MAIN FEATURES OF LONG-TERM VARIABILITY OF STREAMFLOW IN THE CHANGJIANG AND HUANGHE RIVERS IN SUMMER-AUTUMN SEASONS

The main features of long-term variability for the two largest rivers of Changjiang and Huanghe in summerautumn seasons were studied based on observations of stream flow for the last 50 or more years.It is shown that the steamflow in two rivers in summer-autumn seasons occupies more than a half of the annual streamflow; the monthly mean stream flow and standard deviation in the summer-autumn seasons are 2.6 times of or more than that in other months; the average steamflow in Changjiang River is 15.7 times,and the standard deviation is 5.9 times of that in Huanghe River for the summer-autumn seasons.The standard deviation of streamflow in Huanghe River is about 58.8% of the long-term average stream flow,but only about 22.1% in Changjiang.In comparison,the steamflow of Changjiang River is larger and more stable than Huanghe River.The Huanghe steamflow is one order less than,and unstable two times than that in Changjiang River.The streamflow anomalies in Huanghe River were mainly positive during the period from middle of 1930's to 1968;but turned to mostly negative since 1985.The lower reaches of the Huanghe River dried up with zerosteamflow frequently.The marked change points are found based on the calculation of change point proposed by Petitt.A harmonic analysis and a prediction of the streamflow for Huanghe River was carried out based on the dominant Fourier waves.The predictions suggest that the Huanghe steamflow will trend toward increase before 2005.