Ground-based optical measurements at European flux sites: a review of methods, instruments and current controversies

This paper reviews the currently available optical sensors, their limitations and opportunities for deployment at Eddy Covariance (EC) sites in Europe. This review is based on the results obtained from an online survey designed and disseminated by the Co-cooperation in Science and Technology (COST) Action ESO903—“Spectral Sampling Tools for Vegetation Biophysical Parameters and Flux Measurements in Europe” that provided a complete view on spectral sampling activities carried out within the different research teams in European countries. The results have highlighted that a wide variety of optical sensors are in use at flux sites across Europe, and responses further demonstrated that users were not always fully aware of the key issues underpinning repeatability and the reproducibility of their spectral measurements. The key findings of this survey point towards the need for greater awareness of the need for standardisation and development of a common protocol of optical sampling at the European EC sites.
This work was supported by the EU COST Office in the framework of the Action ES0903. C.Nichol’s project on spectral sensing is funded through a UK Natural Environment Research Council standard grant (NE/F017294/1). G.W. acknowledges financial support by the Austrian National Science Fund and the Tyrolean Science Fund through grants P17560-B16 and P19849-B16 and Uni-404/33 and Uni-404/486, respectively, as well as the University of Innsbruck. A.C. acknowledges financial support provide to Foundation CEAM by the programs CONSOLIDER- INGENIO 2010 (GRACCIE), and CARBORED-II (CGL2010-22193-C04-01).