Role of dynamic vegetation in regional climate predictions over western Africa   总被引：2，自引：1，他引：1  

Clement Aga Alo  Guiling Wang 《Climate Dynamics》2010,35(5):907-922

This study examines the role of vegetation dynamics in regional predictions of future climate change in western Africa using a dynamic vegetation model asynchronously coupled to a regional climate model. Two experiments, one for present day and one for future, are conducted with the linked regional climate-vegetation model, and the third with the regional climate model standing alone that predicts future climate based on present-day vegetation. These simulations are so designed in order to tease out the impact of structural vegetation feedback on simulated climate and hydrological processes. According to future predictions by the regional climate-vegetation model, increase in LAI is widespread, with significant shift in vegetation type. Over the Guinean Coast in 2084–2093, evergreen tree coverage decreases by 49% compared to 1984–1993, while drought deciduous tree coverage increases by 56%. Over the Sahel region in the same period, grass cover increases by 31%. Such vegetation changes are accompanied by a decrease of JJA rainfall by 2% over the Guinean Coast and an increase by 23% over the Sahel. This rather small decrease or large increase of precipitation is largely attributable to the role of vegetation feedback. Without the feedback effect from vegetation, the regional climate model would have predicted a 5% decrease of JJA rainfall in both the Guinean Coast and the Sahel as a result of the radiative and physiological effects of higher atmospheric CO₂ concentration. These results demonstrate that climate- and CO₂-induced changes in vegetation structure modify hydrological processes and climate at magnitudes comparable to or even higher than the radiative and physiological effects, thus evincing the importance of including vegetation feedback in future climate predictions.  相似文献   

Scaling of adsorption reactions: U(VI) experiments and modeling     

Vijay A. Loganathan  Mark O. Barnett  T. Prabhakar Clement  Sushil R. Kanel 《Applied Geochemistry》2009,24(11):2051-2060

Iron-coated sands were prepared via two common protocols, a precipitation method, where Fe was precipitated directly onto the sand in a single step, and an adsorption method, where pure goethite was prepared in the first step and then adsorbed onto the sand in a second step. The coated sands from both the systems were characterized using scanning electron microscopy, energy-dispersive spectroscopy, X-ray diffraction, and selective Fe extraction. Although neither of the methods produced a completely crystalline Fe coating, the precipitation method produced sands with larger portions of amorphous Fe than the adsorption method, with the fraction of amorphous Fe decreasing with increasing Fe content. Uranium(VI) adsorption isotherms and pH adsorption edges were measured on three coated sands with Fe contents ranging from 0.04% to 0.3%. Experimentally, the adsorption of U(VI) onto the three sands was more comparable when normalized to surface area than when normalized to Fe content. A surface complexation model, although originally developed for U(VI) adsorption onto amorphous Fe oxide, captured the differences in adsorption when adjusted for the surface area of the coated sand. The findings indicate that surface area is a better scaling parameter than Fe content in predicting U(VI) adsorption to Fe-dominated media. These findings are significant because many common surface complexation models are parameterized on the basis of Fe content rather than specific surface area. Although the interactions of U(VI) and Fe-coated sands were used as representative adsorbate and adsorbent, the general principles may be applicable to other adsorbate–adsorbent systems as well.  相似文献   

Spectrum of climate change and streamflow alteration at a watershed scale     

Dagbegnon Clement Sohoulande Djebou 《Environmental Earth Sciences》2017,76(19):653

Worldwide, evidences of water cycle alteration and fresh water resources depletion are frequently reported with various magnitudes. This alteration in the hydrologic cycle is often regarded as a signal of the actual climate change. However, the debate on climate change seems to have preferentially focused on global-scale patterns such that the rich knowledge gathered in the domain is virtually less integrated to decision making at the watershed level. Indeed, the watershed apprehension of climate change is probably an imperative for sustainable water resources planning. The scope of the present study aligns with that imperative as it aims at conciliating patterns of climate change with observations of hydrologic alterations at the watershed level. Specifically, the paper describes the interplay between land-cover changes and the terrestrial water cycle disturbances under climate change at the global level. Thereafter, it reports a watershed-level analysis of streamflow, land-cover, PET and precipitation alteration. Specially, the case study focused on the Brazos River basin, located in the USA and shared by the states of Texas and New Mexico. From both regional and watershed prospects, signals of hydrologic alteration during the time period 1955–2014 are highlighted and then implications of climate change are discussed. The results show an overall longitudinal gradient of precipitation changes and a latitudinal gradient of PET changes across the Brazos watershed. However, these gradients of changes seem to be driven by regional climate components which extend beyond the physical boundary of the Brazos watershed. Mann–Kendall’s analysis of discharge time series (annual average, minimum and maximum) at 10 different stations exhibits meaningful contrasts from upstream to downstream. An assessment of land-cover changes shows critical patterns of landscape change across the watershed. The analyses depicted signals of urbanization sprawl and land-cover degradation. Specially, the significant statistical relationships observed between the time series of maximum green vegetation fraction (MGVF) and streamflow also indicate that the origin of the observed hydrologic alteration is anthropogenic. Ultimately, the results are discussed within the scope of climate change.  相似文献   

Laboratory and numerical investigation of flow and transport near a seepage-face boundary   总被引：3，自引：0，他引：3  

Simpson MJ  Clement TP  Gallop TA 《Ground water》2003,41(5):690-700

Laboratory and numerical modeling investigations were completed to study the unconfined ground water flow and transport processes near a seepage-face boundary. The laboratory observations were made in a radial sand tank and included measurements of the height of the seepage face, flow velocity near the seepage face, travel time distribution of multiple tracer slugs, and streamlines. All the observations were reliably reproduced with a three-dimensional, axi-symmetric, variably saturated ground water flow model. Physical data presented in this work demonstrate and quantify the importance of three-dimensional transport patterns within a seepage-face zone. The results imply that vertically averaged flow models that employ Dupuit approximations might introduce error in the analysis of localized solute transport near a seepage-face boundary. The experimental dataset reported in this work will also be of interest for those who are attempting to validate a numerical algorithm for solving ground water and contaminant discharge patterns near a surface-water boundary.  相似文献   

Evolution of Hawaiian basalts: a hotspot melting model     

Mian Liu  Clement G. Chase 《Earth and Planetary Science Letters》1991,104(2-4)

Melt generation and extraction along the Hawaiian volcanic chain should be largely controlled by the thermal structure of the Hawaiian swell and the heat source underneath it. We simulate numerically the time- and space-dependent evolution of Hawaiian volcanism in the framework of thermal evolution of the Hawaiian swell, constrained by residual topography, geoid anomalies, and anomalous heat flow along the Hawaiian volcanic chain. The transient heat transfer problem with melting relationships and variable boundary conditions is solved in cylindrical coordinates using a finite difference method. The model requires the lithosphere to be thinned mechanically by mantle plume flow. Melting starts quickly near the base of the plate when the hotspot is encountered. Thermal perturbation and partial melting are largely concentrated in the region where the original lithosphere is thinned and replaced by the mantle flow. The pre-shield Loihi alkalic and tholeiitic basalts are from similar sources, which are a mixture of at least three mantle components: the mantle plume, asthenosphere, and the lower lithosphere. The degree of partial melting averages 10–20%, with a peak value of 30% near the plume center. As a result of continuous compaction, melts are extracted from an active partial melting zone of about 10–20 km thickness, which moves upwards and laterally as the heating and compaction proceed. The rate of melt extraction from the swell increases rapidly to a maximum value of 1 × 10⁵ km³/m.y. over the center of the heat source, corresponding to eruption of large amounts of tholeiitic lavas during the shield-building stage. This volume rate is adequate to account for the observed thickness of the Hawaiian volcanic ridge. Melts from direct partial melting of the mantle plume at depth may be important or even dominant at this stage, although the amount is uncertain. At the waning stage, mixing of melts from the mantle flow pattern with those from low-degree partial melting of the lithosphere may produce postshield alkalic basalts. After the plate moves off the heat source, continuous conductive heating can cause very low degree partial melting (less than 1%) of the lithosphere at shallow depths for about one million years. This process may be responsible for producing post-erosional alkalic basalts. The extraction time for removing such small amount of melts is about 0.4–2 m.y., similar to the time gap between the eruption of post-erosional alkalic lavas and the shield-building stage. Our results show that multi-stage Hawaiian volcanism and the general geochemical characteristics of Hawaiian basalts can be explained by a model of plume-plate interaction.  相似文献   

Beitrag zur feinstmahlung von industriemineralen     

M. Clement  E. Huwald 《International Journal of Mineral Processing》1978,5(2):199-210

Industrial minerals of a high degree of fineness such as kaolinite, calcite and barite are used to an increasing extent in the chemical industries as filling material in the manu-facturing of paper, paints and plastics or as ingredients of ceramic products. Grains smaller than 5 μm can be produced by dry grinding in a jet mill or by wet grinding in drum mills. To avoid impurities due to wear, these mills are lined with rubber or plastic and ceramic balls are used as grinding bodies.Our investigations in a jet mill show that the fineness of the ground product depends on the feed rate and on the air volume. The grain sizes received in the drum mill are influenced by feed rate, pulp density, speed of rotation and by the grinding bodies (density, size).REM-photographs of mill discharge in the grain-size ranges 10-5 μm and below 5 μm indicate that in both the grains mainly show crystal planes as boundaries. After dry grinding the grains show smooth and clean faces; thus is traced back to the impact effect of the air streams. In wet grinding the fine grains are very prone to form agglomerates.  相似文献   

Coulees volcaniques dans le paleozoique superieur des zones internes helleniques (Grece continentale) — environnement sedimentaire et caracterisation petrographique     

Bernard Clement  Henriette Lapierre   《Tectonophysics》1980,70(3-4):261-283

Volcanic flows of Late Paleozoic age in the Pelagonian zone belonging to the internal Hellenic zones, have been studied, for the first time, from petrographical and geochemical viewpoints.

Relevant petrographical data and the geochemical analysis lead us to consider that:

1. (a) the basic lavas can be linked to tholeitic basalts.

2. (b) the acid lavas are associated to metarhyolites.

3. (c) the two magmatic flows do not originate from the same parental magma.

The geochemical results obtained, compared to those from other deposits of nearly the same age outcropping around the Mediterranean, indicate that the latest Hercynian volcanism has been affected by intra-plate distension phenomena preceding the great Lower Mesozoic break-up which caused the formation of the Tethys Ocean.

It is remarkable that in the Mesogean Basin the sedimentary environment is nearly similar to that described in this paper. The volcanic flows end up in a sialic sedimentary basin containing coarse detritic formations that resulted from erosion of the Variscan chain along the southern margin of the European continent.

Abstract

Dans la zone pélagonienne appartenant aux zones internes helléniques, des coulées volcaniques d'âge paléozoïque supérieur sont étudiées, pour la première fois, pétrographiquement et géochimiquement. Les différents agencements pétrographiques et les analyses géochimiques nous apprennent que:

1. (a) les laves basiques peuvent être rattachées á la famille des basaltes tholéitiques.

2. (b) les laves acides s'apparentent aux métarhyolites.

La quantification des différents éléments analyses nous amène á considérer que les deux lignées magmatiques ne sont pas issues d'un même magma parental.

La comparaison des résultats géochimiques avec d'autres gisements, sensiblement contemporains affleurant autour de la Méditerranée actuelle, nous indique que le volcanisme fini-hercynien est assujetti á des phénomènes de distension intraplaques, préparatoires á la grande fracturation mésozoïque inférieure ayant donné naissance á l'océan téthysien.

Ce qui est remarquable dans le bassin mésogéen, c'est que l'environnement sédimentaire est toujours, á peu près, comparable á celui que nous décrivons ici. En effet, les coulées volcaniques s'épanchent dans un bassin sédimentaire á fond sialique au sein de formations détritiques grossières nées de la destruction de la chaîne varisque sur la bordure méridionale du continent europeen.  相似文献   

Oceanic island Pb: Two-stage histories and mantle evolution     

Clement G. Chase 《Earth and Planetary Science Letters》1981,52(2):277-284

Leads in basaltic suites from seven oceanic islands form linear arrays on²⁰⁶Pb/²⁰⁴Pb versus²⁰⁷Pb/²⁰⁴Pb diagrams. These arrays are more reasonably interpreted as secondary isochrons than as mixing lines, because of their systematic relationship. Separate two-stage histories calculated for the leads from each island indicate that the source materials for the magmas were derived from a single primary reservoir with present²³⁸U/²⁰⁴Pb of 7.91 ± 0.04 by secondary enrichment in U/Pb at different times from 2.5 to 1 Ga ago. This is confirmed by a plot of isochron slope versus intercept, on which the points describing each island's Pb-Pb array all lie very near a single straight line. The isochrons for the Canary Islands and Hawaii, at least, are significantly different. The²⁰⁸Pb/²⁰⁴Pb versus²⁰⁶Pb/²⁰⁴Pb relationships are less coherent. The lead isotopic characteristics are consistent with a model in which lead in the oceanic island magmas is derived from ancient subducted oceanic crust. In particular, this explains the close relationship between lead in mid-ocean ridge and oceanic island basalts without invoking mixing.  相似文献   

Untersuchungen zur kreislaufmahlung in einer aufbereitung für komplexe sulfidische erze     

G.B. Schneider  M. Clement 《International Journal of Mineral Processing》1976,3(1):83-96

Circuit grinding was investigated in the Rammelsberg ore dressing plant. From short-interval sampling it was possible to make a statement about the non-stationary and stationary state of grinding. The objective of these investigations was to establish the distribution curves of the quantitative flow, of the grain sizes and of the mineralogical components in the individual streams of the circuit grinding.  相似文献   

Two-Well Test Method for Determining Hydraulic Properties of Aquifers     

T. P. Clement  M. J. Truex  B. S. Hooker 《Ground water》1997,35(4):698-703

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