The impact of sampling rate on eddy-covariance flux estimates

It is shown that sampling rate has no influence on the expected value of an eddy-covariance flux estimate. The uncertainty in the flux estimate, however, increases with decreasing sampling rate. This sampling induced uncertainty is compared with the natural uncertainty in the flux estimate due to the stochastic nature of turbulence. It is shown that the sampling induced uncertainty becomes important when the sampling interval is larger than the integral time scale of the flux time series. Theoretical expressions are evaluated by using 9 days of turbulence data taken over a coniferous forest. For sampling intervals smaller than the integral time scale the sampling induced uncertainties are underestimated by the model because the actual auto-correlation function for the flux time series deviates from the exponential form assumed in the model.