Issued in furtherance of Cooperative Extension work, acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of
Agriculture, James A. Christenson, Director, Cooperative Extension, College of Agriculture and Life Sciences, The University of Arizona.
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Basics of Evaporation and Evapotranspiration
Turf Irrigation Management Series: I
This information has been reviewed by university faculty.
Turf Irrigation Management Series
Local information on evapotranspiration (ET) is now
readily available from on-site weather stations and/or
public weather networks to assist turfgrass profession-
als with irrigation management decisions.
lization of ET information can provide accurate esti-
mates of daily water use and thus can assist irrigation
managers with the all important decisions of when to
apply water and how much water to apply.
cept of ET can be confusing and often is presented in a
highly technical manner.
The objective of this and sub-
sequent bulletins in the
Turf Irrigation Management Se-
is to simplify the subject of ET and thereby increase
the effective utilization of ET in irrigation management.
This bulletin provides some basic background on the
related subjects of evaporation and evapotranspiration.
Water can exist in the natural environment in three
different forms or states — solid (ice), liquid and gas.
The process by which water changes from a liquid to a
gas is known as evaporation.
We are all familiar with
liquid water as we drink, bath and irrigate with it daily.
The gaseous form of water, known as water vapor, is
less familiar since it exists as an invisible gas.
ever, we all have a feel for water vapor during the late
summer months when it is called by the more common
name of humidity.
To the irrigation manager, the most
important points about evaporation are 1) it is the pro-
cess by which most of the liquid water we apply as
irrigation leaves vegetation and 2) that evaporation
requires energy (Fig. 1).
Two common household items — the clothes dryer
and the evaporative cooler — clearly show the energy
requirement of evaporation.
In the case of the dryer, a
gas burner or an electric heating element provides the
heat energy required to evaporate water from the wet
The evaporative cooler works in a somewhat
Energy stored in the hot, dry, outside
air is consumed by the evaporation process as the air
passes through the wet pads.
This energy consump-