Carbon dioxide and latent heat flux measurements in a windbreak-sheltered orchard

An eddy correlation system for carbon dioxide and latent heat flux measurement comprising a twin-channel, closed-path infra-red gas analyzer and sonic anemometer is described. Its performance was examined by comparing latent heat fluxes measured concurrently with an open-path sensor over windbreak-sheltered kiwifruit (Actinidia deliciosia var. deliciosia). Measurements were carried out at the height of surrounding windbreaks (6 m). Excellent average agreement was found between latent heat fluxes measured by either system with no evidence of bandwidth limiting by the closed-path components. This agreement gives confidence that carbon dioxide fluxes were also correctly measured. It also provides validation of the density corrections for eddy fluxes which differ significantly for closed- and open-path sensors. Carbon dioxide fluxes were closely related to solar radiation, reaching 800 μg m^-2 s^-1 towards the canopy early in the afternoon. Water-use efficiency was approximately 7 mg of CO₂ per g of water evaporated and showed little dependence on either radiation or water use. Night-time fluxes were erratic in low wind conditions showing infrequent large excursions which are attributed to the action of occasional gusts sweeping out high levels of carbon dioxide accumulated from canopy and under-storey respiration. During windier conditions, night-time fluxes were typically 200 μg m^-2 s^-1 away from the canopy. The role of windbreaks in isolating the canopy from the free airstream was evident in the large diurnal variation in ambient carbon dioxide concentrations (640-800 mg m^-3) observed 3-4 m above the height of the crop.