Determining Reference Evapotranspiration in Greenhouses from External Climate

Abstract

Estimations of Penman-Monteith reference evapotranspiration (EToPM) in greenhouses can benefit the operation of these intensive agricultural production systems. Meteorological variables required to calculate EToPM within greenhouses are seldom available. In this study, meteorological data for two years from two weather stations (one inside a polycarbonate greenhouse and one outdoors) were used to derive relationships between greenhouse EToPM and outdoor EToPM plus outdoor solar radiation (measured and extraterrestrial). The results were validated in another greenhouse at a different location in a different year. The two locations represent two different climatic conditions (a coastal subhumid climate and an inland semiarid climate). The results show that EToPM in ventilated greenhouses can be predicted from the outdoor EToPM, extraterrestrial solar radiation, and the ventilation rate within the greenhouse; it can also be predicted from measured solar radiation and the square root of greenhouse transmissivity using an equation developed herein. Under typical ventilated conditions, greenhouse EToPM was found equal to 60% of outdoor EToPM, with solar radiation accounting for more than 90% of the variance. Under unventilated conditions, the relationship is more complex; however, EToPM can still be estimated satisfactorily from outdoor solar radiation Rs and greenhouse transmissivity measurements. These findings have potential applications in terms of providing guidelines for predicting greenhouse EToPM from outdoor climate variables or from outdoor EToPM. The estimations could enhance climate-smart agriculture applications in subhumid and semiarid environments while minimizing in-greenhouse meteorological data requirements.