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121.
Ernest Amoussou Yves Tramblay Henri S.V. Totin Gil Mahé Pierre Camberlin 《水文科学杂志》2013,58(11):2060-2071
RésuméLes inondations des dernières années en Afrique de l’Ouest rendent nécessaire la prévision hydrologique pour réduire la vulnérabilité des populations. Nous avons étudié la dynamique des crues dans le bassin du Mono en amont du barrage de Nangbéto (Togo et Bénin) par des méthodes statistiques et déterministes. L’augmentation des précipitations au cours de la décennie 1990 et la mise en eau du barrage de Nangbéto en 1988 ont modifié le régime du Mono, dont les débits d’étiages ont augmenté. Les débits entrants et sortants du barrage sont équivalents, même lors des crues. Les maximums annuels des débits entrants peuvent être modélisés par une distribution des valeurs extrêmes (GEV) de type Fréchet, tandis que ceux des débits sortants, suivent une loi GEV de type Weibull (bornée). Le mécanisme dominant de génération des crues dans le bassin est la montée progressive des eaux associée à l’augmentation de la saturation des sols. L’analyse des débits de crue entre 1988 et 2010 n’a pas montré d’augmentation de la magnitude ou de la fréquence des crues. Le modèle GR4J reproduit bien les débits de crues en période calage, mais ces critères de performance se dégradent lorsque l’on s’en éloigne. Dans ces conditions, l’utilisation de ce modèle pour évaluer les risques futurs de crues serait hasardeuse et il est nécessaire d’envisager d’autres types d’approches. 相似文献
122.
Deepak Jhajharia Brijesh K. Yadav Sunil Maske Surajit Chattopadhyay Anil K. Kar 《Comptes Rendus Geoscience》2012,344(1):1-13
Trends in rainfall, rainy days and 24 h maximum rainfall are investigated using the Mann-Kendall non-parametric test at twenty-four sites of subtropical Assam located in the northeastern region of India. The trends are statistically confirmed by both the parametric and non-parametric methods and the magnitudes of significant trends are obtained through the linear regression test. In Assam, the average monsoon rainfall (rainy days) during the monsoon months of June to September is about 1606 mm (70), which accounts for about 70% (64%) of the annual rainfall (rainy days). On monthly time scales, sixteen and seventeen sites (twenty-one sites each) witnessed decreasing trends in the total rainfall (rainy days), out of which one and three trends (seven trends each) were found to be statistically significant in June and July, respectively. On the other hand, seventeen sites witnessed increasing trends in rainfall in the month of September, but none were statistically significant. In December (February), eighteen (twenty-two) sites witnessed decreasing (increasing) trends in total rainfall, out of which five (three) trends were statistically significant. For the rainy days during the months of November to January, twenty-two or more sites witnessed decreasing trends in Assam, but for nine (November), twelve (January) and eighteen (December) sites, these trends were statistically significant. These observed changes in rainfall, although most time series are not convincing as they show predominantly no significance, along with the well-reported climatic warming in monsoon and post-monsoon seasons may have implications for human health and water resources management over bio-diversity rich Northeast India. 相似文献
123.
124.
During the last decades, several integrated studies of Tethyan Jurassic/Cretaceous boundary
sections from different countries were published with the objective to indicate problems for the selection
of biological, chemical or physical markers suitable for identification of the Jurassic/Cretaceous
boundary e the only system boundary within the Phanerozoic still not fixed by GSSP. Drawing the
boundary between the Jurassic and Cretaceous systems is a matter of global scale discussions. The
problem of proposing possible J/K boundary stratotypes results from lack of a global index fossils,
global sea level drop, paleogeographic changes causing development of isolated facies areas, as well
as from the effect of Late Cimmerian Orogeny. This contribution summarizes and comments data on
J/K boundary interval obtained from several important Tethyan sections and shows still existing problems
and discrepancies in its determination. 相似文献
125.
The different timing results of the magnetar Swift J1822.3—1606 are analyzed and understood theoretically.It is noted that different timing solutions are caused not only by timing noise,but also because the period derivative is decreasing after the outburst.Both the decreasing period derivative and the large timing noise may originate from wind braking associated with the magnetar.Future timing of Swift J1822.3—1606 will help clarify whether or not its period derivative is decreasing with time. 相似文献
126.
We report the Balmer broad absorption lines (BALs) in the quasar SDSS J2220 + 0109 discovered from the SDSS data, and present a detailed analysis of the peculiar absorption line spectrum, including the He I* multiplet at λλ3189, 3889 arising from the metastable 23s-state helium and the Balmer Hα and Hβ lines from the excited hydrogen H I of n = 2 level, which are rarely seen in quasar spectra, as well as many absorption lines arising from the excited Fe II* of the levels 7 955 cm−1, 13 474 cm−1 and 13 673 cm−1 in the wavelength range 3100∼3300 Å. Ca II H, K absorption line doublets also clearly appear in the SDSS spectrum. All absorption lines show a similar blueshifted velocity structure of Δv ≈ − 1500 ∼ 0 km·s−1 relative to the quasar's systematic redshift determined from the emission lines. Detailed analysis suggests that the Balmer absorption lines should arise from the partially ionized region with a column density of NHI ≈ 1021 cm−2 for an electron density of ne ∼ 106 cm−3; and that the hydrogen n = 2 level may be populated via collisional excitation with Lyα pumping. 相似文献
127.
K.. Gabnyi N. Marchili T.P. Krichbaum S. Britzen L. Fuhrmann A. Witzel J.A. Zensus P. Müller X. Liu H.G. Song J.L. Han X.H. Sun 《Astronomische Nachrichten》2007,328(8):863-866
Short time‐scale radio variations of compact extragalactic radio quasars and blazars known as IntraDay Variability (IDV) can be explained in at least some sources as a propagation effect; the variations are interpreted as scintillation of radio waves in the turbulent interstellar medium of the Milky Way. One of the most convincing observational arguments in favor of a propagation‐induced variability scenario is the observed annual modulation in the characteristic time scale of the variation due to the Earth's orbital motion. So far there are only two sources known with a well‐constrained seasonal cycle. Annual modulation has been proposed for a few other less well‐documented objects. However, for some other IDV sources source‐intrinsic structural variations which cause drastic changes in the variability time scale were also suggested. J1128+592 is a recently discovered, highly variable IDV source. Previous, densely time‐sampled flux‐density measurements with the Effelsberg 100‐m radio telescope (Germany) and the Urumqi 25‐m radio telescope (China), strongly indicate an annual modulation of the time scale. The most recent 4 observations in 2006/7, however, do not fit well to the annual modulation model proposed before. In this paper, we investigate a possible explanation of this discrepancy. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
128.
A. Merloni T. Di Matteo † A. C. Fabian 《Monthly notices of the Royal Astronomical Society》2000,318(1):L15-L19
The simultaneous presence of a strong quasi-periodic oscillation, of period ∼10 s, in the optical and X-ray light curves of the X-ray transient XTE J1118+480 suggests that a significant fraction of the optical flux originates from the inner part of the accretion flow, where most of the X-rays are produced. We present a model of magnetic flares in an accretion disc corona where thermal cyclo-synchrotron emission contributes significantly to the optical emission, while the X-rays are produced by inverse Compton scattering of the soft photons produced by dissipation in the underlying disc and by the synchrotron process itself. Given the observational constraints, we estimate the values for the coronal temperature, optical depth and magnetic field intensity, as well as the accretion rate for the source. Within our model we predict a correlation between optical and hard X-ray variability and an anticorrelation between optical and soft X-rays. We also expect optical variability on flaring time-scales (∼tens of ms), with a power-density spectrum similar to that observed in the X-ray band. Finally, we use both the available optical/extreme-ultraviolet/X-ray spectral energy distribution and the low-frequency time variability to discuss limits on the inner radius of the optically thick disc. 相似文献
129.
基于J2EE/XML的分布式WebGIS平台系统设计与实现 总被引:1,自引:1,他引:0
分析了目前WebGIS各种实现模式的优缺点以及利用J2EE构建分布式WebGIS的优越性;提出了一种基于J2EE/XML构建的四层体系结构的分布式DragonMapWeb系统;并就该系统,分别探讨了与其相关的系统构架、EJB类组织、数据模型、关键技术、功能设计等问题. 相似文献
130.
冀北滦平地区中生代火山岩地层锆石U-Pb测年及启示 总被引:26,自引:0,他引:26
在冀北—辽西地区, 冀北的滦平地区是中生代地层最为齐全, 也是唯一的晚中生代沉积作用基本连续的地区.许多研究者认为该地区的土城子组与张家口之间存在着J3—K1界线和中生代的构造转换界面.该地区分布的主要中生代地层自下而上依次为髫髻山组、土城子组、张家口组、大北沟组、大店子组和西瓜园组.通过LAICPMS方法的锆石UPb测年, 获得了该地区中生代火山岩地层的年代格架, 即髫髻山组的顶界年龄为(162.8±3.2)Ma, 土城子组形成的主体年代范围是(142.6±1.3)~(136.4±1.9)Ma, 张家口组形成的年代范围是(135.7±1.8)~(135.2±2.3)Ma, 大店子组顶部安山岩的年龄是(131.4±3.7)~(130.2±3.0)Ma.锆石的稀土元素分析表明: 土城子组中锆石的稀土元素特征与张家口组中锆石的特征一致, 而与髫髻山组中的锆石有较大的区别.综合以上分析数据及野外地质特征可以得出以下结论: (1)该地区的土城子组与髫髻山组之间不仅时间间隔较长, 而且二者中火山岩的源区也有较大的区别; 土城子组与张家口组之间不仅时间上基本连续, 而且二者中火山岩的源区也有明显的一致性.这就表明了滦平地区的土城子组与张家口组是同一地质背景下的产物, 即该地区二者之间应不存在J3—K1界面和构造转换界面.(2)滦平地区大店子组顶部火山岩在年代上与辽西北票—义县地区的义县组底部、辽西凌源地区的张家口组顶部基本相当. 相似文献