LAUSR

Abstract This study introduces a new simplified Penman formulae for estimating reference evapotranspiration, ET0 (mm/d). The new formulae require maximum and minimum air temperatures and relative humidity or in addition local average wind speed. The new ET0 formulae is produced by simplifications to the previously developed by Valiantzas (2006, 2013a) simplified Penman expression that requires a complete set of meteorological data. A calibration based on measurements obtained from a global data base is used for the derivation of the formula. In addition, a relationship previously proposed by the author, connecting solar radiation to temperature and humidity is used. The new formula requiring maximum and minimum temperatures, Tmax (oC) and Tmin (oC) respectively, and humidity data, RH (%) is: ET0 = 0.0118(1-RH/100)0.2(Tmax-Tmin)0.3(Ra(T+10)0.5-40)+0.1(T+20)(1-RH/100) 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.