Theory

The eddy covariance method relies on the combined high-speed measurements of gas concentrations, temperature, and wind speed, followed by data analysis. To date, there is no single uniform step-by-step execution protocol for eddy covariance flux calculations, but much effort is being made by flux measurement networks (e.g., FluxNet, AmeriFlux, ICOS, CarboEurope, Canadian Carbon Program, and iLEAPS) to unify the procedures of flux calculations via the eddy covariance method.

In physical terms, "eddy flux" is computed by measuring how many molecules, moles, or milligrams of gas went up with upward wind movement at one moment and how many went down with downward wind movement in the next moment. In mathematical terms, "eddy flux" is computed as a covariance between the instantaneous deviation in vertical wind speed (w' ) from the mean value (w) and the instantaneous deviation in gas mixing ratio (s' ), from its mean value (s), multiplied by mean air density (ρ_a ). Several mathematical operations and assumptions, including Reynolds decomposition, are involved in getting from physically complete equations of the turbulent flow to practical equations for computing "eddy flux".

For more information on eddy covariance theory view the following poster:

Brief Practical Guide to Eddy Covariance Flux Measurements

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