Study On CH₄ And N₂O Emission Characteristics And Influencing Factors Of Reservoirs In Cold Regions

Reservoirs have long been regarded as green and clean energy.Recent studies show that reservoirs are also an important source of global greenhouse gases,and the role of the greenhouse effect of reservoirs in global climate change has become a controversial issue among scientists.The construction and operation of the reservoir changes the distribution of water resources and nutrients,which are converted into carbon dioxide?CO2?,methane?CH4?and nitrous oxide?N2O?by microbial decomposition.Different from the warm areas in the south of China,the reservoir in the north of China has four distinct seasons.The water body of the reservoir appears thermocline in summer,while the layered water body turns over and mixes in spring and autumn,and there is an ice cover period of more than three months in winter.These obvious seasonal characteristics are also important factors affecting the greenhouse gas emissions of the reservoir.Therefore,this study takes Biliuhe reservoir,Dongfeng reservoir and Tanghe reservoir,three reservoirs in the north of China,as the research objects to explore the cumulative effect of greenhouse gases in the frozen period of reservoirs.Secondly,Biliuhe reservoir was taken as the research object to explore the spatial and temporal changes of CH4and N2O in the reservoir and the influence of environmental factors.Finally,the model evaluation method is adopted to evaluate the CH4 and N2O emissions and the potential of temperature increase at the water-gas interface of the reservoir.The research results of this paper are as follows:?1?In the upstream of Biliuhe reservoir and Tanghe reservoir during the winter ice-covered period,there was an obvious cumulative effect of CH4 in the ice-water interface,and the maximum cumulative amount reached 1318.5nmol/L and 1141.5nmol/L,respectively.There was no cumulative effect of N2O during the ice-covered period.There is no snow cover on the ice of Biliuhe reservoir and Dongfeng reservoir,and there is a phenomenon of oversaturation of DO at the interface of ice water,which oxidizes the accumulated CH4 and reduces the concentration.?2?The spatiotemporal changes of CH₄ and N₂O concentrations in Biliuhe reservoir showed that CH₄ concentration was highest in winter,followed by spring and summer,and lowest in autumn.The N₂O concentration level in summer is the highest,followed by that in winter,spring and autumn,and the concentration level in the three periods is the same.In August,the concentration levels of CH₄ and N₂O in the period of different gravity flow both show sudden increase.In the horizontal direction,the concentration of CH4 gradually decreases from the upstream entrance to the dam,while the concentration of N2O is on the contrary.In the vertical direction,the CH4 ice water interface concentration is the highest in winter,and the N2O bottom layer is the highest.There is no significant difference in the vertical direction concentration between spring and autumn.In summer,the CH4 and N2O concentration in the bottom layer of the vertical layer reservoir is the highest,and gradually decreases to the surface layer.?3?According to the analysis of environmental factors,the temperature stratification in Biliuhe reservoir in summer makes the bottom DO very low,which is conducive to the generation of CH4 and N2O.Therefore,the bottom concentration of the reservoir increases in summer,and the reservoir is in the overturning period in winter and autumn,making the vertical difference between CH4 and N2O smaller.The correlation analysis shows that the CH4 and N2O concentration levels in reservoirs are positively correlated with temperature,and negatively correlated with DO,pH and ORP;the CH4 concentration levels are negatively correlated with water depth,and positively correlated with the nh3-n concentration levels.?4?The spatiotemporal variation analysis of the greenhouse gas emission flux of Biliuhe reservoir shows that the CH₄ release flux of the reservoir varies greatly in different seasons.The CH₄ release flux is the largest in winter,followed by summer and spring,and the lowest in autumn.In addition,the reservoir is basically the source of atmospheric CH₄ in winter,spring and summer.In terms of N₂O,the seasonal fluctuation difference is small,in which the release flux in spring and autumn is the largest,followed by winter and the lowest in summer.The infiltration of the heterogravity current in August makes the release flux of CH₄ in the reservoir lower than that at the beginning of summer in June,while the release flux of N₂O shows a sudden increase.The comprehensive greenhouse gas warming potential of biliuhe reservoir is40.69g CO₂/?m²·a?.