Three years of flux-tower-derived normalized difference vegetation index (NDVI) data were compared with contemporaneous 30 m web-enabled Landsat data (WELD) and with 500 m Moderate-Resolution Imaging Spectroradiometer (MODIS) nadir bidirectional reflectance distribution function-adjusted reflectance (NBAR) NDVI data to assess the relative suitability of these different resolutions of freely available satellite data for phenological monitoring. Comparisons were made at two flux tower sites in the United States with average to above average cloud cover. The WELD 30 m NDVI data were found to have higher correlation with the flux tower NDVI data than the MODIS 500 m NBAR NDVI data. The dates of vegetation green-up onset and maximum-greenness onset, derived using an established phenological metric extraction methodology, were generally closer between the flux tower and WELD NDVI data than between the flux tower and MODIS NBAR data. These results indicate that the WELD NDVI time series is suitable for 30 m scale phenological monitoring.
Available at: http://works.bepress.com/xiaoyang-zhang/30/