Irrigation Training Program


  1. Increase understanding of fundamentals of evapotranspiration (ET).
  2. Increase familiarity with ET resources, including ET Networks and Internet-available data and online tools.
  3. Apply these concepts to optimizing water management in crop production.
Key Points:
  1. Meteorological factors most often used to estimate ET are solar radiation (irradiance), air temperature, humidity, and wind speed.
  2. ET can be limited by soil moisture availability.
  3. Plant factors that affect ET include plant type, plant health, growth stage, plant population, and crop variety (affecting canopy and geometry). Successful application of ET models to irrigation scheduling requires relating the reference crop ET to the target crop ET through use of crop growth information and crop coefficients.
  4. ET is most accurately measured through use of weighing lysimeters.
  5. Alternate methods of estimating ET include water balance estimation techniques, including soil moisture monitoring.
  6. Major ET Networks in the state include the Texas ET Network (primarily central and south Texas), the Texas High Plains ET Network (Texas Panhandle, South Plains, Rolling Plains, and West Texas) and the Precision Irrigators Network (Winter Garden region around Uvalde).
Assess your knowledge:
  1. What is Evapotranspiration?
  2. What is an ET reference crop?
  3. Name the two most commonly used ET reference crops.
  4. Which ET reference crop is used most widely by ET networks in Texas?
  5. How do you calibrate reference crop ET to estimate crop ET?
  6. Why may actual crop use be less than model ET estimates?
  7. How do you access ET information for your area and crop on the internet?
  8. How can you apply ET to the “checkbook method” of irrigation scheduling?
  9. Would you expect cumulative annual reference crop ET to be higher in Lubbock, Texas or Longview, Texas? Why?
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

Recently additional weighing lysimeters have been installed at Uvalde, Texas. Dr. Giovanni Piccinni and others are using these to obtain crop water use information for crops and conditions in the Winter Garden area.

Evapotranspiration networks in Texas may be accessed on the following websites:

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