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1.
Africa’s landscape is dominated by a manifold of second-order epeirogenic structures superimposed on a first-order bimodal topography. Bivariate regression analysis of Africa’s surface topography shows that this is a complexly folded surface with regionally elevated areas in southern and eastern Africa, and a topographically low northern and western Africa. The apparent spatial relationships between these features are analysed using anomaly correlation between surface topography and free-air gravity anomalies. Occurrences of positively correlated features between gravity and topography in Africa are found to be limited to second-order epeirogenic features. Geophysical modelling and geologic evidence indicate that Africa’s bimodal topography is genetically distinct from these second-order features, and linked to sources as deep as the sublithospheric mantle. The age, measured and modelled elevation of the bimodal topography require that topographic uplift of south-central Africa be episodic. We infer from our findings together with relative sea-level changes, that the near-bimodality of Africa’s topography is an ancient feature inherited at least from upper Paleozoic times. Our reconstructed paleotopography suggests that Africa was largely a low-lying continent dominated by its cratons, and that basement distribution disregards the present-day uplift patterns of Africa. 相似文献
2.
Analytical prediction of aggregates' effects on the ITZ volume fraction and Young's modulus of concrete 
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下载免费PDF全文 In order to evaluate analytically the ITZ volume fraction (fITZ) in concrete, a three phase model is proposed for the random concrete microstructure using the Voronoï tessellation. Within this model, the ITZ local thickness is a statistical variable depending on the local paste thickness available between each couple of neighbouring aggregates. The fITZ is found to not exceed 7% for typical concretes. Then, the concrete Young's modulus is predicted analytically using a four‐phase generalized self consistent model but in which the proposed fITZ is considered. It is found that the concrete Young's modulus increases when increasing aggregates volume fraction, aggregates maximum size and the proportion of coarse aggregates and when decreasing the ITZ thickness and Young's modulus. Finally, the validity of the proposed model is discussed based on a comparison between its predictions and three sets of experimental results related to normal and high strength concretes taken from literature. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
3.
Global Climate Change and Tropical Forest Genetic Resources 总被引：4，自引：0，他引：4
Global climate change may have a serious impact on genetic resources in tropical forest trees. Genetic diversity plays a critical role in the survival of populations in rapidly changing environments. Furthermore, most tropical plant species are known to have unique ecological niches, and therefore changes in climate may directly affect the distribution of biomes, ecosystems, and constituent species. Climate change may also indirectly affect plant genetic resources through effects on phenology, breeding systems, and plant-pollinator and plant seed disperser interactions, and may reduce genetic diversity and reproductive output. As a consequence, population densities may be reduced leading to reduction in genetic diversity through genetic drift and inbreeding. Tropical forest plants may respond to climate change through phenotypic plasticity, adaptive evolution, migration to suitable site, or extinction. However, the potential to respond is limited by a rapid pace of change and the non-availability of alternate habitats due to past and present trends of deforestation. Thus climate change may result in extinction of many populations and species. Our ability to estimate the precise response of tropical forest ecosystems to climate change is limited by lack of long-term data on parameters that might be affected by climate change. Collection of correlative data from long-term monitoring of climate as well as population and community responses at selected sites offer the most cost-effective way to understand the effects of climate change on tropical tree populations. However, mitigation strategies need to be implemented immediately. Because many effects of climate change may be similar to the effects of habitat alteration and fragmentation, protected areas and buffer zones should be enlarged, with an emphasis on connectivity among conserved landscapes. Taxa that are likely to become extinct should be identified and protected through ex situ conservation programs. 相似文献
4.
Regions of Agane and Gouérarate represent an ancient fragment of ophiolitic suture localized in the axial area of the Mauritanides Belt. These two regions are characterized by the abundance of completely serpentinized formations. In this study, we present the first use of Raman spectroscopy for identifying the species of serpentine present in the Mauritanides Belt. Serpentinites of Agane are derived from refractory peridotites composed of dunites–harzburgites; however, there are also rare serpentinites derived from ultramafic cumulates. Antigorite represents the dominant species in the serpentinite. Furthermore, chrysotile is found as post-antigorite veins. These veins are post-obduction and mark the final phase of serpentinization. The abundance of antigorite and the absence of lizardite confirm that subduction was the environment of serpentinization in these two regions, and that “the oceanic opening” responsible for the formation of ophiolitic sutures in the Mauritanides Belt was limited. The term “serpentinite” is no longer applicable to the formations of Gouérarate. As a result, these formations correspond to old serpentinites transformed to birbirites which are in phase of transformation into laterites. 相似文献
5.
Interannual variability of rainfall over the Sahel based on multiple regional climate models simulations 总被引：1，自引：0，他引：1
Ismaila Diallo Mouhamadou B. Sylla Moctar Camara Amadou T. Gaye 《Theoretical and Applied Climatology》2013,113(1-2):351-362
We analyse the interannual variability of the averaged summer monsoon rainfall over the Sahel from multiple regional climate models driven by the ERA-interim reanalysis and seek to provide effective information for future modelling work. We find that the majority of the models are able to reproduce the rainfall variability with correlation coefficient exceeding 0.5 compared with observations. This is due to a good representation of the dynamics of the main monsoon features of the West African climate such as the monsoon flux, African Easterly Jet (AEJ) and Tropical Easterly Jet (TEJ). Among the models, only HIRHAM fails to reproduce the rainfall variability exhibiting hence a correlation coefficient of ?0.2. This deficiency originates from the fact that HIRHAM does not properly capture the variability of monsoon flow and the relationship between rainfall and the AEJ dynamic. We conclude that a good performance of a regional climate model in simulating the monsoon dynamical features variability is of primary importance for a better representation of the interannual variability of rainfall over the Sahel. 相似文献
6.
Reliable information on water use and availability at basin and field scales are important to ensure the optimized constructive uses of available water resources. This study was conducted with the specific objective to estimate Landsat-based actual evapotranspiration (ETa) using the Operational Simplified Surface Energy Balance (SSEBop) model across the state of South Dakota (SD), USA for the 1986–2018 (33-year) period. Validated ETa estimations (r2 = 0.91, PBIAS = −4%, and %RMSE = 11.8%) were further used to understand the crop water-use characteristics and existing historic mono-directional (increasing/decreasing) trends over the eastern (ESD) and western (WSD) regions of SD. The crop water-use characteristics indicated that the annual cropland water uses across the ESD and WSD were more or less met by the precipitation amounts in the area. The ample water supply and distribution have led to high rainfed and low percentage of irrigated cropland (~2.5%) in the state. The WSD faced greater crop-water use reductions than the ESD during drought periods. The landscape ETa responses across the state were found to be more sensitive than precipitation for the drought impact assessments. The Mann Kendall trend analysis revealed the absence of a significant trend (p > 0.05) in annual ETa at a regional scale due to the varying weather conditions in the state. However, about 12% and 9% cropland areas in the ESD and WSD, respectively, revealed a significant mono-directional trend at pixel scale ETa. Most of the pixels under significant trend showed an increasing trend that can be explained by the shift in agricultural practices, increased irrigated cropland area, higher productions, moisture regime shifts, and decreased risk of farming in the dry areas. The decreasing trend pixels were clustered in mid-eastern SD and could be the result of dynamic conversion of wetlands to croplands and decreased irrigation practices in the region. This study also demonstrates the tremendous potential and robustness of the SSEBop model, Landsat imagery, and remote sensing-based ETa modelling approaches in estimating consistent spatially distributed evapotranspiration. 相似文献
7.
Har Amrit Singh Sandhu Hemendra Singh Gusain Manoj Kumar Arora Arun Bawa 《Journal of the Indian Society of Remote Sensing》2018,46(11):1835-1840
Dokriani Glacier is regarded as one of the important glaciers of Bhagirathi River basin, which fed river Ganges. The length of the glacier is about 4.6 km, and snout elevation is about 4028 m m.s.l. The mass balance of this glacier was calculated using field-based measurements for few years during 1994 to 2000. However, due to remote and poor accessibility, the field-based measurements could not continue; thus, remote sensing-based methods become useful tool to estimate the long-term mass balance of the glacier. In this study, glacier mass balance has been determined using accumulation area ratio (AAR) method. Remote sensing data sets, e.g. Landsat TM, ETM?+?and OLI, have been used to estimate AAR for different years from 1994 to 2014. An attempt has also been made to develop a mathematical relationship between remote sensing-derived AAR and field-observed mass balance data of the glacier. Further, this relationship has been used to estimate mass balance of the glacier for different years using remote sensing-derived AAR. Estimated mass balance was validated from ground-observed mass balance for few years. The field-observed and remote sensing-derived mass balance data are compared and showed high correlation. It has been observed that AAR for the Dokriani Glacier varies from 0.64 to 0.71. Mass balance of the glacier was observed between ??15.54 cm and ??50.95 cm during the study period. The study highlights the application of remote sensing in mass balance study of the glaciers and impact of climate change in glaciers of Central Indian Himalaya. 相似文献
8.
Off-line solid-phase extraction procedure for the determination of polycyclic aromatic hydrocarbons from aqueous matrices 总被引：1，自引：0，他引：1
I. Limam M. R. Driss 《International Journal of Environmental Science and Technology》2013,10(5):973-982
An off-line solid-phase extraction procedure followed by high-performance liquid chromatography with ultraviolet detection for the determination of 16 priority polycyclic aromatic hydrocarbon pollutants in aqueous matrices was described. Diverse aspects determining extraction efficiency such as packing type (disk or cartridge), elution solvents and addition of organic modifiers to the sample were evaluated. Elution with acetonitrile yields the highest recoveries. Rinsing the sample bottle with acetonitrile and combining the rinse with the sample extract avoids the adsorption of polycyclic aromatic hydrocarbons on the walls of the water containers. The use of isopropanol or methanol 10 % (V/V) was the most appropriate amount for the enrichment of 2- to 6-ring aromatic compounds only on C18 cartridge. The recoveries for all studied polycyclic aromatic hydrocarbons are ranged from 71.4 to 95.2 % for a treated water samples of 500 mL. The proposed method gives very low detection limits (subnanograms per liter) and it has been applied to drinking water, surface water and industrial effluent (oil refinery) samples with good results. 相似文献
9.
In order to protect infrastructures against rockfalls, civil-engineered mitigation measures are widely used. Flexible metallic fences are particularly well suited to stop the propagation of blocks of rock whose kinetic energy can reach 5000?kJ before impact. This paper focuses on the design of highly flexible rockfall fences under the new European guideline ETAG027. The experimental testing and the numerical modeling using the discrete element method (DEM) of a new metallic rockfall fence are presented. Several scales of study were considered; the mesh, the net and the entire structure. The calibration of the DEM models is described and a parametrical study is proposed. The latter aims to underline the type of information that can be obtained from numerical simulations of such a system to enhance its design. 相似文献
10.
Isotopic and geochemical characteristics of groundwater in the Senegal River delta aquifer: implication of recharge and flow regime 总被引：3，自引：3，他引：0
Moctar Diaw Serigne Faye Willibald Stichler Piotr Maloszewski 《Environmental Earth Sciences》2012,66(4):1011-1020
Groundwater and surface water samples were collected to improve understanding of the Senegal River Lower Valley and Delta
system, which is prone to salinization. Inorganic ion concentrations and environmental isotopes (18O, 2H and 3H) in groundwater, river, lake and precipitation were investigated to gain insight into the functioning of the system with
regard to recharge sources and process, groundwater renewability, hydraulic interconnection and geochemical evolution. The
geochemical characteristics of the system display mainly cation (Ca2+ and/or Na+) bicarbonated waters, which evolve to chloride water type; this occurs during groundwater flow in the less mineralized part
of the aquifer. In contrast, saline intrusion and secondary brines together with halite dissolution are likely to contaminate
the groundwater to Na–Cl type. Halite, gypsum and calcite dissolution determine the major ion (Na+, Cl−, Ca2+, Mg2+, SO4
2− and HCO3
−) chemistry, but other processes such as evaporation, salt deposition, ion exchange and reverse exchange reactions also control
the groundwater chemistry. Both surface water and groundwater in the system show an evaporation effect, but high evaporated
signatures in the groundwater may be due to direct evaporation from the ground, infiltration of evaporated water or enriched
rainwater in this region. The stable isotopes also reveal two types of groundwater in this system, which geomorphologically
are distributed in the sand dunes (depleted isotopes) and in the flood plain (enriched isotopes). Consideration of the 3H content reinforces this grouping and suggests two mechanisms of recharge: contribution of enriched surface water in recharging
the flood plain groundwater and, in the sand dunes area where water table is at depth between 8 and 13 m, slow recharge process
characterized the submodern to mixed water. 相似文献