Sayed M. Arafat

Rise and demise of the New Lakes of Sahara

Multispectral remote sensing data and digital elevation models were used to examine the spatial and temporal evolution of the New Lakes of Sahara in southern Egypt. These lakes appeared in September 1998, when water spilled northwestward toward the Tushka depression due to an unusual water rise in Lake Nasser induced by high precipitation in the Ethiopian Highlands. Five lakes were formed in local depressions underlain by an impermeable Paleocene shale and chalk formation. The lakes developed through three stages. (1) A rise stage occurred from September 1998 to August 2001; the area covered by the lakes reached ∼1586 km 2 . In this stage the rate of water supply far exceeded the rate of water loss through evaporation. This stage was characterized by an early phase (August 1998–August 1999) when the area covered by the lakes increased by ∼75 km 2 /month. This was followed by a late phase (August 1999–August 2001), in which area increase averaged ∼28 km 2 /month. (2) A steady-state stage occurred from August 2001 to August 2003, during which the area covered by the lakes remained relatively unchanged and water lost through evaporation was continuously replaced by water supply from Lake Nasser. (3) A demise stage occurred from August 2003 to April 2007, during which water supply from Lake Nasser stopped completely and water was continuously evaporating. The area covered by the lakes decreased to ∼800 km 2 with an average loss of ∼17 km 2 /month. If this trend continues, the New Lakes of Sahara will disappear completely by March 2011. The spatial distribution of the New Lakes of Sahara is strongly controlled by morphologically defined east-, north-, northeast-, and northwest-trending faults. The water recharge of the Nubian aquifer by the New Lakes of Sahara is insignificant; much of the lakes9 area is above an impermeable formation.

Identification and mapping of some soil types using field spectrometry and spectral mixture analyses: a case study of North Sinai, Egypt

This study examines linear spectral unmixing technique for mapping the surface soil types using field spectroscopy data as the reference spectra. The investigated area is located in North Sinai, Egypt. The study employed data from the Landsat 7 ETM+ satellite sensor with improved spatial and spectral resolution. Mixed remotely sensed image pixels may lead to inaccurate classification results in most conventional image classification algorithms. Spectral unmixing may solve this problem by resolving those into separate components. Four soil type end-members were identified with minimum noise fraction and pixel purity index analyses. The identified soil types are calcareous soils, dry sabkhas, wet sabkhas, and sand dunes. Soil end-member reference spectra were collected in the field using an ASD FieldSpec Pro spectrometer. Constrained sum-to-one and non-negativity linear spectral unmixing model was applied and the soil types map was produced. The results showed that linear spectral unmixing model can be a useful tool for mapping soil types from ETM+ images.

Multisensor characterization of subsurface structures in a desert plain area in Egypt with implications for groundwater exploration

A desert plain in Egypt is being investigated to develop new techniques of multisensor data integration for groundwater exploration. A combination of multispectral, thermal and microwave data (ASTER and PALSAR) and supported by ground measurements (GPR, field spectroradiometry and magnetometry) were used to investigate surface sediment characteristics of the El-Gallaba Plain area, NW of Aswan. This desert plain once hosted an ancestral river system long before the Nile even existed. Nowadays the fluvial deposits are largely covered by Aeolian and gravelly sands and thus only detectible with radar and thermal images. Results show two broad strips of thermal cooling anomalies arranged in a linear fashion and diagonally crossing the alluvial basin. Spectral signatures collected along the linear land surface temperature (LST) anomalies show generally higher reflectance values (higher albedo) than the surrounding sediments. Both, the cooler LST and higher albedo, suggest that the surface sediments within the anomaly strips have lower emissivity values (low heat storage capacity). Furthermore, the homogeneity of these sediments was measured with a ground penetrating radar (GPR) using 250 and 100 MHz shielded antenna. The 12 GPR profiles across the LST anomalies confirmed that the near-surface sediments (up to 10 m depth) consist of thin horizontal layers of sandstone with very low gravel content. They show very different textural and compositional characteristics with respect to the surrounding areas suggesting a different depositional environment. Thus 12 magnetic profiles with 1.5 km average length were acquired across the LST anomalies to investigate deep seated structures. The results confirmed the existence of graben-like structures with a maximum depth to the basement of 150 m and shallower depths toward the edges of the LST anomalies. Consequently, these structurally controlled basins could be promising areas for ground water accumulation and exploration in the El-Gallaba Plain of the Western Desert in Egypt.

Mutual influence between climate and vegetation cover through satellite data in Egypt

The effect of vegetation cover on climatic change has not yet observed in Egypt. In the current study, Ismailia Governorate was selected as a case study to assess the impact of the vegetation cover expansion on both land surface and air temperature during twenty-eight years from 1983 to 2010. This observation site was carefully selected as a clear example for the highly rate of reclamation and vegetation expansion process in Egypt. Land Surface Temperature (LST) that were extracted from NOAA/AVHRR satellite data and air temperature (Tair) data that were collected from ground stations, were correlated with the expansion of vegetation cover that was delineated using Landsat TM and Landsat ETM+ data. The result showed that (LST) decreased by about 2.3°C while (Tair) decreased by about 1.6°C with the expansion of the cultivated land during twenty-eight years.