Evapotranspiration is a term that describes crop water demand by combining
evaporation and transpiration. Evaporation is the process through which water is
removed from moist soil and wet surfaces (such as dew on leaves). Transpiration
is the process through which water is drawn up through the plant (roots extract
water from the soil, and water is eventually removed through stomata on the
leaves.)
What is Reference ET (PET)?
Reference crop evapotranspiration, also referred to as Potential
Evapotranspiration (PET), is an estimate of water requirement for a well watered
reference crop. This reference crop (grass or alfalfa) is essentially an
idealized crop used as a basis for the ET model. Reference ET is calculated by
applying climate data (temperature, solar radiation, wind, humidity) in a model
(equation). It is helpful to note that reference ET is only an estimate of the
water demand for this idealized crop, based upon weather station data at a given
location. The Texas High Plains ET Network uses an idealized grass reference
crop.
How is Crop Evapotranspiration calculated?
Crop-specific ET is estimated by multiplying the Reference ET by a crop
coefficient.
Crop ET = Reference ET x Crop Coefficient
The crop coefficient takes into account the crop's water use (at a given growth
stage) compared wth the reference crop. For instance, seedling corn does not use
as much water as the idealized grass reference crop, but during silking the corn
can use more water than the grass reference crop. The crop coefficient is
understood to follow a pattern (curve) of the general shape shown below. Each
crop (wheat, sorghum, etc.) will have its own crop coefficient curve.
The reference crop ET model and the crop coefficient curves were developed from
long-term research at various locations. Actual crop water demand can be
affected by many factors, including soil moisture available, health of the crop,
and likely by plant populations and crop variety traits. These factors are not
taken into account by the models. Hence, ET data provided by on-line networks
are probably best used as guidelines for irrigation scheduling, and (where
applicable) integrated pest management and integrated crop management. The
predicted growth stage and estimated water use should be verified with field
observations. The actual crop water use may be somewhat less than the predicted
value due to less than optimal field conditions
How is estimated ET used to schedule irrigation?
There are a variety of irrigation scheduling methods, models and tools
available. Many are essentially based upon a "checkbook" approach: Water stored
in the soil (in the crop's root zone) is withdrawn by evapotranspiration and
deposited back into the soil through precipitation and irrigation. When soil
moisture storage falls below a given threshold value, irrigation should be
applied to restore the moisture. The threshold value may be determined by crop
drought sensitivity, by irrigation system capabilities, or other farm-level
criteria.
EFFICIENT
IRRIGATION SCHEDULING WITH PET
