Statistical Disasters Of Drought And Flood Disasters In The Yangtze River Basin

As the global climate warming, natural disasters become more frequent, especially droughts and floods. The risk assessment has been a hot issue in the study on droughts and floods, and the key to solve this problem is to accurately determine the occurrence trends of droughts and floods, which the trends and specific years may occur within the next2years to20years of droughts and floods. Droughts and floods are frequent by the combined effect of the southeast and southwest monsoon in the Yangtze Basin.The study on trends and reasons for droughts and floods of the Yangtze Basin has far-reaching significance.The paper conducted a statistical study of the droughts and floods disaster in the Yangtze Basin. Using the methods of the linear trend estimate, Mann-Kendall method, empirical orthogonal function, commensurability, butterfly structure diagram, structural system, correlation analysis, this paper explored the time symmetry on large, medium and small spatial scales of droughts and floods, determined its future trends, the main conclusions showed:(1) The average annual temperature were significant upward trend on large, medium and small spatial scales of the Yangtze Basin during1961to2012, mutations occurred in1994. Spatial variation of annual average temperature showed elevation-dependent, and the tendency rate was the trend of "low in the middle other high". Seasonal temperature in the whole basin showed an upward trend, summer temperatures showed a decline after the first fluctuate upward trend, the remaining season were roughly experienced the process of drop-up-drop. Summer temperatures were rising in typical cities, except Yichang showed a weak decreasing trend. Most sites’ seasonal temperature showed an upward trend in the region.(2) Near52years, precipitation anomalies showed a decreasing trend in the whole basin, upstream and Yichang, while was an increasing trend in midstream and downstream, Chongqing and Nanjing. Spatial distribution pattern of annual precipitation roughly along the southeast-northwest direction, showed a gradual transition from large to small, the tendency of the rate were "low in the middle other high". The temperatures in spring and autumn showed a downward trend, an upward trend in summer and winter of the whole basin and typical cities. Most sites showed decreased precipitation in spring and autumn, an upward trend in summer and winter.(3) Recent52years, it presented drought trend in the whole basin, upstream, Chongqing and Yichang, flood in midstream and downstream, Nanjing. During spring there was a weak drought trend in the whole basin, upstream and downstream, small scale did not change significantly. It was a flood trend during summer in the whole basin, midstream and downstream, Chongqing and Nanjing, was contrary in upstream. There was a sign of drought during autumn in the every basin, Chongqing and Nanjing. In the winter the trend showed a shift to drought in the whole basin, midstream and downstream, was on the contrary in upstream and Nanjing, no change in Chongqing. There had no significant change during summer, autumn and winter in Yichang. Spatial distribution of droughts and floods were:contrary southeast and northwest, southwest and northeast, the Han River, Wujiang Basin and other along the east-west direction, west and east bounded by the Jialing River and Dongting Lake, the first was the most typical distribution.(4) The paper showed that the law of drought disaster symmetry was better on the large, medium and small scale in the Yangtze Basin, except for flood disaster during autumn in the whole basin, the rest had no obvious symmetry rule. The time window of drought and autumn flood were2019,2018in the whole basin,2014and2019years of the summer drought;2014of upstream drought,2014,2015and2016years of autumn drought;2014of summer drought in midstream and downstream;2016of drought in Chongqing,2014and2017of autumn drought;2017,2015years of drought respectively in Yichang and Nanjing.(5) During the period of1961to2012, the majority years of droughts and floods occurred with the ENSO event in Yangtze Basins and typical cities, and the flood year’s proportion was higher than drought when occurred with the ENSO. There were different phase relationships between the sunspot cycles and the droughts and floods in different scales. The drought index showed a negative correlation with the snowstorm frequency in various districts, that was, when the total frequency of the snow in Tibetan Plateau increased, more drought regions of the Yangtze Basins, on the contrary more floods, medium and large scale were more significant compared to small scale.