@Article{GalvãoPetrDala:2024:AnOvAm,
               author = "Galv{\~a}o, L{\^e}nio Soares and Petri, Caio Arlanche and 
                         Dalagnol, Ricardo",
          affiliation = "{Instituto Nacional de Pesquisas Espaciais (INPE)} and {Instituto 
                         Nacional de Pesquisas Espaciais (INPE)} and {California Institute 
                         of Technology}",
                title = "Coupled effects of solar illumination and phenology on vegetation 
                         index determination: an analysis over the Amazonian forests using 
                         the SuperDove satellite constellation",
              journal = "GIScience and Remote Sensing",
                 year = "2024",
               volume = "61",
               number = "1",
                pages = "e2290354",
             keywords = "Satellite constellation, SuperDove, vegetation indices, solar 
                         illumination, vegetation phenology, Amazon.",
             abstract = "Despite the importance of the Amazonian rainforests in the global 
                         carbon cycle, their phenological responses measured by large 
                         field-of-view satellite sensors are still not completely 
                         understood. In this scenario, close-to-nadir observations at high 
                         spatial and temporal resolutions made by satellite constellations 
                         may contribute to improve this knowledge. Here, we investigated 
                         the sensitivity of five vegetation indices (VIs) to canopy shadows 
                         over 15 protected forests of the Amazon, and the possible 
                         existence of coupled effects of solar illumination and vegetation 
                         phenology on the VI determination. The VIs are the Enhanced 
                         Vegetation Index (EVI), Green-Red Normalized Difference (GRND), 
                         Modified Photochemical Reflectance Index (MPRI), Normalized 
                         Difference Vegetation Index (NDVI), and RedEdge Normalized 
                         Difference (REND). They were calculated from 432 images obtained 
                         in 2022 by the Planet's eight-band SuperDove instrument. Few daily 
                         images acquired on the same day with distinct Solar Zenith Angle 
                         (SZA) were used to disentangle the effects of solar illumination 
                         from those of vegetation phenology. The results showed the 
                         presence of coupled effects of solar illumination and vegetation 
                         phenology on the EVI determination regardless of the site 
                         location, especially over dense forests. Such effects were not 
                         observed significantly in the GRND, MPRI, NDVI, and REND data. 
                         When the vegetation phenology was kept fixed in the analysis, 
                         solar illumination generated pseudo-greening effects from the EVI, 
                         even for small differences in SZA between daily observations. As 
                         the most sensitive VI to illumination conditions, the EVI 
                         increased from the beginning to the end of the dry season tracking 
                         solar angles and shade fractions. This dry-season trend was not 
                         observed for GRND, MPRI, NDVI, and REND, which presented low 
                         correlations with SZA and shade fractions. These four VIs were 
                         correlated with each other over most sites, which explained the 
                         agreement observed between their seasonal profiles. From the 
                         analysis of 15 sites distributed throughout the Amazon, our 
                         findings did not confirm patterns of large-scale greening at the 
                         end of the dry season. Local changes in greening (vegetation 
                         productivity) and browning were captured by the VIs over a few 
                         sites but in different periods of the dry season (June to 
                         September). At the high spatial scale of SuperDove observations, 
                         our results highlight the necessity of correcting solar and, in 
                         some cases, terrain illumination effects on the EVI before 
                         retrieving phenological metrics over the Amazon.",
                  doi = "10.1080/15481603.2023.2290354",
                  url = "http://dx.doi.org/10.1080/15481603.2023.2290354",
                 issn = "1548-1603",
             language = "en",
           targetfile = "Coupled effects of solar illumination and phenology on vegetation 
                         index determination an analysis over the Amazonian forests using 
                         the SuperDove sate.pdf",
        urlaccessdate = "02 maio 2024"
}