Encanto Park in central Phoenix has playgrounds, walking paths and ponds, and a golf course, and most of the 222-acre park is covered by grass, according to an article on kjzz.com. 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 in the article.
Perez-Ruiz, a geological sciences graduate student with Arizona State University’s School of Earth and Space Exploration, 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.
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 in the article.
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. 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.”
Gregg Bach with Phoenix’s Parks and Recreation 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 in the article. He added that the department has a good relationship with ASU and will take a close look at Vivoni’s findings and what it may mean for them.
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, sports fields, homeowners’ associations and other large turf areas. Vivoni said more research still needs to be done but that ultimately some small changes to watering schedules might make grass in the desert a little more sustainable.