Evapotranspiration: A Review of the Terminology & the Technology

By Stephen Smith, PhD, FASIC, CAIS, CLIA and Tom Trout, PhD

This graphic illustrates surface irrigation with the associated return flows that are inherent in surface irrigation methods.

Precipitation Irrigation return flows RF Root zone

Subsurface return flows/ deep percolation


Change in soil moisture storage

Irrigation professionals will likely be very familiar with the term evapotranspiration but, as with many terms and technologies, it is generally a good idea to review, contemplate and reflect on the technical background.

Evapotranspiration is the sum total of the evaporative loss from the soil surface and the transpiration through the plant. The term is in common use in both the agriculture and landscape segments of the irrigation industry. However, the relative importance and the day-to-day usage of ET information is somewhat different in the two segments. Why is this?

In landscape water management, the landscape plant pallet is highly varied, from grasses to shrubs to trees. A properly designed irrigation system recognizes the variations of plant water require- ments by identifying and differentiating using hydrozones.

Hydrozones can be irrigated with separate irrigation laterals with separate lateral valving so run times and irrigation amounts can be customized for the plants in the zone. Even within hydrozones there can be varying plant water requirements, so one must often determine a controlling plant for irrigation management purposes.

16 Irrigation TODAY | July 2016

In agriculture, farm crops will vary, but not as dramatically as landscapes. Because of this, irrigation set times or controller run times can be tailored closely to crop water requirements. Growers are motivated to understand evapotranspiration with the intent to meet but not exceed the crop need with the goal to save water, fertilizer and labor. Crops that receive suitable irrigations meeting potential ET, will generally have the highest yields and highest quality — the “more crop per drop” concept.

So, how does a landscape water manager or a grower deter- mine and monitor ET for his or her own irrigation management purposes?


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