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Evapotranspiration
(ET) is a measurement of the total amount of water
needed to grow plants and crops. This term comes from
the words evaporation (i.e., evaporation
of water from the soil) and transpiration
(i.e., transpiration of water by plants). Different
plants have different water requirements, so they have
different ET rates.
Since there are thousands of cultivated plants, we have
tried to simplify matters by establishing a standard
ET rate for general reference and use. The standard
is referred to as the potential evapotranspiration
ETo (pet). This is the potential ET since we are
assuming the crop is in a deep soil and under well watered
conditions. The standard crop we are using is a cool
season grass which is 4-inches tall. The technical term
for this is the "Potential Evapotranspiration of
a Grass Reference Crop" or "ETo" for
short.
ETo depends on the climate and varies from location
to location. Special weather stations are used to collect
the climatic data for calculating ETo,including temperature,
dew point temperature (relative humidity), wind speed,
and solar radiation.
The water requirements of specific crops and turf grasses
can be calculated as a fraction of the ETo. This "fraction"
is the called the crop coefficient (Kc) or turf coefficient
(Tc). Crop coefficients vary depending on the type of
plant and its stage of growth. Detailed information
on crop and turf coefficients and how to use them is
presented at other locations on this Web Site.
We are using the standardized Penman-Monteith
method to calculate ETo from the weather station
data. This is one of a number of methods that can be
used to determine ETo and ET. Several organizations,
such as the International Committee on Irrigation and
Drainage and the Water Requirements Committee of the
American Society of Civil Engineers, have proposed establishing
the Penman-Monteith method as a world-wide standard.
Such a standard would help facilitate the sharing of
ETo data and development of crop coefficients.
In Texas, the Penman-Monteith method is also being used
in the North Plains PET Network, a joint project between
the Texas Agricultural Extension Service (contact: Leon
New ), the Texas Agricultural Experiment Station
(contact: Thomas Marek
) and the USDA-ARS Laboratory in Bushland (contact:
Terry Howell).
For more information on this network call the Texas
A&M Research and Extension Center in Amarillo (806/359-5401)
or send an e-mail message by clicking on the above names.
For more information on the Penman-Monteith equation
see the FAO
website and other methods for determining ETo, see
the book:
Evapotranspiration and Irrigation Water Requirements,
edited by M.E. Jensen, R.D. Burman, and R.G. Allen.
Published by the American Society of Civil Engineers,
New York, NY. 1990. 332pp.
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