Comparison of the Concepts of Potential and Reference Evapotranspiration using Lysimetric Data in Qazvin Province

In order to determine the amount of irrigation water, it is required to calculate the amount of the plant water requirement or evapotranspiration. The equations used to calculate evapotranspiration do not use the same climatic parameters and are not suitable for all climatic conditions due to their experimental nature. Potential evapotranspiration (ETp) and reference evapotranspiration (ETo) are different in concepts, equations, and contexts; however, many researchers have considered the use of the two terms to be the same. This study provided a comprehensive review of the ETp and ETo equations. In this regard, using meteorological data of synoptic station and lysimetric information of the region, reference and potential evapotranspiration were compared and studied based on four experimental groups during 4 years. The equations were calibrated using statistical indicators to select the most ideal model. The results showed that the evapotranspiration potential and the reference are completely different from each other, so that in the same method of the temperature equations of ETo and ETp, the RMSE calculated was 1.17 and 1.1, mm/day, respectively. According to the studies from the ETo and ETo, the temperature group equations had the best performance, showing the superiority of this group of equations in areas with arid and semi-arid climate.