Biogeosciences www.biogeosciences.net 1726-4170 1726-4189 7 8 2010 10.5194/bg-7-2461-2010 http://www.biogeosciences.net/7/2461/2010/ http://www.biogeosciences.net/7/2461/2010/bg-7-2461-2010.html http://www.biogeosciences.net/7/2461/2010/bg-7-2461-2010.pdf 2461 2475 2010-08-17 Contribution of advection to the carbon budget measured by eddy covariance at a steep mountain slope forest in Switzerland S. Etzold sophia.etzold@ipw.agrl.ethz.ch N. Buchmann W. Eugster ETH Zurich, Institute of Plant, Animal and Agroecosystem Sciences, Switzerland We calculated the contribution of advection to the C budget measured by the eddy covariance (EC) technique for a steep and forested mountain site (CarboEurope site CH-Lae, Lägeren, Switzerland) during the growing season 2007 (May to August). Thereby we followed two approaches: (1) the physical correction of the EC data for directly measured advection terms and (2) the <i>u</i>_&lowast; filter approach that replaces periods with <i>u</i>_&lowast; below a site-specific threshold with empirically modelled fluxes. We found good agreement between the two approaches in terms of daily (linear regression slope: 0.78 ± 0.04, intercept: 0.68 ± 0.29 μmol m^−2 s^−1, adj. <i>R</i>²=0.78) and seasonal sums of gross fluxes (difference &le; 12%), when using a <i>u</i>_&lowast; threshold of 0.3 m s^−1 and correcting EC for horizontal advection only. Incorporating also vertical advection into the mass balance equation resulted in unrealistic and highly erratic fluxes. However, on a daily basis vertical advection cancelled out to nearly zero. The <i>u</i>_&lowast; filter seems to account primarily for respiration fluxes, which are mainly affected by horizontal advection. 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