Combining Landsat ETM+ and terrain data for scaling up leaf area index (LAI) in eastern Amazon: an intercomparison with MODIS product

The general aim of this study was to produce a continuous field of LAI to evaluate the LAI surface (MODIS product) derived from the moderate resolution imaging spectroradiometer (MODIS), for the Tapajos region, eastern Amazonia. Our method consisted in generating regression models combining spectral data derived from Enhanced Thematic Mapper Plus (ETM+) sensor (07/30/2001) and terrain slope and altimetry information extracted from a digital terrain model. The spectral variables considered for this study are reflectance, vegetation indices (NDVI and SR) and fraction images. Using a multiple comparison test, we compared the mean LAI estimated by the three models generated in this study (270 m spatial resolution) with the 8 days LAI composition (08/13/2001) derived from MODIS sensor (1 km spatial resolution) and also with field data. The MODIS LAI surface for the Tapajos region showed a more homogenous surface and little information about land cover and land use when we visually compared with our estimations. This fact occurs due to the 1 km resolution from de MODIS LAI against the 270 m resolution used in our approach. The statistics indicated that the mean LAI derived from MODIS sensor is significantly overestimated (P<0.05) in relation to both field and modeled data. We conclude that the approach employed here is promising for generating LAI surfaces, based on field data, for MODIS LAI validation purposes