evaluation and prediction of greenhouse cucumber evapotranspiration at different growth stages

a precise estimation of water consumption throughout a crop's growth season and of the amount of water consumed in each growth stage may play an important role in water resources management, integrated water and soil management, and proper irrigation scheduling. in a greenhouse, this faces with the conditions specific to this environment. this study was conducted to propose a model to make an appropriate and accurate prediction of evapotranspiration (et) for greenhouse cucumber. two same microlysimeters with 28 cm diameter and 30 cm height were deployed simultaneously in the greenhouse for the cucumber culture. amount of et was measured daily by the weighing method in both mycrolysimeters. the data from the first microlysimeter were used to derive, and those from the second to validate the proposed models. the developed models were evaluated by root mean square error (rmse), drawing measured versus predicted et values, and t-statistics. the proposed model was initially developed in the form of a single regression equation with independent variables such as vapor pressure curve slope and relative humidity for the whole growth season; further however, a separate equation was developed for each of the four growth stages, as the initial model did not perform well (rmse= 46.61%). the results showed that the proposed models made appropriate predictions of greenhouse cucumber et. average amount of cucumber et were obtained with proposed models 0.398, 1.79, 3.428 and 2.061 mm for four growth stages. rmse values also were obtained 15.78, 11.48, 9.11 and 7.08 percentage for four growth stages. correlation coefficient from measured and predicted values of cucumber et varied from 0.4 (using single equation) to 0.95 (using variable equations for different growth stages). all of the proposed models were significant (p<0.05).