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Vegetation plays an important role in the carbon cycle. Photosynthesis consumes and respiration produces considerable amounts of CO2, the predominant greenhouse gas in global warming. The monitoring of carbon dynamics is hence a crucial issue in studies of global change and in the analysis of vegetation damage due to climatological anomalies (late frost, long period of drought,...).
The NPP procedure attempts to quantify the carbon fluxes by integrating satellite observations into a simplified carbon budget model. The key element in this approach, is that the evolution of the "amount" and "state" of the vegetation are directly inferred from space observations, and hence no longer need to be estimated by the carbon budget model itself.
This NPP model is based on the Monteith-approach and requires three types of information: a remote sensing variable (VGT S10-NDVI) and two meteorological variables (daily incoming solar radiation and mean air temperature). Estimates of NPP are provided in mgC/m2/d.
NPP = uptake of carbon by photosynthesis - the autotrophic respiration losses by vegetation
= S*c*fAPAR*e*p(T)*CO2fert*(1-r)
where
- S: global solar radiation [MJ/m2/d]
- c: climatic efficiency, Mc Cree K.J. (1972) [dimensionless]
- fAPAR: Fraction of absorbed PAR (photosynthetic active radiation); fAPAR is estimated from the remotely sensed NDVI (Normalised Difference Vegetation Index) by means of a linear equation, suggested by Myneni (1992) [dimensionless]
- e: Photosynthetic efficiency, Wofsy et al. (1993) [gC/MJ(APAR)]
- p(T): Normalised temperature dependency factor, Johnson et al. (1954), and parameterised according to data of Lommen et al. (1971) [dimensionless]
- CO2fert: Normalised CO2 fertilisation factor, Veroustraete (1994) [dimensionless]
- r: fraction of assimilated photosynthates consumed by autotroph repiration; is modelled as a simple linear function of daily mean air temperature, Goward & Due (1987)
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