EM-1 PROJECT -- Measuring CO2 using Eddy Covariance Technique

Background

The scientific community represented by the Intergovernmental Panel on Climate Change is in agreement that the Earth’s climate is warming. It is understood that the rise in the global average temperature is largely due to the increasing emissions of greenhouse gases (GHG), especially through the burning of fossil fuel, but also because of land conversion.

GHG have the ability to absorb infrared radiation, thus warming the Earth’s surface, and contributing to the greenhouse effect. The four major GHG are carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O), and water vapour. One way of reducing GHG concentrations in the atmosphere is through sequestration of carbon in soils and vegetation. Usually, vegetated ecosystems, such as forests, are sinks for atmospheric carbon dioxide (CO₂) because the vegetation absorbs CO₂ from the atmosphere through the process of photosynthesis. When a forest is cut or burned, most of the CO₂ returns to the atmosphere. However, what happens to the carbon when the existing forest gets inundated is still largely unknown. Identifying the impacts of the creation of hydroelectric reservoirs on the GHG emissions is therefore an issue that must be addressed, especially in northern Quebec, where there is a huge potential for the production of hydroelectricity. The Eastmain-1 reservoir in the James Bay region provides a unique opportunity to perform a comprehensive assessment of such impacts.
Many techniques are available for measuring the quantity of greenhouse gases that are exchanged between the atmosphere and a given surface (i.e. how much of the GHG is released to the atmosphere or absorbed by the system). In addition to the static chambers used to quantify GHG emissions in different locations of the EM-1 reservoir surroundings (see terrestrial aspects of the project), the research team is utilizing the eddy covariance (EC) technique. The EC technique is the most direct method used by environmental scientists to determine if trace gases (such as CO₂) are emitted or absorbed by terrestrial and aquatic surfaces.

Figure 1. Example of an eddy covariance system with
the sonic anemometer and infrared gas analyzer.

What is eddy covariance?

The general principle behind the EC technique for measuring GHG fluxes is that as the air flows over a surface, turbulent eddies are created. These eddies transport energy, water vapour and trace gases away from and towards the surface. The instruments used in an EC system are mounted on top of towers and allow the measurement of the concentration of the trace gas, as well as the vertical wind speed continuously over a large area. Knowing how many molecules of a gas are moved away from or towards the surface by the eddies at a given time allows scientists to calculate the upward or downward flux (a flux simply refers to a quantity that is moved through a unit area per unit time). With EC, how much CO₂ is emitted from, or absorbed by, the surface under study is calculated as the covariance between the vertical wind speed and the concentration of the gas.

Figure 2. The EC flux tower located in a black spruce forest near the EM-1 reservoir.

However, because turbulent fluctuations occur at very high frequency, we need fast response instruments. The two main instruments used in the EC systems at EM-1 are the three-dimensional sonic anemometer, which measures wind speed in three dimensions, and an open-path infrared gas analyzer (IRGA), which measures the concentration of CO₂ and water vapour (Figure 1). These concentrations and wind speeds are recorded using fast-response data loggers at 10 Hz (i.e. ten measurements per second).

Why use eddy covariance?

The creation of the EM-1 hydroelectric reservoir required the flooding of over 600 km2 of the boreal forest and peatlands along the Eastmain River. Such a disturbance obviously modifies many ecological, biological and physiological processes, which in turn affect the way GHG are produced and consumed in the flooded area and its surroundings. While CO₂ is absorbed by trees and released through respiration and decomposition in forests, it is mainly being emitted in aquatic systems. Quantifying the impacts of this landscape modification on the GHG emission patterns is an enormous task, but EC is one of the most promising measurement techniques available to achieve this goal.
The EC system offers many advantages compared to other commonly used methods for measuring GHG fluxes. The main benefit of this method is the fact that it allows for the continuous and spatially-averaged measurement of the vertical exchange of carbon, energy and water between the atmosphere and a surface, with minimal disturbance to the environment. The EC technique provides the net balance between uptake and release of CO₂, which is called the net ecosystem exchange (NEE) of CO₂. As opposed to chambers that are useful in identifying discrete and small-scale variations in GHG emission patterns, GHG fluxes measured with the EC technique provide precise measurements over a relatively large area, or footprint (i.e. the area “seen” by the instruments or the area contributing to the measured fluxes) upwind from the tower, which can be 100’s of square metres in area. The exact size of the footprint depends on the measurement height, the roughness of the surface and atmospheric thermal stability. Although the EC technique has some limitations, such as its inability to measure during non-turbulent periods (e.g. during nighttime, under stable atmospheric conditions and low turbulence), the scientific community has developed many ways to deal with periods when data are not reliable.
Furthermore, with the continuous and long-term functioning of the EC systems, it is possible to interpret and compare GHG emission patterns on multiple time scales (hourly, seasonally, annually). Combined with auxiliary measurements of meteorological and ecosystem (soil, plants) variables, the EC technique helps scientists to understand the gas exchange response to changes in environmental conditions. Additional variables measured at the EM-1 study sites include soil and air temperature, relative humidity, solar radiation, photosynthetically active radiation (PAR), wind speed and direction, precipitation, and snow depth; all of which are being recorded continuously. It is therefore made easier to examine the dominant controlling factors on the CO₂ fluxes at the different study sites, and thus to get a better understanding of the present and future contribution of hydroelectric reservoirs to the greenhouse effect. The EC technique is a useful complement to modeling studies, where the continuous flux measurements provide a good means of developing better algorithms for scaling up from ecosystem to regional estimates of GHG fluxes. Therefore, the results obtained in this part of the project will be very useful for predicting future impacts of inundation on carbon budgets within a changing climate, and following the creation of other hydroelectric reservoirs.

Site Location and Tower Setup

In order to assess the impacts of the creation of the reservoir on the GHG emissions, the first challenge the scientific team had to face was to find an appropriate site location that was representative of the pre-flooded (i.e. forest) and post-flooded (i.e. reservoir) environments. Several considerations are required when using the EC technique, primarily, the tower must be installed in a flat and uniform terrain which is large enough to get a reasonable flux footprint and which represents the ecosystem of interest for most (or prevailing) wind directions.
As such, a mature black spruce-dominated forest having similar characteristics to the pre-flooded surface was chosen and became one of the two main study sites in which a flux tower was set up (Figure 2). The second main study site is located on the edge of an island on the reservoir, allowing the scientific team to acquire trace gas measurements from the reservoir itself, thus representing the post-flooding conditions (Figure 3). The results from these two towers will be used to evaluate the pre-flooding vs. post-flooding CO₂ fluxes.
The two main EC systems were installed and were operational by the end of summer 2006. The towers are 21.3 m tall in the forest, and 9.14 m tall on the island with the instruments mounted at the top of each tower. One of the project goals is to compare the various techniques available for trace gas measurement from reservoirs. It will be very interesting to compare EC results to the gas flux measurements obtained from the aquatic teams who look at emission patterns at smaller spatial and temporal scales. The next step will be to integrate the results into a multidisciplinary modeling approach.

Figure 3. The EC flux tower located on an island on the EM-1 reservoir.

Measuring CO2 Emissions at the EM-1 Reservoir Using the Eddy Covariance Technique

Background

What is eddy covariance?

Why use eddy covariance?

Site Location and Tower Setup