PRELIMINARY ANALYSIS OF CHARACTERISTICS OF THE EXCEPTIONAL LOW DISCHARGE AND ITS CAUSE OVER THE YANGTZE RIVER,2006

Exceptional low flow happened over the Yangtze River in 2006 and had some significant influences on water usage and environment.By using the runoff data of main hydrological station and weather data,as well as taking the influence of TGP water storage into account,the runoff characteristics and its cause were studied.It is found that the decrease in discharge this year is mainly due to the significant loss of water in flood season and the discharge is characterized by exceptional low during that time.The maximum reduction of runoff and water level of hydrological stations occur during August and September.Maximum runoff reduction is all beyond 50% and the water level has significant drop at the same time.Compared with that in the same time of history,the water-income from Dongting Lake and Hanjiang River is significantly less while the water from Poyang Lake is plentiful which to some extent lightens the severe low discharge in the main stream.The extreme low precipitation and high temperature over upper reaches(especially the area between Pingshan and Yichang) is the main cause of the exceptional low discharge.There is a good relationship between variation of weather and runoff.Water storage of Three Gorges Project decreases the October discharge of Yichang hydrological station by 50% and the influential index to the discharge decrease of Datong hydrological station from October to November is 18.7%.