Evapotranspiration ET Flips Rainfall Upside Down By Mike Baron

With the advent of weather-based irrigation controllers and their “scheduling engines,” the landscape irrigation industry took its fledgling relationship with evapotranspiration to a new level.

Suddenly, ET moved from the desks of irrigation consultants and off the pages of California’s 1992 Model Water Efficient Landscape Ordinance to the center of conversation between tech-savvy sales people and their customers. Everyone wanted to know what ex- actly ET is, where does the controller get its ET values and how the values are used to determine run times and water days.

Today, most irrigation professionals know that ET information is essential to developing efficient irrigation schedules and that before it can be determined how long and how often sprinklers or driplines should run, it’s important to know how much water has left the landscape over a given period.

When discussing ET with irrigation neophytes, an easy-to- understand analogy linked to rainfall can be used. Think of ET as rainfall — but invisible and in the opposite direction. That direction starts at the landscape level and moves upward into the atmosphere instead of the other way around. With this image in mind, it becomes easier to move to a more scientific explana- tion.

The water, or ET, that is lost from the landscape is a result of two basic processes operating in the landscape: evapora- tion directly from the soil and plant surfaces; and water vapor that is lost from the plants through the biological process called transpiration. So “evaporation” plus “transpiration” equals evapotranspiration. Big word, straight forward meaning.

in the vapor phase. Sound familiar? It should, because photosyn- thesis is a basic process undertaken by green plants and is what makes them so important to humans who breathe in O2 hale CO2

The transpiration process occurs through stomata in the plant. Stomata are tiny pores that can take in CO2

and release O2 and H2 and ex- . This is but one of the many environmental reasons that landscapes are so important.

When ambient temperatures rise, or solar exposure increases, ET rises and plants increase their release of H2

O in order to keep

cool. This is why turf in summer is so much cooler than artificial turf or inorganic mulch. This is why turf, plants and trees act as highly efficient cooling machines when it is hot. In addition to temperature and solar radiation, wind and humidity affect the final mathematical calculation of ET which can be obtained from a weather station, from the internet or effectively estimated by weather sensors that have become part of cost-effective, weath- er-based irrigation control systems. Just remember though, when talking about ET, always specify a time frame. This is because 1" of ET over the course of a week is much, much different from 1" of ET over the course of 24 hours.


Mike Baron is the national specifications manager for Toro’s water management products. In 2008, Mike was named California Landscape Contractors As- sociation’s Associate Member of the Year for his

work on water conservation and in 2014, Mike was inducted into the Green Industry Hall of Fame. 21

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