A new approach for remote sensing of canopy absorbed photosynthetically active radiation. II: Proportion of canopy absorption

The amount of photosynthetically active radiation (PAR) absorbed by canopy (APARCAN) is essential to the productivity of vegetation. Monitoring APARCAN from space has been achieved through the retrievals of two quantities, namely, the PAR incident at the surface (PARSFC↓) and the fraction of PAR intercepted by the canopy, FPAR. We propose a new approach that splits APARCAN into the PAR absorbed in the surface layer below the top of the canopy (APARSFC) and the ratio of APARCAN / APARSFC, RPAR. The method is introduced in two parts. Part I develops a simple parameterization that retrieves APARSFC more readily and accurately than PARSFC↓. Part II, presented in this paper, deals with the retrieval of RPAR. It is shown that RPAR can be derived as accurately and readily as FPAR. Hence, it is envisaged that the new approach offers an easier and more accurate means of estimating APARCAN than the traditional one. As an investigation tool, a one-dimensional multistream and multilayer model of canopy radiative transfer is first formulated. Extensive canopy modeling is conducted with input parameters of large ranges to represent a variety of canopies and ground conditions. For vegetated land, RPAR is found to correlate well with FPAR and thus RPAR can be estimated from FPAR. RPAR is also related with the surface vegetation indices (VIs) such as NDVI, SAVI, and DVI. The relationships between RPAR and VIs are driven by the changes in leaf area index. They are not sensitive to the solar zenith angle and the fractions of direct and diffuse radiation, but to the optical properties of the canopy. The models for inferring RPAR from various VIs are given, together with the correction models to account for the dependencies of RPAR on time and cloud cover.