Observations of equilibrium evaporation from a windbreak-sheltered kiwifruit orchard

Eddy correlation measurements of sensible heat and evaporation fluxes from a kiwifruit orchard confirm earlier, less comprehensive measurements that the spatially-averaged evaporation rate from well-watered and well-sheltered orchard blocks in the northern part of New Zealand occurs at the equilibrium rate (E_q. In this humid coastal climate the rapid onset of dewfall after sunset prevents night-time water use, which contrasts with other work showing that nocturnal transpiration may represent 25% of the daily water economy of a kiwifruit vine. Sensible heat fluxes measured at both shelter height (h) and 2.25h agreed within 5%, implying the existence of a constant flux region above shelter height. Under the low windspeed conditions common in mid-summer, the aerodynamic resistance to scalar transport between the canopy and the height of the windbreaks is shown to be of comparable magnitude (about 65 s m^-1) to both the canopy 'stomatal' and boundary-layer resistances. One consequence of this is that transpiration will be less sensitive to stomatal regulation. Independent of imposed wind and radiative conditions, the effect of the close-spaced windbreaks and crop geometry is to create an orchard environment that is an efficient absorber of radiation but which is relatively poorly coupled with overhead conditions through turbulent exchange.