Changing Trends of Water Level and Runoff during Past 100 Years of the Yangtze River (China)

Changing trends and periodicities of the water level and runoff during past more than 100 years in the Yichang, Hankou and Datong stations are detected with the help of the wavelet analysis, maximum entropy spectrum analysis and Mann-Kendall analysis. The results indicate that the runoff of the Datong station is in upward trend since about 1965; this upward trend is significant at 0.95 confidence level after 1993. The runoff of the Hankou station, however, is in upward trend since 1920, and reached the 0.95 significant level at 1960. The changing features of the runoff in the Yichang station are different from Hankou and Datong stations. The runoff of the Yichang station is in downward trend after 1960. The changing trends of the water level are similar to those of the runoff. As for the periodicities, the changes of the periodicities of the runoff over time is not significant, while periodicities have the obvious changes over time, that is, the periodicities of the water level are shorter over time. This result suggests that changes of the runoff are mainly the results of the climatic changes, less impacted by the human activities. While the water level changes are results of complex factors, that is to say, the changes of the water level are not only influenced by the climatic changes but also influenced by the human activities. Wavelet analysis results, together with the maximum entropy spectrum analysis, demonstrate that the periodicities of the water level become shorter over time, which means that the occurrence possibility of the higher water level is high over time. Therefore we can say that with the development of human activities, intensifying human interference with the river channel evolution and intensified human exploitation of the land in the Yangtze catchments exert increasing influences on the water level changes. Human activities play the increasingly important role in the water level changes in the Yangtze catchments. Wavelet analysis and Mann-Kendall analysis results indicate that, except the Yichang station, the water level and runoff in the Hankou and Datong stations show upward trend. Furthermore excessive precipitation because of global warming will intensify the flood in the Yangtze catchments. Therefore, in the near future, the Yangtze catchments will encounter greater and more serious flood disasters. This result is expected to draw more attention from the local governments.