Comparison of satellite- and ground-based NDVI above different land-use types

In order to evaluate the use of satellite (moderate resolution imaging spectroradiometer: MODIS) and ground-measured (hyperspectral spectrometer and broadband micro-meteorological sensors) normalized difference vegetation index (NDVI), this study compares NDVI derived from five experimental (FLUXNET) field sites (grassland, winter wheat, corn, spruce, and beech) in Germany in June 2006 and April-September 2007. In addition, the spatial variability of ground radiation measured within one specific land-use class (for grass and winter wheat) was investigated to analyze the accuracy of the FLUXNET tower values. Furthermore, the angular dependence of spectrometer values on viewing angles was determined in order to enhance the spatial representativeness of spectrometer measurements which, especially above trees, are affected by soil parts and the tower structure when measured in nadir. The best agreement between the satellite-and ground-measured NDVI was found for winter wheat (2006) with values from 0.79-0.88 followed by grass (2006), showing NDVI values between 0.71 and 0.86. The spatial variability of NDVI within one land-use type was lower than the differences caused by the different NDVI determination methods. Above more open canopies (corn, beech), spectrometer measurements with 60 degrees viewing angle in solar plane direction were found to better correspond to satellite-derived NDVI. Together with broadband NDVI, our ground-based results can complement satellite-derived NDVI.