The fraction of photosynthetically active radiation (PAR) absorbed by green elements (fAPAR) is an essential climate variable in quantifying canopy absorbed PAR and gross and net primary production. Continuous and accurate measurement of the fAPAR is thus critical. Several methods have been proposed in the field measurements, such as the PAR ceptometer (AccuPAR), Plant Canopy Analyzer (LAI-2200), and Digital Hemispherical Photography (DHP). However, the differences in the methods and the impact of solar illumination and non-green elements on the measurement are still unclear.

Dr. LI Wenjuan and Prof. FANG Hongliang at the Institute of Geographic Sciences and Natural Resources Research (IGSNRR), Chinese Academy of Sciences and their cooperators conducted two field campaigns over paddy rice fieldsin northeastern China in 2012 and 2013, and made seasonal continuous fAPAR measurements using, AccuPAR, LAI-2200, and DHP. With the field data collected.

They made a comprehensive comparison of the different fAPAR estimation methods. The results showed that the fraction of intercepted PAR (fIPAR) measured with AccuPAR, DHP, and LAI-2200 was a good proxy of the fAPAR computed with the four-flux method from AccuPAR. They recommended using the downward looking DHP to measure the green fAPAR when canopy becomes senescent because the method was capable of identifying the green pixels automatically.

Fig.1. Comparison of the AccuPAR estimated four-stream fAPAR and the two-stream fAPAR_T under black- (magenta) and white-sky (blue) conditions 

This work was supported by the National Natural Science Foundation of China. The study has been published in a recent issue of Agricultural and forest meteorology. 

Li, W., Fang, H., Wei, S., Weiss, M., Baret, F., (2021). Critical analysis of methods to estimate the fraction of absorbed or intercepted photosynthetically active radiation from ground measurements: application to rice crops. Agricultural and forest meteorology, 297, 108273.