Impact of CO2 storage flux sampling uncertainty on net ecosystem exchange measured by eddy covariance

Abstract

Complying with several assumption and simplifications, most of the carbon budget studies based on eddy covariance (EC) measurements quantify the net ecosystem exchange (NEE) by summing the flux obtained by EC (FC) and the storage flux (SC). SC is the rate of change of CO2, within the control volume below the EC measurement level, given by the difference in the instantaneous profiles of concentration at the beginning and end of the EC averaging period, divided by the averaging period. The approaches used to estimate SC fluxes largely vary, from measurements based on a single sampling point (usually located at the EC measurement height) to measurements based on several sampling profiles distributed within the control volume, either along a single profile or in the three-dimensional space. Measurement accuracy reasonably increases with the sampling intensity, though limited resources often prevent complex measurement systems. In this study we use the experimental dataset collected during the ADVEX campaign in which turbulent and non-turbulent fluxes have been measured in three forest sites by the use of 5 towers. Our main objective is to quantify the impact of SC measurement uncertainty on NEE estimates. Results show that different methods may produce substantially different SC flux estimates, with tolerable consequence on long-term estimates yet problematic in case high frequency (half-hourly) data are needed.