Research On The Multi-scale Characteristics Of Runoff Complexity And It's Driving Mechanism In The Upper Reaches Of The Yellow River

The upper reaches of the Yellow River is the main area to produce stream for whole Yellow River Basin.The dramatic climate change changes and human activities in recent decades have caused the runoff to continue to decline and the hydrology cycle to worse,and brought pressure to sustainable development of water resource system and ecosystem.Most of scholars studied the runoff variation of this area based on single mathematical statistics methods;while there still lack multiple time scale studies to explore the variability characteristics and complexity of the hydrology system,which is difficult to reveal the contribution and drivering mechanism of climate change and human activities in this area under different time scales.This dissertation took the area above Lanzhou station with long-term data series in 12 hydrological stations as objectives investigation and analyzed the characteristics of runoff variation in the region in terms of Mann–Kendall trend analysis and mutation detection method.The flexible sample entropy and its improved multi-scale entropy were further adopted to analyze the runoff complexity characteristics of this area under different time scales.A method to quantify the contribution rate of climate change and human activities to the change of runoff complexity was proposed to reveal the driving mechanism of runoff complexity change at different time and space scales.The results can provide reference for water resources management and water safety countermeasures in the Yellow River Basin under changing environment.Main conclusions were drawn as follows:(1)The spatiotemporal difference of the runoff change trend in the upperreaches of the Yellow River is significant.The annual runoff,flood season runoff,and summer/autumn runoff all show downward trends;while the runoff in the winter and spring of the non-flood season shows increasing trend.Minhe station in the Huangshui basin shows most significant increasing trend.Climate change and human activities decrease the difference of annual runoff after 1990 s in last 20 th Century.(2)The complexity of the runoff in annual,flood-season and non-flood period in the upper reaches of the Yellow River increases overall,only the headwater of the Yellow River and Minhe station show partial decline.The complexity of the low period shows significantly increase in the main-stream controlled by Lanzhou,Xunhua,and Guide stations and the tributary controlled by Xiangtang station,the runoff complexity of the dry season decreases significantly in Minhe and Zheqiao stations,and the changes in other stations are not significant.The runoff complexity of the spring and winter seasons show increases in main stream,and the runoff complexity of the autumn and summer show rises and declines in the tributary respectively.Human activities are the main factors that affect the observed runoff complexity change from the Guide station to Lanzhou station in main stream and the Huangshui in tributary,where observed runoff in annual,flood season and non-flood period,dry season and the non-flood period are changed in stages in these stations.(3)Choose 1986 as the demarcation point to divide base period and variation period,the impact of climate change and human activities on runoff show obviously spatiotemporal differences.Climate change mainly affects flood season,high water period and summer runoff.Human activities mainly affect non-flood season,dry season and autumn and winter runoff.Climate change dominant the mainly status in the headwater river from the Jimai to Tangnaihai station,Datong River,Taohe River and Daxia River Basin during the variation period.Human activities are the main factors affecting the complexity of runoff from the Guide to Lanzhou station in mainstream and Huangshui River Basin in tributary.The main stream area is mainly affected by cascade reservoirs,Huangshui mainly affected by the development of water conservancy projects and changes in the underlying surface.(4)The periods of non-flood season,low season,autumn and winter are the main periods that affecting the complexity of the year,the runoff in annual and these periods are mainly supplemented by base flow,in past decades,this area influence by climate change,undalying surface change and human activities result in the ability to produce flow and water conservation capacity declines,eventually causing a decreasein base flow and driving changes in runoff complexity.