Vegetation water status forms a crucial link between the global carbon and water cycles due
to its control on both photosynthesis and transpiration, through the common pathway of the
two fluxes, the leaf stomata. Since CO2 levels in the atmosphere have been rising
substantially, this tight link has profound impacts on both the carbon uptake and the
evapotranspiration rates of plants.
At present there is still a lack of reliable data on terrestrial biosphere carbon fluxes at the
global scale and model predictions still vary quite widely. Current models determine the
degree of plant water stress, and thus the control on water and carbon fluxes, by the amount of
plant-available water in the soil and the atmospheric demand for water only. The vegetation
water reservoir itself is not taken into account. The relative influence of vegetation water
status on global carbon and water fluxes has thus never been studied before.
Passive microwave remote sensing has the unique ability to provide gravimetric values of
vegetation water content, through measurements of the vegetation optical depth. The proposed
project will combine passive microwave measurements of vegetation optical depth from the
SMOS mission with EVI data from MODIS, in order to obtain a gravimetric vegetation water
content product. The terrestrial biosphere model BETHY, which forms the core of the Carbon
Cycle Data Assimilation System (CCDAS), will be modified in order to include a vegetation
water reservoir. The satellite-derived vegetation water content will subsequently be used in
the CCDAS as a constraint on the parameters in the formulation of the processes regulating
the terrestrial carbon cycle. The results will help to improve estimations of terrestrial carbon
and water fluxes, and increase our understanding of the role of vegetation water in these.
This work concerns a completely novel application of a passive microwave-based vegetation
product, and it will be the first time that the vegetation water reservoir is taken into account in
terrestrial biosphere modelling. The proposed project would be a continuation of ESA’s
currently-running VEGWAC project.
The project will be carried out by a postdoctoral researcher, under supervision of a senior
researcher. The team has a strong network of collaborative partners in place, and extensive
experience in the relevant fields of research.