Working towards a global-scale vegetation water product from SMOS optical depth
Résumé
Currently, global-scale monitoring of vegetation properties by remote sensing is almost always based on the use of optical or infrared sensors. The information provided by these sensors is most often related to chlorophyll content, photosynthetic properties, spectral characteristics or amount and classification of biomass and land cover type. However, passive microwave remote sensing has the potential to offer unique and complementary information, namely on the water status of the terrestrial vegetation. This information can be derived from the passive microwave vegetation optical depth, which is related to vegetation water content, structure and biomass (Kirdyashev et al., 1979). The relationship with vegetation water is of strong interest, as this variable is one of the key controls on photosynthesis and transpiration. Vegetation water status will determine both carbon uptake and water use of the plant, through the common pathway of the two fluxes - the leaf stomata. In this way, vegetation water status forms a crucial link between the carbon and water cycles. Besides having clear applications in the field of agriculture, as it is directly related to plant water stress, it is also of strong interest for e.g. terrestrial biosphere and climate modelling. In this study, gravimetric vegetation water content was obtained from optical depth measurements made by ESA's Soil Moisture and Ocean Salinity (SMOS) mission. This was done for a number of global vegetation classes, with a view towards developing a temporally dynamic, global-scale vegetation water product in the near future. Such a product does currently not exist, and would offer important complementary information to well-known vegetation indices from optical remote sensing such as e.g. NDVI and LAI.