Evaluation of Reference Evapotranspiration Models for a Semiarid Environment Using Lysimeter Measurements

An accurate determination of evapotranspiration is required for many studies that involve estimation of the water balance. One methodology is the use of lysimeters. Considering the semiarid climate of Kerman Province, in southeastern parts of Iran, the only operating electronic weighing lysimeter in the country was used for calculating daily ETo from April 2004 to March 2005 in three different periods, i.e. the entire year, and high and low evaporative demands periods. The measured error was equal to 1 kg mass, which is equivalent to 0.14 mm of water in the field. An automated weather station was used that provided 10-min recordings of the weather data to be used for predicting daily ETo with models. The lysimeter was installed in proximity of the automated weather station and both were located in a field with grass cover. The lysimeteric data were used for the evaluation of six grass evapotranspiration models, including FAO-56 Penman–Monteith, Penman-Kimberly 1996, FAO-24 Blaney-Criddle, FAO-24 Radiation, Makkink, and Hargreaves-Samani. The root mean square error (RMSE) and index of agreement (d) were used for assessing prediction accuracy of different models. Results indicated that for the entire year period, the FAO-24 Radiation equation was the most precise method for calculating ETo, with a RMSE of 1.63 mm day-1 and a d- index of 0.78. During the high evaporative demand period (April to September 2004) the FAO-24 radiation equation was superior compared to the other methods for calculating ETo with a low RMSE value of 1.86 mm day-1 and a d-index of 0.45. During the low evaporative demand period, again, FAO-24 radiation equation was superior compared to the other methods with RMSE of 1.30 mm day-1 and d-index of 0.46. In all of the three periods, the Makkink method showed poor performance and can not be recommended for the region.