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, howev er, increases with decreasing sampling rate. This sampling induced uncertai nty is compared with the natural uncertainty in the flux estimate due to th e stochastic nature of turbulence. It is shown that the sampling induced un certainty becomes important when the sampling interval is larger than the i ntegral time scale of the flux time series. Theoretical expressions are eva luated by using 9 days of turbulence data taken over a coniferous forest. F or sampling intervals smaller than the integral time scale the sampling ind uced 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. (C) 2001 Elsevier Science B.V. All rights reser ved.