Plant community patterns in a gypsum area of NE Spain. II. Effects of ion washing on topographic distribution of vegetation     

Joaquín Guerrero-Campo  Francisco Alberto  Melchor Maestro  John Hodgson  Gabriel Montserrat-Martí 《Journal of Arid Environments》1999,41(4):411

A model, based on the physico-chemical properties of gypsum, is postulated in order to interpret the regular distribution of plant communities associated with the topography and the main environmental stresses to which each vegetation type is subjected in a landscape unit.The leaf chemical composition of 28 representative plant species supports the main predictions of the model: species from low areas present large concentrations of Na, N and ash but low contents of Ca. It is demonstrated that the strong nutrient and salt downwashing in the hilly terrain is an important cause of the vegetation segregation observed on a large spatial scale.  相似文献   

The Temperature–Humidity Covariance in the Marine Surface Layer: A One-dimensional Analytical Model     

Gabriel G. Katul  Anna M. Sempreviva  Daniela Cava 《Boundary-Layer Meteorology》2008,126(2):263-278

An analytical model that predicts how much of the temperature–humidity covariance within the marine atmospheric surface layer (ASL) originates just above the ASL and just near the surface is proposed and tested using observations from the Risø Air Sea Experiment (RASEX). The model is based on a simplified budget for the two-scalar covariance that retains three basic terms: production, dissipation, and vertical transport. Standard second-order closure formulations are employed for the triple moments and the dissipation terms, and the canonical mixing length for the closure model is assumed linear with height (z) from the surface. Despite the poor performance of the gradient–diffusion closure in reproducing the measured triple moment, the overall covariance model was shown to be sufficiently robust to these assumptions. One of the main findings from the analytical treatment is the origin of the asymmetry in how the top and bottom boundary conditions affect the two-scalar covariance in the ASL. The analytical model reveals that ‘bottom-up’ boundary-condition variations scale with \(z^{-\sqrt{a}}\) , while ‘top-down’ variations scale with \(z^{\sqrt{a}}\) , where a is a constant that can be derived from similarity and closure constants. The genesis of this asymmetry stems from the flux-transport term but is modulated by the dissipation, and persists even in the absence of any inhomogeneity in the local production function. It is shown that the local production function acts to adjust the relative proportions of these two boundary conditions with weights that vary with the Obukhov length. The findings here do not provide ‘finality’ to the discussions on the covariance between humidity and temperature or the role of entrainment in modulating the turbulence within the ASL. Rather, they are intended to guide new hypotheses about interpretations of existing field data and identify needs for future field and numerical experiments.  相似文献   

Are fossil groups a challenge of the cold dark matter paradigm?     

Stefano Zibetti  Daniele Pierini  Gabriel W. Pratt 《Monthly notices of the Royal Astronomical Society》2009,392(2):525-536

We study six groups and clusters of galaxies suggested in the literature to be 'fossil' systems (i.e. to have luminous diffuse X-ray emission and a magnitude gap of at least 2 mag R between the first and the second ranked member within half of the virial radius), each having good quality X-ray data and Sloan Digital Sky Survey (SDSS) spectroscopic or photometric coverage out to the virial radius. The poor cluster AWM 4 is clearly established as a fossil system, and we confirm the fossil nature of four other systems (RX J1331.5+1108, RX J1340.6+4018, RX J1256.0+2556 and RX J1416.4+2315), while the cluster RX J1552.2+2013 is disqualified as fossil system. For all systems, we present the luminosity functions within 0.5 and 1 virial radius that are consistent, within the uncertainties, with the universal luminosity function of clusters. For the five bona fide fossil systems, having a mass range  2 × 10¹³–3 × 10¹⁴ M_⊙  , we compute accurate cumulative substructure distribution functions (CSDFs) and compare them with the CSDFs of observed and simulated groups/clusters available in the literature. We demonstrate that the CSDFs of fossil systems are consistent with those of normal observed clusters and do not lack any substructure with respect to simulated galaxy systems in the cosmological Λ cold dark matter (ΛCDM) framework. In particular, this holds for the archetype fossil group RX J1340.6+4018 as well, contrary to earlier claims.  相似文献   

Positional deformation analysis in polygonometric structures     

Gabriel Weiss 《Studia Geophysica et Geodaetica》1991,35(4):251-259

Summary The paper describes the analysis of positional displacements of points on an object, which are connected polygonometrically with assumed stable points of reference. This net, sighted in both the epochs considered, is adjusted jointly first with the elimination of the object points and then with the elimination of the reference points, depending on the purpose of testing the linear hypothesis in the appropriate model.  相似文献   

Estimating the influence of increased nitrate loads from wastewater on phosphorus mobility in river sediments     

Oliver Gabriel  Dagmar Balla  Thomas Kalettka  Christian Schilling  Matthias Zessner 《洁净——土壤、空气、水》2006,34(4):333-345

Research on the sediment‐surface water transition zone in three study site regions, different in hydrological conditions, was conducted to estimate to which extent nitrate in surface water can contribute to riverbed sediment oxidation and thus prevent release of sediment phosphorus to surface water. Consequently, the Du tch Flow Model (DUFLOW) based water quality model “Spreewald” and results from the emission model “ Mo delling N utrient E missions in Ri ver S ystems” (MONERIS) were used to estimate to which extent wastewater treatment plants (WWTPs) could contribute to the NO₃^– concentration in surface waters if they were operated without denitrification. It is demonstrated that an effective phosphorus retention in sediments by means of surface water NO₃^– is possible only under specific conditions, i. e. (i) a sufficient supply of surface water to the sediment by advective fluxes, (ii) a sufficient amount of sediment iron to provide phosphorus binding sites in the case of sediment oxidation, (iii) a redox system not leading to a rapid NO₃^– and FeOOH depletion and to phosphorus release from organic pools by high microbial activities. Model results show that in‐stream denitrification processes counteract a significant increase of NO₃^– surface water concentrations from WWTPs operated without denitrification during summer, when eutrophication risk through phosphorus is highest. The increase of NO₃^– surface water concentrations in winter due to decreased denitrification in the surface water is of minor relevance for phosphorus release from sediments.  相似文献   

The relative importance of ejections and sweeps to momentum transfer in the atmospheric boundary layer     

Gabriel Katul  Davide Poggi  Daniela Cava  John Finnigan 《Boundary-Layer Meteorology》2006,120(3):367-375

Using an incomplete third-order cumulant expansion method (ICEM) and standard second-order closure principles, we show that the imbalance in the stress contribution of sweeps and ejections to momentum transfer (ΔS _o ) can be predicted from measured profiles of the Reynolds stress and the longitudinal velocity standard deviation for different boundary-layer regions. The ICEM approximation is independently verified using flume data, atmospheric surface layer measurements above grass and ice-sheet surfaces, and within the canopy sublayer of maturing Loblolly pine and alpine hardwood forests. The model skill for discriminating whether sweeps or ejections dominate momentum transfer (e.g. the sign of ΔS _o ) agrees well with wind-tunnel measurements in the outer and surface layers, and flume measurements within the canopy sublayer for both sparse and dense vegetation. The broader impact of this work is that the “genesis” of the imbalance in ΔS _o is primarily governed by how boundary conditions impact first and second moments.  相似文献   

Recovering Enhanced Resolution Images And Digital Elevation Models From Compressed Image Sequences     

Gabriel Scarmana  John G. Fryer 《The Photogrammetric Record》2004,19(106):149-162

A device-independent algorithm for the estimation of an enhanced resolution image from a low-resolution compressed sequence is proposed. The algorithm utilises least squares matching to extract the interdependence of the low-resolution images. This algorithm also has the effect of attenuating compression artefacts. Improving the spatial resolution of the image sequence is not the only goal of the enhancement algorithm, as the enhanced images in turn lead to digital elevation models (DEMs) of improved accuracy.
Experimental results illustrate the algorithm's performance as a tool for digital photogrammetric applications. Stereoscopic sets of left and right images were taken of objects of known geometry and DEMs were created using both the original coarse images and compressed images enhanced by the algorithm.  相似文献   

The structure and the phyllosilicates (chemistry, crystallinity and texture) of Talas Ala-Tau (Tien Shan, Kyrgyz Republic): comparison with more recent subduction complexes     

Isabel Abad  Gabriel Gutirrez-Alonso  Fernando Nieto  Igor Gertner  Alex Becker  Ana Cabero 《Tectonophysics》2003,365(1-4):103

Geological mapping and structural analysis of the Talas Ala Tau (Tien Shan, Kyrgyz Republic) have revealed a complex structure composed of folds with axial-plane cleavage and thrust faults verging towards the NE. The main structures of the range correspond to minor Tertiary and Carboniferous–Permian deformation superimposed on the main deformation event that took place during the Baikalian orogeny. The pervasive axial-plane cleavage diminishes in penetrativity from the hinterland to the foreland in both the Uzunakhmat and Karagoin sheets. The main thrusts developed phyllonitic shear-related rocks on the hangingwall immediately above the thrust planes. A crystal-chemical study of the phyllosilicates growth during the Baikalian deformation event along a cross-section revealed changes in the crystallinity, composition and lattice parameters of them. The phyllosilicates present in the Talas Ala Tau rocks were crystallized in very low-grade metamorphic conditions, that is below 300 °C, as indicated by their Kübler Index (KI), which decreases from SW towards the NE. Detailed TEM study of the phyllosilicates reveals a clear textural difference at the lattice level between samples with higher or lower KI parameters. There is also a clear difference in crystal-chemical parameters (KI and b) and composition between the phyllosilicates growth in relation to the axial-plane cleavage and the ones belonging to the thrust-related phyllonites. The first ones are more affected by the ferrimuscovitic vector than the phyllosilicates of phyllonites, closer to the theoretical phengitic component. Huge ranges of values of phengitic content of micas at sample level are interpreted as the result of a decompression path from at least 8 kbar. We propose a subduction geodynamic environment for the regional deformation and the origin of the phyllosilicates, as they are similar to those obtained in more recent accretionary complexes.  相似文献   

Magnitude of Stokes’ drift in coastal waters     

Gabriel Godin 《Ocean Dynamics》1995,47(4):277-286

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Verknüpfung von Wasserbeschaffenheit und Wassermenge für das Grundwasser des Festgesteinsbereiches     

M. Schultze  H. Fischer  B. Gabriel 《洁净——土壤、空气、水》1990,18(6):673-681

Patterns of the typical groundwater quality for 13 hydrogeological units (lithofacies units) of the solid rock and for 5 stages of the contamination of groundwater by the agricultural production are derived from investigations carried out in six regions serving as examples and in evaluation of data series covering many years. The concentration values of the substances contained in water constituting the lower and upper limits of the typical width of variation are regarded as the quality of the groundwater reservoir (long retention time) and a fast runoff component of groundwater. On the basis of this model conception it is possible to determine one of the following four quantities from the other three quantities: groundwater quality, size of the groundwater reservoir, groundwater runoff with the shares of the slow component and of the fast one, removal of substances out of the root zone.  相似文献