Real-world Watering: Why low precipitation rates aren’t always the answer

It’s no secret that many property owners tend to overwater their landscapes. Some overwatering is due to a lack of knowledge about proper watering practices; some results from simple carelessness.

According to the EPA’s WaterSense Program, the average American home uses about 260 gallons of water a day. However, during the hotter months of the year, that number can increase to a whopping 1,000 to 3,000 gallons per day — that’s the same amount as if a garden hose was left running for eight consecutive hours. Even worse, experts estimate that 50 percent of all water used outdoors goes to waste due to evaporation, wind, deep percolation or runoff. Considering this is happening at homes and businesses throughout the country, the amount of irrigation water we’re wasting is staggering.

The low-down on low-precip nozzles

Most water agencies are aware of these numbers, and many are working with city governments to offer rebates for more efficient irrigation system components, such as weather-smart controllers, rain sensors and drip irrigation. Because almost every irrigation system also features sprays, some communities are now also offering rebates to property owners who replace their standard spray nozzles with those offering a lower precipitation rate. These “low-precip” nozzles emit water at a slower rate; this means that, when operated for the same amount of time as a standard nozzle, they don’t apply as much water to turf.

“In addition to peak demand management, there’s another dominant reason why cities and water agencies are encouraging the use of low-precip nozzles,” said Randy Montgomery, Rain Bird Corporation’s product manager for contractor spray nozzles. “The general belief is that homeowners in particular don’t know how to use their irrigation controllers well enough to change their watering schedules as needed. Or, they simply forget to do so. Water agencies believe that if customers switch out their current nozzles for low-precip versions, they’ll use less water — even if they never touch their controllers. This is certainly true. However, it’s a strategy that relies more on behavioral modification than education, and can still result in inefficient water use.”

This sounds logical in theory. And, as most contractors already know, it’s true that low-precip nozzles can be the right answer in certain situations. Low-precip nozzles are typically a solid choice for lawns with slopes and heavy clay soils, allowing water to slowly soak into the soil to discourage run-off and erosion. However, on other landscapes, low-precip nozzles may not have the desired effect — on either the lawn itself or on overall water consumption.

“Switching to low-precip nozzles may be the right choice for some landscapes,” said Montgomery. “However, every landscape is different, and so is every homeowner. Using a low-precip nozzle may decrease water use, but if water is still running off the lawn, onto the street and into the gutter, it’s still being wasted. In other words, a low-precip nozzle can’t fix a system that was poorly designed, out of adjustment or improperly programmed.”

Because landscapes have different watering needs, there’s a chance that installing a low-precip nozzle may now apply too little water to lawns, resulting in dry, brown areas. “When people see brown spots in their yards, they will often increase their watering times,” said Montgomery. “And, once again, they typically end up overwatering — perhaps more so than before they installed the low-precip nozzles.”

Testing in real world conditions

“We’re experiencing legislative pressure that is not only demanding better-performing nozzles, but also concrete data that proves their performance,” said Montgomery. “It’s really all about efficiency. Unfortunately, most nozzle testing is done indoors under zero-wind conditions. In this situation, you may see nozzles achieve very similar results that lead you to believe they’re basically the same. It’s our belief that if you take those same nozzles outside, the test results will be quite different.”

In 2012, Rain Bird approached the Karsten Turf Research Facility at the University of Arizona in Tucson to see if researchers there had interest in designing and performing a study to scientifically prove the effects of real-world conditions on nozzle performance. They did, and thus, the outdoor study began in May 2012. University scientists expect to complete their research in May 2013. Throughout the study, these researchers are using standard irrigation audit procedures and best practices for the highest level of data integrity. The study’s goal is to examine the effects of wind on nozzle application efficiency and distribution uniformity scores.

“’Application efficiency’ represents how much water actually made it into the nozzle’s target zone as compared to the total amount of water applied during an irrigation cycle,” Montgomery explains. “Distribution uniformity, or DU, is a calculated value that shows how evenly water is distributed to avoid excessively wet or dry areas in the landscape. We wanted to plot how quickly the nozzles’ application efficiency and distribution uniformity scores declined as wind speed increased.”

Armed with that information, Rain Bird can then study nozzle differences to determine which nozzle characteristics contribute most to declines in performance. From there, the company could develop more accurate ways to measure a nozzle’s efficiency in the real world and engineer nozzles that maintain high levels of efficiency, even in windy conditions.

“While the study is still underway, we have received a progress report on the tests done between May 16 and August 10, 2012,” said Montgomery. “The report supports our belief that two nozzles which appear identical when tested inside perform very differently when tested outside, and low-precipitation rate is only one of many factors that contribute to a nozzle’s overall efficiency.”

The results thus far

During the summer of 2012, researchers compared Rain Bird’s HE-VAN (high-efficiency variable arc nozzles) to a low-precipitation-rate nozzle. They installed each nozzle into four 12-foot by 12-foot plots with a grid of catch-cans and a water meter to measure how much water the nozzles applied. For 30 days, a weather station measured key data points for each half-inch irrigation cycle. The researchers took readings on the same nozzles under different, naturally occurring wind speeds, allowing them to prove that the effect of wind on test results is consistent and statistically relevant. Using the catch-cans—especially on the downwind edge of the irrigation target zone — they were able to determine the degree to which varying wind speeds disturbed and shifted spray patterns.

Under those conditions, the high efficiency variable arc nozzles had an application efficiency score of 80 percent, compared to the low-precip nozzles’ 63 percent. As wind speed increased, the low-precip nozzles’ lower quarter DU — a measure of how evenly water is applied to an irrigation zone — dropped 13 more percentage points than the HE-VAN’s.

According to Montgomery, when wind is able to push sprays out of their target zone, users typically have to run their sprays longer to compensate for the dry areas that did not receive enough water.

The bottom line?

It’s important to approach the issue of overwatering from all angles rather than just one variable — such as precipitation rate.*

“We’re not discounting the need for low-precipitation-rate nozzles,” said Montgomery. “In certain applications, a low precipitation rate nozzle is very important. On slopes, this type of nozzle fights erosion and run-off. On clay soils, these nozzles put water down slowly enough for the clay to absorb it. But in locations where wind speeds typically exceed three miles per hour, the design of certain low-precip nozzles may make it necessary to run irrigation systems longer to make up for the water that the wind blows away.

“To properly irrigate and maintain the healthiest, most attractive landscape while minimizing water waste, you have to account for more than just precipitation rate. You have to consider nozzle type, vegetation needs, soil types, local environmental conditions and other factors when establishing an irrigation schedule. So far, the University of Arizona’s study proves that there’s no one nozzle that adequately provides a “one-size-fits-all” approach to efficient irrigation.”

Article provided by Rain Bird.

* Rain Bird offers a downloadable Nozzle Solutions Guide at that walks users through all those variables to find the best nozzle for their particular landscape’s needs.

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