Assessment of approaches for estimating fraction of photosynthetically active radiation absorbed by winter wheat canopy

The approaches for estimating the fraction of photosynthetically active radiation absorbed by crop canopy (fAPAR) include the one using the fraction of intercepted photosynthetically active radiation (fIPAR), the one by the leaf area index (LAI) and the Beer-Lambert law and the ones with the different linear or non-linear relationships between the fAPAR and the normalized difference vegetation index NDVI. In this paper, all the above fAPAR estimation methods were tested using the field data of the LAI, fIPAR and the spectral characteristics of winter wheat, with the aims to illustrate the advantages and disadvantages of them and offer references for further study. The results showed that the fAPAR was close to the fIPAR during the vegetative growth period with the green leaves dominated canopy, while a remarkable difference was found between them in the reproductive stage with the increasing leaf senescence. The approach using the fIPAR to estimate the fAPAR gave a good estimation. The approach by the Beer-Lambert law and the LAI underestimated the fAPAR from the heading to dough stages, which may be due to the absorption of photosynthetically active radiation by the green wheat stems and spikes not being included. The empirical fAPAR-NDVI relationships vastly overestimated the fAPAR during the vegetative growth period and the end of reproductive period. The correlation analysis of the fAPAR to the NDVI, the ratio vegetation index (RVI), the soil adjusted vegetation index (SAVI) and the modified soil adjusted vegetation index (MSAVI) of the wheat canopy was performed. It demonstrated that the correlation between the NDVI and the fAPAR was the best, which makes it more useful in the fAPAR estimation