Monitoramento da dinâmica da geomorfologia fluvial da Reserva de Desenvolvimento Sustentável Mamirauá, por meio de técnicas de sensoriamento remoto

Abstract

The dynamics of channel migration and the renewal of the flooded forest by sedimentation and erosion constitute important controls over the ecological diversity of river corridors. Distinct geomorphological features and the hydrological processes determine the dynamic for the ecosystem. In this study, six images of the TM sensor on board the Landsat 5 satellite, acquired for dates between 1984 and 2005, were used to map and analyze the dynamics of vegetated areas on the white-water floodplain of the Mamirauá Sustainable Development Reserve, middle Solimões River. Temporal behaviors of some spectral variables were also appraised (AFRI2.1, NDVI and reflectance of TM bands 3, 4, 5 and 7) over the first 21 years of plant succession. The first step was to undertake procedures of pre-processing: geometric registration, atmospheric correction, transformation to reflectance, radiometric normalization between dates and preparation of a composite mask of clouds or cloud shadows for all dates. The areas that underwent erosion or sedimentation were first mapped through the technique of change detection by subtraction of images of the vegetation index AFRI2.1, for five intervals of time between 1984 and 2005. However, in the AFRI2.1 images water and vegetation could not be distinguished, which made it impossible to undertake change detection by subtraction of images with continuous values. The vegetated areas were instead distinguished from soil and from water by thresholds of AFRI and TM band 7, respectively, both within and between images. Changes of state – from soil or water to vegetation (sedimentation) and from vegetation to soil or water (erosion) – were then mapped by observing changes between these thematic images for the five sequential time intervals. There was no significant tendency of positive or negative sediment deposition over time. The sedimentation and the erosion processes were balanced, representing 51.4% and 48.6%, respectively, of the total area that changed state in any moment of the period. There were no significant relationships between either erosion or sedimentation rates and each of the fluviometric variables examined. The overall rate of lateral migration (erosion + sedimentation) for the Solimões was equivalent to 0.48% of the open-channel surface area per year. The most active migrations were associated with fluvial islands or the confluence of two channels. Thos portions of the várzea along the channels that surround RDSM which did undergo some change generally did so only once over the 21 year. Only 5% of the pixels that changed tae did so two or more times. Pixels undergoing multiple changes of states were more common along the paraná of Aranapu. Regarding spectral changes during vegetation succession, the values of AFRI2.1 and of TM band 4, both showed a significant peak between 2 and 9 years of age. The values of NDVI reached a plateau between 2 and 9 years, without dropping significantly afterwards. The behavior of TM band 3 was complementary to the behavior of NDVI, dropping between 2 and 9 years and remaining stable in the subsequent ages. Between 16 and 21 years a significant drop was observed in bands 4 and 5. TM band 7 oscillated within a narrow range of 1% reflectance over the whole period of plant succession, and these differences were not considered reliable.