A new Approach to Estimate Terrestrial Primary Productivity: A Potential Global Product from Sentinel 3 OLCI Data
Dash, Jadunandan1; Ogutu, Booker2
1University of Southampton, UNITED KINGDOM; 2Department of Geography, University of Leicester, UNITED KINGDOM

Gross primary production (GPP) of terrestrial vegetation is a key component in the carbon cycle and as such an important component of the carbon exchange between the atmosphere and the biosphere. However, currently there exists only one global operational GPP product derived using MODIS data and moreover, the product suffers from uncertainty due to (i) inaccuracies in quantifying light absorbed by photosynthetic element of plant canopy, (ii) errors in land cover classification, (iii) difficulties to characterise light use efficiency and (iv) coarse scale meteorological data. Therefore, there is a pressing need to develop a global GPP product which can overcome the limitation in the current MODIS product and provides a better estimate of carbon uptake by vegetation.
This research develops a new diagnostic model (i.e. MTCIGPP model) for estimating daily gross primary productivity (GPP). The model exploits the existence of distinct quantum yields in the two key photosynthetic pathways (i.e. C3 and C4) to estimate the conversion of absorbed photosynthetic radiation into GPP.
Furthermore, the model uses the MERIS Terrestrial Chlorophyll Index (MTCI) as a surrogate to the fraction of photosynthetically active radiation absorbed by photosynthetic elements of the canopy (i.e. FAPARps) rather than the total canopy FAPAR. The GPP predicted by the MTCIGPP model was comparable to in-situ GPP measurements (R2 > 0.7) in most of the evaluated biomes. The spatial distribution of GPP predicted by the MTCIGPP model over Europe and conterminous USA was comparable to those from the MOD17 GPP product except in regions dominated by croplands. The Sentinel 3 Ocean and Land Colour Instrument (OLCI) will collect data than can be used to derive OLCI Terrestrial Chlorophyll Index (OTCI), a continuation of the MTCI. With further analysis and validation, the new model provides a potential to develop a product to estimate global terrestrial primary productivity using Sentinel 3 OLCI data.