Comparation Different Methods To Measure Greenhouse Gas Fluxes Across Water-Air Interfaces Of Reservoir

The organic carbon in the water of reservoir is decomposed by the microorganisms to generate CH₄ and CO₂,which are discharged to the atmosphere via from the water to the air by diffusion and bubbles.More and more studies have shown that reservoirs are already an important source of greenhouse gases in the atmosphere.Correct evaluation of the contribution of reservoirs to major greenhouse gas sources in the atmosphere is of great significance for evaluating global climate change and environmental change.In this paper,the static chamber technique,diffusion-model approach and gradient method are used to measure the CH₄ and CO₂ fluxes from water-air interface of reservoirs,to compare the CH₄ and CO₂ fluxes which was measured by different approach.From January 2017 to June 2017,taking the Zhuyi River of the Three Gorges Reservoir as the research object,this paper conducts in-situ observations of CH₄ and CO₂ fluxes once a month,and simultaneous monitors environmental factors,and take water samples for indoor analysis.From March 9 to 12,2019,,taking the the Meiziya Reservoir of Yichang City as the research object,this paper conducts in-situ observations of CH₄ and CO₂ fluxes once a month,and simultaneous monitors environmental factors,and take water samples for indoor analysis.Analysis of the results of monitoring data,the following conclusions were obtained(1)In this paper,three different sizes of flux chambers are used to monitor the water-air interface flux.The experimental results show that the flux chamber size is irrelevant factors for estimating the CH₄ and CO₂ fluxes at the water-gas interface.The flux chamber method calculates the gas flux by calculating the rate of change of gas concentration in the flux chamber over time.In this paper,three different regression fitting methods are used to regression fit the concentration-time change rate curve.It is found that the exponential regression fitting can truly reflect the flux of greenhouse gas at the water-gas interface.(2)Sensitivity analysis of different gas diffusion rate models shows that the wind speed and surface water dissolved CH4 and CO2 concentrations are sensitive parameters of the model.The LM98 model,W92 model,RC01 model and CC model were used to estimate the flux of the water-gas interface in the study area.The different models were used to determine the CH4 and CO₂ flux results in the following order:CC model,RC01model,W92 model and LM98 model.The estimating CH₄ and CO₂ fluxes from large to small are:CC model,RC01 model,W92 model and LM98 model.(3)The improved gradient method is used to monitor the CH₄ and CO₂ fluxes at the water-gas interface in the Meiziyu Reservoir.The data results were subjected to trend removal,coordinate rotation and quality evaluation.It was found that the time-average trend removal satisfies the Reynolds average rule and meets the actual monitoring needs.Both the sensible heat flux and the friction wind speed obtained by the coordinate rotation are smaller than those obtained before the rotation.The quadratic coordinate rotation result has a good consistency with the cubic coordinate rotation.And the data quality evaluation results are good and meet the actual requirements.(4)By comparing the flux chamber method and the diffusion model method with the environmental factor conditions of the actual monitoring area,it was found that the flux box method has uncertainty on the water-gas interface flux estimation results of the Zhuyi River.And in the Meizi Ya area,the flux chamber method is more applicable.The gradient method overestimates and underestimates the greenhouse gas fluxes of the water air interface emissions due to the uncertainty of the monitoring area.