Transpiration of Ficus benjamina: comparison of measurements with predictions of the Penman-Monteith model and a simplified version

The rates of transpiration of Ficus benjamina were measured in two greenhouse experiments, one conducted in the UK in May, the other in Spain during July. Transpiration was determined by weighing, and simultaneous measurements were made of solar and net radiation, air speed, wet and dry bulb air temperatures and stomatal resistance. Values of stomatal resistance measured during the experiment in Spain were related to solar irradiance. The leaf external resistance obtained from an energy balance analysis was found to agree more closely with values obtained for forced convection than for free convection. Solar irradiance and net radiation were found to be very similar in the greenhouse during the day. The Penman Monteith equation was used to predict potential transpiration of the plants at the two experimental sites. Over the daylight period, the mean differences between the measured and predicted transpiration rates for the Silsoe and IRTA experiments were 2.8 and 3.2% with root mean square deviations of 20 W m-2. The Penman Monteith equation was simplified by introducing parameter values appropriate to greenhouse conditions, and it was also generalised by explicitly incorporating the temperature dependence of the major temperature sensitive variables. The resulting equation requires values only for the temperature and vapour pressure deficit of the air, global or net radiation, leaf area index and the leaf characteristic dimension. Without adjusting any other parameters this simplified equation predicted the transpiration of F. benjamina in Spain and the UK to within +5%.