Detecting Chlorophyll Fluorescence from Orbit: The Fraunhofer Line Depth Model

Abstract

Modeling of solar-stimulated chlorophyll fluorescence as it would be measured with an orbital instrument using the Fraunhofer line depth method has produced data sets that correlate well with induced stress in vegetation. The models include simulations of data for 24 single solar Fraunhofer lines that span the visible spectrum in the region where chlorophyll fluorescence occurs, and 155 combinations of Fraunhofer lines for red/far-red (R/FR) ratios. Source data for this modeling effort had been obtained from summer and fall bean crops (Phaseolus vulgaris L.) subjected to various levels of nitrogen fertilization. Corrections applied to the derived intensities account for Fraunhofer line depth and atmospheric transmittance. The R/FR ratios and the single Fraunhofer lines were evaluated for correlation with measured leaf chlorophyll and applied nitrogen. Selection criteria were established that yielded 7 ratios determined to be best suited for separating vegetation by levels of stress. Those ratios are: 714.28 / 739.94 nm; 713.09 / 716.44 nm; 713.09 / 744.58 nm; 714.28 / 742.23 nm; 714.28 / 716.44 nm; 703.82 / 716.44 nm; and 656.28 / 738.94 nm Simulated images demonstrate that the statistical differences would be visually recognizable. Based on the findings of this study, it is concluded that a passive fluorescence instrument designed to monitor R/FR chlorophyll fluorescence using the Fraunhofer line depth method from orbit would produce valuable information about the physiological condition of terrestrial vegetation on a global scale.