Retrieving Leaf and Canopy Chlorophyll Content of Potato Crops using Sentinel-2 Bands
Clevers, Jan; Kooistra, Lammert
Wageningen University, NETHERLANDS

Sentinel-2 is planned for launch in 2014 by the European Space Agency. It is equipped with the Multi Spectral Instrument (MSI), which will provide images with high spatial, spectral and temporal resolution. It covers the VNIR/SWIR spectral region in 13 bands and incorporates two new spectral bands in the red-edge region, which can be used to derive vegetation indices using red-edge bands. These are particularly suitable for estimating vegetation chlorophyll and nitrogen content, which are key parameters for understanding plant functioning and status. The band setting in this region is also very similar to the ones of the Ocean and Land Colour Instrument (OLCI) on the planned Sentinel-3 satellite and the Medium Resolution Imaging Spectrometer (MERIS) on Envisat, which operated from 2002 until early 2012.
This paper focuses on the potential of Sentinel-2 in estimating leaf and canopy chlorophyll content by first studying the influence of confounding leaf and crop parameters on the relationship between various vegetation indices and leaf and canopy chlorophyll content using the PROSAIL radiative transfer model at a 1 nm sampling interval. Subsequently, relationships were tested for potato crops by studying in-situ crop variables and radiometer measurements obtained for three different growing seasons (2010 - 2012). Finally, results were scaled up and evaluated using spaceborne Rapideye and WorldView-2 images obtained in the middle of the 2010 growing season for the potato field studied. Currently, these are the only satellite sensors with a high spatial resolution including a red-edge band.
In particular, the red-edge chlorophyll index (CIred-edge), the green chlorophyll index (CIgreen) and the MERIS terrestrial chlorophyll index (MTCI) were found to be accurate and linear estimators of canopy chlorophyll content and the Sentinel-2 bands are well positioned for deriving these indices. For estimating the leaf chlorophyll content the transformed chlorophyll absorption in reflectance index/optimized soil-adjusted vegetation index (TCARI/OSAVI) yielded best results. For both leaf and canopy chlorophyll content, consistent results were obtained both with the PROSAIL simulations and with the experimental data in all three years. Results confirm the importance of the red-edge bands on Sentinel-2 for agricultural applications, in particular in combination with its high spatial resolution of 20 m.