Estimation of Spruce Needle-Leaf Chlorophyll Content Based on DART and PARAS Canopy Reflectance Models

Needle-leaf chlorophyll content (Cab) of a Norway spruce stand was estimated from CHRIS-PROBA images using the canopy reflectance simulated by the PROSPECT model coupled with two canopy reflectance models: 1)discrete anisotropic radiative transfer model (DART); and 2)PARAS. The DART model uses a detailed description of the forest scene, whereas PARAS is based on the photon recollision probability theory and uses a simplified forest structural description. Subsequently, statistically significant empirical functions between the optical indices ANCB⁶⁷⁰-⁷²⁰ and ANMB⁶⁷⁰-⁷²⁰ and the needle-leaf Cab content were established and then applied to CHRIS-PROBA data. The Cab estimating regressions using ANMB⁶⁷⁰-⁷²⁰ were more robust than using ANCB⁶⁷⁰-⁷²⁰ since the latter was more sensitive to LAI, especially in case of PARAS. Comparison between Cab estimates showed strong linear correlations between PARAS and DART retrievals, with a nearly perfect one-to-one fit when using ANMB⁶⁷⁰-⁷²⁰(slope=1.1,offset=11μg.cm^-2)0. Further comparison with Cab estimated from an AISA Eagle image of the same stand showed better results for PARAS (RMSE=2.7μg.cm^-2 for ANCB⁶⁷⁰-⁷²⁰;RMSE=9.5μg.cm^-2 for ANMB⁶⁷⁰-⁷²⁰ than for DART (RMSE=7.5μg.cm^-2 for ANCB⁶⁷⁰-⁷²⁰;RMSE=23μg.cm^-2 for ANMB⁶⁷⁰-⁷²⁰). Although these results show the potential for simpler models like PARAS in estimating needle-leaf Cab from satellite imaging spectroscopy data, further analyses regarding parameterization of radiative transfer models are recommended.