Comparing Chambers for Measuring Greenhouse Gas Emissions from Reservoirs

The action of damming waterways has altered the natural courses of water, resulting in large volumes help up in reservoirs. Previous studies (1, 2) have shown that these reservoirs, along with other inland water sources, are greenhouse gas (GHG) contributors; specifically CO2 and CH4. Precise methods for quantifying reservoir fluxes are not well-documented, though it is observed throughout the literature that different research groups tend to adopt different protocols for measuring gas fluxes.

Through a collaboration held at the Three Gorges Dam in China, researchers from four groups compared measurements of GHG fluxes from a nearby reservoir using varied chamber-based monitoring techniques. Each of the four groups implemented the use of a different chamber design for the flux measurements, as described in a paper by Zhao et al. The results from each chamber design should remain consistent if all designs are built to measure the same parameters. The goal of the project was to determine if flux measurements vary with chamber design or if variance in the data can be attributed to external factors, such as environmental conditions.

The study was conducted at the Three Gorges River in the province of Hubei, China (Figure 1), located six kilometers upstream from the Three Gorges Dam and the Yangtze River. This reservoir attracted researchers as it contains well-mixed waters that ensure a more uniform composition and decrease error attributed to dissolved gases. Additionally, there is a rapid flow rate through the dam and little solar impact due to the high walls of the gorge, which both work to enhance reproducible results and to reduce stratification in the river compared to other reservoirs of similar sizes.

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Figure 1: Three Gorges Dam, Hubei province, China

Each of the four groups used cavity ring-down spectrometers (CRDS) from Picarro (Figure 2), which provide sensitive measurements of the two GHG’s studied, CO2 and CH4, as they absorb predominantly in the near-infrared region. The most common gas flux monitoring techniques are floating chambers (FC) and eddy covariance systems, however, for this study only FCs were used. Three different chamber designs were utilized throughout the experiment, the first being used by two groups, but with different chamber depths, i.e. the component resting below water. A description of each of the three types is as follows:

  1. Rectangular aluminum chambers with polyvinyl chloride (PVC) floats as supports
  2. Rectangular plastic chamber with sloping side walls and inflatable rubber tubing as supports
  3. Cylindrical steel chamber with inflating rubber tubing as supports
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Figure 2: Schematic of the experimental design

 

The fluxes of the two GHG’s were measured over a period of two days, however Zhao et al. recommend that measurements be taken over a longer period of time to correct for environmental interferences such as wind speed, water turbulence, and temperature changes. Based on this study, it can be concluded that measurements of reservoir fluxes can be completed using a floating chamber system of any design, connected to a gas analyzer such as those equipped with CRDS technology. The pairing of high-precision gas sensors containing CRDS technology with an FC system enables the precise and accurate detection of gases that diffuse quickly and are usually retained at low concentrations at the reservoir surface.

It was observed that each of the four chamber types measured the same trends in gas fluxes and that the chamber design did not significantly impact the results; therefore correction factors for different chamber types were not required. The CO2 and CH4 fluxes measured in this study were well-correlated with previous measurements of the same gases also measured in the Three Gorges Reservoir. Continued studies of this type are necessary for robust conclusions to be drawn about reservoir GHG emissions, however this work is a preliminary step in establishing a consistent protocol for the measurement of reservoir gas fluxes.