Greenhouse micrometeorology and estimation of heat and water vapour fluxes

Abstract Micrometeorological measurements were made within and above a potted stand of chrysanthemums in a conventional glasshouse, using high frequency sensors to detect airflow, temperature and humidity, quantum sensors to measure radiation and a digital weighing lysimeter to quantify transpiration. Analyses were conducted to evaluate the aerodynamic and thermal effects on heat and mass transfer from vegetation canopies to greenhouse air, and the validity and comparability of resistance formulae for delineating the aerial effects. The differences of temperature and velocity between air within and above the canopy were found to be very significant. Thus it is necessary to distinguish aerodynamic and thermal parameters within these two distinct domains since the "big-leaf and perfectly-stirred-tank" assumption will be less satisfactory in estimating heat and water vapour fluxes from the greenhouse canopy. Three resistance terms were parameterized using the micrometeorological data, a stomatal resistance describing the physiological control, a boundary layer resistance controlling the exchange processes at leaf surfaces, and an aerodynamic resistance governing the vertical aerial transport processes. The parameterization of the resistances supported the hypothesis that, while the boundary layer resistance can be well formulated using similarity numbers, the aerodynamic resistance is largely dependent upon the aerodynamic and thermal conditions of the air column. It was shown that the similarity numbers should be determined using climatic parameters measured within the plant canopy, not above. Calibration of the aerodynamic resistance should be made with air temperatures measured at two vertical locations, not the difference between air and leaf temperatures. Further, such calibrated aerodynamic resistance is conceptually different and not comparable with the boundary layer resistance.