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Crop
Coefficients
Potential
Evapotranspiration (PET) is an estimate of the water
requirements of a 4-inch grass in a deep soil growing
under well watered conditions. The water requirements
of specific crops are determined from pET through crop
coeffecients (Kc). The following equation is used:
PET
x Kc = crop water requirement
Crop
coefficients vary for different crops, as well as for
the region crops are grown in. In addition, they change
based on the growing stage of the crop. Unfortunately,
for Texas, we only have verified crop coefficients for
the North High Plains. These coefficients were developed
by the North Plains pET Network Project Team.
FAO
(Food and Agriculture Organization of the United Nations)
has published generalized crop coefficients which are
used throughout the world where local values are not
available.
Irrigation
Efficiency - To adjust for irrigation system efficiency,
we use the follwoing equation
PET
x Kc / Eff = crop water requriement
FAO
Crop Coefficients
North
Plains Crop Coefficients for Sorghum
North
Plains Crop Coefficients for Cotton
North
Plains Crop Coefficient for Corn
North
Plains Crop Coefficient for Winter Wheat
Turf
Coefficients
Potential Evapotranspiration, ETo (also abreviated as PET), is an estimate
of the water requirements of a 4-inch grass growing
in a deep soil under well-watered conditions. A turf
coefficent (Tc) is needed to relate pET to the water
requirements of a specific turf. For established lawns,
the Tc remains constant throughout the active growing
season of the grass. For warm season grasses, such
as St. Augustine, the Tc is 0.6 throughout much of
the year, while the Tc for cool season grasses, such
as rye, is 0.8.
However, we seldomly apply this amount of water to
lawns. Why? Because we do not want maximum production
of grass clippings. Instead, we want to maintain a
healthy, attractive turf with as little water as possible.
Thus, we modify the coefficent by specifying a Quality Factor.
The complete equation is:
ETo x Tc x Qf = turf water requirement
Turf Coefficient Values (Tc)
| Warm
Season |
0.6 |
| Cool
Season |
0.8 |
Quality Factor (Qf)
| No
Stress |
1.0 |
| Low
Stress |
0.8 |
| Normal
Stress |
0.6 |
| High
Stress |
0.5 |
| Very
High Stress |
0.4 |
Full Turf Coefficient Charts
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