Soil moisture active and passive microwave products: intercomparison and evaluation over a Sahelian site

This paper presents a comparison and an evaluation of five soil moisture products based on satellite-based passive and active microwave measurements. Products are evaluated for 2005–2006 against ground measurements obtained from the soil moisture network deployed in Mali (Sahel) in the framework of the African Monsoon Multidisciplinary Analysis project. It is shown that the accuracy of the soil moisture products is sensitive to the retrieval approach as well as to the sensor type (active or passive) and to the signal frequency (from 5.6 GHz to 18.8 GHz). The spatial patterns of surface soil moisture are compared between the different products at meso-scale (14.5° N–17.5° N and 2° W–1° W). A general good consistency between the different satellite soil moisture products is shown in terms of meso-scale spatial distribution, in particular after convective rainfall occurrences. Comparison to ground measurement shows that although soil moisture products obtained from satellite generally over-estimate soil moisture values during the dry season, most of them capture soil moisture temporal variations in good agreement with ground station measurements

An Intercomparison of ERS-Scat, AMSR-E Soil Moisture Observations with Model Simulations over France

This paper presents a study undertaken in preparation of the work leading up to the assimilation of Soil Moisture and Ocean Salinity (SMOS) observations into the land surface model (LSM) Interaction Soil Biosphere Atmosphere (ISBA) at Météo-France. This study consists of an intercomparison experiment of different space-borne platforms providing surface soil moisture information [Advanced Microwave Scanning Radiometer for Earth Observing (AMSR-E) and European Remote Sensing Satellite Scatterometer (ERS-Scat)] with the reanalysis soil moisture predictions over France from the model suite of Système d'analyse fournissant des renseignements atmosphériques à la neige (SAFRAN), ISBA, and coupled model (MODCOU; SIM) of Météo-France for the years of 2003-05. Both modeled and remotely sensed data are initially validated against in situ observations obtained at the experimental soil moisture monitoring site Surface Monitoring of the Soil Reservoir Experiment (SMOSREX) in southwestern France. Two different AMSR-E soil moisture products are compared in the course of this study - the official AMSR-E product from the National Snow and Ice Data Center (NSIDC) and a new product developed at the Vrije Universiteit Amsterdam and NASA (VUA-NASA) - which were obtained using two different retrieval algorithms. This allows for an additional assessment of the different algorithms while using identical brightness temperature datasets. This study shows that a good correlation generally exists between AMSR-E (VUA-NASA), ERS-Scat, and SIM for low altitudes and low-to-moderate vegetation covers (1.5-3 kg

Pour une cartographie plurielle des continuités écologiques

La politique Trame verte et bleue (TVB), actuellement en cours de déploiement en France, s’inscrit dans un contexte de changement de perspective, au moins dans le discours, en matière d’action environnementale. corridors, trames et réseaux écologiques proposent de renouveler la vision aréale de la gestion de la biodiversité par une vision réticulaire. Il ne s’agit plus de se focaliser seulement sur les espaces et les espèces emblématiques, mais de considérer également la biodiversité ordinaire et de prendre en compte toutes les échelles. A une logique d’exceptionnalité, de localisme et de partition de l’espace (l’aire protégée, la réserve naturelle, etc.) se juxtapose désormais, une logique où tous les espaces, y compris les plus anthropisés, peuvent être pris en compte sous l’angle des continuités et fonctionnalités écologiques. Cette démarche suit les recommandations du Grenelle de l'environnement en associant les populations concernées la mise en oeuvre des politiques publiques.Ces mutations du cadre de référence de la conservation devraient permettre de repenser un appareillage conceptuel et méthodologique impliquant de nouvelles connaissances, de nouveaux outils, de nouveaux acteurs à tous les échelons territoriaux (du réseau écologique continental au document d’urbanisme local). Or, en dépit des discours, les connaissances mobilisées, les acteurs sollicités et les méthodes employées n’évoluent guère. es outils semblent faire défaut pour appréhender cette complexité et les pratiques demeurent marquées par une forte inertie. Les données, souvent inadaptées, sont généralement recyclées et doivent satisfaire des standards de la planification peu propices l’innovation. Face à ces enjeux, il nous semble pourtant possible de renouveler les méthodes en concevant de nouveaux outils avec les acteurs du territoire. Nous avons donc initié une démarche exploratoire visant favoriser la mobilisation et l’appropriation des enjeux par les acteurs locaux. Cette démarche s'insère dans le cadre de l’élaboration des Trames vertes et bleues de la région Poitou-Charentes et concerne les continuités écologiques bocagères.Le croisement d’une approche paysagère par enquête photographique et d’une démarche de déconstruction-reconstruction cartographique a permis de mettre en dialogue différents systèmes de représentation et d’interprétation : savoirs experts, savoirs scientifiques, savoirs vernaculaires, pour ensuite proposer un dispositif de médiation entre les constructions institutionnelles et les représentations vernaculaires de la biodiversité à l’échelon local. e recours au traitement d’images et au SIG offre le matériau exploité au sein d’ateliers participatifs et permet la traduction projectionnelle de représentations paysagères tangentielles. Les résultats de cette co-production cartographiques sont nécessairement multiples, l’argumentaire étant directement issu de la confrontation de points de vue des acteurs. a mise en forme spatialisée d‘argumentaires développés en ateliers offre un socle informationnel riche pour une cartographie plurielle des continuités écologiques. Une telle approche fondée sur des dispositifs participatifs, favorise une plus grande richesse des représentations cartographiques. La démarche montre que traiter des continuités écologiques ne consiste pas seulement à résoudre des questionnements scientifiques et techniques mais aussi à prendre en compte des enjeux de gouvernance. Elle replace également les dimensions sociales et culturelles au coeur du débat et interroge la pertinence de la carte comme support de médiatio

Multi-scale soil moisture measurements at the Gourma meso-scale site in Mali

This paper presents the ground soil moisture measurements performed over the so-called Gourma meso-scale site in Mali, Sahel, in the context of the African Monsoon Multidisciplinary Analysis (AMMA) project. The Gourma meso-scale soil moisture network is part of a complete land surface processes observing and modelling strategy and is associated to vegetation and meteorological field measurements as well as soil moisture remote sensing. It is spanning 2 degrees in latitude between 15 degrees N and 17 degrees N. In 2007, it includes 10 soil moisture stations, of which three stations also have meteorological and flux measurements. A relevant spatial sampling strategy is proposed to characterise soil moisture at different scales including local, kilometer, super-site and meso-scales. In addition to the local stations network, transect measurements were performed on different coarse textured (sand to sandy-loam) sites, using portable impedance probes. They indicate mean value and standard deviation (STD) of the surface soil moisture (SSM) at the kilometer scale. This paper presents the data set and illustrates soil moisture spatial and temporal features over the Sahelian Gourma meso-scale site for 2005-2006. Up-scaling relation of SSM is investigated from (i) local to kilometer scale and (ii) from local to the super site scale. It is shown to be stable in space and time (2005-2006) for different coarse textured sites. For the Agoufou local site, the up-scaling relation captures SSM dynamics at the kilometer scale with a 0.9% accuracy in volumetric soil moisture. At the multi-site scale, an unique up-scaling relation is shown to be able to represent kilometer SSM for the coarse textured soils of the meso-scale site with an accuracy of 2.2% (volumetric). Spatial stability of the ground soil moisture stations network is also addressed by the Mean Relative Difference (MRD) approach for the Agoufou super site where five soil moisture stations are available (about 25 km x 25 km). This allows the identification of the most representative ground soil moisture station which is shown to be an accurate indicator with low variance and bias of the soil moisture dynamics at the scale of the super site. Intensive local measurements, together with a robust up-scaling relation make the Gourma soil moisture network suitable for a large range of applications including remote sensing and land surface modelling at different spatial scales

Evaluation of AMSR-E soil moisture product based on ground measurements over temperate and semi-arid regions

Soil moisture (SM) products provided by remote sensing approaches at continental scale are of great importance for land surface modeling and numerical weather prediction. Before using remotely sensed SM products it is crucial to validate them. This paper presents an evaluation of AMSR-E (Advanced Microwave Scanning Radiometer - Earth Observing System) SM products over two sites. They are located in the south-west of France and in the Sahelian part of Mali in West Africa, in the framework of the SMOSREX (Surface Monitoring Of Soil Reservoir Experiment) and AMMA (African Monsoon Multidisciplinary Analysis) projects respectively. The most representative station of the four stations of each site is used for the comparison of AMSR-E derived and in-situ SM measurements in absolute and normalized values. Results suggest that, although AMSR-E SM product is not able to capture absolute SM values, it provides reliable information on surface SM temporal variability, at seasonal and rainy event scale. It is shown, however, that the use of radiometric products, such as polarization ratio, provides better agreement with ground stations than the derived SM products

The SMOS Soil Moisture Retrieval Algorithm

International audienceThe Soil Moisture and Ocean Salinity (SMOS) mission is European Space Agency (ESA's) second Earth Explorer Opportunity mission, launched in November 2009. It is a joint program between ESA Centre National d'Etudes Spatiales (CNES) and Centro para el Desarrollo Tecnologico Industrial. SMOS carries a single payload, an L-Band 2-D interferometric radiometer in the 1400-1427 MHz protected band. This wavelength penetrates well through the atmosphere, and hence the instrument probes the earth surface emissivity. Surface emissivity can then be related to the moisture content in the first few centimeters of soil, and, after some surface roughness and temperature corrections, to the sea surface salinity over ocean. The goal of the level 2 algorithm is thus to deliver global soil moisture (SM) maps with a desired accuracy of 0.04 m3/m3. To reach this goal, a retrieval algorithm was developed and implemented in the ground segment which processes level 1 to level 2 data. Level 1 consists mainly of angular brightness temperatures (TB), while level 2 consists of geophysical products in swath mode, i.e., as acquired by the sensor during a half orbit from pole to pole. In this context, a group of institutes prepared the SMOS algorithm theoretical basis documents to be used to produce the operational algorithm. The principle of the SM retrieval algorithm is based on an iterative approach which aims at minimizing a cost function. The main component of the cost function is given by the sum of the squared weighted differences between measured and modeled TB data, for a variety of incidence angles. The algorithm finds the best set of the parameters, e.g., SM and vegetation characteristics, which drive the direct TB model and minimizes the cost function. The end user Level 2 SM product contains SM, vegetation opacity, and estimated dielectric constant of any surface, TB computed at 42.5$^{circ}$, flags and quality indices, and other parameters o- interest. This paper gives an overview of the algorithm, discusses the caveats, and provides a glimpse of the Cal Val exercises

The AMMA-CATCH Gourma observatory site in Mali: Relating climatic variations to changes in vegetation, surface hydrology, fluxes and natural resources

The Gourma site in Mali is one of the three instrumented meso-scale sites deployed in West-Africa as part of the African Monsoon Multi-disciplinary Analysis (AMMA) project. Located both in the Sahelian zone sensu stricto, and in the Saharo–Sahelian transition zone, the Gourma meso-scale window is the northernmost site of the AMMA-CATCH observatory reached by the West African Monsoon. The experimental strategy includes deployment of a variety of instruments, from local to meso-scale, dedicated to monitoring and documentation of the major variables characterizing the climate forcing, and the spatio-temporal variability of surface processes and state variables such as vegetation mass, leaf area index (LAI), soil moisture and surface fluxes. This paper describes the Gourma site, its associated instrumental network and the research activities that have been carried out since 1984. In the AMMA project, emphasis is put on the relations between climate, vegetation and surface fluxes. However, the Gourma site is also important for development and validation of satellite products, mainly due to the existence of large and relatively homogeneous surfaces. The social dimension of the water resource uses and governance is also briefly analyzed, relying on field enquiry and interviews. The climate of the Gourma region is semi-arid, daytime air temperatures are always high and annual rainfall amounts exhibit strong inter-annual and seasonal variations. Measurements sites organized along a north–south transect reveal sharp gradients in surface albedo, net radiation, vegetation production, and distribution of plant functional types. However, at any point along the gradient, surface energy budget, soil moisture and vegetation growth contrast between two main types of soil surfaces and hydrologic systems. On the one hand, sandy soils with high water infiltration rates and limited run-off support almost continuous herbaceous vegetation with scattered woody plants. On the other hand, water infiltration is poor on shallow soils, and vegetation is sparse and discontinuous, with more concentrated run-off that ends in pools or low lands within structured endorheic watersheds. Land surface in the Gourma is characterized by rapid response to climate variability, strong intra-seasonal, seasonal and inter-annual variations in vegetation growth, soil moisture and energy balance. Despite the multi-decadal drought, which still persists, ponds and lakes have increased, the grass cover has largely recovered, and there are signs of increased tree cover at least in the low lands