EVALUATION OF SEVERAL REFERENCE EVAPOTRANSPIRATION METHODS: A COMPARITIVE 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.