Practical Approaches for Normalizing Directional Solar-induced Fluorescence at both red and far-red bands

Abstract:

Recent advances in remote sensing of solar-induced chlorophyll fluorescence (SIF) have improved our capabilities of monitoring large-scale terrestrial photosynthesis. However, SIF observations are subject to directional effects which can lead to considerable uncertainties in various applications. Practical approaches for normalizing off-nadir SIF observations (at both red and far-red bands) to nadir view have not been well studied. Here we developed two practical and physically-based approaches for removing the directional effects of anisotropic SIF observations at both red and far-red bands: one is based on near-infrared or red reflectance of vegetation (NIRv and Redv), and the other is based on the kernel-driven model with multi-angular SIF measurements. The first approach uses surface reflectance and the second approach leverages multi-angular SIF measurements. The performance of the two approaches was evaluated using a dataset of multi-angular measurements of SIF and reflectance collected with a high-resolution field spectrometer for different plant canopies. Results show that the relative mean absolute errors between the normalized nadir SIF and the observed SIF at nadir decrease by 3-6% (far-red) and 6-8% (red) for the first approach, and by 7-13% and 6-11% for the second approach, compared to original data, respectively. The effectiveness and simplicity of our proposed approaches provide great potentials to develop long-term and consistent SIF data records with minimized directional effects and further improve global estimates of gross primary productivity (GPP).