Carbon Dioxide and Water Vapour Flux Densities over a Grassland Area in the Netherlands

Measurements of carbon dioxide and water vapour flux densities were carried out for a grassland area in the centre of the Netherlands during a 12 month continuous experiment using the eddy-covariance technique. Lolium perenne and Poa trivialis mainly dominated the vegetation of the area. The leaf area index (LAI) of the grassland varied between 2.5 and 3.4, depending on the mowing scheme at the site. Fluxes are also calculated by using a canopy model in which the canopy resistance is related to the assimilation rate (A–gs model). The calculated evapotranspiration fluxes agree within 7% of the measured fluxes (R2 = 0.93;SEE = 23 W m−2). The calculated net carbon dioxide fluxes agree within 12% of the measured fluxes (R2 = 0.79;SEE = 0.16 mg m−2s−1). The net carbon dioxide fluxes consist of two opposite components: the uptake via assimilation and release via soil and plant respiration. It appears that nearly the whole year round the assimilation term dominates, which means an accumulation of carbon dioxide if no other exchange processes are present. For 2002 a net carbon uptake was found of 6.8 tonnes ha−1. Copyright  2003 Royal Meteorological Society.