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

Reference evapotranspiration (${\mathrm{ET}}_{\mathrm{o}}$) estimates in greenhouses require the measurement of several meteorological variables. These measurements are not always conducted by water managers. The ${\mathrm{ET}}_{\mathrm{o}}$ 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 (${\mathrm{ET}}_{\text{gauge}}$) in greenhouses under ventilated and unventilated conditions as a surrogate for the reference evapotranspiration estimated by two grass-reference ${\mathrm{ET}}_{\mathrm{o}}$ equations: the Penman-Monteith equation (${\mathrm{ET}}_{\text{oPM}}$) and the Hargreaves-Samani equation (${\mathrm{ET}}_{\mathrm{oH}}$). Two versions of the Hargreaves-Samani equation were compared with the ${\mathrm{ET}}_{\text{gauge}}$; one used the measured solar radiation (${\mathrm{ET}}_{\text{oHm}}$), and the other used the calculated solar radiation (${\mathrm{ET}}_{\text{oHc}}$). The results showed that the ventilation had a great impact on the relationships between the ${\mathrm{ET}}_{\text{gauge}}$ and the reference equations. Under ventilated conditions, the daily ${\mathrm{ET}}_{\text{gauge}}$ and both the ${\mathrm{ET}}_{\text{oPM}}$ and ${\mathrm{ET}}_{\text{oHm}}$ were linearly correlated. The daily ${\mathrm{ET}}_{\text{gauge}}$ measurements were 13% lower than the ${\mathrm{ET}}_{\text{oPM}}$ and 57% higher than the ${\mathrm{ET}}_{\mathrm{oH}}$. Under unventilated conditions, the ${\mathrm{ET}}_{\text{gauge}}$ measurements were higher than the ${\mathrm{ET}}_{\text{oPM}}$ and ${\mathrm{ET}}_{\mathrm{oH}}$ by 68 and 50%, respectively. With the proposed air temperature–based and relative humidity–based adjustments for the ${\mathrm{ET}}_{\text{gauge}}$ readings, the performance of the atmometer inside the greenhouse improved (the relative error decreased by 40% 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.