DROUGHTS, FLOOD DISASTERS, AND CLIMATE CHANGE DURING THE LAST 2000 YEARS IN HEIHE RIVER BASIN

The Heihe River is one of the largest inland rivers in Northwest China. Over the past 100 years there has been a trend of climatic warming and drying since the end of Little Ice Age in Northwest China. Over this period, annual air temperature has risen about 1-1.3Cº, precipitation has been reduced by 50-78mm, and runoff has been reduced by about 14% in the middle sector of Tianshan and eastern sector of Qilianshan. Because of abundant historical data on droughts, floods, and changes in temperature in the past 2000 years in the Heihe River basin, we were able to construct a drought and flood sequence. Statistical and power spectrum analyses were used to determine the periods and phases of droughts and floods over the past 2000 years. Droughts, flood disasters, and precipitation show a good negative correlation over the past 40 years with a correlation coefficient of 0.892. This allows us to build a relation between the drought and flood sequences and percentage anomalies of annual precipitation. From this we use the drought and flood sequence to reconstruct a precipitation sequence for the past 2000 years in Heihe River basin. Integrating information on droughts and floods with historical data on changes in temperature, we conclude that:(1) On a century time scale, drought periods tend to be cold, while wet periods tend to be warm over the past 2000 years; (2) Cycles in frequency of droughts and floods have 240 and 200 periodicities, respectively, but also short periods of 4 and 2-3 years, respectively. On a century scale, climatic changes of droughts and floods fluctuate, with times of droughts longer than those of floods; (3) The reconstructed precipitation sequence over the past 2000 years in Heihe River basin shows that precipitation during cold periods is about 200mm, while precipitation during warm periods is about 240mm; (4) During the last 2000 years we conclude that six cold periods and the six warm periods occurred. Other means can be used to distinguish between warm and cold stages and identify the warm or cold events in each stage, including advances of glaciers and periglacial phenomenon in northwestern China, ice cores, tree_rings, and lake sediments records.