Assessment of nine different equations for ETo estimation using lysimeter data in a semi-arid environment

Abstract

There are many equations for the estimation of reference crop evapotranspiration (ETo) by using climatic data measured in weather stations. Also, different types of lysimeter are used to measure ETo directly. However, lysimeters are not available for ETo measurement everywhere; therefore, an appropriate equation should be used for ETo estimation. Furthermore, there is a sharp fluctuation or noise in the daily recorded weather data that should be smoothed for proper application. In this research, daily, smoothed daily, mean 10-day and mean monthly ETo were estimated by Penman-FAO, Penman-Monteith, Hargreaves-Samani, Jensen-Haise, Turc, Priestley-Taylor, FAO-Blaney-Criddle, FAO-Radiation and Pan Evaporation equations and the results of these equations compared with ETo data from a weighing type lysimeter. To indicate the most appropriate estimation equation, they are ranked according to statistical and error analysis. The results indicate that the FAO-Radiation and Hargreaves-Samani are the most appropriate methods and the Priestley-Taylor method is the least appropriate. The Penman-Monteith ranked in third to fifth place according to the duration of mean values. It is also concluded that smoothing weather data are preferred for estimation of daily ETo. Therefore, it is concluded that the FAO-Radiation method is the most suitable for estimation of ETo for irrigation planning and scheduling in regions where radiation and temperature data are available. However, the Hargreaves-Samani method is the most appropriate method when only temperature data are available.