Analysis on evolution of drought-flood and its abrupt alternation in typical year from 1951 to 2015 in Dongting Lake area

Abstract: Drought-flood disaster is one of vital factors restricting Dongting Lake area's economic development. In order to predict the future drought-flood change tendency, using the monthly precipitation data of 36 meteorological stations in Dongting Lake and its surrounding areas, this paper attempted to describe the characteristics of evolution of drought-flood and its abrupt alternation in typical year from 1951 to 2015 in Dongting Lake area. Standardized precipitation index (SPI) and long cycle drought-flood abrupt alternation index (LDFAI), Morlet wavelet analysis and other climate diagnosis method were adopted. The results showed that the variation of drought and flood could be roughly divided into 5 periods. The first period was from 1951 to 1955, which was relatively humid. The second period was from 1956 to 1970. In this period, the alternation of flood and drought was frequent and the calamity degree was not severe. The third period was from 1971 to 1990. The alternation was also frequent and the calamity degree was severer than the previous period. The fourth was a humid period from 1991-2003. The last was an arid period after 2003. In this period, drought disaster has showed an obvious increasing trend. In the study area, flood and drought occurred frequently, and its alternation was also frequent during 1951-2015. The average frequency of different time scales of drought and flood events was 42.62%. The distribution of precipitation was uneven and concentrated in summer. The highest frequency of flood events was 33.33%, which occurred in summer. In winter, the frequency of drought events was 41.54%. Flood events showed an increasing trend in spring and summer in the 1990s. But in recent years, the rainfall has kept decreasing in 4 seasons. On an interdecadal scale, the 1990s was the wettest decade and had the highest frequency of flood events which was 30%. The precipitation was the least and the frequency of drought event was 22.99% in 2001-2015. In this period, the frequency of extreme drought event was the highest. The highest frequency of drought events was 25.83% in the 1970s. In this period, the precipitation was also low. Variation of drought-flood had 4 characteristic time scales, respectively, 4, 10, 25 and 31 a, which revealed that Dongting Lake area would be have more precipitation in the next few years. The variations of drought-flood in the 3 areas including East, West and South Dongting Lake area were basically the same, but had some differences in different periods. The variation of drought-flood even showed an obviously reverse phase in South and West Dongting Lake area in 1990s. There were 15 years with the absolute value of LDFAI is greater than 1 in recent 65 years in this area, which meant drought- flood abrupt alternation phenomenon was frequent. For either the variation of drought-flood or its abrupt alternation, West Dongting Lake area and East Dongting Lake area were basically the same. The variation of drought-flood abrupt alternation in South Dongting Lake area was obviously different from East and West Dongting Lake area.