Eman Ghoneim

DEM-optical-radar data integration for palaeohydrological mapping in the northern Darfur, Sudan: implication for groundwater exploration

North‐western Sudan, as a part of the eastern Sahara, is among the driest places on earth. However, the region underwent drastic climatic changes through the alternation of dry and wet conditions in the past. During humid phases, when the rain was plentiful over a prolonged time period, the surface was veined by rivers and dotted by large lakes. The new Shuttle Radar Topography Mission data (SRTM ∼90 m) revealed a large endorheic drainage basin, which is centred by a large terminal palaeolake, in the northern Darfur State. The use of GIS methods allowed the delineation of the drainage basin and its associated palaeorivers. The SRTM data along with the Landsat (ETM+) and Radarsat‐1 images corroborate the presence of segments of palaeoshorelines associated with the palaeolake highstands. These constitute a convincing argument of the long‐term existence of a possible pre‐Holocene large water body in the region in the past. The remains of the highest palaeoshoreline have a constant altitude of 573±3 m asl. At its maximum extent, the mega Lake occupied an area of about 30 750 km2 (the same size as the Great Bear Lake, Canadas largest lake), which would have contained approximately 2530 km3 of water. This, ancestral lake, which we named the Northern Darfur Megalake (ND Megalake), represents indisputable evidence of the past pluvial conditions in the eastern Sahara. The discovered palaeoshorelines will have significant consequences for improving our knowledge of continental climate change and regional palaeohydorology, and should be taken into consideration in studies of past human habitation in the region. Much of the water carried by the Northern Darfur palaeorivers and the ND Megalake would have percolated into the underlying rocks feeding the Nubian Sandston aquifer. These findings show that the used approach of space‐data integration can help significantly in the groundwater exploration efforts in the Darfur region, where freshwater access is essential for refugee survival, and can be successfully adopted in other parts of Sudan and arid lands in general.

Radar topography data reveal drainage relics in the eastern Sahara

The Shuttle Radar Topographic Mission (SRTM) elevation data reveal segments of inactive drainage channel systems in the eastern Sahara. Four sites are investigated and their drainage networks are delineated and compared with processed Synthetic Aperture Radar (SAR) and Enhanced Thematic Mapper Plus (ETM+) images. These include parts of the drainage lines that lead to the Kufra Oasis in Libya; the eastern Gilf Kebir drainage system that straddles the border region between Egypt and Sudan; the lower reach of Wadi Howar, the extinct western Nile tributary in Sudan; and a reversed flow channel of Wadi Kubbaniya northwest of Aswan in Egypt. These features are delineated using SRTM data and a Geographic Information System (GIS) hydrologic routine. The results show that the SRTM signals penetrate the desert‐sandy surfaces and map ancient drainage systems in the same way as SAR. The study also demonstrates that SRTM data can be used to confirm or modify pathways of channel courses mapped from radar images alone. Because of this capability, it is also recommended that SRTM data be used in place of GTOPO30 elevation data in hydrologic modelling of sand‐covered deserts.

Optimum groundwater locations in the northern United Arab Emirates

Due to the increase in urban and agricultural activities in arid regions, the exploration of new locations of possible groundwater discharge and accumulation is required to augment the limited water resources. In order to locate such discharge areas, it is necessary first to identify zones of high recharge potentials. In such an arid region, like the northern United Arab Emirates (UAE), one of the ways to predict areas of potential groundwater recharge is by understanding the hydrological response of its drainage basins to rainfall events. Due to the scarcity of basic hydrological data, a hydrological model driven mainly by information on the physiographic characteristics, drainage network properties (generated from DEM), and surface cover distribution (generated from satellite images) was used to comprehend the dynamics of surface runoff through hydrographs, and hence water loss in the study area. Results show that the northern UAE is drained by 48 drainage basins emerging from the Oman Mountains. Two‐thirds of these basins drain easterly toward the Gulf of Oman, and one‐third drain westerly toward the Arabian Gulf. These basins are found to be structurally controlled by three major fault trends, which are the NE trend (Dibba zone), NW trend (Ham Zone), and WNW trend (Hatta zone). The hydrological response of a basin is correlated with its morphological characteristics. Based on these characteristics, and through the application of a cluster analysis, it was feasible to classify the largest basins in the region into four groundwater potentiality groups in accordance with the magnitude of their peak discharges. From this study, it became evident that the downstream area of the two major basins of Ar‐Rafiah and Limhah, and their vicinities are the most probable sites for groundwater accumulation. The drainage systems of these two basins, especially those controlled by major fault lines, play a vital role in transmitting surface–subsurface rainwater from the Oman Mountains, the recharge zone, into the western desert plain, the discharge zone, where freshwater accumulates underground. The study also revealed that a large volume of groundwater is dissipated into the sea along the eastern coast. A detailed examination of MODIS thermal data supports this by revealing cool surface anomalies issuing from the mountain range toward both the western desert plain and the Gulf of Oman following major rainfall events. Thus, the technique used facilitates the prediction of new locations of optimum groundwater resources in the northern UAE. Such a technique could be adopted, with appropriate modifications, elsewhere in arid regions, where groundwater is restricted and subject to greater complexity.

Largest crater shape in the Great Sahara revealed by multi-spectral images and radar data

This Letter communicates the discovery of an exceptionally large, double‐ringed crater in the eastern part of the Great Sahara, North Africa. The crater is centred at 24.40° N 24.58° E, straddling the boarder between Egypt and Libya. It is the 15th and largest impact crater identified in the Sahara. Landsat Enhanced Thematic Mapper Plus (ETM+) images as well as Radarsat‐1 data reveal a discontinuous outer rim, 31 km in diameter, and a group of prominences forming an inner ring. The Nubian sandstone surface in which the crater was formed has undergone severe erosion. Thus, the crater morphology was affected by both aeolian and fluvial processes. Courses of a major river and smaller streams, now dry, have eroded much of the craters outer rim as revealed by Shuttle Radar Topography Mission (SRTM) data. The probable impact that created the crater, named Kebira, meaning large in Arabic, is possibly the source of the silica glass fragments that abound on the desert surface between giant linear dunes of the Great Sand Sea in southwestern Egypt.

Groundwater prospect map of Egypt's Qena Valley using data fusion

Groundwater is a precious resource of limited extent in arid and hyper-arid regions. Over the last few years, the demand for water supply has increased because of increasing population, urbanisation and agricultural activities. This is particularly true in developing countries like Egypt. In this article, an integrated approach of remote sensing (RS) and geographic information systems (GIS) methodologies was used to delineate groundwater potential zones. The selected location is Qena Valley (Wadi Qena) in the Eastern Desert of Egypt. Landsat enhanced thematic mapper and Shuttle Radar Topography Mission data were employed to prepare a groundwater prospection map. This was done by integrating slope, drainage, geology–geomorphology and lineament density maps through GIS techniques. These thematic maps were integrated after assigning weight factors to identify features in each case depending on infiltration properties. This resulted in classifying the region into six categories of groundwater potentiality from excellent to very low. The alluvial and flood plains consisting of thick sand and gravel strata were proven to be the most favourable zones for groundwater exploration. Field information and existing geophysical data were employed to validate the groundwater potential map. The overall results demonstrate that RS and GIS techniques provide powerful tools for better assessing, planning and monitoring of water resources in arid regions.

Characterization of an Oil Spill Along the Lebanese Coast by Satellite Images

The Lebanon population witnessed a severe environmental problem when one of the countrys largest coastal power stations in Jiyeh area was bombed on July 13, 2006. Several million gallons of fuel oil were released into the Mediterranean Sea, forming a huge oil spill. To assess the extent of the spill, two types of satellite images were used. First, the moderate resolution imaging spectroradiometer (MODIS)-Terra images were obtained in near real-time immediately following the event and second advanced space-borne thermal emission and reflection radiometer (ASTER) images were taken about 1 month later. Results showed an oil plume with areal extent of approximately 3,100 km2 shortly after the event, reaching the northern coast of Lebanon. However, after 2 months, satellite monitoring showed the geographic distribution of oil was reduced to 185 km2. The bio-environmental impact of this oil spill, due to its size and hydraulic dynamics, makes a major disaster.

Mapping Water Basins in the Eastern Sahara by SRTM Data

RTM (~90 m) data and Radarsat-1 images of northern Sudan were processed for exploring potential groundwater accumulation sites in the region. Data analysis unveiled three extensive ancient river courses named as Howar, El-Qaab and Arbain. These eastward flowing rivers, which are now partly hidden beneath the sand, are extinct tributaries of the Nile Valley hydro-system. The main channels of these rivers were mapped from Radar data and compared with those delineated by the GIS hydrologic routines. The beds of these river courses, which are now void of moisture, must have hosted a great volume of surface water in the past and played a significant role in recharging the sandstone groundwater aquifers in the region. An ongoing data investigation will provide additional information as to the specific nature of these ancient rivers that will help in future water prospecting in the region.