Coordinated Regulation Of Water And Sediment In The Key Reservoirs In The Main Stream Of The Yellow River

The Yellow River is the "mother river" of the Chinese nation and is famous for its high sediment content.The large-scale soil and water conservation work on the Loess Plateau has resulted in significant changes in the runoff and sediment of the Yellow River.Aiming at the changes of water and sediment caused by terrace,wich is less researched,this paper quantitatively analyzed the impact of changing slope to terrace measure on runoff through numerical simulation,which can provide a scientific basis for better evaluating the impact of basin governance measures on changes in the relationship between water and sediment in the Yellow River.Based on the current situation of water and sediment changes and the new requirements for the regulation of water and sediment from the ecological protection and high-quality development strategy of the Yellow River Basin,four key reservoirs in the main streams of the Yellow River,which are Longyangxia,Wanjiazhai,Sanmenxia and Xiaolangdi,were taken as the research objects to construct water and sediment operation objective functions that comprehensively consider ecological impact,social demand and economic benefits,and applied them to the researches on optimal operation of reservoirs.The main work and research results are as follows:(1)The rainfall-runoff process of terrace on the Loess Plateau was simulated and analyzed using the physical process-based distributed hydrological model In HM.The study found that terrace has a good ability to reduce runoff(almost all runoff can be intercepted under heavy rainfall condition with a total rainfall of 120 mm,lasted for one hour),and ridges play an important role in this process.Considering the extensive construction of terrace on the Loess Plateau since "returning farmland to forest",it can be inferred that the conditions of water and sediment in the main stream of the Yellow River must have changed significantly;(2)The measured incoming and outgoing sediment data of Longyangxia Reservoir from 2007 to 2014,Wanjiazhai Reservoir from 1998 to 2017,Sanmenxia Reservoir from 2000 to 2017,and Xiaolangdi Reservoir from 2000 to 2017 were analyzed.On the annual and monthly scale,the maximum,average and minimum values of runoff amount,sediment transport amount,sediment content and silt volume in the reservoir area were calculated.The fluctuations of the above indicators were analyzed,the relationship between the inflow and sediment of the four reservoirs was also revealed,as well as the differences in their respective responsibilities for water and sediment control;(3)The framework of the objective function and the basic operation rules of the reservoirs were constructed.The driving-response relationship between the main control factors and the control targets of the single-reservoir water and sediment operation of Longyangxia,Wanjiazhai,Sanmenxia and Xiaolangdi reservoirs and the combined water and sediment operation of Sanmenxia and Xiaolangdi were analyzed through different weight allocation ratios.It was found that the contradictory relationship between the silt reduction benefit and the sediment discharge benefit always exists,but only within a specific weight range,while the synergistic or contradictory relationship between the power generation benefit,flood control benefit and other benefits manifested differently in different reservoirs;(4)Determining the weight value of the objective function based on the objective weight calculated by the driving-response relationship and the subjective weight got from the reservoir scheduling task,and then using the genetic algorithm for optimization to obtain the operation result with the least impact on the ecology under the unit economics.Compared with the actual operation results,the optimization results of Wanjiazhai and Sanmenxia reservoirs significantly improved in economic and ecological benefits,the optimization result of Longyangxia Reservoir slightly decreased economic benefits but weakened the ecological impact,the optimization result of Xiaolangdi Reservoir slightly decreased the ecological benefit but improved the economic benefit.Compared with the single-reservoir optimization,the joint operation slightly lower economic benefit but lower ecological impact at the same time,that is,the ecological benefit is higher;(5)Through the analysis of the changes of the optimal operation parameters of the reservoirs under the representative annual runoff and sediment scenarios,it is found that the optimal operation parameters of the reservoirs fluctuated greatly with the changes of water and sediment,but do not show a simple increase or decrease trend with the increase or decrease of the amount of incoming runoff or sediment.The same parameters also have different changes in different reservoirs.It is pointed out that the changes of reservoir optimization operation parameters are not only determined by the situation of incoming water and sediment.It is necessary to update the operation threshold value for each reservoir under the changing situation such as the water and sediment conditions,the sediment storage capacity of each reservoir and the downstream river conditions.The knowledge of water and sediment change is a necessary precondition to determine the threshold;The research on terrace in this paper not only confirms the importance of terrace for soil and water conservation work,but also provides a methodological basis for the follow-up study to further explore the changes of water and sediment in the Yellow River Basin under large-scale terracing,and data support for the research on changes in reservoir operation patterns.The objective functions proposed for the regulation of water and sediment of reservoirs comprehensively consider both the theoretical priorities and the actual demands of the reservoirs,and have certain feasibility and effectiveness.The obtained optimal operation results can provide a certain theoretical reference for the actual reservoir operation.