The changing patterns of floods in Poyang Lake, China: characteristics and explanations

Poyang Lake, directly connected with the Yangtze River, is one of the most frequently flooded areas in China. The frequent large floods have caused huge damages to the environment and economy and threatened the life of approximately 10 million people. Understanding the changing characteristics of floods as well as the affecting factors is an important prerequisite of flood disaster prevention and mitigation. In this study, the characteristics of historical floods in Poyang Lake were identified and examined based on several widely used indices and Mann-Kendall test. The study also analyzed the related driving forces and discussed their relationships with Poyang Lake floods. The results show that the floods in Poyang Lake mainly occurred in mid-and late July. The inter-annual variation of highest flood stages and duration showed a long-term increasing linear trend. Also, a slightly increasing linear trend in the timing of highest stages indicated the floods have occurred later and later during the last 60 years. At the decadal scale, the flood situations were most severe in 1990s while gentle in 2000s in terms of the occurrence frequency and average duration. The climate change was the primary influence factor for changing of flood characteristics in Poyang Lake; i.e., the abnormally large rainfall during the flood season and subsequent large discharges of Yangtze River and runoff inflow from the basin were mainly responsible for the severe floods in 1990s. Also, the smallest storage capacity of Poyang Lake in 1990s due to the intensive human activities such as the great floodplain occupancy and levee construction further increased the severity of floods. While the rare floods in 2000s can be attributed to, on the one hand, the decrease in rainfall over the middle reaches of Yangtze River which caused the low streamflows of Yangtze River and runoff inflow from the Poyang Lake basin. On the other hand, the return land to lake policy, intensive sand mining in the lake and the flood control of Three Gorges Dam also played an important role in mitigation of flood frequency and severity.