Potential Evapotranspiration–A Water Management Tool

Ag Biz News Column
Chad Gulley
County Extension Agent—Ag/NR
Smith County


Potential Evapotranspiration a Water Management Tool

Water is essential for life.  All living things need water to survive.  In our lawns, landscapes, and farms, we can use the potential evapotranspiration to calculate the water requirements of our crops and plants.  What is Potential Evapotranspiration?

Potential Evapotranspiration or PET represents the combined loss of water through:  1) the plant’s process of transpiration via its vascular system, and 2) evaporation of water from the earth’s surface.  Both are influenced by temperature, humidity, sunlight, and wind.  PET values indicate the amount of water that has been lost, and thus needs to be replaced, through irrigation and/or rainfall.

Using the PET data, we can apply the correct amounts of water to our crops and landscape.  This allows us to conserve water, reduce waste, and save money.  Overwatering can be a bigger problem in our lawns than under watering.

So how can I use PET in my irrigation schedule?  Historical PET values for East Texas can be found by going to the following website:  http://etweather.tamu.edu/pet . An online calculator for PET can be found by going to this website:  http://texaset.tamu.edu/ .  The online site gives the user options to choose either home watering, turf/landscape irrigation, or crop irrigation.

Water is important even during winter months.  While our plants may have gone dormant and turned brown during this winter period, the roots still need water to stay alive.  With regular rainfall, that could be the amounts needed to keep these roots alive.

Soil type can play an important role in the amount of water a lawn needs.  Water moves very slowly into some soil types especially fine-textured clay and loam soils.  If a sprinkler system is applying water faster than the water can seep into the soil, significant water loss in the form of run-off may result.  Sandy soils, however, have less water holding capacity thus much of the water leaches or filters right through the soil.

It is also important to know what your goals are and what species of plants you will be watering to irrigate efficiently.  A strong root system in our plants is important and proper watering techniques can help develop that strong root system.  If we water too much, the roots will stay close to the surface of the soil, thus making a weaker root system that cannot handle stresses well.

Watering deeper and less frequent can help our root system grow as it should.  Monitoring the soil moisture can be done by using a soil probe to see how deep the water is moving through the soil profile.  Some are even using soil moisture sensors to determine moisture levels in their lawn or landscape.  Mulching flower beds can help with moisture retention.

The squeeze test can also be helpful in determining moisture in our soil.  The squeeze test will vary from soil type but can be another useful tool.  To perform a squeeze test, obtain a handful of soil.  Take this handful of soil, grip it and make a ball squeezing the soil in your hand.  Once the soil forms a ball, open your fingers and see how the ball of soil stays together.  Ideal soil moisture is present when a wet outline of the soil remains on your hands.  If water drips from the ball, your soil is too wet.  If the soil is dry in your hand and crumbles having a hard time staying together, the soil is too dry.  Again, the various soil types will vary with moisture holding capacity so knowing what type or types of soil you have is important.

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