Can removing grass cause temperatures to rise? UNLV study shows impact of plants on heat

For years, people in southern Nevada have been encouraged to tear up their lawns and install drought-friendly desert landscaping to combat the crippling drought along the Colorado River.

But a new study examines whether this push to conserve water will cause temperatures to rise in desert cities such as Las Vegas, Phoenix and others in the American Southwest.

In a study published this week in the journal Hydrology, researchers from the Desert Research Institute, UNLV and Arizona State University analyzed data from three plots of land on the Arizona State campus on the hottest day of 2011, each providing a type of different landscape and different water needs. This included water-intensive trees and grass, called mesic; low water native desert landscaping on drip irrigation, called xeric; and a mix of low and high water use plants such as acacia and ghost gum trees and various shrubs that use sprinklers and drip, called oasis landscaping.

The oasis landscape’s mix of water-intensive and water-intensive plants provided the best of both worlds by keeping daytime temperatures cooler and providing denser shade than desert landscaping while requiring less water than grass and trees, according to the study. Desert landscaping used the least amount of water, but it also produced temperatures an average of 5.4 degrees Fahrenheit higher than other landscape types.

“While these strategies may appear to have immediate benefits, such as reduced per capita water consumption, their long-term benefits are questionable,” the researchers wrote.

Balance between preservation and heat

The landscape study’s lead author, Rubab Saher, a research associate at the Desert Research Institute, said she hopes the results will help homeowners, commercial users and city planners better understand the impact of the turf removal on temperatures in urban areas while balancing the water dilemma that has gripped the Southwest.

“We are in the middle of a crisis and we have to be aware,” Saher said. “I hope we can all find solutions to improve our conditions.”

Since 1999, the Southern Nevada Water Authority has encouraged residents and businesses to rip up lawns, with the current program offering $3 for every square foot of sod converted to desert landscaping. Since then, over 4,700 acres of grass have been replaced across the valley.

To step up conservation efforts in the area, Nevada passed a law in 2021 that requires the removal of an estimated 4,000 additional acres of so-called decorative grass in the Las Vegas Valley by 2027, a move that , according to the water authority, would save about 10% of Nevada. 300,000 acre-feet Colorado River subdivision.

Managing the urban heat island effect is something the water authority is well aware of, said Colby Pellegrino, the authority’s deputy chief resource officer.

To receive the water authority rebate, the converted area must be at least 50% covered in plants once they mature, and Pellegrino said they encourage people to plant trees in plots converted to offset the warming trend that accompanies grass removal.

“Despite the fact that we have these programs to get out and remove the turf, we have a lot of systems in place to make sure that what they put in place will withstand the rise in temperature,” Pellegrino said.

concrete jungle

The biggest driver of these warming trends, Pellegrino said, is the urban design of cities like Las Vegas, where heat is trapped in concrete and asphalt during the day and is slowly released into the air at night. .

Average annual temperatures in Las Vegas have risen 5.9 degrees since 1970, according to the latest research from Climate Central, ranking as the second fastest warming city behind Reno. And the nights have gotten even warmer, with average Las Vegas summer nights now 9.5 degrees warmer than they were in 1970, according to Climate Central.

“Landscaping cannot overcome the impacts of the built world in cities,” Pellegrino said. “But the proper use of shadow is our best tool. And I don’t think it makes sense to sacrifice water use, especially in the Mohave Desert, when there are other suitable alternatives to get us to the same goal.

Contact Colton Lochhead at [email protected] Follow @ColtonLochhead on Twitter.

Denise W. Whigham