Simplified forms for the standardized FAO-56 Penman–Monteith reference evapotranspiration using limited weather data

New simple algebraic expressions equivalent in accuracy to the “standardized” FAO-56 Penman–Monteith daily reference crop evapotranspiration (ET0) computation procedure are derived. The suggested formulas are based on extensions made to a previously developed simple algebraic formula for the Penman evaporation equation. The derivation of the new formulas is based on simplifications made and the systematic analysis on the correspondence between the FAO-56 Penman–Monteith equation and the standardized Penman’s equation. The ET0 calculated by the new formulas is easy to use for routine hydrologic applications requiring routine weather records usually available at standard weather stations: air temperature, T (°C), solar radiation RS (MJ/m2/d), relative humidity, RH (%), and wind velocity, u (m/s). For places where not all these data are available (or reliable), new expressions which does not require wind speed and/or solar radiation data are proposed. A simplified formula for estimating reference crop evapotranspiration requiring Tmax and Tmin (the maximum and minimum air temperatures), and Tdew (the dew point temperature) or RH data alone is derived. The performance of the new derived formulas was tested under various climatic conditions using a global climatic data set including monthly data as well as daily data obtained from weather stations.