Data processing

Eddy covariance technique is used to calculate flux densities of momentum, sensible heat, water vapor and CO by measuring the mean covariance between vertical velocity and the other quantity in question. The method is based on the Navier-Stokes equations of motion, after application of Reynolds' averaging procedure. The vertical flux density of one quantity with density thus has the form: . The advantage of the method is that the values inferred by the profile method (, and , see section for abbreviations) can be measured directly. Turbulent fluctuations are determined from the differences between the instantaneous and mean quantities.

Eddy covariance provides a more accurate way to determine vertical fluxes compared to the fluxes calculated by means of the semi-empirical similarity theory. It is extremely important to take care of the quality of the measurement and the data processing since errors (most seriously selective systematic errors) can cause serious bias in the integrated, long term net ecosystem exchange (Moncrieff et al., 1996). The main reason for the complicated data correction and quality control described below is to minimize the error term in the measurement.