Éditeur(s) : T. Fisher Unwin T. Fisher Unwin ( London )
Résumé : (Statement of Responsibility) explained by J.J. Thomas.

Droits : Applicable rights reserved.
06150384
01011139
http://ufdc.ufl.edu/AA00000555/00001

North America -- United States Virgin Islands
Caribbean
1970-2005

Éditeur(s) : HAL CCSD Society for Range Management
Résumé : Grasslands and agroecosystems occupy one-third of the terrestrial area, but their contribution to the global carbon cycle remains uncertain. We used a set of 316 site-years of CO(2) exchange measurements to quantify gross primary productivity, respiration, and light-response parameters of grasslands, shrublands/savanna, wetlands, and cropland ecosystems worldwide. We analyzed data from 72 global flux-tower sites partitioned into gross photosynthesis and ecosystem respiration with the use of the light-response method (Gilmanov, T. G., D. A. Johnson, and N. Z. Saliendra. 2003. Growing season CO(2) fluxes in a sagebrushsteppe ecosystem in Idaho: Bowen ratio/energy balance measurements and modeling. Basic and Applied Ecology 4:167-183) from the RANGEFLUX and WORLDGRASSAGRIFLUX data sets supplemented by 46 sites from the FLUXNET La Thuile data set partitioned with the use of the temperature-response method (Reichstein, M., E. Falge, D. Baldocchi, D. Papale, R. Valentini, M. Aubinet, P. Berbigier, C. Bernhofer, N. Buchmann, M. Falk, T. Gilmanov, A. Granier, T. Grunwald, K. Havrankova, D. Janous, A. Knohl, T. Laurela, A. Lohila, D. Loustau, G. Matteucci, T. Meyers, F. Miglietta, J.M. Ourcival, D. Perrin, J. Pumpanen, S. Rambal, E. Rotenberg, M. Sanz, J. Tenhunen, G. Seufert, F. Vaccari, T. Vesala, and D. Yakir. 2005. On the separation of net ecosystem exchange into assimilation and ecosystem respiration: review and improved algorithm. Global Change Biology 11: 1.424-1439). Maximum values of the quantum yield (alpha = 75 mmol.mol(-1)), photosynthetic capacity (A(max) = 3.4 mg CO(2) . m(-2).s-1), gross photosynthesis (P(g,max) = 1.16 g CO(2) . m(-2).d(-1)), and ecological light-use efficiency (epsilon(ecol) = 59 mmol . mol(-1)) of managed grasslands and high-production croplands exceeded those of most forest ecosystems, indicating the potential of nonforest ecosystems for uptake of atmospheric CO(2). Maximum values of gross primary production (8 600 g CO(2) . m(-2).yr(-1)), total ecosystem respiration (7 900 g CO(2) . m(-2).yr(-1)), and net CO(2) exchange (2 400 g CO(2) . m(-2).yr(-1)) were observed for intensively managed grasslands and high-yield crops, and are comparable to or higher than those for forest ecosystems, excluding some tropical forests. On average, 80% of the nonforest sites were apparent sinks for atmospheric CO(2), with mean net uptake of 700 g CO(2) . m(-2).yr(-1) for intensive grasslands and 933 g CO(2) . m(-2).d(-1) for croplands. However, part of these apparent sinks is accumulated in crops and forage, which are carbon pools that are harvested, transported, and decomposed off site. Therefore, although agricultural fields may be predominantly sinks for atmospheric CO(2), this does not imply that they are necessarily increasing their carbon stock.
ISSN: 1550-7424

hal-01190754
https://hal.archives-ouvertes.fr/hal-01190754
DOI : 10.2111/rem-d-09-00072.1
PRODINRA : 214043

Résumé : En 2003 des mesures au spectromètre IR à Transformée de Fourier (FTIR) ont permis d'identifier et de mesurer les COV émis par la décharge à ciel ouvert de la Gabarre, principale de l'île Guadeloupe, située entre une zone urbaine et une mangrove. Ces COV ont été retrouvés (2004) la nuit dans les cités, justifiant les plaintes des riverains. Dans le cadre de cette thèse, des mesures au spectromètre de masse portatif MS 200 ont validé ces résultats du FTIR. De nouvelles mesures au MS 200 ont été menées dans toute la zone de la décharge. Les cités concernées étant à l'opposé du flux synoptique d'Alizés-Est, les facteurs météorologiques permettant la dispersion et le transport des COV de la décharge vers la zone urbaine ont été recherchés. La diminution nocturne de l'intensité des Alizés au dessus de l'ile peut laisser place à des phénomènes locaux tels les brises. L'idée d'une brise terre-mer a été éliminée. Un maillage autou~ de la décharge (cités et mangrove) avec 8 thermomètres a révélé un îlot de chaleur urbain nocturnegénérant une brise thermique d'environ Ims- I (mesurée et calculée). Avec les radiosondages Météo France et un SODAR installé dans la décharge, une très forte stabilité dans les basses couches atmosphériques de la couche limite nocturne avec une inversion de surface d'environ 120mvv apparait. Ces facteurs expliquent la pollution des cités par les COV de la décharge, Un modèlevGaussien en tenant compte a été validé par les mesures de COV.vCette étude peut être étendue à d'autres décharges à ciel ouvert et à d'autres types de traitement de polluants de décharge.
In 2003, the VOC emissions coming from "La Gabarre", the main open landfill in Guadeloupe, located in-between an urban area and a mangrove, were identified and quantified with a portable FTIR spectrometer. In 2004, COVs found at nighttime in the urban area nearby confirmed why residents complain about. As part of this thesis, portable mass spectrometer MS 200 measurements validated these FTIR figures. New systematic SM measurements have been carried on around the landfill. Since the polluted urban area stands on the opposite way of the East Trade winds synoptic flux, aIl the weather factors likely to scatter and transport the dump COVs were scrutinized. At night, the strength of the Trade winds decreases over Guadeloupe, which may give way to local phenomena such as breezes. The occurrence of land/sea breeze was eliminated. A close surveying surrounding the landfill with 8 thermometers both in the projects and in the mangrove revealed an urban heat island causing thermal breezes of about lms-l (measured and calculated). Using soundings from Meteo France, and a SODAR inside the dump, we found a great stability of the night boundary layer with a surface inversion near 120m. Pollution of the nearby urban area with landfill COVs is elucidatcd by the above factors. A transport Gaussian model is in agreement with COV measurements. This study can be extended to different open landfills and different types of polluting matters processes in dumps

http://www.theses.fr/2013AGUY0700/document

Éditeur(s) : HAL CCSD American Geophysical Union
Résumé : International audience
Time-lapse microgravity surveying combined with absolute gravity measurements is used to investigate water storage changes in a karst aquifer of similar to 100 km(2) area. The survey consists of 40 gravity stations measured with a relative gravimeter; absolute gravity is measured at three stations for each survey. In total, four gravity surveys are performed over a 2 year time period during consecutive wet and dry periods. Survey precisions range between 2.4 and 5 mu Gal, enabling statistically significant detection of 10 mu Gal change, i.e., similar to 0.25 m equivalent water level change. Observed gravity changes are coherent between consecutive survey periods, i.e., net water withdrawal and net water recharge is observed, reaching changes as high as 22 mu Gal. Observed gravity changes allow refining evapotranspiration estimates, which may serve to improve the water budget of the aquifer. High-and low-gravity amplitude zones characterize the karst system, demonstrating spatially variable storage behavior. Geomorphologic considerations are invoked to explain the location of preferential zones of water storage, and a conceptual model of water storage is discussed for the studied karst.
ISSN: 0148-0227

hal-00496400
https://hal.archives-ouvertes.fr/hal-00496400
DOI : 10.1029/2009JB006616

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
We use 3 yr of surface absolute gravity measurements at three sites on the Larzac plateau (France) to quantify the changes induced by topography and the building on gravity time-series, with respect to an idealized infinite slab approximation. Indeed, local topography and buildings housing ground-based gravity measurement have an effect on the distribution of water storage changes, therefore affecting the associated gravity signal. We first calculate the effects of surrounding topography and building dimensions on the gravity attraction for a uniform layer of water. We show that a gravimetric interpretation of water storage change using an infinite slab, the so-called Bouguer approximation, is generally not suitable. We propose to split the time varying gravity signal in two parts (1) a surface component including topographic and building effects (2) a deep component associated to underground water transfer. A reservoir modelling scheme is herein presented to remove the local site effects and to invert for the effective hydrological properties of the unsaturated zone. We show that effective time constants associated to water transfer vary greatly from site to site. We propose that our modelling scheme can be used to correct for the local site effects on gravity at any site presenting a departure from a flat topography. Depending on sites, the corrected signal can exceed measured values by 5-15 μGal, corresponding to 120-380 mm of water using the Bouguer slab formula. Our approach only requires the knowledge of daily precipitation corrected for evapotranspiration. Therefore, it can be a useful tool to correct any kind of gravimetric time-series data.
ISSN: 0956-540X

hal-00795590
https://hal.archives-ouvertes.fr/hal-00795590
DOI : 10.1093/gji/ggs007

Éditeur(s) : HAL CCSD Springer
Résumé : International audience
Thiswork examines the relevance of the inclusion of ground-based gravity data in the calibration process of a global rainfall-discharge reservoir model. The analysis is performed for the Durzon karst system (Larzac, France). The first part of the study focuses on the hydrological interpretation of the ground-based gravity measurements. The second part of the study investigates further the information content of the gravity data with respect to water storage dynamics modelling.The gravity-derived information is found unable to either reduce equifinality of the single-objective, dischargebased model calibration process or enhance model performance through assimilation.
ISSN: 1866-6280

hal-00812624
https://hal-brgm.archives-ouvertes.fr/hal-00812624
DOI : 10.1007/s12665-012-1856-z

Éditeur(s) : HAL CCSD
Résumé : Karst systems are the most important groundwater reservoirs in Mediterranean areas. Their structure and functioning are complex and not well known because of the high heterogeneity created and organised by groundwater flow. Water storage entities and water storage changes within remain difficult to assess by direct measurement and modelling. Geodetic monitoring is here used to constrain and bring original insights on karst system functioning. The Durzon karst system (Larzac plateau, Southern France) is monitored by several geodetic techniques: - Monthly absolute gravity measurements at three sites - Time-lapse surface to depth gravity measurements - Time-lapse microgravity surveying - Continuous long base hydrostatic tiltmeter monitoring at two sites Absolute gravity monitoring complemented with relative gravity surveys allows quantification of water storage changes on the studied karst system. Interannual water storage changes as detected by gravity are ~400 mm in amplitude. These storage changes are shown to occur in the epikarst and infiltration zone in certain areas. Water storage variations spatial heterogeneity is also demonstrated on the karst system. Tilt measurements monitor fast and slow infiltration processes within the karst fractured media. Karst system geodetic monitoring brings both quantitative and integrated hydrologic knowledge. Such methods could be applied to other karst aquifer studies.
Les aquifères karstiques constituent l'essentiel des ressources en eau souterraine sur le pourtour de la Méditerranée. Leur structure et leur fonctionnement sont complexes et mal connus à cause du processus de karstification qui hiérarchise la circulation hydrodynamique en leur sein. La localisation du stockage de l'eau et de ses variations au sein de tels aquifères est difficile à mesurer et à modéliser. Nous utilisons les outils de la géodésie pour apporter de nouvelles contraintes sur le fonctionnement hydrologique et les variations de stock d'eau en milieu karstique. Ainsi, le système karstique du Durzon, (Causse du Larzac, France) fait l'objet d'un suivi par plusieurs méthodes géodésiques incluant: - Des mesures de gravimétrie absolue en trois sites répétées de manière mensuelle ; - Des mesures répétées de gravimétrie relative entre la surface et la profondeur du karst ; - Des mesures répétées de microgravimétrie ; - Des mesures inclinométriques longue base en deux sites. Nous démontrons que le couplage entre la gravimétrie absolue et relative permet d'appréhender les variations de stock d'eau sur le bassin d'alimentation étudié. Les variations inter-anuelles observées sont de l'ordre de 400 mm, et nous montrons que celles-ci s'opèrent dans certaines zones au sein des horizons supérieurs du karst : épikarst et zone d'infiltration. Une forte hétérogénéité spatiale des variations de stockage est aussi mise en évidence à l'échelle du bassin d'alimentation. Les mesures inclinométriques permettent de suivre les phénomènes d'infiltration rapide et lente dans la masse karstique fracturée. Le suivi géodésique de ce système karstique permet d'apprécier quantitativement et de façon intégrée son fonctionnement hydrologique et pourra être transposé pour l'étude d'autres aquifère karstiques.
https://tel.archives-ouvertes.fr/tel-00404851

tel-00404851
https://tel.archives-ouvertes.fr/tel-00404851
https://tel.archives-ouvertes.fr/tel-00404851/document
https://tel.archives-ouvertes.fr/tel-00404851/file/THESE_jacob.pdf

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
In this study we attempt to understand the water storage variations in a karst aquifer on the Larzac Plateau (South of France) using ground-based gravimetry. Surface to 60 m depth gravity measurements are performed three times a year since 2006 down a pothole, in complement to monthly absolute gravity (AG) measurements at three sites. The time variations of the surface to depth (STD) gravity differences are compared with the AG variations. Using a simple Bouguer plate model, we find that the STD gravity differences are very similar to the AG variations. The STD gravity differences are then used to determine apparent density values. These integrative density values are compared with measured grain densities from core samples to obtain an apparent porosity and saturation change representative of the investigated depth. The apparent porosity ranges from 4.8 to 7.3 per cent. We then discuss on the repartition of the apparent physical properties with respect to the epikarst and infiltration zone karst structures. We argue that AG and STD differences monitor epikarst water storage variations. Within this scope, we discuss the fact that seasonal scale water storage variation occurs predominantly in the epikarst.
ISSN: 0956-540X

hal-00413528
https://hal.archives-ouvertes.fr/hal-00413528
DOI : 10.1111/j.1365-246X.2009.04118.x

Éditeur(s) : HAL CCSD American Geophysical Union
Résumé : International audience
The aim of this study is to bring new information on water storage dynamics in karst systems from tiltmeter measurements. Newly developed long-base hydrostatic tiltmeters are installed at two sites on the Larzac plateau (France) in a karst aquifer of similar to 100 km(2) recharge area. Each site is located within a karst cave where two tilt directions are monitored. Significant reversible tilt deformation reaching amplitudes of 10(-6) to 10(-5) rad was observed at both sites following heavy precipitation. Elastic parameters were determined with an experiment in which a tiltmeter site was loaded by up to 25 t of weight at strategic locations at the ground surface. Mechanisms responsible for the observed tilt were then examined by means of finite element modeling. Deformation induced by water pressure changes in fractures is our preferred interpretation as the most plausible mechanism. Within this scope, we used the tiltmeter responses to extract time constants using lumped parameter modeling. Time constants are interpreted to be associated with the filling and emptying of fractures nearby the tiltmeters. These instruments therefore appear as useful tools to study the local dynamics of water infiltration in karst systems.
ISSN: 0043-1397

hal-00534461
https://hal.archives-ouvertes.fr/hal-00534461
DOI : 10.1029/2009WR008082

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
We investigate the contribution of atmospheric and its induced non-tidal oceanic loading effects on surface time-varying gravity and tilt measurements for several stations in Western Europe. The ocean response to pressure forcing can be modelled accordingly to the inverted barometer, i.e. assuming that air pressure variations are fully compensated by static sea height changes, or using ocean general circulation models.Wevalidate two runs of theHUGO-mbarotropic ocean model by comparing predicted sea surface height variations with hundred tide-gauge measurements along the European coasts. We then show that global surface pressure field, as well as a barotropic high-resolution ocean model forced by air pressure and winds allow in most cases a significant reduction of the variance of gravity residuals and, to a smaller extends tilt residuals. Wefinally show that precise gravity measurements with superconducting gravimeters allow the observation of large storm surges, occurring in the North Sea, even for inland stations. However, we also confirm that the continental hydrology contribution cannot be neglected. Thanks to their specific sensitivity feature, only tiltmeters closest to the coast can clearly detect the loading due to these storm surges.
ISSN: 0264-3707

hal-00708046
https://hal.archives-ouvertes.fr/hal-00708046
https://hal.archives-ouvertes.fr/hal-00708046/document
https://hal.archives-ouvertes.fr/hal-00708046/file/ETS2008_BOY.pdf
BIBCODE : 2009JGeo...48..182B

Éditeur(s) : HAL CCSD Elsevier
Résumé : TAG LITHO
In this study we attempt to understand the water storage variations in a karst aquifer on the Larzac Plateau (South of France) using absolute gravimetry. On this karst system, water input is exclusively rainfall and draining occurs at the Durzon perennial spring in a karstic valley. Our basis assumption is that water storage in the vadose zone (epikarst and infiltration zone) has a significant effect on surface gravity through Newtonian attraction. The karst aquifer is hence being monitored since January 2006 with monthly absolute gravity measurements at three sites. The gravity measurements are corrected for regional scale gravity using water storage models in order to obtain gravity variations related exclusively to the local water storage variations. The gravity variations exhibit an important seasonal component (10-15 mu gal corresponding to a variation of equivalent water stab thickness of similar to 24-36 cm). Water storage variation seen by gravimetry is not spatially uniform on the studied karst system, corroborating current geomorphologic observations and interpretations. Finally, we use a global mass balance consideration linking rainfall, evapotranspiration and spring discharge to provide a conceptual framework for the understanding of observed gravity variations.
ISSN: 0022-1694

hal-00411884
https://hal.archives-ouvertes.fr/hal-00411884
DOI : 10.1016/j.jhydrol.2008.06.020