GPP, NPP, Chlorophyll and Remote Sensing

According to NASA, terrestrial biological productivity (or primary productivity) is the single most fundamental measure of “global change” of practical interest for humankind. Primary productivity is the measure of carbon intake by plants during photosynthesis, and this measure is an important indicator for studying the health for plant communities.
Net Primary Productivity (NPP) is the amount of carbon uptake after subtracting Plant Respiration (RES) from Gross Primary Productivity (GPP). GPP is the total rate at which the ecosystem capture and store carbon as plant biomass, for a given length of time.
NPP = GPP – RES
Photosynthesis is the process in which the energy from the sun converts carbon dioxide (CO2) from the atmosphere and water (or water vapor) to organic sugar molecules (carbohydrates), which are stored in the plants, and oxygen, which we, and other life on earth, consume. The extra water molecules which are derived in photosynthesis are reused by the plant or transpired into the atmosphere. Below is the chemical formula for photosynthesis:
6CO2 + 12H20 (+sunlight) C6H12O6+6O2+6H2O
NPP measures the mass of the new plant growth (chemically-fixed carbon) produced during a given interval. Change in NPP may change with vegetation health, so NPP rates were used to analyze the overall trend of carbon uptake in this region over the past ten years. 

Gitelson, A. A., A. Viña, S. B. Verma, D. C. Rundquist, T. J. Arkebauer, G. Keydan, B. Leavitt, V. Ciganda, G. G. Burba, and A. E. Suyker (2006), Relationship between gross primary production and chlorophyll content in crops: Implications for the synoptic monitoring of vegetation productivity, J. Geophys. Res., 111, D08S11, doi: 10.1029/2005JD006017.

Gitelson, A. A., A. Viña, V. Ciganda, D. C. Rundquist, and T. J. Arkebauer (2005), Remote estimation of canopy chlorophyll content in crops, Geophys. Res. Lett., 32, L08403, doi: 10.1029/2005GL022688.
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