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| The NEP 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. The parametric Production
Efficiency Model (PEM) C-Fix (Veroustraete et al., 2002) estimates carbon
mass fluxes from local, regional to continental scales using a Monteith
approach. C-Fix is a mass balance model based on the parameterization
of fAPAR derived from remotely sensed NDVI. The daily net ecosystem production
(NEP) is the balance of daily gross uptake of carbon by photosynthesis
(GPP) subtracted with the autotrophic respiratory loss by vegetation and
subtracted with the soil respiratory flux originating from heterotrophic
decomposition of soil organic matter.
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NEP = NPP - Sresp
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| where |
- Sresp: soil respiration, Lommen et al. (1971)
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Soil Respiration Sresp is difficult to model, because it depends
on many interacting factors. We can cite soil carbon content, soil humidity,
soil temperature and microbiological activity of the soil. This model
only copes with the impacts of temperature on soil respiration:
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Sresp = ks* p(T) with:
- p(T): Normalised temperature dependency factor, Johnson et al. (1954),
and parameterised according to data of Lommen et al. (1971) [dimensionless]
- ks: a site-specific rate constant which is estimated using
the condition that the yearly NEP per pixel is zero
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