EVALUATION OF SEVERAL REFERENCE EVAPOTRANSPIRATION METHODS: A COMPARITIVE STUDY OF GREENHOUSE AND OUTDOOR CONDITIONS

Abstract

Precise estimates of reference evapotranspiration (ET0) are necessary for theapplication of irrigation design and scheduling. Numerous empirical methods for predicting ET0are available, but their accuracy under different environmental conditions is uncertain. Greateruncertainty exists under greenhouse conditions because these methods were designed to apply tofield situations, and greenhouses have an effect on the temperature, humidity and wind, etc. In thisstudy, the results of 13 different common daily ET0 estimation methods, namely FAO56 Penman –Monteith, Hargreaves-Samanι, FAO-24 Blaney-Criddle, FAO-24 Radiation, Priestley-Taylor,Makkink, Turc, Linacre, Jensen-Haise, Copais, Pan Evaporation, Rn-radiation and Rs-radiation arecompared with lysimetric measurements in an area of Fars (Badjgah) in a plastic greenhouse toprovide helpful information for selecting the appropriate ET0 equation to use. In addition to dailyvalues, smoothed daily and mean 10-day ET0s were estimated to study the effect of daily weatherdata fluctuations on the precision of predictions. Performances of ET0 methods are evaluated byfour statistical criteria along with regression indices. The results indicate that FAO Penman-Monteith and Linacre are the most and the least appropriate methods for estimating daily ET0 ingreenhouse conditions, respectively. For outdoor conditions the best and worst results wereobtained from FAO24- Radiation and Copias methods, respectively. Smoothing weather data, gavebetter regression fits for FAO Penman-Monteith and FAO24-Radiation methods for bothgreenhouse and field conditions than those for daily weather data. Better predictions were obtainedfor field than greenhouse conditions. The total ET0 values in greenhouse were about 0.85 of thosemeasured in outdoor lysimeters.