Farouk El Baz* explains how radar images indicating accumulated groundwater in Northern Darfur are helping resolve a humanitarian crisis The Great Sahara of North Africa constitutes the largest desert belt on earth, extending for nearly 6,000 kilometres from east to west. Its eastern part includes some of the driest regions of the planet, where the received solar radiation is capable of evaporating 200-times the amount of rainfall it receives. This hyper-arid condition necessitates complete dependence on groundwater resources for human consumption and agricultural activities. Increase of populations and the attendant food and fibre requirements have exasperated the situation. Severe droughts over the past two decades initiated years of unrest and a vicious war in the Darfur province of northwestern Sudan. A recent report by the United Nations Environment Program (UNEP) indicates that water shortages underlie the conflict between Darfur's sedentary and nomadic populations. Thus, there is a need to develop new and innovative techniques to locate additional water resources to satisfy urgent needs. Satellite images represent excellent tools for this activity and have been used to analyse the regional setting of parts of Egypt and Sudan in the search for groundwater resources. These include Landsat images that display surface features, radar data that penetrates sand cover to reveal underlying topography, and Shuttle Radar Topography Mission (SRTM) data that allows three- dimensional viewing. Although the Sahara is now dry and is subject to the action of strong winds from the north, geological and archaeological data indicate that it hosted much wetter climates in the past. Surface water during past moist climates led to the formation of lakes in topographic basins. These basins would have stored much of the water in the underlying porous sandstone rocks. When the climate dried up, the wind resulted in the formation of sand dunes and sand sheets. The wind regime in the eastern Sahara traces a pattern that emanates from the coastal zone of the Mediterranean Sea. This pattern changes from southward in the north to southwestward along the borders with the Sahel. Weather satellite images such as those of Meteosat of the European Space Agency (ESA) helped greatly in deciphering the details of this regional pattern. Erosion scars throughout the desert suggest that this wind regime was effective during much of the last one million years. Another important observation is that sand accumulations in the eastern Sahara occur within or near topographic depressions. This must be explained in any theory regarding the origin of the sand and the evolution of dune forms in space and time. A notable second observation is that dune sand is composed mostly of well- rounded quartz grains. Radiocarbon dating and geo-archaeological investigations show that the eastern Sahara experienced a period of greater effective moisture 10,000 to 5,000 years ago (10-5 ka). Uranium-series technique was used to date lake carbonates from the western Desert of Egypt and Northern Darfur in Sudan. Results indicate that five palaeo-lake forming episodes occurred since about 350,000 years. These wet episodes correlate with major interglacial stages. This information suggests that groundwater resources may be inferred from large accumulations of sand during moist climate episodes. This involves the down-gradient transport of sand grains toward low areas, where sand was deposited in horizontal laminae. As dry climates set in, the wind mobilised the sand and shaped it into various dune forms. This implies that sand was born by water and sculpted by the wind. Development of this theory was based on the analysis of satellite images of the eastern Sahara. These data are easily grouped in a Geographic Information System (GIS) to allow superposition and correlation of notable features. This combination results in an ability to suggest a sequence of events in space and time that would explain present features. Southwest Egypt and Northern Darfur Two cases to be considered include one in southwest Egypt and another in Northern Darfur. In southwest Egypt, a 300 kilometre flat, sand-covered area straddles the border between southwest Egypt and northwest Sudan. This region is called the Great Salima Sand Sheet, after the Salima Oasis on its eastern border. Morphologically, the area is a depressed basin, covered by sand deposits with a few exposures of solid rocks. Radar images revealed the courses of five rivers and streams leading to it from highlands to the west and southwest. I interpreted this setting to indicate groundwater accumulation in its eastern, lower-most area. Hence, the government of Egypt in 1995 drilled exploratory wells. The wells were monitored for five years to assure the presence of large amounts of groundwater. Starting in 2000, plots of 10,000 acres were offered for agricultural development by the private sector in Egypt. Today, agricultural farms thrive where wheat, chickpeas, peanuts and other crops are profitably raised. The groundwater is pervasive in the underlying porous sandstone. Its salinity is only 200 ppm, which makes it sweeter than Nile River water. The proven resources in the drilled area are enough to support agriculture in 150,000 acres for 100 years. Similarly, interpretations of the space-borne data relating to Darfur suggest that water remained in a lake-like expanse of 30,750 kilometres square for long periods of time. This is indicated by horizontally layered sediments at the highest level attained by the lake water: 573 cubic metres above sea-level. Based on the SRTM data, the volume of water in that former lake would have been approximately 2,530 kilometres square. During the residence time of the water in the Northern Darfur depression, for thousands of years before the lake dried up, much of it would have seeped into the substrate. This seepage would occur through the primary porosity of the underlying sandstone or secondary porosity caused by fractures in the rock. Upon completion of mapping of the lake boundaries by the space data, I thought it was important to convey this to the government of Sudan. Thus, I briefed President Omar Al-Bashir in the presence of Kamal Ali, minister of irrigation and water resources. President Al-Bashir recognised the potential of the find and adopted the initiative of "1,000 Wells for Darfur." Upon hearing the news, Osman Kebir, governor of North Darfur, declared "this brings hope for a better future." Shortly thereafter, Mahmoud Abu Zeid, Egyptian minister of water resources and irrigation, responded by offering to drill 20 wells to satisfy urgent needs in Darfur, not only for the sedentary farmers, but also for nomadic tribes that were badly affected by draughts. In addition, a hybrid UN-African Union force of 26,000 will be deployed in Darfur for peacekeeping. This force will require water resources. Thus, Ban Ki Moon, secretary general of the UN, asked me to brief him at the UN's headquarters in New York. He also recognised the significance of the discovery and was amenable to place the "1,000 Wells" initiative under the auspices of the UN, which would assure both expediency and accountability. At this time, efforts are being made to select the best sites for well drilling to seek new water resources. In the final analysis, the planned well drilling programme will be a tangible illustration of using advanced space technology in resolving a humanitarian crisis. * The writer is a veteran of NASA's Apollo program, director of the Boston University Centre for Remote Sensing, and adjunct professor in the Faculty of Science at Ain Shams University.
