Simplified versions for the Penman evaporation equation using routine weather data

At standard weather stations the routine weather records usually available are air temperature, T (°C), solar radiation RS (MJ/m2/d), relative humidity, RH (%), and wind velocity, u (m/s). A simple algebraic formula, equivalent in accuracy to the Penman equation is derived for computing evaporation from readily available measured data. The derivation of the formula is based on simplifications made to the “standardized” form of the Penman equation. The two components of the standard Penman equation (radiation and aerodynamic) were computed indirectly from the available routine weather data using the standardized calculation procedure recommended by Shuttleworth [Shuttleworth, W.J., 1993. In: Maidment, D.R. (Ed.), Evaporation, McGraw-Hill, New York, pp. 4.1–4.53 (Chapter 4)] and Allen et al. [Allen, R.G., Pereira, L.S., Raes, D., Smith, M., 1998. Crop evapotranspiration: guidelines for computing crop water requirements. FAO Irrigation and Drainage Paper, 56, Rome, 300 pp]. In addition, another, more simplified formula, easy to use for routine hydrologic applications, is also developed. On the other hand, at many places reliable wind speed data are rarely available. For such cases, an expression, which does not require wind speed data, is proposed. The simplified formula for the Penman equation proposed to estimate potential evaporation from open-water without recourse to wind data is image where RA (MJ/m2/d) is the extraterrestrial radiation. A simplified expression for the calculation of RA is also given. The simplicity of the formulas is demonstrated in a computational example applied for monthly climatic data. 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 a weather station. The new open water evaporation formulas were also adapted for calculating reference crop evapotranspiration.