The Effects of Sun-Viewer Geometry on Sun-Induced Fluorescence and Its Relationship with Gross Primary Production

Abstract

Gross primary production (GPP) from photosynthesis by terrestrial vegetation is the largest sink of atmospheric CO 2 . Sun-induced chlorophyll fluorescence (SIF) has been shown a powerful proxy for photosynthetic activity and used to estimate GPP. However, both non-physiological and functional factors controlling the emission of canopy SIF. The non-physiological factors, especially the sun-viewer geometry, impact the relationships between SIF and GPP. In this study, we did near-surface observations of both carbon flux and multi-view-angle spectra above a wheat canopy. The carbon flux was used to calculate GPP and the canopy spectra were used to retrieve SIF. SIF is significantly correlated with the angle between sun and viewer than SIF (R2=0.63). The relationships of SIF with GPP are also changing with different view azimuth angles. Generally, SIF observed at 180o (pointing north) are more correlated to GPP than that at the other two angles. A model recently developed by He et al. (2017) was used to normalize multi-angle SIF to the hotspot direction (SIF h ) and to compute the canopy-level total SIF (SIF t ), in order to reduce the effects of sun-viewer geometry on SIF. Compared to the correlation of GPP with the original observed SIF (SIF obs ), the coefficients of determination (R2) increase by ~0.1 and ~0.05 for those of GPP with SIF h and SIF t , respectively. These results would be helpful in estimating GPP of crops at large scales using remote sensing techniques.