Aridity-humidity Changes in the Borderland of Shaanxi,Gansu and Ningxia

More than 240 items of climatic information were chosen from the official historical books,local chronicles,annals and regional meteorological disaster yearbooks.By using Moisture Index and Flood-drought Index obtained from the above information,we studied the historical climate change,namely aridity-humidity conditions in the borderland of Shaanxi Province,Gansu Province and Ningxia Hui Autonomous Region.The results showed that during the period of 1208-1369,it tended to be dry in the region and the ratio of drought to flood disasters was 85-38.According to the frequency of drought-flood disasters,the period can be identified into three stages.(1) 1208-1240:drought dominated the stage with several flood disasters occasionally;(2) 1240-1320:Long-term drought disasters and the extreme drought events happened frequently;and(3) 1320-1369:The drought disasters were less severe but the flood and drought disasters happened alternately.Besides,the reconstructed aridity-humidity experenced an abrupt change as well as a periodicity in the Mongol-Yuan Period.The turning points were 1230 and 1325.There were 10-year and 23-year quasi-periods in the aridity-humidity changes,which were consistent with solar cycles,indicating that solar activity affected the aridity and humidity conditions of the study region in the Mongol-Yuan Period.The reconstructed results are consistent with two other study results rebuilt by natural evidences,and are also similar to another study result rebuilt by historical documents.