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11.
Access to fresh water is one of the major issues of northern and sub-Saharan Africa. The majority of the fresh water used for drinking and irrigation is obtained from large ground water basins where there is minor contemporary recharge and the aquifers cross national borders. These aquifers include the Nubian Aquifer System shared by Chad, Egypt, Libya, and Sudan; the Iullemeden Aquifer System, extending over Niger, Nigeria, Mali, Benin, and Algeria; and the Northwest Sahara Aquifer System shared by Algeria, Libya, and Tunisia. These resources are subject to increased exploitation and may be severely stressed if not managed properly as witnessed already by declining water levels. In order to make appropriate decisions for the sustainable management of these shared water resources, planners and managers in different countries need an improved knowledge base of hydrological information. Three technical cooperation projects related to aquifer systems will be implemented by the International Atomic Energy Agency, in collaboration with the United Nations Educational, Scientific and Cultural Organization and United Nations Development Programme/Global Environmental Facility. These projects focus on isotope hydrology studies to better quantify ground water recharge and dynamics. The multiple isotope approach combining commonly used isotopes 18O and 2H together with more recently developed techniques (chlorofluorocarbons, 36Cl, noble gases) will be applied to improve the conceptual model to study stratification and ground water flows. Moreover, the isotopes will be an important indicator of changes in the aquifer due to water abstraction, and therefore they will assist in the effort to establish a sustainable ground water management. 相似文献
12.
Niko Lahajnar Martin G. Wiesner Birgit Gaye 《Deep Sea Research Part I: Oceanographic Research Papers》2007,54(12):2120-2144
Settling particles collected by sediment traps deployed between 1987 and 1999 in the northern, central and southwestern South China Sea (SCS) were analysed to study seasonal, interannual and spatial variations in the composition and flux of labile particulate matter. Results were combined with remote-sensing and surface-sediment data in order to describe the factors controlling the preservation of organic matter en route from the upper ocean to the seafloor. Organic carbon, amino acid and hexosamine fluxes generally follow the fluxes of total particulate matter, with maxima during the SW and NE monsoon periods. During non-El Niño conditions spectral amino acid distributions show that degradation of organic matter in the water column decreases as the flux rates increase. This is suggested to be the combined result of enhanced primary productivity, greater input of lithogenics serving as ballast to increase settling rates, and sorption of labile components to clay minerals. During El Niño conditions, in contrast, the degree of organic matter degradation is at very high and comparable levels at all trap sites. Flux component seasonality is strongly reduced except for the coastal upwelling areas, particularly off central Vietnam, which show significantly higher fluxes of organic carbon and lithogenic matter as compared to the open SCS. This suggests that the fluxes are affected by lateral advection of reworked organic matter from riverine sources or resuspended sediments from the nearby shelf/slope. Comparison of the measured organic carbon fluxes in 1200 m depth with those accumulating in surface sediments results in a more than 80% loss of organic matter before final burial in the sediments. The degree of organic matter preservation in the surface sediments of the deep SCS is distinctly lower than in other monsoonal oceans. This may be due to varying lithogenic input and almost complete dissolution of protective biogenic mineral matrices at greater water depth. 相似文献
13.
Mineralization processes and water quality of Quaternary to Mio-Pliocene sanstone aquifers near Cotonou, Benin have been studied using wells sampled during May 1991, August 1991 and April 1992. Anthropogenic pollution, indicated especially by high concentrations of nitrate, P and K, has been detected in the upper aquifer. In contrast, the lower aquifer has acceptable concentrations of solutes, with the exception of the Godomey pumping area, where incipient saline intrusion is detected. Some suggestions are made for aquifer protection based on the careful understanding of groundwater flow directions. 相似文献
14.
We develop a probabilistic model for estimating the tsunami hazard along the coast of New Zealand due to plate-interface earthquakes
along the South American subduction zone. To do this we develop statistical and physical models for several stages in the
process of tsunami generation and propagation, and develop a method for combining these models to produce hazard estimates
using a Monte-Carlo technique. This process is largely analogous to that used for seismic hazard modelling, but is distinguished
from it by the use of a physical model to represent the tsunami propagation, as opposed to the use of empirical attenuation
models for probabilistic seismic hazard analysis. 相似文献
15.
Gaye Bayrakci Carla Scalabrin Stéphanie Dupré Isabelle Leblond Jean-Baptiste Tary Nadine Lanteri Jean-Marie Augustin Laurent Berger Estelle Cros André Ogor Christos Tsabaris Marc Lescanne Louis Géli 《Marine Geophysical Researches》2014,35(3):211-229
A rotating, acoustic gas bubble detector, BOB (Bubble OBservatory) module was deployed during two surveys, conducted in 2009 and 2011 respectively, to study the temporal variations of gas emissions from the Marmara seafloor, along the North Anatolian Fault zone. The echosounder mounted on the instrument insonifies an angular sector of 7° during a given duration (of about 1 h). Then it rotates to the next, near-by angular sector and so forth. When the full angular domain is insonified, the “pan and tilt system” rotates back to its initial position, in order to start a new cycle (of about 1 day). The acoustic data reveal that gas emission is not a steady process, with observed temporal variations ranging between a few minutes and 24 h (from one cycle to the other). Echo-integration and inversion performed on the acoustic data as described in the companion paper of Leblond et al. (Mar Geophys Res, 2014), also indicate important variations in, respectively, the target strength and the volumetric flow rates of individual sources. However, the observed temporal variations may not be related to the properties of the gas source only, but reflect possible variations in sea-bottom currents, which could deviate the bubble train towards the neighboring sector. During the 2011 survey, a 4-component ocean bottom seismometer (OBS) was co-located at the seafloor, 59 m away from the BOB module. The acoustic data from our rotating, monitoring system support, but do not provide undisputable evidence to confirm, the hypothesis formulated by Tary et al. (2012), that the short-duration, non-seismic micro-events recorded by the OBS are likely produced by gas-related processes within the near seabed sediments. Hence, the use of a multibeam echosounder, or of several split beam echosounders should be preferred to rotating systems, for future experiments. 相似文献
16.
Seyni Salack Alessandra Giannini Moussa Diakhaté Amadou. T. Gaye Bertrand Muller 《Climate Dynamics》2014,42(1-2):189-201
Intra-seasonal drought episodes (extreme dry spells) are strongly linked to crop yield loss in the West African Sahel, especially when they occur at crop critical stages such as juvenile or flowering stage. This paper seeks to expose potentially predictable features in the sub-seasonal to inter-annual occurrence of “extreme dry spells” (extDS) through their links to sea surface temperature anomalies (SSTAs). We consider two kinds of extreme dry spells: more than 2 weeks of consecutive dry days following a rain event (often found at the beginning of the rainy season, after the first rain events) and more than a week (observed towards the end of the rainy season, before the last rain events). We extract dry spells from daily rainfall data at 43 stations (31 stations in Senegal over 1950–2010 and 12 stations in Niger over 1960–2000) to identify the intra-seasonal distribution of extDS and their significant correlation with local rainfall deficits. Seasonality of distribution and high spatial coherence are found in the timing and the frequency of occurrence of extDS in different rainfall regions over Niger and Senegal. The correlation between the regional occurrence index (ROI), necessary to capture the spatial extent of extDS, and observed global sea surface temperature anomalies (SSTAs) sheds light on the influence of the external factors on the decadal, interannual and sub-seasonal variability of extDS over the West African Sahel. When the global tropics and the Atlantic are warmer than normal, more coherent and delayed June–July extDS are observed after onset of rainy season, as well as early cessation type in August–September. When the Indo-Pacific is cooler and the equatorial south Atlantic is warmer than normal little to no extDS are found in the onset sub-period of the monsoon season. Mostly late types of extDS occur in October as a result of late cessation. These results show potential predictability of extreme dry spells after onset and before cessation of monsoonal rain based on global patterns of sea surface temperature anomalies. 相似文献
17.
Ismaïla Diallo Caroline L. Bain Amadou T. Gaye Wilfran Moufouma-Okia Coumba Niang Mame D. B. Dieng Richard Graham 《Climate Dynamics》2014,43(3-4):575-594
The performance of the Hadley Centre Global Environmental Model version 3 regional climate model (HadGEM3-RA) in simulating the West African monsoon (WAM) is investigated. We focus on performance for monsoon onset timing and for rainfall totals over the June–July–August (JJA) season and on the model’s representation of the underlying dynamical processes. Experiments are driven by the ERA-Interim reanalysis and follow the CORDEX experimental protocol. Simulations with the HadGEM3 global model, which shares a common physical formulation with HadGEM3-RA, are used to gain insight into the causes of HadGEM3-RA simulation errors. It is found that HadGEM3-RA simulations of monsoon onset timing are realistic, with an error in mean onset date of two pentads. However, the model has a dry bias over the Sahel during JJA of 15–20 %. Analysis suggests that this is related to errors in the positioning of the Saharan heat low, which is too far south in HadGEM3-RA and associated with an insufficient northward reach of the south-westerly low-level monsoon flow and weaker moisture convergence over the Sahel. Despite these biases HadGEM3-RA’s representation of the general rainfall distribution during the WAM appears superior to that of ERA-Interim when using Global Precipitation Climatology Project or Tropical Rain Measurement Mission data as reference. This suggests that the associated dynamical features seen in HadGEM3-RA can complement the physical picture available from ERA-Interim. This approach is supported by the fact that the global HadGEM3 model generates realistic simulations of the WAM without the benefit of pseudo-observational forcing at the lateral boundaries; suggesting that the physical formulation shared with HadGEM3-RA, is able to represent the driving processes. HadGEM3-RA simulations confirm previous findings that the main rainfall peak near 10°N during June–August is maintained by a region of mid-tropospheric ascent located, latitudinally, between the cores of the African Easterly Jet and Tropical Easterly Jet that intensifies around the time of onset. This region of ascent is weaker and located further south near 5°N in the driving ERA-Interim reanalysis, for reasons that may be related to the coarser resolution or the physics of the underlying model, and this is consistent with a less realistic latitudinal rainfall profile than found in the HadGEM3-RA simulations. 相似文献
18.
Nitrogen sources in the South China Sea, as discerned from stable nitrogen isotopic ratios in rivers, sinking particles, and sediments 总被引：2，自引：0，他引：2
Stable nitrogen isotopic ratios were measured in sinking particles and surface sediments from the South China Sea (SCS) in order to study recent nitrogen sources and degradation. Average δ15N values of 16 sediment traps deployed at seven locations in the northern, central and southern SCS were uniformly low, ranging between 2.7 and 4.5‰ with a winter minimum in the northern and central SCS. Enhanced nitrogen contents and δ15N values were noted in samples affected by swimmers, comprising between 5 and 20% of total nitrogen fluxes. Nitrate sources were subsurface waters from the western Pacific, which were isotopically depleted due to the remineralization of nitrogen from nitrogen fixation in surface waters. Nitrogen fixation in the SCS contributed up to 20% to the settling particles. In the southern SCS, resuspended matter close to the shelf added to the sinking particulates. The long-term trap record from the central SCS revealed decreasing δ15N values during the 1990s, which correspond with findings from the North Pacific Subtropical Gyre and may be attributable to increased nitrogen fixation due to global warming-related stratification. This trend may be restricted to the 1990s but could also persist due to the projection of more frequent occurrence of El Niño conditions.The δ15N increase from swimmer-free trap averages of 2.7–3.6‰ to values of 5–6‰ in underlying deep-sea sediments was in the same range as in other deep ocean areas. Similar to results from the northern Indian Ocean, this increase could be related to isotopic enrichment during amino acid degradation. The lowest sedimentary δ15N values characterize the Pinatubo ash layer deposited off Luzon in an event of mass sedimentation in 1991. The fast deposition of organic matter drawn from the surface waters with the ash in the form of vertical density currents evidently preserved the planktonic δ15N signal. 相似文献