PhD Thesis, 2007 - University College Cork

Chapter 3
Materials and Methods
3.4 The Eddy Covariance methodology
3.4.1 Eddy Covariance technique
A flux is the transfer of a quantity per unit area per unit time. An eddy flux is a
transfer of a quantity by the turbulent motion of air. Eddy covariance is a
micrometeorological technique that measures the turbulent flux across the
vegetation canopy-atmosphere layer to determine the net difference of material
moving across this interface (Lenschow, 1995; Baldocchi, 2003). Two fast
response instruments are necessary for EC measurements: a 3-D sonic anemometer
and an infrared gas analyser, both operating at high frequency (generally at 10 Hz),
to cover the full range of the turbulent motion.
In the Glencar Atlantic blanket bog, we used an open-path infrared gas analyser
(Chapter 5 and 6, Appendix 1 and 2). Compared to close-path sensors, the openpath
gas analyser has easier computational requirements and causes less
aerodynamic disturbance, due to its open structure (Baldocchi, 2003). The main
disadvantage of an open-path sensor is its poor performance during rainy
conditions. The flux data were logged on a CR23X data logger (Campbell
Scientific, UK). Since this data logger does not have a very powerful memory, the
10 Hz data were averaged online over a 30-minute period based on a running-mean
approach (Aubinet et al., 2000) and 10 Hz data were then discarded, reducing the
possibility for further analysis of 10 Hz measured data.
The EC approach is based on the existence of the wind spectral gap; this allows the
statistical separation of the mean term of the wind velocity or trace gas
concentration (varying over several-hour periods) from their turbulent parts
(varying over periods of tens of seconds), using the Reynold’s rule of averaging
(Stull, 1988):
ξ +
'
= ξ ξ
(3.1)
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