Dry/Wet Changes and Their Standing Characteristics in China During the Past 531 Years

Time series of the dryness-wetness（DW） index of 531 yr（AD 1470-2000） at 42 stations in regions A（most of North China and the east of Northwest China） and B（the Yangtze-Huaihe River valley） in China are applied to investigating the historical DW characteristics over various periods of the series with a relatively stationary average value using Bernaola-Galvan（BG） algorithm.The results indicate that region A/B underwent three drought-intensive periods（DIP;1471-1560,1571-1640,and 1920-2000/1501-1540,1631-1690,and 1911-1960） in the last 531 years.In the DIP of the last 130 years,the frequency of DW transition has increased in region A,but not obviously changed in region B in comparison with the other two historical DIPs.The dry period started in about 1920 in region A with severe drought events occurring from the late 1970s to the early 1980s.It lasted for about 50-70 yr in this century,and then a DW shift took place.The wet period in region B might maintain for the coming several decades.The variations of DW in region A are positively correlated with changes in temperature,but in region B,the correlation with temperature is weaker.It is found that the number of DW indices of various categories within a running window is an exponential function of the running window length.The dryness scale factor（DSF） is defined as the reciprocal of the characteristic value of the exponential distribution,and it has a band-like fluctuation distribution that is good for the detection of extreme drought（flood） clustering events.The results show that frequencies of the severe large-scale drought events that concurrently occurred in regions A and B were high in the late 12th century,the early 13th century,the early 17th century,and the late 20th century.This provides evidence for the existence of the time-clustering phenomena of droughts（floods）.