Distribution Of Dissolved CH₄ And CO₂ And Diffusion Flux At Water-air Interface In Cascade Reservoirs Of Huangbai River Basin

The increase of greenhouse gas emissions leads to global warming and poses a threat to human survival and the sustainable development of social economy.Reservoir is an important source of global greenhouse gases,but the estimation of CO₂ and CH₄ emissions from reservoir is uncertain.Due to the heterogeneity of topography and sediment,the carbon flux of the same reservoir is highly different.Different from plain reservoir,channel reservoir has the dual characteristics of river and reservoir.Generally,it can be divided into three areas from reservoir tail to reservoir head:River area,transition area and Lake area.In this paper,Three River reservoirs(Xuanmiaoguan reservoir,Tianfumiao reservoir and Xibeikou reservoir)in the upper reaches of the East Branch of Huangbai River Basin were taken as an example.In January,March,August and October 2020,a new type of fast water gas balance device independently developed was connected with a portable greenhouse gas analyzer to monitor the dissolved CO₂and CH₄ concentrations in the surface and vertical water of the reservoir,the main work and understanding of this paper are as follows:(1)Spatially,the change trend of the dissolved CH₄ concentration in the surface water of each reservoir is consistent,and the overall performance is that the concentration gradually decreases from the end of the reservoir to the head of the reservoir,and the change along the way conforms to the form of a logarithmic function.The seasonal difference is large,the overall performance is the largest in winter and the smallest in spring.In spring,the concentration range of Xuanmiaoguan Reservoir is 0.023?0.193?mol/L,the concentration range of Tianfumiao Reservoir is 0.055?0.092?mol/L,and the concentration range of Xibeikou Reservoir is0.010?0.116?mol/L.The average surface concentration shows that Tianfumiao Reservoir is best,next is Xuanmiaguan Reservoir,while Xibeikou Reservoir is the worst.In summer,the concentration range of Xuanmiaoguan Reservoir is 0.014?0.263?mol/L,the concentration range of Tianfumiao Reservoir is 0.024?0.096?mol/L,and the concentration range of Xibeikou Reservoir is 0.028?0.131?mol/L.The average concentration shows that Xuanmiaoguan Reservoir is the best,next is Xibeikou Reservoir,while Tianfumiao Reservoir is the worst.In autumn,the concentration range of Tianfumiao Reservoir is 0.067?1.394?mol/L,the concentration range of Xibeikou Reservoir is 0.022?0.913?mol/L,and the average concentration shows that Tianfumiao is more than Xibeikou Reservoir.In winter,the concentration range of Xuanmiaoguan Reservoir is 0.008?0.157?mol/L,the concentration range of Tianfumiao Reservoir is 0.035?0.302?mol/L,the concentration range of Xibeikou Reservoir is 0.027?3.607?mol/L,and the average surface concentration shows that Xibeikou Reservoir is the best,next is Tianfumiao Reservoir,while Xuanmiaoguan Reservoir is the worst.Dividing each reservoir into a transition zone and a lake zone,the difference in the concentration of surface dissolved CH₄ in the transition zone is significantly greater than that in the lake zone.The dissolved concentration of the surface layer has a significant negative correlation with the distance from the end of the backwater and the water depth,and a significant positive correlation with p H,dissolved oxygen,and chlorophyll.(2)Spatially,the concentration of dissolved CO₂ in the surface water of each reservoir shows a gradual increase from the end of the reservoir to the head of the reservoir.The concentration of chlorophyll is used as the independent variable and the concentration of dissolved CO₂ in the surface water is the dependent variable.In spring,the concentration range of Xuanmiaoguan Reservoir is 1.436?3.529?mol/L,the concentration range of Tianfumiao Reservoir is 7.098?11.482?mol/L,and the concentration range of Xibeikou Reservoir is5.913?6.716?mol/L.The average concentration shows that Tianfumiao Reservoir is the best,next is Xibeikou Reservoir,while Xuanmiaoguan Reservoir is the worst.In summer,the concentration range of Xuanmiaoguan Reservoir is 4.943?5.172?mol/L,the concentration range of Tianfumiao Reservoir is 15.600?18.878?mol/L,and the concentration range of Xibeikou Reservoir is 8.995?12.167?mol/L.The average concentration shows that Tianfumiao is more than Xibeikou Reservoir.In autumn,the concentration range of Tianfumiao Reservoir is 22.086?37.603?mol/L,the concentration range of Xibeikou Reservoir is49.152?56.000?mol/L,and the average concentration shows that Xibeikou is more than Tianfumiao Reservoir.In winter,the concentration range of Xuanmiaoguan Reservoir is36.780?54.792?mol/L,the concentration range of Tianfumiao Reservoir is37.376?54.367?mol/L,and the concentration range of Xibeikou Reservoir is49.378?56.000?mol/L.The average concentration shows that Xuanmiaoguan Reservoir is the best,next is Xibeikou Reservoir,while Tianfumiao Reservoir is the worst.The sequence of the overall surface concentration in the whole year from high to low is winter,autumn,summer,spring.Dividing each reservoir into a transition zone and a lake zone,the difference in the concentration of surface dissolved CO₂in the transition zone is significantly greater than that in the lake zone.On the whole,the concentration of dissolved CO₂ has a significant negative correlation with chlorophyll,dissolved oxygen,and p H.(3)In the vertical direction,the dissolved CH₄ concentration appears as subsurface layer>bottom layer>surface layer in spring.The concentration gradually increases from the surface layer to the bottom layer in summer and autumn.In winter,the water body is overturned and mixed,and there is no significant difference in the vertical concentration.Dissolved CO₂concentration has obvious concentration stratification in spring,summer and autumn,and has a significant negative correlation with water temperature,chlorophyll and dissolved oxygen as a whole.No obvious concentration stratification is seen in winter.The range of dissolved methane concentration in vertical water is 0.022?4.621?mol/L,and the range of dissolved carbon dioxide concentration is 0.311?56.833?mol/L.(4)The methane diffusion flux of the reservoir ranges from 0.003 to 0.097 mg·m²·h^-1,which is the"source"of atmospheric methane in the whole year.The variation range of carbon dioxide diffusion flux is-7.182 to 23.616 mg·m²·h^-1,which is the"source"of atmospheric carbon dioxide in winter and the"sink"in other seasons.Spatially,along the direction from the reservoir tail to the reservoir head,the methane diffusion flux decreases and the carbon dioxide flux increases along the way.k_600CH4 and k_600CO2 increased slowly.From Xuanmiaoguan reservoir to Xibeikou reservoir,the TOC of water body decreases step by step,and the cascade reservoirs have obvious carbon sequestration effect.