| Dam construction and reservoir creation will result in changing the relationship between source and sink of greenhouse gases in the basin or region,which has been broad consensus on additional greenhouse gas emissions caused by damming.However,the change of greenhouse gas source and sink in the reservoir is a complicated process,for multi-dimensional and multivariate causing a large uncertainty to the release of greenhouse gases.Based on the statistical analysis of the bibliometrics,the present situation of greenhouse gas research in reservoirs at home and abroad is systematically analyzed in this study.The analysis result showed that it has become an important means to study the greenhouse gases in reservoirs by using the model to evaluate the flux of greenhouse gases in reservoirs.However,the insufficient in-situ monitoring data cannot cover the changing situation of the reservoir greenhouse gas flux in all cases,which leads to uncertainty in the results of model evaluation.Therefore,in-situ detection of greenhouse gases in typical reservoirs of different regions is still an important basic work in reservoir greenhouse gas research.In this study,Xiluodu Dam and Xiangjiaba Dam,the cascade reservoirs in the lower reaches of Jinsha River,were taken as research objects.On the basis of a clear understanding of the influence of river cascade development on physical and chemical indexes of water,the study explored the spatial and temporal distribution of CH4 and CO2,analyzed the composition of organic carbon sources.and explored the applicability of the G-res Tool(GHG Reservoir Tool)on the assessment of greenhouse gas flux in cascade reservoirs in the lower reaches of Jinsha River.This paper has the following main conclusions:(1)The annual CH4 concentration of water body was between 0.012～0.137μmol·L-1 in the lower reaches of Jinsha River.Specifically,it reaches the lowest in January 2018 with an average value of 0.029 μmol·-L-1 and the highest in July with an average value of 0.057 μmol·L-1.Along the river,the CH4 concentrations in the Xiluodu and Xiangjiaba reservoirs had a similar variation rule throughout the year,but it’s higher in Xiangjiaba reservoir than in Xiluodu reservoir due to the impact of damming and human activities.There was no significant difference of CH4 concentration of the surface and bottom water.The annual CO2 concentration of water body was around 0.014～0.084 mmol·L-1.It reaches the lowest in March with an average value of 0.033 mmol·L-1 and reaches the highest in July with an average value of 0.054 mmol·L-1.There was no obvious change rule along the river.On the vertical section of water body,the CO2 concentration of the surface water of the Xiluodu and Xiangjiaba dams was significantly lower than that of the bottom water in March.In other months,the difference of surface and bottom CO2 concentration was not obvious.(2)The annual CH4 diffusion flux at water-air interfaces was 0.009～0.299 mmol·m-2·d-1 in the lower reaches of Jinsha River,which showed carbon source characteristic.Compared with typical global reservoirs,the CH4 diffusion flux was at a very low level.The annual CO2 diffusion flux at water-air interfaces was between-3.415》161.820 mmol·m-2·d-1,which showed carbon source characteristic.Compared to subtropical reservoirs,the CO2 diffusion flux was at a high level.(3)Our research found that the factors affecting CH4 and CO2 concentrations in different regions are different.The concentration of CH4 in water was significantly affected by temperature and DO.It had a highly significant positive correlation with temperature and a highly significant negative correlation with DO.Throughout the year,the C02 concentration of the surface water body significantly reduced due to the exuberant growth of algae in March,and the CO2 concentration of the bottom water body was more significantly affected by respiration of microorganisms.In addition,the biological endogenous respiration was the main factor to control the CO2 concentration of the water during high water stage in winter,while both exogenous carbon and biological endogenous respiration formed the main factors to control the CO2 concentration of the water in the flood season of summer.(4)The cascade reservoirs in the lower reaches of the Jinsha River had certain interception of particulate organic carbon and nitrogen.In addition,the analysis of particulate organic carbon sources showed that plankton and C3 soil were the main sources of POC in the cascade reservoirs in January 2018.However,only plankton was the main source of POC in other months of the year.In March,the concentrations of particulate organic carbon and nitrogen were the highest and the δ13CPOC was the heaviest,which were mainly affected by algal vigorous growth.Carbon isotope fractionation factor during phytoplankton DIC assimilation was estimated to be 18.198‰in March based on the linear relation between δ13CPOC and 1/[CO2].(5)G-res Tool is a model to estimate the net GHG flux change in a freshwater reservoir developed by the International Hydropower Association.In this study,the model was used to estimate the net GHG flux before and after impounding in the Xiluodu and Xiangjiaba reservoirs.The evaluation results of the post-impoundment showed that the CH4 flux was 44 gCO2eq·m-2·yr-1 and the CO2 flux was 60 gCO2eq·m-2·yr-1 in Xiluodu reservoir,while the CH4 flux was 66 gCO2eq·m-2·yr-1 and CO2 flux was 63 gCO2eq·m-2·yr-1 in Xiangjiaba reservoir.Comparing with the on-site monitoring results,the model underestimated the high CO2 emissions of the Xiluodu and Xiangjiaba reservoirs,but overestimated the extremely low emissions of CH4.As an empirical model,G-res Tool has certain limitations in model principle and module design,so it can’t be fully applicable to the Xiluodu and Xiangjiaba cascade reservoirs.According to the high CO2 diffusion flux and the extremely low CH4 diffusion flux of the cascade reservoirs in the lower reaches of the Jinsha River,it is still the key to carry on the long-term follow-up observation and mechanism study for researches of typical reservoir greenhouse gases in the river cascade development process in future. |
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