Considering ET rates

With more restrictions placed on water use, irrigation is one of the most expensive and most important jobs of today’s turf bosses.

About 35 percent of superintendents schedule irrigation times by the rate of evapotranspiration (ET), according to the Environmental Institute for Golf’s report "Golf Course Environmental Profile, Volume II: Water Use and Conservation Practices on U.S. Golf Courses." ET is a combination of water transpired from vegetation and evaporated from the soil and plant surfaces.

While ET-based scheduling is theoretically a precise way to water, the EIFG report shows that most facilities use direct observations of turfgrass and soil conditions to determine irrigation schedules. It appears most turf managers believe that irrigation is an art, not a science.

Art of Irrigation
Ask a sampling of superintendents and consultants, though, whether scheduling irrigation times by evapotranspiration rates is the ideal way to water turf and most say not necessarily.

"More people are using ET, but only as a tool, and in conjunction with soil probes, moisture sensors, looking at the turf and using their experience," says Brian Vinchesi of Irrigation Consulting. "It’s one way to look at water use – not the best or the greatest."

In addition to measuring ET rates or relying on computerized weather stations, superintendents need to make their own adjustments based on their specific climate conditions.

David Homme, superintendent at The Falls Golf Resort in Chilliwack, British Columbia, has looked into using ET. He doesn’t believe it’s foolproof.

"Watering is and will always be an art," he says. "Using ET rates is a politically sound approach and is possibly a good guideline to helping a superintendent schedule water. At the end of the day, though, the best way to know how much and when to apply water is to be out there and constantly adjusting times, adding, reducing and hand watering.

"I can’t imagine that two courses across the street from one another would be able to use the same ET rate, and then apply the same amount of water and it would have the same effect," Homme adds.

Vinchesi says if you use the ET value as a tool and adjust this value accordingly, it is a great way to schedule irrigation.

"But, if you take the number directly from the weather station and automatically change your schedule without oversight of the value and run times calculated, then it’s a good way to get fired," he says. "The weather station, which provides the ET, could be located improperly, need maintenance or have the wrong crop coefficients in use, so the number should always be screened before adjusting run times."

ET scheduling leaves some questions
Bryan Howell, a former superintendent, says everyone who irrigates does so with some reaction to ET, but the problem is ET rates don’t answer all the questions.

"Most ET calculators are incapable of making the adjustments that must be done," he comments. "Soil condition, root-zone depth, etc., all play vital factors in how long the irrigation must run, or in how much water must be applied in any given cycle. How well does that hump drain? How much water is gained or lost through drainage and run off from the irrigation cycle?

"Since every irrigation zone on a golf course has a micro-climate that is somewhat unique, a moisture meter is an irrigation scheduler’s best friend. The more data and the more management that can be put into an irrigation program, the more conservation minded the facility becomes in respect to water stewardship."

Howell believes computerized ET water management is sales buzz intended to sell controllers and software.

"It is not the end-all that many would have us believe," he says. "Take any golf course that’s a minimum of 5-years old. The putting greens should be the most consistent soil on the course. Take a nine-grid sample on one green and have a complete soil analysis done on all nine samples. You’re going to find differences between samples in adjoining grids, not just opposite sides of the green. This is part of the reason we have such a struggle eliminating hand watering. If we can’t uniformly water and maintain a 10,000- to 15,000-square-oot area without hand applications, how do we propose that a computer can take basic weather information and transform it into a watering schedule for 200 acres?"

No matter how advanced irrigation technology gets, golf courses will still need knowledgeable human decision-makers, he says.

Not to mention, Homme says, turf managers need to consider irrigation system inefficiencies.

"Is each sprinkler head operating at the pressure it’s supposed to? Is the spacing correct and so on," he says, adding most golf courses only get 55 to 60 percent efficiency from their systems. "So, how does the ET rate recommendations make up for these things? The longer a superintendent operates his own course, the more he/she gets to know their site specifics, how the wind will hit certain areas, what dries up the quickest, what never needs water, and so on."

Homme says that ET is also a political issue. "It provides justification to even watering at all, especially in today’s world of dwindling potable water supplies, watering restrictions, and so on."

Thom Charters, superintendent at Bayview Golf and Country Club, in Thornhill, Ontario, agrees with his peers that while irrigation scheduling by ET rates is useful, he relies just as much on his instincts.

"I will adjust the run times on sprinklers to match the daily ET or the cumulative ET over a period of time, but I always want to have the final say of whether I’m going to irrigate," he explains. "I know what’s coming tomorrow in terms of play on the golf course and tomorrow’s weather. ET can’t factor that in; it can only look at recent history to determine what irrigation practices are going to be employed."

Charters says ET gives a historical perspective over the last week of the amount of rain that has fallen, but your soil moisture levels may be dwindling long before your turf shows signs of significant stress.

"Depending on what you’re trying to accomplish with course conditioning over the next few days, you may choose to water or not water," he says. "There are times you can afford not to water and dry things up a bit. Where ET-based irrigation is a lot more prevalent and computer-programming is trusted is in desert situations where you know you are not going to get rain."

At the end of the day, ET rates give superintendents the ability to be more precise, says Mike Huck – owner of Irrigation and Turfgrass Services, a consulting firm.

"I’m from the old-school where I would go around with a soil tube and take three or four samples on a green, see what areas tended to get drier than others, feel them, and then make my adjustments based on that," he explains. "ET is a value, but it’s not the be-all, end-all. We are asking a machine to replicate what a plant is actually doing."