Applications géodésiques du système DORIS à l'Institut géographique national

IGN is in charge of the installation and maintenance of the DORIS orbit determination network. More recently, in collaboration with JPL, precise geodetic computations were performed. The goal of this paper is to recall the various historic contributions of IGN to the DORIS system in their international context and then to describe a new estimation technique developed for a multi-satellite mode, making full profit of a better modeling for satellites and ground clocks as well as tropospheric correction parameters. Derived geodetic results demonstrate a precision in the order of 1 cm for station positions. To cite this article: P. Willis et al., C. R. Geoscience 337 (2005).     

Régimes des flux des matières solides en suspension au Cameroun: revue et synthèse à l’échelle des principaux écosystèmes; diversité climatique et actions anthropiques / Regimes of suspended sediment flux in Cameroon: review and synthesis for the main ecosystems; climatic diversity and anthropogenic activities

Abstract

Abstract Since the end of the 1950s, suspended matter measurements have been carried out in the streams and rivers of Cameroon. Despite the fact that these are often point measurements, they provide a framework for a global approach towards studying the regimes of suspended sediment transport in these rivers. The objective here is to assess the intensity of sediment transport and to determine the principal factors which influence it, according to the main climatic units. The influence of human activities is pointed out. It appears that steep slopes, population density, soil cultivation and cattle grazing are the essential factors. The sediment load in these rivers is increasing with latitude with 20–40 g m^-3 at the Equator, 80–100 g m^-3 in the transition zones and 150–160 g m^-3 in the dry tropical zones. The choice of drainage basin size for the characterisation of the rate of effective erosion is indispensable. In fact, in large drainage basins (5 × 10⁴ km²), there is an integration of heterogeneous geomorphological, phytogeographical, pedological and anthropogenic characteristics into average characteristics which do not show the influence of local conditions on sediment transport.     

Distribution d'un élément trace (Cr) dans un sol développé sur roches métamorphiques : variabilité à l'échelle d'un versantDistribution of chromium in soils developed into metamorphic rocks: variability at the hillslope scale

The influence of pedogenetic factors onto spatial variability of chromium in soil surface horizon was studied in a metamorphic area (Massif Central, France) at the hillside scale. A grid survey was performed. Surface soil horizons were sampled at each point and analysed in Cr. Results show the determinant influence of the rock type on Cr content distribution along the slope. However, surface horizons of these soils exhibit a lower Cr content heterogeneity than the deeper horizons. This was explained by soils' material erosion and deposition along the slope. Tillage and hedges also influence the Cr distribution in surface horizon along the slope. To cite this article: S. Salvador-Blanes et al., C. R. Geoscience 334 (2002) 51–58     

Soil moisture estimation using ERS 2 SAR data: a case study in the Solani River catchment/Estimation de l’humidité du sol grâce à des données ERS-2 SAR: étude de cas dans le bassin de la rivière Solani

Abstract

Abstract The information regarding spatial and temporal variation of soil moisture in a catchment is of utmost importance in hydrological, as well as many other studies. Point measurements from gravimetric and other methods for soil moisture determination are insufficient to understand the spatial behaviour of soil moisture in a region. Microwave remote sensing data from active sensors on board various satellites are increasingly being used to map spatial distribution of soil moisture within the 0–10 cm top surface. The northern part of India has a network of large rivers and canals and, therefore, spatial and temporal distribution of soil moisture in this region has a significant bearing on the hydrology of the region. In this paper, results on estimation of soil moisture from an ERS-2 SAR image in the catchment of the Solani River (a tributary to the River Ganga) in and around the town of Roorkee, India, have been presented. The radar backscatter coefficient for each pixel of the image has been modelled from the digital numbers of the SAR image. Gravimetric measurements have been made simultaneously during the satellite pass to determine the concurrent value of volumetric soil moisture at a large number of sample points within the satellite sweep area. The backscatter coefficient is found to vary from –30 dB to –42 dB for a variation in soil moisture from 30 to 75%. Regression analyses between volumetric soil moisture and both the digital numbers and backscatter coefficients were performed. Strong correlations between volumetric soil moisture and digital number were observed with R ² values of 0.84, 0.75 and 0.83 for bare soil, vegetative and combined surfaces, respectively. A similar trend was observed with the relationship between backscatter and volumetric soil moisture with R ² values of 0.60, 0.89 and 0.67 for bare soil, vegetative and combined surfaces, respectively. These results demonstrate the utilization of SAR data for estimation of spatial distribution of soil moisture in the region of the present study.     

CAMAM: A Miniature Laser Ablation Ionisation Mass Spectrometer and Microscope‐Camera System for In Situ Investigation of the Composition and Morphology of Extraterrestrial Materials

Performance studies of a microscope‐camera system (MCS) and a laser ablation/ionisation mass spectrometer (LIMS) instrument (referred to here as a laser mass spectrometer or LMS) are presented. These two instruments were designed independently for in situ analysis of solids on planetary surfaces and will be combined to a single miniature instrument suite for in situ chemical and morphological analysis of surface materials on planetary bodies. LMS can perform sensitive chemical (elemental, isotope and molecular) analyses with spatial resolution close to micrometre‐sized grains. It allows for studies with mass resolution (M/ΔM) up to 800 in ablation mode (elemental composition) and up to 1500 in desorption mode (molecular analysis). With an effective dynamic range of at least eight orders of magnitude, sensitive and quantitative measurements can be conducted of almost all elements and isotopes with a concentration larger than a few ppb atoms. Hence, in addition to the major element composition, which is important for the determination of mineralogical constituents of surface materials, trace elements can also be measured to provide information on mineral formation processes. Highly accurate isotope ratio measurements can be used to determine in situ geochronology of sample material and for investigations of various isotope fractionation processes. MCS can conduct optical imagery of mm‐sized objects at several wavelengths with micrometre spatial resolution for the characterisation of morphological surface details and to provide insight into surface mineralogy. Furthermore, MCS can help in the selection of sample surface areas for further mass spectrometric analysis of the chemical composition. Surface auto‐fluorescence measurements and images in polarised light are additional capabilities of the MCS, to identify either fluorescing minerals or organic materials, if present on the analysed surface, for further investigation by LMS. The results obtained by investigations of NIST reference materials, amino acid films and a natural graphite sample embedded in silicate rock are presented to illustrate the performance of the instruments and their potential to deliver chemical information for mineral and organic phases in their geological context.     

Investigation of the rainfall variability in central Tunisia / Investigations sur la variabilité pluviométrique en Tunisie centrale

Abstract

The decrease in runoff of the Merguellil basin, Tunisia recorded during the decade 1989–1998 led to the study of several time series derived from daily rainfall. It was found that variability characteristics related to daily rainfall greater than 30 mm are significantly different among successive time periods. In addition, the probability distributions of these rains, recorded at different raingauge sites, are significantly different between the period 1976–1989 and the prior and posterior observation periods. Furthermore, the surface covered by daily rains greater than 30 mm decreased between 1976 and 1989. It was also noted that the probability distributions of the surfaces covered changed significantly after 1976. The combination of human action and pluviometric variability (rainfall increase in the period 1989–1998, notably the increase in the number of days of rainfall greater than 30 mm) may explain the decrease in runoff of Merguellil Wadi.     

Structures spatiales de l'évapotranspiration de référence et des variables climatiques corrélées en Tunisie

Résumé

Nous étudions sur le territoire Tunisien, transition entre les climats sub-humide Méditerranéen et aride Saharien, l'organisation spatiale de l'évapotranspiration de référence décadaire interannuelle (1986–2003) et des variables climatiques utilisées pour son estimation: température de l'air, humidité relative de l'air, vitesse du vent et durée d'ensoleillement. L'étude est basée sur la modélisation du variogramme et la détermination de ses paramètres (pépite, pente ou palier et portée). Nous considérons également des indicateurs sur le comportement à l'origine du variogramme et sur la variation de la structure spatiale des variables étudiées. L'étude montre que les modèles des variogrammes changent d'une variable à une autre et d'une période de l'année à une autre. Cependant, ils sont de type linéaire dans la plupart des cas. Le variogramme de l'évapotranspiration de référence est sans pépite, quelle que soit la période de l'année. Pour les autres variables, notamment la température maximale de l'air, l'humidité relative de l'air et la durée d'ensoleillement, la pépite est plus importante pendant la période chaude de l'année. Le variogramme de la vitesse du vent est sphérique à grande portée sur toute l'année, et la structure de l'ensoleillement est pépitique pendant l'été et linéaire le reste de l'année. L'évapotranspiration de référence présente un gradient structural semblable à celui de l'humidité relative durant toute l'année, mais différent de ceux des autres variables climatiques.

Editeur Z.W. Kundzewicz; Editeur associé G. Mahé

Citation Baccour, H., Slimani, M. et Cudennec, C., 2012. Structures spatiales de l'évapotranspiration de référence et des variables climatiques corrélées en Tunisie. Hydrological Sciences Journal, 57 (4), 818–829.     

Analyse multi-échelles de la variabilité spatiale de l’équivalent en eau de la neige (EEN) sur le territoire de l’Est du Canada

La présente étude a pour objectif d’analyser la variabilité spatiale multi-échelles de l’équivalent en eau de la neige (EEN). Dans un premier temps, la variabilité spatiale de l’EEN par rapport à la latitude et à la longitude a été analysée. Des indices locaux ont été utilisés pour caractériser les différentes structures spatiales. Par la suite, les structures spatiales homogènes ont été délimitées à l’aide de l’approche de segmentation spatiale multi-résolutions en utilisant des méta-variables physiographiques. La segmentation a été validée à l’aide du test non paramétrique de Kruskal-Wallis appliqué aux données de l’EEN de chaque paire de zones adjacentes. À l’échelle régionale, la segmentation spatiale a permis d’identifier six zones géographiques différenciées par leur position par rapport aux modes de circulations atmosphériques et la disposition du relief. À l’échelle locale, la segmentation spatiale montre le rôle de la pente, de la courbure, etc. dans la variabilité spatiale du couvert nival.

Editeur Z. W. Kundzewicz; Editeur associé E. Gargouri     

Determining Spatially-Distributed Annual Water Balances for Ungauged Locations on Shikoku Island,Japan: A Comparison of Two Interpolators/Détermination de Bilans Hydriques Spatialisés pour des Sites Non-Jaugés de L'Īle de Shikoku,au Japon: Comparaison de Deux Interpolateurs

Abstract

Methods were evaluated for interpolating precipitation (P), evapotranspiration (ET), and runoff (RO) at ungauged points on Shikoku Island, Japan, using data gathered from gauged stations on the same island. Two methods were examined: a “local” cubic spline interpolator, which, for a given point, fitted the function exactly to nearby gauged data points; and a “global” multivariate regression interpolator, which fitted the function to all gauged data points based on their topographic positions (i.e. latitude, longitude, altitude). Local and global interpolators did not generate similar results for P and temperature (T). The spatial density of gauged data points used in the interpolation affected the performance of the interpolators. With any given density of gauged data points included in the interpolation, the local interpolator outperformed the global interpolator. The findings indicate that local interpolators are more accurate predictors of the spatial distribution of water balance components in mountainous regions such as Shikoku Island.     

Comportement de l'oscillateur DORIS/Jason au passage de l'anomalie sud-atlantique

We point out an acceleration of the DORIS clock on-board the Jason satellite during passes over the South Atlantic Anomaly (SAA). When this effect is ignored in the current geodetic positioning of the DORIS stations, derived coordinates show almost linear trends in time, corresponding to anomalous horizontal and vertical velocities of the order of 1 m yr^?1. We propose a simple scientific explanation of this physical phenomenon that is corroborated by direct Jason/TOPEX clock comparisons with respect to the DORIS master beacons in Kourou and Toulouse. To cite this article: P. Willis et al., C. R. Geoscience 336 (2004).