Monitoring seasonal and interannual variations of gross primary productivity, net primary productivity and net ecosystem productivity using a diagnostic model and remotely-sensed data

A new diagnostic model for the estimation of net primary productivity (NPP) is presented. It is derived from the Kumar and Monteith model for remote sensing of crop growth, with a separate parameterization of autotrophic respiration. The NPP model is coupled to a soil respiration model, calibrated in order to balance the resulting net ecosystem productivity (NEP) over a year. This model is run over the years 1986 to 1991 with NOAA-AVHRR derived normalized difference vegetation index (NDVI) as inputs, for which the radiometric calibration drift has been accounted for. The outputs of the model are analysed in terms of interannual variations: the model seems to be able to simulate the effects of events such as El Niño on the terrestrial net primary productivity. The NEP outputs are compared to CO₂ concentration measurements in the atmosphere, on a zonally averaged basis, without taking into account atmospheric transport. Seasonal evolution of NEP and atmospheric CO² are well in phase in latitudes of the temperate northern hemisphere, but the correspondence is weaker or absent in the tropics and southern hemisphere.