A comparison of methods for the measurement of CO₂ and CH₄ emissions from surface water reservoirs: Results from an international workshop held at Three Gorges Dam, June 2012

Abstract

Fluxes of carbon dioxide (CO2) and methane (CH4) from hydroelectric and water supply reservoirs are receiving increasing attention around the world with a number of research groups having undertaken measurements of these emissions across a range of lakes and reservoirs located in different climates and landscapes. The use of floating chambers (aka flux chambers) is the most common technique for direct measurement of these fluxes. However, the relative performance of different measurement systems, especially different chamber designs, is not well documented. We report the results of an international workshop held in June 2012 at Three Gorges Dam, China, to compare measurements performed by four groups with extensive chamber monitoring experience: the Chinese Academy of Science (China), CSIRO (Australia), SINTEF (Norway), Hydro‐Québec/Environnement Illimité (Canada). A fifth group, Eawag (Switzerland), performed hydroacoustic surveys to detect ebullition in the water column. We recommend CH4 as a more suitable trace gas for comparing methodologies due to its relative stability in the surface layer of the water column, for example, it is not subject to significant diurnal changes due to photosynthesis and respiration. Measured fluxes agreed to within 20% between the four teams suggesting that the shape and dimensions of the floating chambers and the chamber gas flow rates (i.e., chamber residence time) did not have an appreciable systematic effect on the measured fluxes for the relatively low wind speeds prevalent at the reservoir. The CO2 and CH4 fluxes measured during the workshop agree well with previous measurements in Three Gorges Reservoir.

Greenhouse gas fluxes from reservoirs determined by watershed lithology, morphometry, and anthropogenic pressure

“…We calculated the daily (from 10 am to 4 pm) average and the standard error from these measurements. We obtained the flux calculation using the equation (1) (Zhao et al 2015):…”

Assessment of methane and carbon dioxide emissions in two sub‐basins of a small acidic bog lake artificially divided 30 years ago

“…Zhao et al. () suggested such a method as a tool for assessing diffusion and ebullition, while Grasset et al. () was one of the first studies to include ebullition data from these newer gas analysers.…”

“…Therefore, when no linear regression trend was observed to determine the flux, we used the initial and end points to determine the sum of diffusive and ebullitive CH 4 fluxes, as discussed by Goodrich, Varner, Frolking, Duncan, and Crill (2011), Zhao et al (2015) and Grasset, Abril, Guillard, Delolme, and Bornette (2016). We acknowledge that our sampling procedure may not accurately measure diel gas emissions, as previous studies have found large variations in diel CH 4 and CO 2 fluxes (Liu et al, 2016;Natchimuthu, Selvam, & Bastviken, 2014;Natchimuthu et al, 2016).…”

“…Newer gas analysers and the combination with established techniques may help to determine CH 4 diffusion and ebullition in freshwater ecosystems. Zhao et al (2015) suggested such a method as a tool for assessing diffusion and ebullition, while Grasset et al (2016) was one of the first studies to include ebullition data from these newer gas analysers.…”

Mesocosms with wavemakers: A new device to study the effects of water mixing on lake ecology

“…Fifth, studies on buoyant and sinking phytoplankton could continue with a focus on the intensity of water mixing in interaction with its timing and duration (Huisman et al 2002;Huisman et al 2004;Blottière et al 2014;Zhou et al 2015). Sixth, gas dynamics in the water column and gas exchanges at the air-water interface are of major importance to understand freshwater community dynamics (see for instance Verspagen et al 2014;Hanson et al 2015;Zhao et al 2015). Gas exchanges between the atmosphere and water depend on wind velocity and wave height among other parameters (Vachon and Prairie 2013).…”