The research of durative characteristics of dry/wet series of China during the past 1000 years

Abstract

Using dry/wet index series of China during the past 531 years and 1041 years, by defining the dry (wet) grade and making running calculation of the appearance times of each dry (wet) grade in the original index series, it was shown that there is a exponential distribution P(x)=A*e-γx of the appearance times of each grade with respect to the window size. We defined 1/γ as the scale factor to characterize the dry (wet) durative and used it to analyze the dry (wet) durative of north China and the area between Yangtze and Huai rivers. Results show that the dry factor has the semi-belt wave distribution from north to south, the dry durative in north China is somewhat longer than that between Yangtze and Huai rivers, the average dry factors of area A, B and C are 1.87, 1.62 and 1.82, respectively. The dry (wet) durative is rather obvious in those intervals in which dry or wet are comparatively concentrated, so the large spatial scale and synchronized extreme dry accidents are easier to happen during the end of 12th century, the beginning of 13th century, the beginning of 17th century and the end of 20th century, which verifies the existence of cluster phenomena in the dry/wet index. The influence to dry durative in north China caused by the dry/wet index during 1260—1280 (the climate back ground being the end of the middle ages warm) was much lavger than that caused by the index during 1980—2000 (the climate background being the global warming). Furthermore, the number of dry years during 1260—1280 was also larger than that of 1980—2000. Obviously, serious, larger spatial scale and synchronized extreme dry accidents are easy to happen in warm periods, and the drought since 1970s in north China might be caused by co-operation of human and natural factors, in which the natural factor might have played the more important role.