Wetland hydrodynamics using interferometric synthetic aperture radar, remote sensing, and modeling.

摘要

The wetlands of low-land rivers and lakes are massive in size and in volumetric fluxes, which greatly limits a thorough understanding of their flow dynamics. The complexity of floodwater flows has not been well captured because flood waters move laterally across wetlands and this movement is not bounded like that of typical channel flow. The importance of these issues is exemplified by wetland loss in the Lake Chad Basin, which has been accelerated due primarily to natural and anthropogenic processes. This loss makes an impact on the magnitude of flooding in the basin and threatens the ecosystems. In my research, I study three wetlands: the Amazon, Congo, and Logone wetlands. The three wetlands are different in size and location, but all are associated with rivers. These are representative of riparian tropical, swamp tropical and inland Saharan wetlands, respectively. First, interferometric coherence variations in JERS-1 (Japanese Earth Resources Satellite) L-band SAR (Synthetic Aperture Radar) data are analyzed at three central Amazon sites. Lake Balbina consists mostly of upland forests and inundated trunks of dead, leafless trees as opposed to Cabaliana and Solimoes-Purus which are dominated by flooded forests. Balbina has higher coherence values than either Cabaliana or Solimoes-Purus likely because the dead, leafless trees support strong double-bounce returns. Flooded and nonflooded wetland coherence varies with the season whereas terre-firme and open water do not have similarly evident seasonal variations. Second, interferometric processing of JERS-1 SAR data from the central portions of both the Amazon and Congo Basins provides centimeter-scale measurements of water level change (∂h/∂t). Despite being large, low-relief, tropical river systems, the floodplains and wetlands of the Amazon and Congo Basins show markedly different surface water flow hydraulics. Amazon patterns of ∂h/∂t are well defined with clear boundaries whereas the Congo patterns are not well defined and have diffuse boundaries. Amazon floodplain channels, lakes and pans are well interconnected, whereas the Congo wetlands are expanses with few boundaries or flow routes. Third, flood inundation maps in Logone floodplain, Lake Chad Basin are generated from 33 multi-temporal Landsat Enhanced Thematic Mapper Plus (ETM+) images. The maximum flooding extent in the study area increases up to ~5.8K km2 in late October 2008. Coefficients of determination between flooding extents and water height variations are greater than 0.91 with 4 to 36 days in phase lag. Fourth, the spatial and temporal distribution of water level and storage changes are quantified in the central Congo wetland using spaceborne data and the LISFLOOD-FP hydrodynamic model. This model provides 1-D diffusive channel flow and 2-D dynamic floodplain flow. The model shows meter scale water level changes on the main stem Congo River and in its tributaries (e.g. Ubangi, Sangha, Likouala-aux-Herbes, and Likouala Rivers) at 500-meters/pixel spatial resolution. In this dissertation, my research improves the characterization of wetland surface water dynamics by making inter-comparisons of the three wetlands.