| Dongting Lake, the second largest freshwater lake in China, is one of the most important wetlands in the world, with various ecological functions such as material production, water storage and regulation, as well as biodiversity conservation. Therefore, the Dongting Lake is an important ecological barrier of the Yangtze River for social and economic sustainable development. Hydrological regime is one of the most important contents in the study of wetland ecology. The research on the hydrological regime in the Dongting Lake is beneficial for us to understand the changing rhythms of runoff and sediment, to reveal the trend of wetland successions and the underlying hydrological mechanisms of water regulation function, and to provide theoretical basis for management and conservation of Dongting Lake wetlands. In this thesis, the characteristics of water and sediment balance in the Dongting Lake, the succession of hydrological regime of four rivers and three channels, the changes of hydrological regime and its responses to major projects (take Three Gorges Reservoir as a example), landscape changes and underlying reasons in the Dongting Lake, and the succession of the functions of water regulation and storage of Dongting Lake were studied based on more than50years of hydrological data with various statistical methods such as multi-year average values, moving average values, Mann-Kendall test, double mass analysis, and interpretation of remote sensing images. The results are as following:(1) The amount of annual runoff, annual sediment input and output, the amount of sedimentation and the ratio of total sedimentation to total sediment input decreased significantly along with the time in the recent50years. During1951-2009, the amount of sedimentation and the ratio of total sedimentation to total sediment input was negative, indicating the increase of the ratio of sediment output and the status of erosion. Further analysis indicated that the decrease of annual runoff was mainly caused by the decrease of the runoff from three channels. The inter-annual changes of runoff from four rivers had no significant changes. The decrease of sediment input was mainly caused by the decrease of the sediment input both from three channels and four rivers. The ratio of the runoff and sediment input from three channels to the total runoff and sediment input of Dongting Lake decreased significantly along with time, while that of from four rivers increased continuously.(2) The annual runoff of the four rivers had insignificant changes during1951-2009 years, even though there were several up-down processes. The volumes of annual sediment load from the Four Rivers showed a significantly decreasing trend generally. Moreover, the time of abrupt change was different among the four rivers, with Xiang River in1996, Zi River in1973-1974, Yuan River in1997-1998and Li River in1998. The trending changes of annual sediment input were related to the construction of the largest reservoir in the river. The comprehensive impacts of the reduction of annual runoff, increase of vegetation cover and construction of medium-and large-sized reservoirs were the main reasons accounting for the great reduction of annual sediment load after2001.(3) The annual runoff and the volumes of annual sediment load of the three channels decreased significantly during1956-2009. Among them, the change of the Ouchi Channel was most significant and time of abrupt change was the earliest. In Songzi Channel, the time of tendency change of annual runoff and sediment discharge was1980and2003respectively. In Taiping Channel, the time of tendency change was1972and1986for annual runoff, and1998and2003for annual sediment discharge. In Ouchi Channel, the time of tendency change was1967,1972and1989for annual runoff, and1998and2003for annual sediment discharge. The reduction of annual runoff and the volumes of annual sediment load were mainly caused by the decrement of runoff from upper mainstream and various hydrological project, especially the cutoff of Jingjiang River and the Three George Reservoir. However, the way that the two projects impacted the annual sediment input was different. The cutoffs deceased the sediment input was mainly through the reduction of runoff, while the Three Gorges Reservoir was mainly through the deposition of sediment in the reservoir and therefore decrease the concentration of suspended sediment.(4) During1955-2008, the annual mean water level and the annual lowest water level in Chenglingi increased apparently. The mean monthly water level increased during Jan.-Mar., while was relative stable in other months. After the operation of the Three Gorges Reservoir, the mean monthly water level was slightly higher during Jan.-April, and was lower during May-Dec, especially in July, Oct. and November. The runoff from Three Gorges Reservoir has decisive roles on water levels of Chenglingji. The significant decrease of water level of Dongting Lake was induced by the decrease of precipitation, the change of water level-runoff relationship due to the erosion of Jingjiang section, and the reduction of runoff from the reservoir due to the water storage of Three Gorges Reservoir. The large decease of runoff from the reservoir enlarged the water level difference inside and outside the lake, which has an "empty effect’. Therefore, the speed of water discharging from Dongting Lake into Yangtze River increased. Meanwhile, the erosion of Jingjiang section deepened the river channel. The same volume of runoff from reservoir maintained a lower water level of Dongting Lake, which means only a higher runoff from the Three Gorges Reservoir can has the same effect of "backup" to water level of Dongting Lake.(5) The grassland area increased continuously during1982-2010, which increased497.76km2totally. The speed of expansion was fast during1995-2010, and was relatively slow during1995-2010. The grassland area increased mainly in the following area, including the margins of the lake basin of Changzhou, Xinshengzhou and Piaoweizhou, large areas of Niutouzhou, Wuguang-Chaixiazhou and zhongzhou-tuanzhou. The grassland decreased only in the following area, Fengchengguai, Beizhou, Wugangzi and Liugangzi. The area of grassland that was increasing was consistent with the area where sedimentation occurred before1995and the expansion of grassland was driven by sedimentation. After2005, the expansion of grassland was caused by rapid decrease of water level and sedimentation in some regions. Since1981, the submergence time of areas between altitude of22-24m decreased continuously. Therefore, sand deposition and the decrease of submergence time caused by water level were the main reasons for expansion of grasslands.(6) For the51years, the changes of lake area and water storage capacity of Dongting Lake could be divided into three phases. Lake area and water storage capacity sharply decreased significantly from1951to1958and from1960-1989. However, lake area and water storage capacity increased sharply from1995to2002. Significant correlation between water storage capacity and lake area, lake area and impoldering area were found. After the operation of Three Gorges Reservoir, the accumulation of sand deposition decreased seriously, and which was mainly due to the decrement of sand deposition in three channels. The decreased sedimentation enlarged the lake volume by0.31×10m. Both lake area and accumulated volume of sand deposition are significantly correlated with lake volume. The formula was:lake volume=0.115×area-0.987×accumulated sedimentation-1.119E-5×area2. Therefore, water storage capacity decreased along with increasing sand deposition and with decreasing lake area. |
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