Integrating chlorophyll fAPAR and nadir photochemical reflectance index from EO-1/Hyperion to predict cornfield daily gross primary production

The concept of light use efficiency (ε) and the concept of fraction of photosynthetically active ration (PAR) absorbed for vegetation photosynthesis (PSN), i.e., fAPARPSN, have been widely utilized to estimate vegetation gross primary productivity (GPP). It has been demonstrated that the photochemical reflectance index (PRI) is empirically related to ε. An experimental US Department of Agriculture (USDA) cornfield in Maryland was selected as our study field. We explored the potential of integrating fAPARchl (defined as the fraction of PAR absorbed by chlorophyll) and nadir PRI (PRInadir) to predict cornfield daily GPP. We acquired nadir or near-nadir EO-1/Hyperion satellite images that covered the cornfield and took nadir in-situ field spectral measurements. Those data were used to derive the PRInadir and fAPARchl. The fAPARchl is retrieved with the advanced radiative transfer model PROSAIL2 and the Metropolis approach, a type of Markov Chain Monte Carlo (MCMC) estimation procedure. We define chlorophyll light use efficiency (εchl) as the ratio of vegetation GPP as measured by eddy covariance techniques to PAR absorbed by chlorophyll (εchl = GPP/APARchl). Daily εchl retrieved with the EO-1 Hyperion images was regressed with a linear equation of PRInadir (εchl = α × PRInadir + β). The satellite εchl-PRInadir linear relationship for the cornfield was implemented to develop an integrated daily GPP model [GPP = (α × PRInadir + β) × fAPARchl × PAR], which was evaluated with fAPARchl and PRInadir retrieved from field measurements. Daily GPP estimated with this fAPARchl-PRInadir integration model was strongly correlated with the observed tower in-situ daily GPP (R2 = 0.93); with a root mean square error (RMSE) of 1.71 g C mol− 1 PPFD and coefficient of variation (CV) of 16.57%. Both seasonal εchl and PRInadir were strongly correlated with fAPARchl retrieved from field measurements, which indicates that chlorophyll content strongly affects seasonal εchl and PRInadir. We demonstrate the potential capacity to monitor GPP with space-based visible through shortwave infrared (VSWIR) imaging spectrometers such as NASA's soon to be decommissioned EO-1/Hyperion and the future Hyperspectral Infrared Imager (HyspIRI).