Temperature-and humidity-based simplified Penman's ET0 formulae. Comparisons with temperature-based Hargreaves-Samani and other methodologies

where T = (Tmax+Tmin)/2 is the average temperature (oC), and Ra is the extraterrestial radiation (MJ/m2/d). The new formula is applicable for stations under well-watered conditions. An extension of the new formula incorporating local average wind value as an additional input is also proposed. The performance of the new derived formula is tested with daily weather data obtained from 32 stations in California, Florida, Arizona and Greece. The proposed formula is not as generalized as the Hargreaves-Samani equation requiring only widely available Tmax and Tmin, as it is restricted to locations where good quality RH data is measured. However, incorporating RH as additional input to the suggested T-RH formula increases dramatically the accuracy of the ET0 estimation compared to the Hargreaves-Samani method. It is concluded that the proposed formula is more accurate than Hargreaves-Samani by 46%. In addition, the T-RH formula performed better than the reduced-set FAO-56 Penman-Monteith method that required T and RH by 21%. The results of the formula are further improved when local average wind speed is incorporated as an additional input.