Potential Evapotranspiration Calculation Program of FAO (ET0 Calculator)

ETo Calc is a software tool developed by the Land and Water Division of FAO under the MS-DOS operating system. Its main function is to calculate reference evapotranspiration (ETo) according to FAO standards. The software is meant as a practical tool to help agro-meteorologists, agronomists, and irrigation engineers to carry out standard calculations for ETo, to be later used in crop water use studies. The functionality of ETo Calc has now been incorporated into the CROPWAT software.

ETo Calc assesses ETo from meteorological data by means of the FAO Penman-Monteith equation. This method has been selected by FAO as the reference because it closely approximates grass ETo at the location evaluated, is physically based, and explicitly incorporates both physiological and aerodynamic parameters.

The program can handle daily, ten-day and monthly climatic data, that can be provided by the user in a wide variety of units. Data can also be imported from FAO’s climatic databases CLIMWAT and FAOCLIM. When data for some weather variables are missing, procedures are used for estimating missing climatic data from temperature data or from specific climatic conditions according to methodologies outlined in the FAO Irrigation and Drainage Paper No 56.: "Crop Evapotranspiration. Even where the dataset contains only maximum and minimum air temperature, it is still possible to obtain reasonable estimates for ten-day or monthly ETo. By selecting appropriate lower and upper limits for meteorological data, the program applies a quality check when specifying or importing data.

Specified and derived climatic data, including ETo, can be exported in text files compatible with AquaCrop software or plotted in various ways specified by the user.