Evaluating Atmometer Performance for Estimating Reference Evapotranspiration in Ventilated and Unventilated Greenhouses

Abstract

Reference evapotranspiration (ETo) estimates in greenhouses require the measurement of several meteorological variables. These measurements are not always conducted by water managers. The ETo estimates also depend on the aerodynamic conditions in the greenhouse. The objective of this study is to evaluate the use of atmometer-based evapotranspiration estimates (ETgauge) in greenhouses under ventilated and unventilated conditions as a surrogate for the reference evapotranspiration estimated by two grass-reference ETo equations: the Penman-Monteith equation (EToPM) and the Hargreaves-Samani equation (EToH). Two versions of the Hargreaves-Samani equation were compared with the ETgauge; one used the measured solar radiation (EToHm), and the other used the calculated solar radiation (EToHc). The results showed that the ventilation had a great impact on the relationships between the ETgauge and the reference equations. Under ventilated conditions, the daily ETgauge and both the EToPM and EToHm were linearly correlated. The daily ETgauge measurements were 13% lower than the EToPM and 57% higher than the EToH. Under unventilated conditions, the ETgauge measurements were higher than the EToPM and EToH by 68 and 5%, respectively. With the proposed air temperature–based and relative humidity–based adjustments for the ETgauge readings, the performance of the atmometer inside the greenhouse improved (the relative error decreased by 4% on average). Our findings indicate that atmometers can be used in greenhouses in lieu of the calculated Penman-Monteith reference evapotranspiration (but not the Hargreaves-Samani’s) if the proper calibration is performed, especially under unventilated conditions.