@Article{PetriGalv:2019:SeSeMO,
               author = "Petri, Caio Arlanche and Galv{\~a}o, L{\^e}nio Soares",
          affiliation = "{Instituto Nacional de Pesquisas Espaciais (INPE)} and {Instituto 
                         Nacional de Pesquisas Espaciais (INPE)}",
                title = "Sensitivity of seven MODIS vegetation indices to BRDF effects 
                         during the amazonian dry season",
              journal = "Remote Sensing",
                 year = "2019",
               volume = "11",
               number = "14",
                pages = "e1650",
                month = "July",
             keywords = "BRDF, vegetation indices, Amazon, dry season, greening, 
                         viewing-illumination effects, MODIS.",
             abstract = "We used Moderate Resolution Imaging Spectroradiometer (MODIS) 
                         data, processed by the multi-angle implementation of atmospheric 
                         correction (MAIAC) algorithm, to investigate the sensitivity of 
                         seven vegetation indices (VIs) to bidirectional reflectance 
                         distribution function (BRDF) effects in the dry season 
                         (June-September) of the Brazilian Amazon. The analysis was first 
                         performed over three sites, located from north to south of the 
                         Amazon, and then extended into the entire region. We inspected for 
                         differences in viewing-illumination parameters and pixel quality 
                         retrievals during MODIS data acquisition over the region. By 
                         comparing and correlating corrected and non-corrected data for 
                         bidirectional effects, we evaluated monthly changes in reflectance 
                         and VIs (2000-2014). Finally, we computed the effect size of the 
                         BRDF correction using non-parametric Mann-Whitney tests and 
                         Cohen's r metrics. The results showed that the most anisotropic 
                         VIs were the enhanced vegetation index (EVI), photochemical 
                         reflectance index (PRI), and shortwave infrared normalized 
                         difference (SWND). These VIs presented the largest relative 
                         changes and the lowest correlation coefficients, between corrected 
                         and non-corrected data, because of the large effect size of the 
                         BRDF. The least anisotropic VI was the normalized difference water 
                         index (NDWI). The anisotropy of these VIs was stronger in the 
                         northern Amazon. It increased from the beginning to the end of the 
                         dry season, following changes in the relative azimuth angle (RAA) 
                         toward the BRDF hotspot in September. The modifications in the 
                         relative proportions of backscattering observations used in 
                         composite products caused a reflectance increase in all MODIS 
                         bands at the end of the dry season, especially in the near 
                         infrared (NIR). The reflectance decreased after BRDF correction. 
                         Because of the atmospheric effects, the view zenith angle (VZA) of 
                         the pixels selected in composite products decreased toward the 
                         south of the Amazon. In the southern Amazon, the seasonal 
                         amplitude in the solar zenith angle (SZA) reached values close to 
                         18 degrees. For the most anisotropic index, the BRDF correction 
                         removed, on average, 30% of the EVI signal in June, and 60% of the 
                         EVI signal in September, reducing dry season variations over time. 
                         The results reinforce the need for bidirectional correction of 
                         MODIS data before the seasonal and inter-annual analyses of the 
                         most anisotropic VIs.",
                  doi = "10.3390/rs11141650",
                  url = "http://dx.doi.org/10.3390/rs11141650",
                 issn = "2072-4292",
             language = "en",
           targetfile = "remotesensing-11-01650.pdf",
        urlaccessdate = "27 abr. 2024"
}