Analysis of the periodicity of yearly extreme water level in the tidal reaches of the Yangtze River

The periodic characteristics of the time series of yearly highest water level and yearly lowest water level on the lower tidal reaches of the Yangtze River are analyzed by using periodogram analysis and maximum entropy spectrum analysis according to the data(1964-1987) of Wuhu,Nanjing and Jiangyin hydrologic stations.The results show that the period obtained in the yearly highest water level of Wuhu,Nanjing and Jiangyin is all 3.14 years,that obtained in the yearly highest water level of Wuhu and Nanjing is 7.33 and 2.2 years respectively,and that obtained in the yearly highest water level of Jiangyin is 5.5 years.The period obtained in the yearly lowest water level of Wuhu,Nanjing and Jiangyin is 2.44~3.14 years,and that obtained in the yearly lowest water level of Wuhu,Nanjing and Jiangyin is 2.2,5.5 and 11 years respectively.These periods not only reflect the periodic characteristics of runoff of the Yangtze River,but also represent those of ocean tide.It provides the foundation for further research of evolution of the lower tidal reaches of the Yangtze River.