Comparison of three evapotranspiration models for a greenhouse cooling strategy with natural ventilation and variable high pressure fogging

Even though several models to predict evapotranspiration (ET) of greenhouse crops have been developed, previous studies have evaluated them under fixed greenhouse conditions. It is still not clear which model is more appropriate, accurate, and best suited for applications such as inclusion in greenhouse cooling strategies for different crops, climatic conditions and greenhouse cooling settings. This study evaluated three theoretical models (Stanghellini, Penman-Monteith and Takakura) to simulate the ET of two crops (bell pepper and tomato), under two greenhouse cooling settings (natural ventilation with fog cooling and mechanical ventilation with pad and fan), and for three growing seasons (spring, summer, fall). Predictions of ET from the models were compared to measured values obtained from sap flow gauges. Inputs of internal and external crop resistances for Stanghellini and Penman-Monteith models were calibrated separately by crop and by model. Even though Stanghellini model produced the smallest deviations of the predicted ET from the measured ET, having the best overall performance under all conditions evaluated, an analysis of variance of the daily mean square errors did not show significant differences (α= 0.05) between the three models. This suggested that any of the three models could be used for inclusion in a greenhouse cooling climate control strategy. However, parameter adjustments such as stomatal and aerodynamic resistances, and the need of leaf area index (LAI) in the models of Penman-Monteith and Stanghellini represent a limitation for this application. The Takakura model was found to be easier to implement; however as the crop grows, careful adjustments on the height of the solarimeter used for this approach are required. Such adjustments determine the field of view of the solarimeter and play a significant role on the determination of radiation balances and the average apparent temperature of the evaporative surface.