Estimation of crop gross primary production (GPP): fAPARchl versus MOD15A2 FPAR

Photosynthesis (PSN) is a pigment level process in which antenna pigments (predominately chlorophylls) in chloroplasts absorb photosynthetically active radiation (PAR) for the photochemical process. PAR absorbed by foliar non-photosynthetic components is not used for PSN. The fraction of PAR absorbed (fAPAR) by a canopy/vegetation (i.e., fAPAR(canopy)) derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) images, referred to as MOD15A2 FPAR, has been used to compute absorbed PAR (APAR) for PSN (APAR(PSN)) which is utilized to produce the standard MODIS gross primary production (GPP) product, referred to as MOD17A2 GPP. In this study, the fraction of PAR absorbed by chlorophyll throughout the canopy (fAPAR(chl)) was retrieved from MODIS images for three AmeriFlux crop fields in Nebraska. There are few studies in the literature that compare the performance of MOD15A2 FPAR versus fAPAR(chl) in GPP estimation. In our study MOD15A2 FPAR and the retrieved fAPAR(chl) were compared with field fAPAR(canopy) and the fraction of PAR absorbed by green leaves of the vegetation (fAPAR(green)). MOD15A2 FPAR overestimated field fAPAR(canopy) in spring and in fall, and underestimated field fAPAR(canopy) in midsummer whereas fAPAR(chl) correctly captured the seasonal phenology. The retrieved fAPAR(chl) agreed well with field fAPAR(green) at early crop growth stage in June, and was less than field fAPAR(green) in late July, August and September. GPP estimates with fAPAR(chl) and with MOD15A2 FPAR were compared to tower flux GPP. GPP simulated with fAPAR(chl) was corroborated with tower flux GPP. Improvements in crop GPP estimation were achieved by replacing MOD15A2 FPAR with fAPAR(chl) which also reduced uncertainties of crop GPP estimates by 1.12-237 g C m(-2) d(-1). (C) 2014 Elsevier Inc. All rights reserved.