Measurements of carbon sequestration by long‐term eddy covariance: methods and a critical evaluation of accuracy

Abstract

The turbulent exchanges of CO2 and water vapour between an aggrading deciduous forest in the north‐eastern United States (Harvard Forest) and the atmosphere were measured from 1990 to 1994 using the eddy covariance technique. We present a detailed description of the methods used and a rigorous evaluation of the precision and accuracy of these measurements. 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 environment, and (3) sampling uncertainty results when summing an incomplete data set to calculate long‐term exchange. Analysis of the surface energy budget indicates a uniform systematic error in the turbulent exchange 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 interval due to sampling uncertainty of ±0.3 t C ha−1 y−1 determined by Monte Carlo simulation. Sampling uncertainty may be reduced by estimating the flux as a function of the physical environment during periods when direct observations are unavailable, and by minimizing the length of intervals without flux data. These analyses lead us to place an overall uncertainty on the annual carbon sequestration in 1994 of ‐0.3 to +0.8 t C ha−1 y−1.

Heat storage and anthropogenic heat flux in relation to the energy balance of a central European city centre

“…These uncertainties and errors have been discussed in detail elsewhere (e.g. Goulden et al, 1996;Moncrieff et al, 1996;Vickers and Mahrt, 1997;Wilson et al, 2002;Su et al, 2004). Here, issues of particular importance in the urban environment are highlighted.…”

Scaling Gross Primary Production (GPP) over boreal and deciduous forest landscapes in support of MODIS GPP product validation

“…Each site is approximately centered on an eddy covariance flux tower that makes continuous measurements of temperature, precipitation, solar radiation, humidity and NEE of carbon (Goulden et al, 1996a). Within the 25 km 2 , 100 plots are established that sample most intensively around the flux tower and more randomly over the remainder of the area.…”

“…Law, Ryan, & Anthoni, 1999) is a complex undertaking. Eddy covariance flux towers measure GPP indirectly as the difference between net ecosystem exchange (NEE) and ecosystem respiration (R e ) during daylight periods (Goulden, Munger, Fan, Daube, & Wofsy, 1996a;Turner et al, 2003). For these estimates, R e is either scaled from chamber measurements of soil and plant respiration (Ham & Knapp, 1998) or from the relationship of air temperature to NEE during nighttime periods above a threshold friction velocity (Goulden et al, 1997).…”

Ecosystem fluxes of hydrogen: a comparison of flux-gradient methods

“…Eddy covariance measurements represent the entire ecosystem flux and chamber measurements just the soil flux. Studies comparing nocturnal CO 2 eddy flux with chamber measurements often report significant discrepancies of 20 to over 50 % (e.g., Goulden et al, 1996;Lavigne et al, 1997;Phillips et al, 2010). We did not expect exact agreement between eddy and chamber fluxes because of the mismatch in measurement footprints, but we expected to see fluxes of a comparable magnitude with similar diel patterns.…”