An improved fapar-p model for cloudy conditions

The fraction of absorbed photosynthetically active radiation (FAPAR) is directly linked to the estimation of canopy gross primary production and is a key input parameter in terrestrial ecosystem models. Diffuse FAPAR calculation is necessary for cloudy conditions. Direct and diffuse FAPAR require distinct simulation methods due to different transfer paths, yet most models do not consider diffuse FAPAR independently. In this study, an improved FAPAR-P model ( FAPAR-Pro) based on the spectral invariant theory, which considers direct and diffuse FAPAR separately, was proposed to simulate all-sky vegetation FAPAR. Based on the proposed FAPAR-Pro model, along with the hourly ratio of diffuse radiance data calculated from Himawari-8 products, the all-sky vegetation FAPAR was calculated. Validation performed with ground measurements showed that the overall RMSE and MAE values are 0.056 and 0.067 for diverse types of vegetation canopies and different diffuse radiation conditions. The results indicate that the model was applicable to diverse types of vegetation canopies and different radiation conditions, and therefore, may provide support for continuous FAPAR time series simulation and various ecological applications in the future.