Rearch On The Effects Of Reservoir Operation On Self-organized Criticality Of River Flows And River Ecosystem

Widespread dam constructions have played an important role in supporting global social and economic development;however,they also have significantly altered natural hydrologic processes and are considered as one of the most significant factors threatening the integrity and diversity of river ecosystem.Recovering the natural flow regime is the basic premise and the most effective way for river ecosystem protection and restoration.Thus,how to describe the basic characteristics of natural flows based on the latest scientific theories,evaluate the impacts of hydrologic modification on aquatic ecosystem and then put forward ecological flow schemes,are significant for river ecosystem protection and reservoir economic operation.In view of this,this study focuses on the effects of reservoir operation on characteristics of river flows and river ecosystem.The influence of reservoir regulation on the self-organized criticality behavior of river flows,the lateral connectivity of river-floodplain systems and the downstream fish habitat suitability are analyzed,respectively.And then,based on the SOC behavior of river flows,an ecological flow evaluation framework is proposed for downstream ecological restoration.The major study contents and conclusions are as follows:(1)The concept of self-organized criticality(SOC)implies the ability of a system to create and maintain its own function and has important implications for the entire ecosystem,thus can be used to assess anthropogenic disturbances.Focusing on how reservoir operations affect the SOC behavior of river flows and taking five large-sized reservoirs in China as examples,the Maximum Likelihood Method,Kolmogorov Smirnov testing and Detrended Fluctuation Analysis(DFA)are used to examine the typical SOC fingerprints for daily river flow series,including their power law frequency-magnitude distributions and long-range correlations.The results show that with increasing reservoir-modified pressure,the frequency-magnitude relationships of natural flows gradually deviate from heavy-tailed power laws and tend to behave as normal or lognormal distributions,characterized by vanishing tails and narrower spans.Furthermore,dam-influenced flows tend to display delayed scaling break times,decreased fluctuations and increased long-range persistence,and their variations demonstrate more anisotropic.The results indicate that a loss of hydrologic diversity is gradually revealed due to reservoir regulation,and further emphasize restoration of the distributional and temporal characteristics of natural flows rather than a focus on simple eco-flow constraints.(2)Based on the two-dimensional hydrodynamic model downstream Niuling Reservoir in Hainan Province(the section from Jiaji to the estuary),the effects of reservoir regulation on the lateral river-floodplain connectivity are studied.The results of magnitude,frequency and fluctuation analysis under different regulation scenarios and different confluence conditions show that the reservoir regulation transforms the natural hydraulic transition zones into the submerged areas or the dry areas,and their peaks and thick tails are weakened,which greatly reduces the lateral hydraulic connection between the floodplain and the main channel.The daily fluctuation significantly decreased and the long-range correlation increased,implying the alternation periods between the submerged areas and the dry areas become longer.The reservoir operation mode and the inter-zone inflows have great influence on the submerged regime.It is important to control the reservoir size or consider moderate flooding discharge to restore the lateral connectivity of the downstream system.(3)A more general fish habitat suitability model is established and coupled with the two-dimensional hydrodynamic model to analyze the effects of hydrologic regulation on fish habitat suitability and spawning events.Two representative freshwater fishes in China are seceleted as the target species,including migratory fish(grass carp,silver carp,bighead carp)and setting fish(cyprinoid).Taking flow velocity,water depth and water temperature as the main environmental factors,the habitat suitability criteria for the target species is established,considering the whole life processes of fish(migration,spawning,juvenile and adult).The habitat suitability index(HSI)and the habitat usable area(UA)are used as the main evaluation indicators,in which HSI is calculated based on Analytic Hierarchy Process(AHP)and UA is carried out according to HSI classification.The analysis results for the section from Jiaji to the estuary downstream of Niululing Reservoir show that the reservoir operation is beneficial to fish migration and adult stage,but it is disadvantageous for spawning and juvenile stage.The present reservoir operation schemes have significantly reduced high flow pulses and flood events in the spawning period,which makes the suitable spawning area decrease notably.Furthermore,the dry flows,which favor the growth of juvenile fish,also decrease significantly,making a wide range of suitable habitats for juveniles disappear.(4)Based on the SOC behavior of natural flows,low flow characteristics and ecological demands in different periods,an ecological flow assessment framework is proposed in this paper and then applied to Niuling Reservoir.This framework is mainly based on the principle of "3+3+3",which considers three flow components(low flows,high flow pulses,floods),three quantitative indicators(magnitude,frequency,timing)and three typical years(wet,normal,dry).It includes four steps: problem diagnosis,high flow pulses and flood events recovery,low flows recovery,implementation,monitoring and feedback,and emphasizes adaptive management of ecological flow schemes.High flow pulses and flood events recovery is on the basis of self-organized criticality of river flows and the recovery schemes are developed according to the natural power law tail template.And low flow recovery takes the annual frequency of low flows as the main optimization index.