Frequency analysis of annual maximum suspended sediment concentrations in Abiod wadi,Biskra (Algeria)          下载免费PDF全文

A. Benkhaled  H. Higgins  F. Chebana  A. Necir 《水文研究》2014,28(12):3841-3854

Sediment transport processes in the Mediterranean semi‐arid areas have interested a great number of hydrologists and statisticians. Frequency analysis (FA) procedures are commonly applied for several hydrological events such as floods and droughts. However, in general, FA is not widely applied to treat suspended sediment concentration (SSC), especially in semi‐arid regions. In the present study, an FA was performed on SSC data from 1979 to 1991 observed upstream from the Foum El Gherza dam, which is located at Biskra in the South–East of Algeria. This dam is problematic in terms of silting. Therefore, this study is mainly motivated by providing an FA model and risk evaluation in order to assist dam managers to better evaluate the potential of silting resulting from SSC transport to the reservoir. Probability distributions commonly used in hydrology were tested to SSC data recorded at the M'chounech station on Abiod wadi located upstream the dam. All the FA steps were considered; including classical techniques (e.g. goodness‐of‐fit tests, statistical criteria) as well as recently developed tools (tail distribution classification). Their application led to the selection of the lognormal distribution (LN2) to fit the considered data, and hence an accurate risk assessment could be obtained. Because of the presence of a slight trend, non‐stationary models are also considered in the present study. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

A Simple Model for the Afternoon and Early Evening Decay of Convective Turbulence Over Different Land Surfaces     

Daniel F. Nadeau  Eric R. Pardyjak  Chad W. Higgins  Harinda Joseph S. Fernando  Marc B. Parlange 《Boundary-Layer Meteorology》2011,141(2):301-324

A simple model to study the decay of turbulent kinetic energy (TKE) in the convective surface layer is presented. In this model, the TKE is dependent upon two terms, the turbulent dissipation rate and the surface buoyancy fluctuations. The time evolution of the surface sensible heat flux is modelled based on fitting functions of actual measurements from the LITFASS-2003 field campaign. These fitting functions carry an amplitude and a time scale. With this approach, the sensible heat flux can be estimated without having to solve the entire surface energy balance. The period of interest covers two characteristic transition sub-periods involved in the decay of convective boundary-layer turbulence. The first sub-period is the afternoon transition, when the sensible heat flux starts to decrease in response to the reduction in solar radiation. It is typically associated with a decay rate of TKE of approximately t ⁻² (t is time following the start of the decay) after several convective eddy turnover times. The early evening transition is the second sub-period, typically just before sunset when the surface sensible heat flux becomes negative. This sub-period is characterized by an abrupt decay in TKE associated with the rapid collapse of turbulence. Overall, the results presented show a significant improvement of the modelled TKE decay when compared to the often applied assumption of a sensible heat flux decreasing instantaneously or with a very short forcing time scale. In addition, for atmospheric modelling studies, it is suggested that the afternoon and early evening decay of sensible heat flux be modelled as a complementary error function.  相似文献   

Paleoenvironmental implications of taxonomic variation among δN values of chloropigments     

Meytal B. Higgins  Felisa Wolfe-Simon  Rebecca S. Robinson  Mak A. Saito 《Geochimica et cosmochimica acta》2011,75(22):7351-7363

Natural variations in the ratios of nitrogen isotopes in biomass reflect variations in nutrient sources utilized for growth. In order to use δ¹⁵N values of chloropigments of photosynthetic organisms to determine the corresponding δ¹⁵N values of biomass - and by extension, surface waters - the isotopic offset between chlorophyll and biomass must be constrained. Here we examine this offset in various geologically-relevant taxa, grown using nutrient sources that may approximate ocean conditions at different times in Earth’s history. Phytoplankton in this study include cyanobacteria (diazotrophic and non-diazotrophic), eukaryotic algae (red and green), and anoxygenic photosynthetic bacteria (Proteobacteria), as well as environmental samples from sulfidic lake water. Cultures were grown using N₂, NO₃⁻, and NH₄⁺ as nitrogen sources, and were examined under different light regimes and growth conditions. We find surprisingly high variability in the isotopic difference (δ¹⁵N_biomass − δ¹⁵N_{chloropigment}) for prokaryotes, with average values for species ranging from −12.2‰ to +11.7‰. We define this difference as ε_por, a term that encompasses diagenetic porphyrins and chlorins, as well as chlorophyll. Negative values of ε_por reflect chloropigments that are ¹⁵N-enriched relative to biomass. Notably, this enrichment appears to occur only in cyanobacteria. The average value of ε_por for freshwater cyanobacterial species is −9.8 ± 1.8‰, while for marine cyanobacteria it is −0.9 ± 1.3‰. These isotopic effects group environmentally but not phylogenetically, e.g., ε_por values for freshwater Chroococcales resemble those of freshwater Nostocales but differ from those of marine Chroococcales. Our measured values of ε_por for eukaryotic algae (range = 4.7-8.7‰) are similar to previous reports for pure cultures. For all taxa studied, values of ε_por do not depend on the type of nitrogen substrate used for growth. The observed environmental control of ε_por suggests that values of ε_por could be useful for determining the fractional burial of eukaryotic vs. cyanobacterial organic matter in the sedimentary record.  相似文献   

Effects of magnesium ions on near-equilibrium calcite dissolution: Step kinetics and morphology   总被引：1，自引：0，他引：1  

Man Xu  Steven R. Higgins 《Geochimica et cosmochimica acta》2011,75(3):719-6592

Dissolution kinetics at the aqueous solution-calcite interface at 50 °C were investigated using in situ atomic force microscopy (AFM) to reveal the influence of magnesium concentration and solution saturation state on calcite dissolution kinetics and surface morphology. Under near-equilibrium conditions, dissolved Mg²⁺ displayed negligible inhibitory effects on calcite dissolution even at concentrations of . Upon the introduction of , the solution saturation state with respect to calcite, , acted as a “switch” for magnesium inhibition whereby no significant changes in step kinetics were observed at Ω_calcite<0.2, whereas a sudden inhibition from Mg²⁺ was activated at Ω_calcite?0.2. The presence of the Ω-switch in dissolution kinetics indicates the presence of critical undersaturation in accordance with thermodynamic principles. The etch pits formed in solutions with exhibited a unique distorted rhombic profile, different from those formed in Mg-free solutions and in de-ionized water. Such unique etch pit morphology may be associated with the anisotropy in net detachment rates of counter-propagating kink sites upon the addition of Mg²⁺.  相似文献   

Reactions of radium and barium with the surfaces of carbonate minerals     

Mark J. Jones  Laura J. Butchins  John M. CharnockRichard A.D. Pattrick  Joe S. SmallDavid J. Vaughan  Paul L. WincottFrancis R. Livens 《Applied Geochemistry》2011,26(7):1231-1238

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An XMM–Newton observation of Ton S180: constraints on the continuum emission in ultrasoft Seyfert galaxies     

S. Vaughan  Th. Boller  A. C. Fabian  D. R. Ballantyne  W. N. Brandt  J. Trümper 《Monthly notices of the Royal Astronomical Society》2002,337(1):247-255

We present an XMM–Newton observation of the bright, narrow-line, ultrasoft type 1 Seyfert galaxy Ton S180. The  0.3–10 keV  X-ray spectrum is steep and curved, showing a steep slope above 2.5 keV  (Γ∼ 2.3)  and a smooth, featureless excess of emission at lower energies. The spectrum can be adequately parametrized using a simple double power-law model. The source is strongly variable over the course of the observation but shows only weak spectral variability, with the fractional variability amplitude remaining approximately constant over more than a decade in energy. The curved continuum shape and weak spectral variability are discussed in terms of various physical models for the soft X-ray excess emission, including reflection off the surface of an ionized accretion disc, inverse Compton scattering of soft disc photons by thermal electrons, and Comptonization by electrons with a hybrid thermal/non-thermal distribution. We emphasize the possibility that the strong soft excess may be produced by dissipation of accretion energy in the hot, upper atmosphere of the putative accretion disc.  相似文献   

Ridge-trench collision-induced switching of arc tectonics and magma sources: clues from Antarctic Peninsula mafic dykes     

J.H. Scarrow  A.P.M. Vaughan  P.T. Leat 《地学学报》1997,9(5-6):255-259

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The iron line in MCG–6-30-15 from XMM–Newton: evidence for gravitational light bending?     

A. C. Fabian  S. Vaughan 《Monthly notices of the Royal Astronomical Society》2003,340(3):L28-L32

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‘Opening up’ geoengineering appraisal: Multi-Criteria Mapping of options for tackling climate change     

Rob Bellamy  Jason Chilvers  Naomi E. Vaughan  Timothy M. Lenton 《Global Environmental Change》2013,23(5):926-937

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Conodont paleoecology | C.R. Barnes (editor), 1976. The geological association of Canada special paper, 15, 324 pp., $ 12.00     

Alan C. Higgins 《Earth》1978,14(1):85-86

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