Analysis on Short-term Variation of Runoff and Sediment Load in the Upper and Middle Reaches of Yellow River

According to the statistic to long-term observation data of the main stream and main branch in the upper and middle reaches of Yellow River,the variation characteristic of runoff and sediment load from 1997 to 2006 was analyzed.The results show that,comparing with the data from 1970 to 1996,the pecipitation in the upper and middle reaches of Yellow River decreases universally,the value is from 3% to 11%,the relation between precipitation and runoff has not obvious change in the upper reaches,the relation between precipitation and runoff has obvious change in the middle reaches,the quantity of runoff and sediment in the main stream and branch decrease,the decrease value becomes big little by little,the value of runoff decreases from 20% in the upstream to 40% in the downstream,the decrease value of sediment is above 27% for sediment transport at every station in the main stream except source area with no obvious change in Yellow River,and the decrease vlaue is from 57% to 87% from Toudaoguai to Tongguan,the number of flood is universally decreasing in the main strean and branch,and the decrease value of flood quantity is from 8% to 46%,and the decrease value of sediment quantity is 57% to 64%,the composition of sediment changes with the quantity of sediment transport decreasing,the mediangrain size of sediment exists three states at every area,such as becoming big,becoming thin,or becoming little.