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Torbern Tagesson Jonas Ardö Idrissa Guiro Ford Cropley Cheikh Mbow Stephanie Horion 《Geografisk tidskrift / udgivet af Bestyrelsen for Det Kongelige danske geografiske selskab》2016,116(2):93-109
Africa is a sink of carbon, but there are large gaps in our knowledge regarding the CO2 exchange fluxes for many African ecosystems. Here, we analyse multi-annual eddy covariance data of CO2 exchange fluxes for a grazed Sahelian semi-arid savanna ecosystem in Senegal, West Africa. The aim of the study is to investigate the high CO2 exchange fluxes measured at the peak of the rainy season at the Dahra field site: gross primary productivity and ecosystem respiration peaked at values up to ?48 μmol CO2 m?2 s?1 and 20 μmol CO2 m?2 s?1, respectively. Possible explanations for such high fluxes include a combination of moderately dense herbaceous C4 ground vegetation, high soil nutrient availability and a grazing pressure increasing the fluxes. Even though the peak net CO2 uptake was high, the annual budget of ?229 ± 7 ± 49 g C m?2 y?1 (±random errors ± systematic errors) is comparable to that of other semi-arid savanna sites due the short length of the rainy season. An inter-comparison between the open-path and a closed-path infrared sensor indicated no systematic errors related to the instrumentation. An uncertainty analysis of long-term NEE budgets indicated that corrections for air density fluctuations were the largest error source (11.3% out of 24.3% uncertainty). Soil organic carbon data indicated a substantial increase in the soil organic carbon pool for the uppermost .20 m. These findings have large implications for the perception of the carbon sink/source of Sahelian ecosystems and its response to climate change. 相似文献
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Cheikh Mbow Ole Mertz Awa Diouf Kjeld Rasmussen AnetteReenberg 《Global and Planetary Change》2008,64(3-4):210
Environmental change in the Sahel–Sudan zone of West Africa has been a major issue in development debates over the last decades. Using remote sensing based land cover change analysis, archival data, national and international statistical data, and household interviews, we analyze the drivers of environmental change in Eastern Saloum in Central East Senegal as well as the local perceptions of these changes and adaptation. Being part of the ground nut basin, Eastern Saloum has witnessed rapid environmental degradation caused by the conversion of forest and savanna areas to agricultural land during the last 20–30 years and by a combination of decline in precipitation, soil degradation, a diversity of policies with little concern for the environment, fluctuating markets and population pressure. Farmers perceive the environmental change mainly as land degradation and poor soil fertility, though recent extensification of agriculture counters this effect and has led to increased vegetation cover in marginal areas. They identified erratic climate, agricultural policies, insufficient food production and desire to increase income as the main drivers of change in the area. We conclude that while climate variability has influenced environmental change in the area, various types of State interventions in agriculture and global market fluctuations appear to have been the main underlying causes of environmental degradation. 相似文献
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Isotope techniques for monitoring groundwater salinization 总被引：4，自引：0，他引：4
Cheikh B. Gaye 《Hydrogeology Journal》2001,9(3):217-218
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Catastrophic flooding of an aeolian dune field: Jurassic Entrada and Todilto Formations, Ghost Ranch, New Mexico, USA 总被引：2，自引：0，他引：2
Surveyed outcrops of the Middle Jurassic Entrada Sandstone at Ghost Ranch, New Mexico, show the unusual occurrence of preserved aeolian dune palaeotopography buried beneath subaqueous strata. The preserved dune remnants have relief up to 35 m, trend NNW, and show internal scalloped cross-strata dipping to the WSW, with small sets occurring as both topsets and bottomsets. Outcrop data are best satisfied in computer models by 50 m high, sinuous bedforms that migrated to the WSW, while the sinuosity migrated alongcrest to the NNW. Superimposed small dunes occurred upon the stoss slope, and at the basal lee of the main bedform where they migrated alongslope to the NNW. Remnant dune palaeotopography is buried by onlapping, subaqueous, largely structureless sandstones believed to be derived by mass wasting of the upper portions of the dunes and deposited as sediment-gravity flows that infilled between the dunes. Preservation of dune palaeotopography beneath mass-flow deposits, with no evidence for gradually rising water, argues that flooding of the Entrada dune field was geologically instantaneous. The thickness and lithology of the overlying Todilto Formation conform to slight remnant palaeotopography on the Entrada surface. The Todilto is a laminated limestone and thinnest over remnant dune crestal areas, but thickens and increases in gypsum content downslope until it abruptly yields to a gypsum mound positioned over a remnant interdune hollow. The Todilto laminations are interpreted as seasonal varves deposited below wave base in a density-stratified water body. The flooding event that gave rise to the controversial Todilto water body occurred during Entrada time, with Todilto deposition occurring within an already substantial water body. 相似文献
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In this paper, we present a numerical simulation of a water jet impacting a new aeronautical material Ti555-03 plate.The Computational Fluid Dynamics(CFD) behavior of the jet is investigated using a FV(Finite Volume) method.The Fluid–Structure Interaction(FSI) is studied using a coupled SPH(Smoothed Particle Hydrodynamics)-FE(Finite Element) method. The jets hit the metal sheet with an initial velocity 500 m/s. Two configurations which differ from each other by the position(angle of inclination) of the plate relatively to the axis of revolution of the jet inlet are investigated in this study. The objective of this study is to predict the impact of the fluid produced at high pressure and high speed especially at the first moment of impact. Numerical simulations are carried out under ABAQUS. We have shown in this study that the inclination of the titanium alloy plate by 45° stimulates the phenomenon of recirculation of water. This affects the velocity profile, turbulence and boundary layers in the impact zone. The stagnation zone and the phenomenon of water recirculation are strongly influenced by the slope of the plate which gives a pressure gradient and displacement very important between the two configurations. Fluctuations of physical variables(displacement and pressure) prove the need for a noise and vibratory study. These predictions will subsequently be used for the modeling of the problem of an orthogonal cut in a high-speed machining process assisted by high-pressure water jet. 相似文献
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Mads Olander Rasmussen Frank-M. Göttsche Doudou Diop Cheikh Mbow Folke-S. Olesen Rasmus Fensholt Inge Sandholt 《International Journal of Applied Earth Observation and Geoinformation》2011
A tree survey and an analysis of high resolution satellite data were performed to characterise the woody vegetation within a 10 × 10 km2 area around a site located close to the town of Dahra in the semi-arid northern part of Senegal. The surveyed parameters were tree species, height, tree crown radius, and diameter at breast height (DBH), for which allometric models were determined. An object-based classification method was used to determine tree crown cover (TCC) from Quickbird data. The average TCC from the tree survey and the respective TCC from remote sensing were both about 3.0%. For areas beyond the surveyed areas TCC varied between 3.0% and 4.5%. Furthermore, an empirical correction factor for tree clumping was obtained, which considerably improved the estimated number of trees and the estimated average tree crown area and radius. An allometric model linking TCC to tree stem crosssectional area (CSA) was developed, which allows to estimate tree biomass from remote sensing. The allometric models for the three main tree species found performed well and had r2-values of about 0.7–0.8. 相似文献
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Mohamed Yacine Tebbouche Djamel Machane Souhila Chabane El-Hadi Oubaiche Aghiles Abdelghani Meziani Dalila Ait Benamar Hakim Moulouel Ghani Cheikh Lounis Rabah Bensalem Abderrahmane Bendaoud 《Arabian Journal of Geosciences》2018,11(7):154
The original version of this paper was published with incomplete affiliation. Given in this article are the complete affiliations. 相似文献
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Résumé L'étude des bassins côtiers de Sfax et de la Djeffara de Gabès Nord, basée sur l'interprétation des données hydrochimiques (éléments majeurs) et isotopiques (18O, 2H et 14C), a permis une meilleure compréhension du fonctionnement hydrodynamique de l'aquifère Miocène le long de la frange côtière. En effet, d'après les données géochimiques, apparaît la présence de deux types de faciès chimiques: chloruré sodique dans le bassin de Sfax et mixte à chloruré sodique dans le bassin de Gabès Nord. Cette transition d'un faciès à un autre souligne la variabilité des origines de minéralisation des eaux. L'utilisation des outils isotopiques a permis l'identification d'une eau relativement ancienne à l'exception de quelques poches de recharge récente matérialisées à partir du relief de Zemlet El Beida en amont du bassin de Gabès Nord. Ceci a été vérifié par les fortes activités en14C des eaux dans cette région. Par conséquent, on peut noter que le passage du Sud au Nord le long de la côte est marqué par une grande variation latérale de lithologie et d'épaisseur de la formation aquifère, soulignant ainsi l'indépendance de chacun des deux réservoirs de Sfax et de la Djeffara de Gabès en absence de toute continuité hydrodynamique entre les deux bassins. Editeur Z.W. Kundzewicz; Editeur associé S. Faye Citation Ben Cheikh, N., Zouari, K., et Abidi, B., 2012. Application des outils chimiques et isotopiques à l'étude de la relation hydrodynamique entre les aquifères profonds de Sfax et de la Djeffara de Gabès Nord (Sud-Est tunisien). Hydrological Sciences Journal, 57 (8), 1662–1671. 相似文献