Radar [SIR-C) and Radarsat reveal sand- buried courses of sand in inland basins (El-Baz, 1998). It follows that these palaeo-channels that lead to depressions, which enclose major basins would have stored much of the water in the underlying sand accumulations. Interpretation of these data suggests that porous sandstone rocks and their fracture zones (Bisson and the sand originated in the southern part of the Sahara and El-Baz, 1991). During dry conditions that alternated with the was carried northward in river courses during past wet climates wet climate episodes, aeolian activities resulted in the forma- to be deposited within inland lakes. Prior to the onset of tion of a variety of sand dunes and sand sheets. dryness, and the resulting aeolian forms, much of the water The objective of this study was to investigate the causes of would have seeped into the substrate through primary (rock) localization of large accumulations of sand in the Great Sahara. and/or secondary (fault-induced) porosity to be stored as Interpretation of space-borne images of varying scales provided ground water. Therefore, the exploration for ground water in the necessary indicators of the relationship between sand the Sahara should consider depressions with large accumulations and fluvial processes. The latter would have accumulations of sand. been responsible for the concentration of ground water in the substrate. Therefore, the close relationship between the poten- tial of ground-water occurrence beneath depressions with large accumulations of sand is an important factor in the explo- The interplay between aeolian and fluvial processes in the ration for this increasingly scarce resource. Great Sahara of North Africa is revealed by the interpretation of satellite images. These include the regional views from the Image Dataset Advanced Very High Resolution Radiometer (AVHRR)instru- It was important to establish the effectiveness of the major sur- ments onboard the National Oceanic and Atmospheric Admin- face processes on a regional scale. Therefore, NOAA satellite istration (NOAA)satellites. Further details are provided by data (Table I) were studied to understand the regional setting of photographs obtained by astronauts on manned missions, the total area of the Great Sahara. The results were compared images of the Landsat Multi-spectral Scanner (MSS) and The- to, and were found to agree with, maps based on images of Met- matic Mapper (TM),as well as radar data from NASA'sSpace- eosat, the weather satellite of the European Space Agency, borne Imaging Radar (SIR) and Radarsat of the Canadian Space which is geostationary over North Africa. Agency. These data provide unique perspectives that allow the Emphasis in this study was placed on the Western Desert of recognition of regional influences on wind action and surface Egypt, where most of the field confirmations of satellite image features that result in ground water concentration. interpretations were made. Therefore, it was necessary to estab- The Great Sahara includes some of the driest regions on the lish the setting of large accumulation of sand in that desert. A Earth. In its eastern parts, the received solar radiation is capa- mosaic of Landsat Mss images (80-m resolution) provided a ble of evaporating over 200 times the amount of rainfall (Hen- base for this part of the study. Additional insights were ob- ning and Flohn, 1977). In this desert, precipitation is ex- tained by the study of color photographs obtained by the astro- tremely variable and unpredictable; in some parts it rains only nauts of the Gemini (Cortright, 1968) and Apollo-Soyuz (Gif- once in 20 to 50 years. This condition necessitates a complete ford et al., 1979)missions. dependence on ground-water resources for human consump- Furthermore, the high spatial resolution of Landsat TM tion and agricultural activities. images (30 m) allowed the interpretation of distribution pat- In the past three decades, ground-water levels in the oases terns of sand deposits and their relationships to topographic depressions throughout the desert have decreased and the features. These data also served as the base upon which to known resources are becoming scarce (El-Baz and El-Ashry, superimpose radar image strips, and to allow interpretation of 1991).Population growth and attendant food and fiber require- zones not covered by these strips (Seto et al., 1997). ments threaten to exacerbate the situation in the future. There- fore, the need exists to pursue innovative approaches for the location of additional ground-water resources for future use (El-Baz and El-Ashry, 1991). Photogrammetric Engineering & Remote Sensing Vol. 66, No. 6, June 2000, pp. 777-782. 0099-lll2/00/6606-777$3.00/0 Center for Remote Sensing, Boston University, Boston, MA O 2000 American Society for Photogrammetry 02215 ([email protected]). and Remote Sensing TABLE1. SOURCESOF DATA USED IN THE STUDY Global AVHRR 10-day composite data. Image processing con- sisted of a linear stretch combined with sharpening using a SystemIMission Dataset and Location Information high pass convolution. Apollo-Soyuz 1975 strip of color photographs of the West- Used SIR-cdata included 49 scenes covering southwestern Egypt ern Desert of Egypt by a bracket-mounted and northwestern Sudan. The cE0.S-~eadersoftware (obtained camera (ASTP-16-1246 to 1257) from NASA'sJet Propulsion Laboratory) was used to convert the Gemini 7 1965 color photographs of eastern Algeria by SIR-c data into displayable images. Their processing included a hand-held camera (particularly 65-HC- histogram equalization, Gaussian and lee filters, and edge filters. 2526) A Radarsat image of southeastern Libya was directly read from Landsat MSS 1972-1984 Multi-Spectral Scanner (MSS) a CD-ROM using PC1 EasiIpACE programs. The processing followed image mosaic of the Western Dessert of Egypt the same procedures as that of the SIR-c data. (33 scenes) Landsat TM 1986-1992 Thematic Mapper (TM) image mosaic (ten scenes) of southern Egypt and Observations and Discussion northern Sudan Prevailing Wind Direction NOAA satellite 1995 ten-day (01-10 October) composite Regional views of the Great Sahara indicate that surface winds AVHRR images; one kilometer resolution (p trend in an arcuate pattern that emanates from the direction of 127-b2) Radarsat 1998 Radarsat scene (47-139D; 16344.4332) the Mediterranean Sea. This is based on the fact that dune lines of area in southeast Libya centered at 22" indicate the direction of the prevailing wind. AVHRR data show 16'N; 24" 12'E that this pattern changes from southward in the northern part of SIR-C 1994: (a) Spaceborne Imaging Radar scenes the desert to westward along the borders with the Sahel (Figure (49) in ten strips within the coverage of the 1). Landsat TM mosaic listed above; (b) one strip This regional pattern was first suggested for the eastern of the Kufra Oasis region of southeast Libya Sahara based on field mapping by Bagnold (1941).It was also mapped in the rest of the Sahara from Meteosat data by Main- guet (1992; 1995) (Figure 2). Wind-produced erosional scars throughout the desert suggest that this regime was effective Radar data critical to this study were provided by the during much of the Pleistocene. Sand dune chains in the West- imaging radar instrument of the Space Shuttle, which was first ern Desert of Egypt prove the consistency of this regime, at least flown in November 1981 as SIR-A (Elachi et al., 1982). One during the past 5,000 years (El-Baz, 1998). image strip covered part of the eastern Sahara in northwestern The sand-carrying wind in this desert moves toward the Sudan. Among the revealed features in that strip were dry south during most of the year. However, the direction may be courses of wide channels of rivers and streams. One of these locally affected by topographic prominences (Mainguet, 1992). rivers measured up to 20 kilometers across; field work indi- Embabi (1982) measured the rate of motion of dunes in the cated that these courses were covered by up to five meters of Kharga depression of the Western Desert of Egypt to vary from dry, fine-grained, wind-blown sand (McCauley et al., 1982). 20 mlyr for the large barchans to 100 mlyr for the smallest (1-m Roth and Elachi (1975)had first theorized that radar can high) dunes. Seasonal winds from the south do occur, particu- reveal subsurface topography in arid lands and that longer larly in mid-Spring, but these are not significant transporters wavelength (L-band)images have the best subsurface imaging of sand. abilities. The theory gained widespread recognition after the Throughout the Sahara, as shown by the global 1-km DEM SIR-A mission. The SIR-B mission encountered problems, but the data and orbital photographs, sand accumulations occur renamed Spaceborne Imaging Radar (SIR-C)instrument, which within topographic depressions (e.g., Figure 3). This must be was flown on Shuttle missions in April and October 1994, pro- explained in any theory regarding the origin of the sand and vided other evidence of paleo-channels throughout the Sahara the evolution of the dune forms in space and time.