CLIMATIC BACKGROUND ANALYSIS OF FLOW-BREAK EPISODE OF LOWER REACHES OF YELLOW RIVER IN RECENT YEARS

The successive dry episodes in the lower reaches of Yellow River in recent yearsari causing great concern of our society.On the basis of monthly precipitation data of nearly 47 years gained from 160 basic observatories of China and by means of rotating principal component annual variations of average precipitation along Yellow River are studied,and the anomalies of rainfall and their accumulation extent are compared with other regions of the country,and contrasting analysis is conducted with historic heavy droughts and dry episodes in the lower reaches.The results show that the past 20y earsis a severe successive drought period encount eredper hundredy ears,and the total amount of decreased rainfal in recent 10 years is equal to the runoff of 4 years of the river,15 years out of the recent 20 years are dry,in which 5 years are severe and 10 years are moderate.The accum ulating effect of these droughts is not only the heaviest in the coutry inrecent one hundredy ears but also heavier than the hist oric "Chongzhen Catatrophe(1640)" and the drought and flow-break episode in 1372.