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

Precise estimates of reference evapotranspiration (ET0) are necessary for the application of irrigation design and scheduling. Numerous empirical methods for predicting ET0 are available, but their accuracy under different environmental conditions is uncertain. Greater uncertainty exists under greenhouse conditions because these methods were designed to apply to field situations, and greenhouses have an effect on the temperature, humidity and wind, etc. In this study, 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 are compared with lysimetric measurements in an area of Fars (Badjgah) in a plastic greenhouse to provide helpful information for selecting the appropriate ET0 equation to use. In addition to daily values, smoothed daily and mean 10-day ET0s were estimated to study the effect of daily weather data fluctuations on the precision of predictions. Performances of ET0 methods are evaluated by four 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 in greenhouse conditions, respectively. For outdoor conditions the best and worst results were obtained from FAO24- Radiation and Copias methods, respectively. Smoothing weather data, gave better regression fits for FAO Penman-Monteith and FAO24-Radiation methods for both greenhouse and field conditions than those for daily weather data. Better predictions were obtained for field than greenhouse conditions. The total ET0 values in greenhouse were about 0.85 of those measured in outdoor lysimeters.