Multi-time scale analysis of runoff at the Yangtze estuary based on the Morlet Wavelet Transform method

Runoff series of the Yangtze River presents an intricate variation tendency under the reinforced influence of human activities. The Morlet Wavelet Transform method has been applied to analyze the annual runoff data from 1950 to 2011 at the Yangtze River Estuary. It can clearly reveal the multi-time scales structure, break point, change and distribution of periodic variation in the different time scales of the runoff series. The main conclusions are that: 1) Repeated periodic oscillations accompanied by an extremely large fluctuation are presented in the runoff series with an obvious difference between wet and dry years, and the major periods of the time series are about 3, 8, 16 and 23 years respectively. Among them, the presented maximum periodic oscillation is 23 years scale. 2) In the 23-year time scale, the wet periods are 1950–1958, 1969–1980 and 1992–2003, and the dry periods are 1959–1968, 1981–1991 and 2004–2011. 3) It can be predicted from the view of long time scales that the low annual runoff will likely occur in the near future.