| Wetland is an important ecosystem on earth, has its unique ecological functionsand values, and the research and protection of wetland is of great significance.Dongting Lake wetland has significant environmental and socio-economic benefits.It has a variety of vegetation and abundant water resources, is not only a goodhabitat for migratory birds, but also an important base for agricultural and fisheryproduction. Dongting Lake wetland has a close relationship with the Yangtze River,since the operation of the Three Gorges Dam, the runoff and sediment dischargefrom the Yangtze River to Dongting Lake declined sharply. The changes of river-lakerelation led to the changes of ecological environment in Dongting Lake wetland. Inthis paper the water level changes and vegetation cover changes of Dongting Lakewetland were studied and discussed, especially after the running of Three GorgesDam. Meanwhile, the wetland vegetation succession and species diversity and someother ecological environmental changes caused by the Three Gorges Dam wereanalyzed. This research will play an important role in the Dongting Lakeenvironmental protection and ecological restoration. At the same time, the researchwill also provide certain references for the build of dam on the outlet of DongtingLake.Firstly, Anxiang, Xiaohezui and Chenglingji the three hydrologic stations’water-level time series were analyzed by boxplot statistical method. Especially, thewater-level time series of Chenglingji hydrologic station were analyzed based on theMann-Kendall (MK) test and continuous wavelet transform. Through the researchesof variation point, the periodic and multi time-scale of water-level time series,making a better understand of the Dongting Lake wetland water environment. Andhow the Three Gorges Dam impacted on the water environment of the Dongting Lakewetland was analyzed. The results indicated that: the water-level change amplitudesof the3hydrologic stations were Chenglingji> Anxiang> Xiaohezui, thewater-level fluctuation of each hydrologic station enhanced obviously after1990.The annual water-level of Dongting Lake wetland mainly fluctuates between24m to26m from1959to2010. In a whole, the annual maximum water-level and the annual minimum water-level in an upward trend, but there was an obvious decrease after2003. And the annual minimum water-level keep rising, the increasing was about2m. The water-level changed process can be divided into4stages:1959-1972,1973-1990,1991-2003and from2004to2010. There were3main characteristic timescales, respectively were5a,12a and24a. The periodic change of long time scalewas relatively obvious, but the periodic change of short time scale was from weak tostrong, and a significant change occurred after1990.Secondly, the normalized differrence vegetation index (NDVI) of the DongtingLake wetland extracted by MODIS with the remote sensing and GIS technologieswas analyzed. The Dongting Lake wetland vegetation were driven by the water levelof the lake, the growth status and the coverage of vegetation changed with the waterlevel, and the NDVI values in the south of East Dongting wetland varied mostly inthe running years of the Three Gorges Dam. The characteristic time scales of NDVItime series were4a and7a. The cycle of4a time scale was2.5years, and theperiodic changes from strong to weak. While the7a time scale had a period of about5years, and the periodic changes relative stable. Dongting Lake wetland had a largesediment siltation, and the running of Three Georges Dam reduced the amount ofrunoff led to a larger and longer bare area of the wetland. The NDVI values in theflood period after2003were higher than the period before2003. The reduction ofwater body in the flood period led to the expanded of vegetation coverage. And thewater-level of24m was the critical point of occurring maximum NDVI value in theDongting Lake wetland.Finally, the research studied how Three Gorges Dam impacts on the ecologicalenvironment of Dongting Lake wetland. The climate changes, the runoff andsediment variation, the vegetation succession and biodiversity changes of DongtingLake wetland were analyzed after the runing of the Three Gorges Dam. Theprecipitation of Dongting Lake wetland rose slightly before2003, after that time theprecipitation maintain a relatively low level. Affected by the global warming and thewater body reduction, bare land increased and the interference of human activitiesthe temperature of Dongting Lake wetland rose about1℃. The amount of runoff andsediment of Dongting Lake in a multiple reduced after2003when the operation ofthe Three Gorges Dam, the reduced rate of sediment even reached a magnitude level. The storage water-level of Three Gorges Dam went up from135m to175m, the rankcorrelation coefficient ρsof the study areas increased to0.641,0.687and0.518,respectively. The correlation between vegetation cover and water-level enhanced.The changes of water ecological environment led to the changes of wetlandvegetation types and coverage area. The aquatic succession of wetland vegetationaccelerated, and in succession tended to the slow deposition direction. The birds andfish diversity and numbers of Dongting Lake wetland decreased rapidly in the firstfew years after the operation of the Three Gorges Dam, but with the stable runningof Three Gorges Dam the biodiversity gradually became rich. |
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