Eddy Covariance | Lowland Peatlands

'Flux Towers' & Eddy Covariance

Eddy Covariance (EC) is a crucial method for measuring the exchange (the flux) of energy, water vapour and other gases like carbon dioxide & methane, between the Earth's surface and the atmosphere.

The system detects turbulent air flows ['eddys'] using sonic anemometers, which calculate vertical wind velocity, and gas analysers that measure scalar entities like CO₂. With clever science and lots of maths we can use these measurements to work out how much Greenhouse gases (GHG's) is being emitted from the immediate local area of the instrument.

The instruments need to be the correct height above the ground surface or the upper surface of the vegetation so they're often mounted on masts or towers. We tend to use the term flux tower no matter what the actual structure is. The one pictured is a few metres high as the vegetation is expected to grow to about two metres. But in a woodland the instruments might be mounted on a very tall scaffolding structure in order to be above the tops of the trees.

The particular flux tower pictured here is powered by a solar panel bank (in the background on the left). It has storage batteries for operation at night and the setup includes a rain gauge, a water-table sensor under the ground and also soil moisture and temperature monitors.

Straight into the terminology: In the context of Eddy Covariance "scalar entities" refer to quantities like heat, water vapour, and gases (e.g., carbon dioxide or methane) that are transported by turbulent airflows but don't have a directional component. Unlike wind velocity, which has direction, scalar entities are properties measured at a specific point without directionality, representing the concentration or intensity of the substance being studied. EC systems track how these entities move between the Earth's surface and the atmosphere to understand fluxes like evapotranspiration and carbon exchange.

The micrometeorological measurements made by the EC method allow researchers to quantify and understand the spatial and temporal variations in CO₂ storage in plants, soils and the atmosphere and to monitor exchanges between them. This is vital for understanding how peatlands and other ecosystems, capture, store and release greenhouse gases under changing conditions such as climatic variations or differing management regimes.

In contrast to local sensors, EC captures data from a dynamic "footprint," which varies based on wind, vegetation, and terrain. Careful site selection and sensor alignment are therefore essential to minimise errors caused by surface irregularities or nearby objects.

Key Challenges:

Click each numbered section to expand

1. Site Selection:

The ideal location is a flat, homogeneous surface. Slopes, hills, or dense vegetation can affect the wind vector, leading to skewed data. Positioning the sensors at the "blending height" helps to minimise terrain-induced turbulence. The blending height in eddy covariance refers to the altitude above the surface at which the turbulent airflow becomes uniform, effectively smoothing out variations caused by terrain features or surface irregularities. Below this height, the turbulence is more influenced by local features like vegetation or hills, which can distort measurements. Above the blending height, the turbulence represents a larger, more homogeneous area, making it ideal for accurate flux measurements of gases, heat, and other scalar entities in EC systems.

2. Fetch and Footprint:

Eddy Footprint 'Fetch' visualisation — *Visualisation of an eddy footprint 'fetch'*

The footprint of the area sampled by EC is sensitive to the sensor height and atmospheric stability. Rough terrains reduce fetch length, while smoother surfaces increase the distance contributing to the measurement. Researchers often use a 100:1 rule-of-thumb (1 m height = 100 m fetch).

3. Sensor Configuration:

The placement of sensors, particularly the sonic anemometer, is critical. Misaligned setups can distort airflow, compromising data integrity. Co-locating concentration sensors near the sonic anemometer helps, but distance must be managed to prevent turbulence from affecting measurements.

4. Power and Structure Interference:

EC systems are often deployed in remote areas, powered by solar panels or wind turbines. These components must be placed away from the sensor to avoid interference with air flow.

A more detailed explanation and discussion of these challenges is well explained in this paper: Colin R. Lloyd. A Path to successful eddy covariance measurements: a field deployment handbook.

Flux towers and eddy covariance systems in general are indispensable for peatland ecosystem and climate research. However proper deployment is critical to obtaining accurate flux measurements. Researchers must account for terrain, sensor configuration and environmental conditions to ensure robust data collection.