Research On Drought/Flood Temporal-spatial Characteristics And Extreme Drought Events In The Eastern Northwest China From 1470 To 1912

Under the background of global climate anomalies,drought and flood disasters occurred frequently in China.Collecting and classifying the data about drought and flood disasters in the eastern Northwest China from 1470 to 1912,according to the criterion of drought and flood grade,this paper reconstructed the sequence of drought/flood grade.All the results of the research have important practical significance for the development of disaster prevention and mitigation measures in eastern Northwest China.The study will be very important to discover change law of drought and flood disaster especially accurately forecast the future climate.The main conclusions are:(1)According to the comparison and analysis,reconstruction of drought/flood grad sequence has high consistency with other precipitation sequence,the three curves had several similar trends where peaks and troughs were appeared at the same time.They corresponded to stalagmites oxygen isotopes records during the same period.All these could be an indirect proof of reliability and feasibility of our recovered historical flood and drought grade sequence based on the historical document data.(2)The change characteristics of drought/flood disasters over the past 443 years in all the regions were analyzed by employing the methods such as moving average,accumulative anomaly,moving t-test technique,and wavelet analysis.We can found the change of the decade of a century and frequency,periodic and mutability,periodicity.The results showed that drought disasters took place frequently and seriously in the eastern Northwest China.The alternate phenomenon about drought and flood was obvious,they were comparable balance.Probability of drought and the partial drought were higher than flood disasters.The probability of flood years was the lowest.The drought and flood disaster which had obvious features of phases could be divided into four stages.At the basis of four stages,we could further divide them into twelve drought-partial or flood-partial periods.Using moving t-test technique,we found that there were 12 abrupt changes on decade scale and 5 abrupt changes on 50 years scale.The reconstructed sequences displayed a periodic variability in multiple scales,and the obvious ones were on the timescale of 130-110a,70-80a,50-52a and 28a,10-15a,5a,in which 10-15a and 28a were relatively dominant.The first to fourth primary periods were 28a,15a,73a and 5a respectively.(3)Seeing from the space,the frequency of drought disasters occurrence is highly distributed in northern Qinghai?parts of the middle east Gansu and northern Shanxi among which Qinghai is the highest.The hotspots of flood disasters in mainly concentrated in southern Qinghai,northern Shanxi and parts of the southeast Gansu.(4)Define heavy drought event,build the factor model of extreme drought,and reconstruct the extreme events of eastern northwest areas for years.It is found that there are 22 extreme drought events,among which extreme drought of type I is the main part,while type II is second to it and type III is the least,it is found that the results of the model has significant similarity with records through the contrast verification.Power spectrum is used to analyze the main periodicity of series of the extreme drought events which displays a period of 7?8a?4a?6a?10a,and 7?8a is the first significant period.According to the principle of time fractal,fractal dimension characteristics of series of the extreme drought events can be mined.The temporal fractal dimension of the drought disasters of the whole series reduces with the increasing of extreme drought intensity,it proves the inverse correlation with extremely drought period.Different types of extreme drought events have their own repeating law in time domain,the drought degree lighter the scale-free area is wider,the fractal dimension is higher,the short period is rather obvious.(5)Do a case analysis on the extreme drought years(1876-1878).Analyze this extreme drought event thoroughly about its climate background,cause and situation of it,its dynamic evolution process and the famine,locust plague,epidemic disease and the associated disasters it brought;Do research on the drought degree of this extreme event through the records of days without rain,casualties as well as the records of die out rivers,lakes,springs and wells.