MEASUREMENTS OF CARBON SEQUESTRATION BY LONG-TERM EDDY COVARIANCE - METHODS AND A CRITICAL-EVALUATION OF ACCURACY

The turbulent exchanges of COP and water vapour between an aggrading d eciduous forest in the north-eastern United States (Harvard Forest) an d the atmosphere were measured from 1990 to 1994 using the eddy covari ance technique. We present a detailed description of the methods used and a rigorous evaluation of the precision and accuracy of these measu rements. We partition the sources of error into three categories: (1) uniform systematic errors are constant and independent of measurement conditions (2) selective systematic errors result when the accuracy of the exchange measurement varies as a function of the physical environ ment, and (3) sampling uncertainty results when summing an incomplete data set to calculate long-term exchange. Analysis of the surface ener gy budget indicates a uniform systematic error in the turbulent exchan ge measurements of -20 to 0%. A comparison of nocturnal eddy flux with chamber measurements indicates a selective systematic underestimation during calm (friction velocity < 0.17 m s(-1)) nocturnal periods. We describe an approach to correct for this error. The integrated carbon sequestration in 1994 was 2.1 t C ha(-1) y(-1) with a 90% confidence i nterval due to sampling uncertainty of +/-0.3 t C ha(-1) y(-1) determi ned by Monte Carlo simulation. Sampling uncertainty may be reduced by estimating the flux as a function of the physical environment during p eriods when direct observations are unavailable, and by minimizing the length of intervals without flux data. These analyses lead us to plac e an overall uncertainty on the annual carbon sequestration in 1994 of -0.3 to +0.8 t C ha(-1) y(-1).