| Summary: | 碩士 === 國立中央大學 === 土木工程學系 === 105 === Water level (WL) and water volume (WV) of surface-water bodies are among the most crucial variables used in water-resources assessment and management. They fluctuate as a result of climatic forcing, and they are considered as indicators of climatic impacts on water resources. Quantifying riverine WL and WV, however, usually requires the availability of timely and continuous in-situ data, which could be a challenge for rivers in remote regions, including the Mekong River basin. As one of the most developed rivers in the world, with more than 20 dams built or under construction, Mekong River is in need of a monitoring system that could facilitate basin-scale management of water resources facing future climate change. This study used spaceborne sensors to investigate two dams in the upper Mekong River, Xiaowan and Jinghong Dams within China, to examine river flow dynamics after these dams became operational. We integrated multi-mission satellite radar altimetry (RA, Envisat and Jason-2), satellite laser altimetry ICESat, Landsat-5/-7/-8 Thematic Mapper (TM)/Enhanced Thematic Mapper plus (ETM+)/Operational Land Imager (OLI) optical and Sentinel-1A synthetic aperture radar (SAR) remote sensing (RS) imageries to construct composite WL time series with enhanced spatial resolutions and substantially extended WL data records. An empirical relationship between altimetry WL and water extent was first established for each dam and 6 checkpoints, and then the combined long-term WL time series from Landsat/Sentinel-1A images are reconstructed for all study sites. The R2 between altimetry WL and Landsat water area measurements is >0.9. Next, the Tropical Rainfall Measuring Mission (TRMM) data were used to diagnose and determine water variation caused by the precipitation anomaly within the basin. Finally, the impact of hydrologic dynamics caused by the impoundment of the dams is assessed. The discrepancy between satellite-derived WL and available in-situ gauge data, in term of root-mean-square error (RMSE) is at 2–5 m level at upstream dams, and 1 m at downstream checkpoints. Estimated WV variations derived from combined RA, RS imageries and shuttle radar topography mission (SRTM) data are consistent with results from in-situ data with a difference at about 3%. We concluded that the river level downstream is affected by a combined operation of these two dams after 2009, which has increased WL by 0.18±0.08 m•yr-1 in dry seasons and decreased WL by 0.32±0.14 m•yr-1 in wet seasons.
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