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.     

A viscoelastic-viscoplastic constitutive equation and its finite element implementation

A viscoelastic-viscoplastic constitutive model for isotropic materials undergoing isothermal infinitesimal deformation is proposed. The model is based on the assumption that the total strain rate is decomposable into a viscoelastic and a viscoplastic portion. Consequently, the model consists of a linear viscoelastic model in series with a modified plasticity model. This modified plasticity model adopts the classical Drucker-Prager yield surface with isotropic hardening and the associative flow rule of the invicid theory of plasticity. However, hardening is assumed to be a function of both the viscoplastic strain as well as the total strain rate. In this manner, the proposed model acquires the advantage of having both the initial and the subsequent yield surfaces to be a function of the strain rate, a property which has its experimental supportive evidence for viscoplastic materials such as polymers and some metals at highly elevated temperatures.A finite-element algorithm is developed to implement the constitutive equation derived in Part I. This algorithm adopts a combination of the tangent stiffness matrix and the initial load approach. The method of treating the transitional region between viscoelastic and viscoelastic-viscoplastic behavior is given. The details of implementation is described. Convergence of the computation scheme is discussed.Two examples are calculated numerically to demonstrate the strain rate and the pressure effects on the mechanical behavior of some viscoelastic-viscoplastic material. Results show that essential features in the stress-strain diagram obtained experimentally are exhibited by the model.     

Binding of thiopental to plasma proteins: effects on distribution in the brain and heart

Thiopental-14C (30 mg and 10 muCi/kg) was injected intravenously into rats 36-48 hours following bilateral nephrectomy and one minute after pretreatment with sulfadimethoxine (30 mg/kg, iv). Control groups of normal and sham-operated animals were used. The distributions of radioactivity in plasma, brain, and heart 1, 5, and 30 minutes after injection were examined. Uremic and sulfonamide-pretreated rats showed significantly higher levels of 14C in brain and heart and more free thiopental in plasma at each time than did control animals. There was a significant correlation between the free thiopental in plasma and total drug concentrations in the brain and heart. Uremic rats bound less thiopental in plasma compared with controls in spite of normal total plasma protein and albumin concentrations. It is concluded that reduced protein binding of thiopental leads to accelerated distribution and increased drug concentrations in the brain and and heart.     

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, Canada's 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.     

Corrosion characterization of new tin-silver binary alloys in nitric acid solutions

Electrochemical techniques were used to characterize the corrosion behavior of four new binary alloys xSn-Ag (x = 26, 50, 70 and 96.5 wt%) alloys and their individual metal components in nitric acid solutions. The experimental data were collected by using open-circuit potential, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). Over the concentration range studied (0.075-4.5 M), each of the two corrosion parameters (E_corr and i_corr) shows a regular dependency on both the alloy composition and the solution concentration. In general, for all studied samples, especially pure Ag and those with lower Sn contents (26 and 50 wt%), increasing the acid concentration increases i_corr, meanwhile causes a shift of the corresponding E_corr towards more positive values. This is probably due to the increase in the effect of cathodic depolarizer as the nitric acid concentration is increased. EIS results at the free corrosion potential confirmed well this behavior, where at concentrations ?1.5 M the thickness of the surface film increases while its resistance decreases with increasing tin wt%, indicating formation of less protective thicker film. However, at higher concentrations all samples exhibit identical behavior.