| China is known for her vast territory and abundant resources, the physical environment is complicated and varied, human activity has a long history and far-reaching. The floods and droughts is one of the disasters which academia paying close attention to among the study of natural disasters possesses such characters in high frequency, wide affected area and heavy situation of disaster. This paper took Chaohu Lake Basin as its study object, which is located in the areas along the middle and lower reaches of the Yangtze River. With a large number of textual research of documents, local records as well as analysis of natural disasters in the historical periods, to study floods and droughts in Chaohu Lake Basin during the past 600 years. A graded series of floods/droughts has been rebuilt. Basined on statistics analysis, continuous power spectrum analysis, wavelet analysis rules and cycles of floods/droughts change within the basin has been revealed. Mann-kandell test methods and t-test methods are used to test abrupt climate change during past 600 years in Chaohu Lake Basin. There is comparative analysis between the study of this paper and the results have been found by our discussion group. The main conclusions are as follows:1. The history records of Chaohu Lake Basin of floods and droughts ascended to Ming Dynasty , more than 600 years now. At the beginning of Ming Dynasty, there is heavy lack of records of unusual years till 1470a with the work of local records complication rising up and developing. Based on listing systematically the historical records of Chaohu Lake Basin's flood and drought, a graded series of dryness/wetness during past 600 years and integrating flood drought indexes and grades has been rebuilt. 2. In the past 600 years,the disaster of floods and droughts usually occurred within Chaohu Lake Basin, the closer the higher. The periods from 1949 to 1985 are most serious, almost occurred every year. The frequency of floods is nearly equivalent to that of drought, and its average frequency is once-in-2.28-2.40 years. All kinds of grades occurred within Chaohu Lake Basin. Among them, grades as partials drought has the highest frequency, its average frequency is once-in-5.58 years. Grade as"floods"is second average frequency with once-in-6.21 years.3. Concurrent disaster of floods and droughts has been a tendency in Chaohu Lake Basin, it totally happened 48 times during the past 600 years. But in recent 100 years, the average frequency has reached once-in-3.8years. Heavy drought (flood), great drought (flood) appeared continuously is another of tendency, once it happens, usually last 2-3 years. And the frequency of heavy and great flood is higher than that of heavy and great drought.4. The result of continuous power spectrum analysis demonstrates that short-term oscillation of flood and drought in Chaohu Lake Basin is in evidence as well as wavelet analysis. There are the 10-11 years, 2-4 years, 7-9 years, 20 years, 50 years, 6 9-73 years fluctuations among them within Chaohu Lake Basin.5. The coupling of flood or drought and change of dry or wet climate environment are very well during the past 600 years within Chaohu Lake Basin. (1)1951-1980a A.D. is the period which flood lasted the longest duration, the highest frequency and index of strength, and at the same time it's also the period which drought disaster lasted the longest duration, secondly highest frequency and high index of strength. The period was just in 1871a A.D. to now which is proven to be warm and wet period. (2) 1831-1850a A.D. is the period which flood lasted the longest duration, secondary highest frequency and secondary highest index of strength. It's just in 1818-1871a A.D. which is proven to be cold and wet period. (3) 1561-1570a A.D. is the period lasted secondary longest duration, high index of strength , it's just in 1271-1635a A.D. which is proven to be mild and wet period. (4) 1711-1720a A.D. is the period which drought lasted the longest duration, high index of strength, it's just in 1635-1753a A.D. which is proven to be cold and dry period.6. Climate transition with in Chaohu Lake Basin approximately is: 1520's A.D., wet to dry; 1550's-1560's A.D., dry to wet; 1620's A.D., wet to dry; 1720's-1730's A.D. dry to wet; 1780's-1790's, wet to dry; 1820's A.D., dry to wet; end of 1800's to beginning of the 1900's wet to dry; 1960's A.D., dry to wet. There are environment mutation event in 1627, extreme drought event in 1750's A.D. which indicates the climatic characteristics of the Little Ice Age. In the later 100 years, it became warm and wet. 1750a A.D. also is an important boundary of Chaohu Lake Basin environment change history. There was extreme drought event in 1930's A.D.. At the beginning of 21th century, wetness will be the main climate in Chaohu Lake Basin.7. Coupling with environmental demonstrated that the records of flood and drought, the form of flood and drought during the past 600 years within the Chaohu Lake Basin mainly resulted from the drive of climatic environment change. The human's activity imposed on environment is making flood and drought more and more complex.
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