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Since 2003, Yangtze River has been in the continuous dry lever periods, the Three Gorges Project has been running. The water lever of dry season in the main channel of Yangtze River and the Chinese biggest fresh-water lake-poyang lake created new low limits, and the duration prolong. At 2006, the water lever of dry season below10m in Xingzi station which is the representation of Poyang Lake continued three months, the duration of water lever below 9m or 8m continued more than 1963, at 2007, the water lever below 8m continued almost one month. The lake water quality was deteriorated, wetland plants were degenerated, fish and migratory birds were reduced by continued low water lever, this situation also influence the social development included service water、irrigation、navigation and so on. According to this problem, in this paper, we focused on analysis of hydro long-sequence and application of remote sensing, aiming to explain the reason of continued low water lever and its influence to Poyang lake wetland ecosystems, the main content and concludes as follows:(1)By using methods of coefficient of variation, anomaly analysis, sliding average least square (SALS), standard deviation classification and Empirical mode decomposition (EMD),to analysis the long-sequences include rainfall of poyang lake, water lever of Xingzi station and stream flow of Hankou station, we can explain the phenomenon of poyang lake low water lever:first, in the aspect of distribution in years, the coefficient of variation of both the river runoff of Hankou station and rainfall of poyang lake were minished,1940s was a turning point of Hankou station river runoff, human service water of Yangtze river drainage area were increased, management of conservancy projects to river was reinforced. Second, in the aspect of trend between years, middle and lower reaches of Yangtze River was in dry years, the trend of water lever in Xingzi station of poyang lake in 54 years was circuity reduced, main reason is that water has translated between poyang lake and Yangtze river, and that drainage area of poyang lake in Jiangxi province human service water was increased, subsidiary reason is that return cropland to water extended water areas, and that disorder sand mining changed floor of lakes. Third, in the aspect of periodicity, Poyang Lake and Yangtze River have consistent cycle in middle and short period of floody or dry years, but this cycle has many different years.(2) Based on characteristics of each band of remote sensing date in Lake District of Poyang lake, used TM4,3,2 and TM4,5,3 as the basis band to human interpretation. According to the classification system of Ramsar Convent and china classification system, built classification system and interpret symbol of Poyang lake wetland. By using that built seven different classifications templates for seven different water lever, then made unsupervised classification, gained classification pictures and date of water lever include 7.90m,9.83m,12.10m,12.87m,13.82m,14.93m,16.09m. Evaluated seven different templates, all classification accuracy were above 90%, kappa coefficient were above 85%, satisfied demand. Analysis gained date include the areas of lake water, sad, mud, swamp, exposed grass, billabong,10-14m is the favor water lever of poyang lake wetland ecosystem,14m is the turning point, the time of dry water lever appearance earlier or duration extend made wetland plants degenerated, floating plant decreased, some high-ranking species in community of submerged plant changed, influenced the fishes and migratory bird.
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