Effect Of Damming On CO₂/CH₄ Production And Discharge In Tail Tributary Of The Three Gorges Reservoir Area

In order to meet the development needs of the economic and social development of mankind,the phenomenon of damming on rivers is very common.Although dam building improve the efficiency of water use,it blocks the natural continuity of river water,resulting in significant changes in the physical,chemical and ecological properties of the river.And then the process of carbon emission in river ecosystem is affected so that it has become the focus of research on CO₂ and CH₄ emission of global greenhouse gas in recent years.However,the carbon emissions from river ecosystems are the result of the combined effect of carbon on the biogeochemical processes of land,river sediments,water bodies and atmospheric environment composite systems.At present,the internal influence mechanism of damming on the multi-interface production and discharge process of river carbon in the above system has not been clear yet.Taking the Yulin river,the dam-building tail tributary in the Three Gorges Reservoir as subject,during stable storage period(January 2019,hereinafter referred to as the storage period)and stable drainage period(August 2019,hereinafter referred to as the drainage period)of reservoir in the Three Gorges Reservoir,this paper samples and determines sediment,vertical stratified water body and water-gas release at the upper and lower reaches of the ecological regulation dam of the Yulin River.In combination with the change of water environment factors,this paper explores the influence mechanism of damming on the whole process of CO₂,CH₄ generation,migration,transformation and discharge in the upstream and downstream reaches of the Yulin River under the condition of periodic change of water level.Through monitoring the water environment factors in the upper and lower reaches of the Yulin River ecological regulation dam,it shows that the regulating dam changes the spatial and temporal pattern of environmental factors and the nutritional status of the Yulin river body.Since the Yulin river is affected by the backwater of the main stream of the Yangtze River during the storage period,the water level of upstream and downstream of ecological regulating dam is consistent(174.5m).There is small difference between hydrodynamic condition and space of physical and chemical characteristics but the blocking effect of regulating dam on nitrogen and phosphorus nutrient is obvious.Average retention rates of TN?NO₃^--N?NH₃-N?TP concentrations in upstream water are 22.6%,55.0%,7.8%,and 17.2%,respectively.And the upstream and downstream water bodies of the regulating dam are all in medium-nutritional state.During the drainage period,the water level of upstream is high(172.20 m)with weak hydrodynamic conditions,and water body thermal stratification is obvious and Chl-a,DO,p H and other physical and chemical indexes of water bodies also show obvious vertical stratification phenomenon,and the degree of eutrophication of water body is heavy.The water level downstream of the dam is consistent with the Yangtze River main stream(159.0m),and the hydrodynamic conditions are slightly stronger.There is no obvious vertical stratification of chl-a,DO,p H and other physical and chemical indicators of the water body,and the eutrophication degree of the water body is relatively light.During this period,the regulation dam significantly changed the spatial distribution pattern of water nutrients.Through monitoring the carbon concentration of each form of water in the upper and lower reaches of the ecological regulation dam,it shows that the influence mechanism of regulating dam on the spatial and temporal distribution pattern of water carbon is different under different dispatching stages in reservoir area.During the storage period,the ecological regulation dam of the Yulin River mainly influences the natural transport process of river DOC and POC through physical interception,with average rejection rates of 53.9%and 27.9%for DOC and POC concentrations in upstream water,respectively,but it has less impact on DIC and PIC.In terms of the influence mechanism of the ecological regulation dam on the various forms of carbon during the drainage period,except for the direct physical interception,the development of eutrophication in the upper reaches caused by its blocking and impounding effect indirectly changes the spatial distribution pattern of water carbon through chemical and biological processes.Through monitoring the distribution of CO₂/CH₄ in sediment and water and release of water-gas interface,the influence mechanism of damming on the production and discharge process of CO₂ and CH₄ in the Yulin River is obviously different in different dispatching periods in the Three Gorges Reservoir area.During the storage period,the ecological regulating dam mainly affects the"endogenous"generation of sediment CO₂ and CH₄ by blocking the downward transport of nutrients from upstream sediments and water bodies,and blocks the CH₄ that is dissolved in water from transporting to downstream(CH₄ retention rate of water bodies reaches 76%).And then it affects the atmospheric emissions of upstream and downstream CO₂ and CH₄,without obvious effect on the vertical transport and consumption process of the CO₂ and CH₄ in the water column.The main influencing factors of water CO₂ partial pressure in this period are sediment CO₂ concentration level and microbial metabolic intensity in water.The main influencing factors of water c CH₄are N and P nutrients and organic matter concentration.CO₂ and CH₄ are emission to the atmosphere in the form of molecular diffusion from the water-air interface,and major influencing factors of emissions are water temperature and surface water CO₂and CH₄ concentration.During the drainage period,the interception of water body and source material by the ecological regulating dam makes the hydrological conditions and habitat characteristics of its upstream and downstream very different,which significantly affect the whole process of CO₂ endogenous production,water column and river transport,photosynthesis consumption and diffusion release.It significantly affect the whole process of CH₄ generation,vertical water column and river transport,oxidation consumption,bubbling release and diffusion release.CO₂partial pressure of water body during this period is mainly affected by Chl-a and SRP concentration while the water CH₄ concentration is affected by many factors,including river flow,water temperature,CH₄ environmental release mode(ebullition and diffusion)and exogenous CH₄ input.During this period,CO₂ is release to the atmosphere mainly in the form of diffusion,which is mainly affected by sediment CO₂ concentration and microbial metabolic intensity in water.CH₄ is release to the atmosphere mainly in the form of ebullition,which is affected by the river depth,velocity and sediment CH₄ concentration.