| Mapping and monitoring the extent and dynamics of land surface water with remote sensing technology is an important means for water resources management and climate-related research.Because of the characteristics on long time series and high spatial resolution,Landsat has been widely used in remote sensing application.However,the lack of data information caused by noise such as cloud coverage is an inescapable problem for remote sensing image interpretation of earth information.Therefore,this paper constructs a lakeshore elevation model by inverse distance weighted(IDW)interpolation method,using water-land boundaries extracted from Landsat images with less cloud cover and water level observation data of hydrological station.Then the water-land boundaries affected by the cloud cover were extracted based on the digital elevation model and the relationship between long-term lake surface water and the main environmental factors were analyzed.In this study,the lakeshore elevation model can be a great reference for the extraction of long-time surface water in high cloud-covered areas,and the results of long-term surface water changes and driving forces in the Tonle Sap Lake will be of great significance to sustainable use of the water resources and the formulation of relevant management policies.This study is divided into two parts,including the construction of lakeshore elevation model of the Tonle Sap Lake based on water-land boundary and corresponding water level,extracting the surface water affected by cloud cover and the analysis of surface water change in1988-2017 and its relationship with driving forces.In the first part,based on Landsat images,the edge detection method and the method based on distance deviation were used to obtain the optimum water index for extracting the surface water of Tonle Sap Lake from these existing 33 water indices(Chapter 3).On this basis,a lakeshore elevation model was constructed based on water-land boundaries extracted by the optimum index and corresponding period water level,and it was used to reconstruct missing water surface under cloud cover(Chapter 4).The second part,long-term lake surface water coverages were extracted based on the elevation model.and the change and main driving forces of the surface water area of Tonle Sap Lake from 1988 to 2017 are confirm,by analyzing the trend of surface water area variation and the correlation with the precipitation in the Tonle Sap Lake basin,the water level and the dam construction of the upstream Mekong River mainstream(Chapter 5)The results show that:(1)NIR single-band index,Green/NIR ratio index,WRI index and CIWI composite index are the most suitable for extracting the surface water extent of the Tonle Sap Lake compared with other water indices,among which WRI index is the water index with stable time and the best extraction effect.(2)Combining the Landsat image water-land boundaries and lakeshore elevation model constructed by the water boundaries of images with less cloud cover can extract the surface water coverage of the lake for a long period of time,thus the problem of surface water extraction in the large cloud cover period is solved,with the extracted water-land boundary error controlled within two pixels.(3)Between 1988 and 2017,the surface water area of Tonle Sap Lake showed a non-significant(a=0.05)increase trend in wet season and a non-significant(α=0.05)decrease trend in dry season.The change of monthly average surface water area in the lake had a significant(a=0.05)positive correlation with the monthly precipitation delayed for 3 months and the monthly average water level of Pakse and Kratie hydrological station delayed for 2 months in Lower Mekong River.The correlation coefficient is 0.71.,0.88 and 0.88.respectively.The study found that surface water area of the Tonle Sap Lake decreased by 11%and 8%on average during the dry season after 2008 and 2011,respectively.However,the water level of Pakse and Kratie hydrological station increased by 39%and 41%.6%and 4%during the dry season after 2008 and 2011.respectively.The water level changes at the two hydrological station are opposite to the change in the surface water area of the Tonle Sap Lake,which indicates that the change of the water level in the Mekong main stem doesn’t directly cause the change of the surface water area of the Tonle Sap Lake.Therefore,there is non-significant correlation between the construction of the hydropower station in the upper reaches of the Mekong River and the change of surface water area in the Tonle Sap Lake. |
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