where, Kpar is the function of extinction and scattering co-efficient.
Experimental evidence indicated that the growth rate of several agricultural crop species increases linearly with increasing amounts of APAR, when soil water nutrients are not limiting (Myneni and Choudhury, 1993).
Therefore, integration of APAR over period e.g. growth cycle (i.e. emergence to maturity) represents total photosynthetic capacity of crops :
where, e is the radiation use efficiency, defined as the ratio of canopy net photosynthesis to incident PAR; f (NDVI) is a function relating NDVI with f PAR and Rs is the incoming solar radiation. Field et al. (1995) developed a global ecology model for net primary production in which e is calculated as :
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