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ASU Research Finds Surprising Inefficiencies In A Phoenix Park's Irrigation Plan

Encanto Park in central Phoenix has playgrounds, walking paths and ponds, not to mention a big golf course. And most of the 222-acre park is covered by grass.

In the middle of the golf course is chain link fence box surrounding meteorological equipment.

"With those two instruments we can obtain the vertical flux of CO2 and water vapor that is moving from the surface to the atmosphere," explained Eli Perez-Ruiz, a geological sciences graduate student with ASU’s School of Earth and Space Exploration. 

Perez-Ruiz was part of a team led by hydrologist Enrique Vivoni. The researchers have been paying close attention to the grass in Encanto Park for more than a year. 

Their findings were published recently in the journal  Geophysical Research Letters. They found a system of irrigating meant to save water was actually causing a lot of evaporation and contributing to carbon emissions.

Vivoni said Encanto Park, like a lot of parks, gets watered at night.

“It’s the idea that if you irrigate when the sun is not out, then you’re potentially conserving more of that water against evaporation," Vivoni said. 

But Phoenix has an exceptionally hot, dry climate, especially this year, when the city had a record 48 excessive heat warning days. On those excessively hot days, Vivoni found all that heat built up in the Encanto neighborhood and got drawn into the cool, moist park at night.

“To our surprise, we find that the park loses a lot of water, not so much because of the solar radiation it receives, but more because the surrounding urban area adds heat to the park," Vivoni said. 

The phenomenon is called the “oasis effect” and it caused a lot of evaporation, even after dark.

But Vivoni’s team noticed something else too. When the soil under the grass got wet from that nighttime irrigation, it started releasing CO2 into the atmosphere.

"This really surprised us," Vivoni said. “When we irrigate at night, we convert this park into a net source of CO2." 

Phoenix’s Parks and Recreation Department prides itself on careful watering. Gregg Bach with the department said the 185 parks in the city get watered by a smart, centralized computer system meant to limit waste.

“If it has rained and we can save a few days where we don’t need to water or water as much, we can adjust it that way," Bach said, adding the department has a good relationship with ASU and will take a close look at Vivoni’s findings. “We’re certainly interested in what the results are and what that might mean for us."

Vivoni said his team’s findings would apply just to hot dry climates like Phoenix’s and mostly just to excessively hot summer days. He said the research also applies more to large parks than to a small, residential backyard. But he says schools with sports fields, homeowners' associations or other managers of large turf areas could be impacted by this research.

“The prior evidence in the literature didn’t have any suggestion that this was a problem, so this is by no means the fault of park managers," Vivoni said. 

Vivoni said more research still needs to be done. He said Phoenix doesn’t need to get rid of all of its big turf fields, which have many other environmental and societal benefits. But, he said, some small changes to watering schedules might make grass in the desert a little more sustainable.

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Katherine Davis-Young is a senior field correspondent. She has produced work for NPR, New England Public Radio, Southern California Public Radio, PRI's The World, Washington Post, Reuters and more.She has a master’s degree in radio journalism from the USC Annenberg School of Journalism.She lives in central Phoenix with her husband, two daughters, and ill-behaved cat and dog. Her side-passions include photography, crosswords and hot sauce.