What is evapotranspiration (ET)?

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.

Where can I find additional information on ET and related topics?

One of the best sources for ET and other related water use information is available from the USDA-ARS Conservation and Production Research Laboratory, Soil and Water Management Research Unit at Bushland, Texas, near Amarillo. The water management unit is directed by Dr. Terry Howell, who is responsible for the large weighing lysimeter facility at Bushland. In laymen’s terms, lysimeters are extremely large “flower pots” (weighing on the order of 100,000 pounds or so) that rest upon an extremely sensitive scale whereby Dr. Howell’s group can measure water used through a crop’s evaporation and transpiration throughout the growing season. Much of these data from various crops have been incorporated into the TXHPET network water use and crop growth models. Some of Dr. Howell’s research data and associated efforts are available at http://www.cprl.ars.usda.gov/wmru.htm.