ETo represents the evapotranspiration
rate from a reference surface, not short
of water. A large uniform grass field is
considered worldwide as the reference surface.
The reference crop completely covers the
soil, is kept short, well watered and is
actively growing under optimal agronomic
The ETo calculator 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. The data can be given
in a wide variety of units and data specified
in commonly used climatic parameters can
be processed. 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 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
Specified and derived climatic data, including
ETo, can be exported in text files compatible
with AquaCrop or plotted in various ways
specified by the user.
ETo calculator 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.