Passive microwave sensors such as ESA’s Soil Moisture and Ocean Salinity (SMOS) mission have the potential to provide new vegetation information which is complementary to that commonly obtained from optical or infrared sensors. Passive microwave observations are more representative of the whole vegetation canopy, rather than just the top, as most vegetation becomes semi-transparent at these higher frequencies. Information on the vegetation layer can be derived from the passive microwave ‘vegetation optical depth’, one of the land products provided by SMOS.

Vegetation optical depth is related to vegetation water content, structure and biomass. The relationship with vegetation water is of strong interest, as plant water status is one of the key controls on photosynthesis and transpiration. It determines both the carbon uptake and the 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. Thus, information on vegetation water status could be used in terrestrial ecosystem models in order to improve future climate predictions, besides also being essential information in agricultural applications related to water stress and droughts.

The VEGWAC project aims to develop a novel satellite product consisting of global-scale vegetation water maps. This will be done based on SMOS optical depth observations in combination with complementary satellite data, model simulations and ground measurements. The vegetation water product will subsequently be used in a global terrestrial ecosystem model, modified to include the vegetation water reservoir. Finally, data assimilation will provide a better understanding and quantification of the influence of vegetation water status on water, carbon and energy fluxes in the soil-vegetation-atmosphere system.