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Abstract
Progress in deriving land surface biophysical parameters in a spatially explicit manner using remotely sensed data has greatly enhanced our ability to model ecosystem processes and monitor crop development. A multitude of satellite sensors and algorithms have been used to generate ready-to-use maps of various biophysical parameters. Validation of these products for different vegetation types is needed to assess their reliability and consistency. While most of the current satellite biophysical products have spatial resolution of one kilometre, a recent effort utilizing data from the Medium Resolution Imaging Spectrometer (MERIS) provided leaf area index (LAI), Fraction of Absorbed Photosynthetically Active Radiation (FAPAR), and other canopy parameters in a resolution as fine as 300 m over the European continent. This resolution would be more appropriate for application at the regional scale, particularly for crop monitoring. This higher-resolution MERIS product has been evaluated in a limited number of studies to date. This article aims to validate LAI and FAPAR from the MERIS 10-day composite BioPar BP-10 product over winter wheat fields in northeast Bulgaria. The ground measurements of LAI and FAPAR were up-scaled and 30 m resolution reference raster layers were created using empirical relationships with Landsat TM (RMSE = 0.06 and RMSE = 0.22 for FAPAR and LAI, respectively). MERIS FAPAR and LAI were found to have significant correlation with FAPAR and LAI from the reference raster layers (R2 = 0.84 and R2 = 0.78, respectively). When MERIS Green LAI was calculated (incorporating the fraction of vegetation and brown vegetation cover from the BioPar BP-10 product), better correspondence with LAI values from the reference raster layer was achieved, with RMSE and bias reduced by 30–35%. The results from this study confirm the findings of previous validations showing that MERIS Green LAI tends to overestimate LAI values lower than 1. As a conclusion of the study, the BioPar BP-10 product was found to provide reliable estimates of FAPAR and acceptably accurate estimates of LAI for winter wheat crops in North-East Bulgaria.
Acknowledgements
We are greatly obliged to the US Geological Survey’s Land Processes Distributed Active Archive Center (LP DAAC) for the Landsat TM data. We appreciate the effort of Ms Lubomira Kraleva, MSc, who carried out the final English language correction of the manuscript. Special thanks are due to the farmer, Mr Kiril Zhendov, for his dedicated support during the field experiments.
Funding
The field data used in this study were collected within the scientific research project Testing PROBA-V and VEGETATION Data for Agricultural Applications in Bulgaria and Romania (PROAGROBURO), Contract Ref. No CB/XX/16, financed by the Belgian Federal Science Policy Office (BELSPO) under the PROBA-V Preparatory Programme. The MERIS BioPar BP-10 product is produced within the geoland2 project (http://www.geoland2.eu/).