Comment et pourquoi mesurer l’humidité des sols

L’humidité superficielle a toujours été une variable de surface très recherchée, mais son obtention de façon mondiale s’est toujours avérée une gageure. La quête pour cette humidité a commencé à trouver une solution avec l’avènement des mesures de l’espace (télédétection). Cependant toutes les techniques tentées à ce jour ont trouvées leurs limitations, et ce n’est qu’avec l’apparition de nouvelles approches (en télédétection hyperfréquences passives) que de véritables solutions commencent à pouvoir être envisagées. Le but de cette présentation est de donner un aperçu des techniques existantes, de leurs avantages et inconvénients respectifs ainsi que des possibilités futures à moyen ou court terme avec des missions prévues

Examination of the difference between radiative and aerodynamic surface temperatures over sparsely vegetated surfaces

A four-layer hydrologic model, coupled to a vegetation growth model, has been used to investigate the differences between aerodynamic surface temperature and radiative surface temperature over sparsely vegetated surface. The rationale for the coupling of the two models was to assess the dependency of these differences on changing surface conditions (i.e., growing vegetation). A simulation was carried out for a 3-month period corresponding to a typical growth seasonal cycle of an herbaceous canopy in the Sahel region of West Africa (Goutorbe et al., 1993). The results showed that the ratio of radiative-aerodynamic temperature difference to radiative-air temperature difference was constant for a given day. However, the seasonal trend of this ratio was changing with respect to the leaf area index (LAI). A parameterization involving radiative surface temperature, air temperature, and LAI was then developed to estimate aerodynamic-air temperature gradient, and thus sensible heat flux. This parameterization was validated using data collected over herbaceous site during the Hapex-Sahel experiment. This approach was further advanced by using a radiative transfer model in conjunction with the above models to simulate the temporal behavior of surface reflectances in the visible and the near-infrared spectral bands. The result showed that sensible heat flux can be fairly accurately estimated by combining remotely sensed surface temperature, air temperature, and spectral vegetation index. The result of this study may represent a great opportunity of using remotely sensed data to estimate spatiotemporal variabilities of surface fluxes in arid and semiarid regions. (Résumé d'auteur

A combined modeling and multipectral/multiresolution remote sensing approach for disaggregation of surface soil moisture: Application to SMOS configuration

A new physically based disaggregation method is developed to improve the spatial resolution of the surface soil moisture extracted from the Soil Moisture and Ocean Salinity (SMOS) data. The approach combines the 40-km resolutionSMOSmultiangular brightness temperatures and 1-km resolution auxiliary data composed of visible, near-infrared, and thermal infrared remote sensing data and all the surface variables involved in the modeling of land surface–atmosphere interaction available at this scale (soil texture, atmospheric forcing, etc.). The method successively estimates a relative spatial distribution of soil moisture with fine-scale auxiliary data, and normalizes this distribution at SMOS resolution with SMOS data. The main assumption relies on the relationship between the radiometric soil temperature inverted from the thermal infrared and the microwave soil moisture. Based on synthetic data generated with a land surface model, it is shown that the radiometric soil temperature can be used as a tracer of the spatial variability of the 0–5 cm soil moisture. A sensitivity analysis shows that the algorithm remains stable for big uncertainties in auxiliary data and that the uncertainty in SMOS observation seems to be the limiting factor. Finally, a simple application to the SGP97/AVHRR data illustrates the usefulness of the approach

An approach to couple vegetation functioning and soil-vegetation- atmosphere-transfer models for semiarid grasslands during the HAPEX-Sahel experiment

This paper presents a model which has been developed to simulate the major land surface processes occurring in arid and semiarid grasslands. The model is composed of a hydrological submodel which describes the water and energy budgets, and a vegetation growth submodel which groups the processes associated with biomass production. Emphasis has been placed on developing a realistic representation of the interaction between these subprocesses taking account of the different time scales involved. The hydrological submodel couples the energy balance of the soil/canopy with the soil moisture and thermal dynamics. It interacts with the vegetation growth submodel by exchanging information needed to account for the influence of the vegetation canopy on the boundary layer within which transport processes are taking place. The model has been tested with meteorological, biomass and energy flux measurements made on a grassland site during the HAPEX-Sahel experiment, Niger, in 1992. Model simulations of biomass over the growing season are all found to be within a 15% error margin allowed on biomass measurements. Hourly values of net radiation, as well as latent and sensible heat fluxes, are simulated with an RMSE of less than 50 W/m2. Given the relative simplicity of the model and the long period of uninterrupted simulation, these results are considered satisfactory. Overall, the results show that the model behaves consistently at different stages of vegetation growth, and satisfactorily reproduces the interdependence of vegetation growth with the physical processes giving rise to the water and energy balances. (Résumé d'auteur