Estimation of leaf chlorophyll content and the fraction of light absorption by chlorophyll using MODIS images and a radiative transfer model

Photosynthetically actively radiation (PAR) absorbed by leaf chlorophyll is used for photosynthesis, therefore, there is a need to quantify the fraction of PAR absorbed by leaf chlorophyll (FAPARchl) within a vegetation canopy. We recently developed a theoretical and modeling framework to estimate leaf chlorophyll content and FAPARchl.in a vegetation canopy The modeling framework coupled a leaf radiative transfer model (PROPSECT) with a canopy radiative transfer model (SAIL-2) and incorporated a Markov Chain Monte Carlo (MCMC) method (the Metropolis algorithm) for model inversion, which provides posterior distributions of retrieved variables. Our two-step procedure is: (1) to invert biophysical and biochemical variables (including leaf chlorophyll content) with the coupled PROSPECT+SAIL-2 (PROSAIL-2) model and multiple daily images (five spectral bands) from MODerate resolution Imaging Spectroradiometer (MODIS) sensor , (2) to reproduce MODIS spectral reflectance and to calculate FAPARchl with the inverted variables from step (1). We ran the PROSAIL-2 model for a deciduous broadleaf forest, an evergreen forest and an agricultural site using MODIS data in 2001-2003. The PROSAIL-2 -reproduced reflectance agrees well with observed MODIS reflectance for the five MODIS spectral bands. The retrieved variables (leaf area index, leaf chlorophyll content) were also evaluated with available literature data. Estimated FAPARchl is substantially lower than the fraction of PAR absorbed by leaf (FAPARleaf) and by vegetation canopy (FAPARcanopy). Biogeochemical models that use either the fraction of PAR absorbed by canopy (FAPARcanopy) or the fraction of PAR absorbed by leaf (FAPARleaf) in estimating gross and net primary production are likely to overestimate the amount of PAR used in photosynthesis process, which represents one large source of error and uncertainty. The results of this study highlight the needs of field works and the potential of a radiative transfer model (PROSAIL-2) for quantifying FAPARchl and leaf chlorophyll content within a vegetation canopy.