Numerical Simulation of Methane Emission from an Artificial Reservoir

In the context of the Paris Agreement, the inventory of greenhouse gases emissions by various sectors of the economy becomes especially important. Artificially flooded areas are well known as sources of CO 2 , CH 4 , and N 2 O for the atmosphere, while the existing inventory methods for such objects do not explicitly take into account many important physical and biogeochemical mechanisms responsible for the formation of these emissions. A new version of the one-dimensional (in vertical) physical and biogeochemical model LAKE (version 2.3) is adapted for water bodies with significant through flow; i.e., it takes into account the sources/sinks of all prognostic variables due to inflows and effluent streams, as well as the average vertical speed and level fluctuations. The model reproduces the processes of vertical transfer of heat and radiation in the water column, the formation of ice and snow in winter, the thermal conductivity and phase transitions in bottom sediments at different depths, generation, diffusion, and the bubble transport of methane from bottom sediments to the surface; the model also calculates a complex of other biogeochemical variables, including dissolved oxygen, carbon dioxide, the content of phyto- and zooplankton, dissolved organic carbon, dead particles, etc. Using the model, we calculated one annual cycle (2016–2017) of the thermodynamic and hydrochemical state of the Mozhaisk artificial reservoir, forced by the data of meteorological measurements. The simulated horizontally averaged vertical distribution of water temperature, dissolved oxygen, and methane content agrees satisfactorily with the observational data for the summer of 2017. At the same time, significant horizontal inhomogeneity is noticeable in the measurement data, especially in the methane concentration. The parameterization of horizontal heterogeneity effects is an important task for the future development of the model. According to the simulation results during the period from August 2016 to August 2017, the total methane flux from the reservoir to the atmosphere was 570 Mg/year, of which 80% is ebullition from the surface of the water body, 15% is the surface diffusion flux, and 5% of methane leaves the reservoir through the dam. Thus, the proposed model can be considered a tool for inventorying the emission of greenhouse gases from artificial water bodies.