Developing equations for estimating reference evapotranspiration in Australia

Quantifying reference evapotranspiration (ET0) is essential in water resources management. Although, many methods have been developed with different level of accuracy, in this study, two new equations were developed and optimized for estimating ET0 using Honey-Bee Mating Optimization (HBMO) algorithm. The firs eq. estimates ET0 from extraterrestrial radiation (Ra), relative humidity (RH) and mean daily temperature (Tmean), while the second uses the same parameters except that mean daily temperatures is replaced with maximum daily air temperature (Tmax). Both equations were developed using climatic data from eight weather stations in Western Australia and subsequently verified using data from ten sites across Australia. The estimated ET0 values from both equations versus the FAO56-Penman-Monteith have a coefficient of determination, R2, of larger than 0.96. Moreover, the performance of six commonly used methods of estimating ET0 including Hargreaves-Samani, Thornthwaith, Hamon, McGuinness-Bordne, Irmak and Jensen-Haise were assessed and the Hargreaves-Samani method performed better than others. An attempt was made to calibrate the Hargreaves-Samani equation; however, its overall performance did not improve and the two newly proposed equations are suggested to be used in Australi