Global Soil Moisture Patterns Observed by Space Borne Microwave Radiometers and Scatterometers

Within the scope of the upcoming launch of a new water related satellite mission (SMOS) a global evaluation study was performed on two available global soil moisture products. ERS scatterometer surface wetness data was compared to AMSR-E soil moisture data. This study pointed out a strong similarity between both products in sparse to moderate vegetated regions with an average correlation coefficient of 0.83. Low correlations were found in densely vegetated areas and deserts. The low values in the vegetated regions can be explained by the limited soil moisture retrieval capabilities over dense vegetation covers. Soil emission is attenuated by the canopy and tends to saturate the microwave signal with increasing vegetation density, resulting in a decreased sensor sensitivity to soil moisture variations. It is expected that the new low frequency satellite mission (SMOS) will obtain soil moisture products with a higher quality in these regions. The low correlations in the desert regions are likely due to volume scattering or to the dielectric dynamics within the soil. The volume scattering in dry soils causes a higher backscatter under very dry conditions than under conditions when the sub-surface soil layers are somewhat wet. In addition, at low moisture levels the dielectric constant has a reduced sensitivity in response to changes in the soil moisture content. At a global scale the spatial correspondence of both products is high and both products clearly distinguish similar regions with high seasonal and inter annual variations. Based on the global analyses we concluded that the quality of both products was comparable and in the sparse to moderate vegetated regions both products may be beneficial for large scale validation of SMOS soil moisture. Some limitations of the studied products are different, pointing to significant potential for combining both products into one superior soil moisture data set. © The Author(s) 2008

Modeling forest emissivity at L-band and a comparison with multitemporal measurements

This letter describes recent advances in modeling forest emissivity at L-band. The formulation is based on a previously developed discrete model and includes a new representation of forest litter. Comparisons with multitemporal radiometric data collected in the framework of the "Bray 2004" experiment, which was carried out within Les Landes forest, are shown and discussed. Input variables are given by using detailed ground measurements. In general, the model reproduces both absolute values and temporal variations of measured brightness temperature. The contribution of the litter to overall emission was found to be important

Simulating L-band emission of coniferous forests using a discrete model and a detailed geometrical representation

A discrete model, based on the radiative transfer theory, is used to simulate coniferous forest emissivity at L-band. Inputs to the model are. given by using a detailed geometrical representation of Les Landes forest. Simulated emissivities are compared against EuroSTARRS campaign measurements, which were made over the same forest at nominally vertical polarization and several angles. The model has also been used to investigate the sensitivity of L-band radiometers to soil moisture under forests. Results of this investigation indicate that the soil contribution to emission is potentially appreciable, even under developed forests. This may. be a useful result, in view of future satellite missions, such as SMOS and HYDROS

Monitoring energy and mass transfers during the Alpilles-ReSeDA experiment

International audienceThe Alpilles-ReSeDA program was initiated to develop and test methods for interpreting remote sensing data that could lead to a better evaluation of soil and vegetation processes. This article presents the experiment that was set up in order to acquire the necessary data to achieve this goal. Intensive measurements were performed for almost one year over a small agricultural region in the South of France (20 kilometers square). To capture the main processes controlling land-atmosphere exchanges, the local climate was fully characterized, and surface energy fluxes, vegetation biomass, vegetation structure, soil moisture profiles, surface soil moisture, surface temperature and soil temperature were monitored. Additional plant physiological measurements and a full characterization of physical soil parameters were also carried out. After presenting the different types of measurements, examples are given in order to illustrate the variability of soils and plant processes in the area in response to the experienced climate.Le programme Alpilles-ReSeDA a été mis en place pour développer et tester des méthodes permettant une meilleure utilisation des données de télédétection pour le suivi du fonctionnement des sols et des cultures. Cet article présente l'expérimentation qui a été réalisée pour acquérir un jeu de données permettant cette analyse. Des mesures intensives ont été réalisées pendant presque une année sur une petite région agricole du Sud de la France (20 kilomètres carrés). De façon à suivre l'ensemble des processus contrôlant les échanges surface-atmosphère, l'ensemble des paramètres climatiques locaux ont été mesurés, ainsi que les flux d'énergie de surface, les caractéristiques de structure de la végétation et du sol l'humidité et les températures du sol, la température de surface. Des mesures des paramètres physiologiques des plantes et des caractéristiques physiques des sols ont également été entreprises. Après avoir présenté les différents types de mesures réalisées, des exemples présentant la variabilité des couverts végétaux et des sols dans la zone d'étude sont présentés

Satellite passive microwaves reveal recent climate-induced carbon losses in African drylands

International audienceThe African continent is facing one of the driest periods in the past three decades as well as continued deforestation. These disturbances threaten vegetation carbon (C) stocks and highlight the need for improved capabilities of monitoring large-scale aboveground carbon stock dynamics. Here we use a satellite dataset based on vegetation optical depth derived from low-frequency passive microwaves (L-VOD) to quantify annual aboveground biomass-carbon changes in sub-Saharan Africa between 2010 and 2016. L-VOD is shown not to saturate over densely vegetated areas. The overall net change in drylands (53% of the land area) was −0.05 petagrams of C per year (Pg C yr$^{−1}$) associated with drying trends, and a net change of −0.02 Pg C yr$^{−1}$ was observed in humid areas. These trends reflect a high inter-annual variability with a very dry year in 2015 (net change, −0.69 Pg C) with about half of the gross losses occurring in drylands. This study demonstrates, first, the applicability of L-VOD to monitor the dynamics of carbon loss and gain due to weather variations, and second, the importance of the highly dynamic and vulnerable carbon pool of dryland savannahs for the global carbon balance, despite the relatively low carbon stock per unit area