Using reflectance to reduce solar angle effects on the scattering of far-red solar-induced chlorophyll fluorescence to better track the diurnal dynamics of gross primary productivity

Solar-induced chlorophyll fluorescence (SIF) is linked to photosynthesis and provides reliable estimation of Gross Primary Productivity (GPP). However, SIF observed (SIF _obs) at the top-of-canopy is theoretically less accurate in predicting GPP compared with the total emitted SIF (SIF _total). Converting remotely sensed SIF _obs to SIF _total is therefore crucial for improving GPP estimation. SIF _obs is a portion of SIF _total and their ratio is termed the fluorescence escape ratio (f _esc), describing the scattering and re-absorption processes. f _esc is affected by canopy structure and sun-observer geometry. Vegetation indices, such as the Fluorescence Correction Vegetation Index (FCVI) and the near-infrared reflectance of vegetation (NIR _v), have been developed to estimate f _esc and SIF _total with reasonable accuracy particularly under varying viewing angles scenarios. However, their performance in estimating f _esc and SIF _total has not been well-studied for a diurnal scale, in which solar angles vary. Solar angles affect both f _esc and SIF _total, while viewing angles affect f _esc but not SIF emission. In this study, we analyzed the solar angle effects on far-red SIF scattering and mitigated the effects by leveraging these indices to estimate f _esc, facilitating the derivation of SIF _total for improving GPP estimation. Numerical experiments were employed to support the use of FCVI and NIR _v for f _esc estimation under varying chlorophyll content (C _ab), leaf angle distribution (LAD) and leaf area index (LAI) conditions. SIF _total was derived to enhance the accuracy of GPP estimation at the diurnal scale in filed measurements. Our results indicated that both FCVI and NIR _v reliably estimated f _esc under varying solar angles. Moreover, SIF _total improved the SIF-GPP relationship, with R² increasing to 0.43 – 0.80, compared to SIF _obs (0.12 < R ² <0.52), and its diurnal pattern was more consistent with that of GPP. Our study suggested that the use of FCVI and NIR _v for f _esc mitigates solar angle effects on SIF _obs and thus improves GPP estimation.