Fluxes (F) determined with the eddy-covariance method depend on the covariance between vertical wind velocity (w) and the concentration, or mixing ratio, of a gas (s). A sonic anemometer (Campbell Scientific CSAT3) measures the vertical wind velocity 2m above the ground. A variety of sensors can be used to measure the mixing ratio of CO2 and N2O in situ.
A technique called Reynold’s decomposition is used to separate the average and fluctuating parts of a signal. By removing the average of each signal, the moment-to-moment fluctuations in vertical wind (w’) and concentration (s’) can be multiplied together to find the covariance. Both vertical wind and the mixing ratio are measured at a rate of 10 Hz, then the high-frequency covariance of the signals is averaged for every half-hour.