Influences of Longyangxia and Liujiaxia Reservoirs Joint Operation on Runoff of Upper Reaches of Yellow River

Based on the runoff data from 1950 to 2009of eight stations in the upper reaches of the Yellow River,the impacts of Longyangxia and Liujiaxia reservoirs joint operation on runoff were analyzed.The runoff series after reservoir operation were restored by water balance.The annual runoff characteristics of observed and restored series in both Lanzhou and Xiaheyan stations were analyzed by Mann-Kendall trend analysis,sudden change test and wavelet analysis method.The results show that significant downward trend exists in observed and restored runoff series;the abrupt change in observed and restored runoff series occurred in 1987 and 1990,respectively;the periodic oscillations in observed and restored runoff series had occurred at the temporal scales of 3~5,6~9,14~17and 23~27years;observed runoff series of Lanzhou and Xiaheyan stations had main periods of 25,16,8and 4years;restored runoff series of Lanzhou station had main periods of 23,8,and 4years;restored runoff series of Xiaheyan station had main periods of 24,4and 8years.It demonstrates that Longyangxia and Liujiaxia reservoirs joint operation will aggravate the downward trend,move up the abrupt change year and affect the periodic variation of runoff series in some extent.