Sun-Induced Chlorophyll Fluorescence III: Benchmarking Retrieval Methods and Sensor Characteristics for Proximal Sensing

Cendrero-Mateo Cendrero-Mateo,Celesti Celesti,Guanter Guanter,Damm Damm,Sabater Sabater,Cogliati Cogliati,Aasen Aasen,Pacheco-Labrador Pacheco-Labrador,Alonso Alonso,Arthur Arthur,Rossini Rossini,Pinto Pinto,Rascher Rascher,Julitta Julitta,Moreno Moreno,Goulas Goulas,Wieneke Wieneke

doi:10.3390/rs11080962

Abstract

The interest of the scientific community on the remote observation of sun‐induced chlorophyll fluorescence (SIF) has increased in the recent years. In this context, hyperspectral ground measurements play a crucial role in the calibration and validation of future satellite missions. For this reason, the European cooperation in science and technology (COST) Action ES1309 OPTIMISE has compiled three papers on instrument characterization, measurement setups and protocols, and retrieval methods (current paper). This study is divided in two sections; first, we evaluated the uncertainties in SIF retrieval methods (e.g., Fraunhofer line depth (FLD) approaches and spectral fitting method (SFM)) for a combination of off-the-shelf commercial spectrometers. Secondly, we evaluated how an erroneous implementation of the retrieval methods increases the uncertainty in the estimated SIF values. Results show that the SFM approach applied to high-resolution spectra provided the most reliable SIF retrieval with a relative error (RE) ≤6% and <5% for F687 and F760, respectively. Furthermore, although the SFM was the least affected by an inaccurate definition of the absorption spectral window (RE = 5%) and/or interpolation strategy (RE = 15%–30%), we observed a sensitivity of the SIF retrieval for the simulated training data underlying the SFM model implementation.

Highlights

Plant photosynthesis is the primary process in terrestrial ecosystems
The following Section describes the impact of the different sensor specifications (i.e., spectral resolution (SR), SSI, and signal-to-noise ratio (SNR)) on the precision, accuracy, and relative error of four different retrieval methods: sFLD, 3FLD, iFLD, and SFM
As described by Damm et al [32], the strong overestimation of F760 by the sFLD method is caused by a violation of the assumption that R and F are spectrally constant over the O2A band

Summary

Introduction

Plant photosynthesis is the primary process in terrestrial ecosystems. Accurate estimates of photosynthesis and its dynamics are pivotal to understand complex feedbacks and exchange interactions in the land–atmosphere system [1,2]. Where λ is the wavelength, R is the actual reflectance, E↓ is the down-welling irradiance incident to the surface, and F the top of the canopy SIF radiance in the direction of observation. The method relies on general mathematical functions representing canopy R and F within narrow spectral windows centered at oxygen absorption features. The parameters of the functions employed to represent F and R are optimized by non-linear least square optimization process, comparing instrument observations with radiance computed with Equation (1). In this way, the a-priori F and reflectance functions can be spectrally decoupled on the base of their contribution at the different spectral lines. SIF is generally represented as peak-like functions such as Gaussian, Lorenzian, or Voigt; whereas canopy reflectance is usually described as second- or third-order polynomial or more complex piecewise cubic splines

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Conclusion