Spectral Invariant Provides a Practical Modeling Approach for Future Biophysical Variable Estimations

Yelu Zeng,Wanjuan Song,Shengbiao Wu,Kai Yan,Baodong Xu,Bin Yang,Jing Li,Guoqing Hu,Gaofei Yin

doi:10.3390/rs10101508

Abstract

This paper presents a simple radiative transfer model based on spectral invariant properties (SIP). The canopy structure parameters, including the leaf angle distribution and multi-angular clumping index, are explicitly described in the SIP model. The SIP model has been evaluated on its bidirectional reflectance factor (BRF) in the angular space at the radiation transfer model intercomparison platform, and in the spectrum space by the PROSPECT+SAIL (PROSAIL) model. The simulations of BRF by SIP agreed well with the reference values in both the angular space and spectrum space, with a root-mean-square-error (RMSE) of 0.006. When compared with the widely-used Soil-Canopy Observation of Photochemistry and Energy fluxes (SCOPE) model on fPAR, the RMSE was 0.006 and the R2 was 0.99, which shows a high accuracy. This study also suggests the newly proposed vegetation index, the near-infrared (NIR) reflectance of vegetation (NIRv), was a good linear approximation of the canopy structure parameter, the directional area scattering factor (DASF), with an R2 of 0.99. NIRv was not influenced much by the soil background contribution, but was sensitive to the leaf inclination angle. The sensitivity of NIRv to canopy structure and the robustness of NIRv to the soil background suggest NIRv is a promising index in future biophysical variable estimations with the support of the SIP model, especially for the Deep Space Climate Observatory (DSCOVR) Earth Polychromatic Imaging Camera (EPIC) observations near the hot spot directions.

Highlights

Canopy reflectance models have been widely used as the forward model for biophysical variable estimations, e.g., leaf area index (LAI), fractional vegetation cover (FVC) and the fraction of absorbed photosynthetically active radiation [1,2,3,4,5]
NIRv had a similar shape to DASFbs, while DASFt shows a different trend, especially the obvious high values around the hot spot directions
The fraction of absorbed photosynthetically active radiation (fPAR) estimation by spectral invariant properties (SIP) at different canopy structures (e.g., FVC, LAI and leaf angle distribution (LAD)), solar angles, and soil backgrounds was evaluated by the widely-used Soil-Canopy Observation of Photochemistry and Energy fluxes (SCOPE) model (Figure 5)

Summary

Introduction

Canopy reflectance models have been widely used as the forward model for biophysical variable estimations, e.g., leaf area index (LAI), fractional vegetation cover (FVC) and the fraction of absorbed photosynthetically active radiation (fPAR) [1,2,3,4,5]. Spectral invariant theory simplifies the radiative transfer process by decoupling the canopy structure and leaf optics [17]. There are three key spectral invariant parameters to describe the canopy structure: the canopy interceptance i0, recollision probability p and escape probability ρ [18,19,20]. There are still several aspects of the spectral invariant that need to be improved for future biophysical variable estimations

Methods

Results

Discussion

Conclusion