Measurement and modeling of Bidirectional SIF Emissions in Wheat Canopies

Numerous observations and modeling results have shown that there is noticeable directional variation in the Solar-Induced chlorophyll Fluorescence (SIF), while this has not been well investigated. In this study, sixteen multi-angular spectral observations were carried out on winter wheat to assess the bidirectional SIF emission. First, the bidirectional SIF emission was retrieved from the spectral measurements made by a high performance QE-Pro spectrometer and an automatic multi-angle observation system using the 3FLD algorithm. The bidirectional shape of the SIF emission was found to be similar to that of the canopy reflectance in the solar principal plane, with a mean correlation coefficient of 0.94 and 0.97 at the O2-B and O2-A band, respectively. The Modified Rahman-Pinty-Verstraete (MRPV) model, a semi-empirical Bidirectional Reflectance Distribution Function (BRDF) model, was then employed to describe the bidirectional variation in the SIF and reflectance with a mean Root Mean Square Error (RMSE) value of 0.036 and 0.041 mW m-2sr-1nm-1 for the SIF at the O2-B and O2-A band, respectively. Finally, both the bidirectional reflectance and SIF were BRDF-corrected to nadir using the MRPV model. Most of the directional variation was successfully corrected by this method - the mean correction ratios were 87% and 81% for the reflectance at the O2-B and O2-A band, and 84% and 72% for the SIF at the O2-B and O2-A band, respectively. Therefore, the SIF emission cannot be regarded as isotropic and the high similarity between the bidirectional SIF and reflectance, together with the BRDF correction results, indicates that the bidirectional SIF emission can be adjusted using either the BRDF reflectance models or prior knowledge.