Removing grass may increase urban heat, study finds

The Southern Nevada Water Authority runs what is likely the longest-running program to motivate homeowners to replace water-thirsty grass with desert landscaping, but a new study says that while the move may save water, the price could be a superheated city.

In a new study, a team of researchers investigated the microclimate effects of three common landscape types in an arid region of Phoenix, Arizona.  Scientists found that desert landscaping had the lowest water requirement but the highest temperatures. Air temperatures in the desert landscape plot averaged 5.4 degrees Fahrenheit  higher than in the other two landscape types.

“Removing turf grass from the landscape is an excellent approach for saving water, but if we remove all the turf grass, the temperature will go up,” said the study’s lead author Rubab Saher, a postdoctoral research associate at the Desert Research Institute.

While the potential water savings from converting turf grass to more water-efficient landscapes has been extensively analyzed by water managers—including the SNWA—little research has been conducted into other possible impacts, said Saher.

In most cases, high-water-use plant species, such as turf grass, are replaced by “gravelscape,” a cheap and easy water-saving landscape typically made up of light colored crushed rock dotted with drought tolerant plants.

These desert landscapes, otherwise known as xeriscaping, need little or no water beyond what the natural climate provides. Water is further conserved in xeriscaping using drip irrigation which feeds minimal amounts of water directly to the base of drought resistant plants and prevents water evaporation.

Plants cool the environment by releasing excess water back into the atmosphere, however, drought resistant plants have adaptations meant to retain water, making them poor at cooling their surroundings. 

“Think of it as the mist, big restaurants in the Las Vegas valley have to cool the air, that’s the same kind of system plants have,” Saher said.

With temperatures rising an average of 5.76% since 1970, Las Vegas is one of the fastest-warming cities in the U.S. and is projected to warm by as much as 10 degrees Fahrenheit by the end of the century, according to the Southern Nevada Water Authority.

Removal of water intensive trees and turf-grass, or “mesic,” landscaping will likely lead to higher air or surface temperatures for the city.

It takes time to research appropriate plants to install a drought tolerant yard while preserving cooler temperatures, said Saher.

Finding the balance between water conservation and protecting communities from soaring temperatures led Saher and other researchers from Arizona State University and the University of Nevada, Las Vegas to conduct their study.

The study provides a glimpse of how improving landscaping choices can reduce water use and provide daytime cooling, said Saher.

While the “mesic” tree and turf-grass landscape had the lowest surface and air temperatures, both in the daytime and nighttime, it also showed the highest water consumption rate, according to the study. 

“Vegetation has a very significant role in reducing the urban heat island effect but we need to be smart about our water resources,” Saher said. “No vegetation is not a great idea for any type of city.”

In the last two decades, Lake Mead has dropped 180 feet due to the rapid growth of cities drawing water from the Colorado River and the ongoing megadrought in the Southwest, making every drop of water conservation count.

Grass consumes about 73 gallons per square foot, per year, much more than the 18 gallons needed to sustain drip-irrigated landscapes, according to SNWA. As drought increasingly threatens water supplies to Las Vegas, there is a consensus that ornamental grass is no longer sustainable.

“Are we going to get to the point where there’s going to be no landscaping outside, no grass? That’s very possible,” said Nevada Gov. Steve Sisolak last month during a tour of Southern Nevada Water Authority’s $1 billion dollar low lake level pumping station at Lake Mead.

Last year, Nevada lawmakers passed a law that essentially banned ornamental grass from most public spaces by the end of 2026.

Park strips — the narrow space between the sidewalk and the street typically filled with grass — will need to be removed across Southern Nevada, a move that may increase temperatures in pockets of the valley that benefited from turf’s cooling effect, according to the study.

Researchers, however, say there are strategies that can mitigate temperature increases as a result of removing water intensive grass and trees.

Researchers analyzed what they called “an oasis landscape,” a lot that used a mix of high- and low-water-use plants. The mixed landscape provided both lower irrigation water requirements than the “mesic” site but more daytime cooling than the xeric desert landscape.

The study found that the average surface temperature of the mixed oasis landscape was comparable to the water intensive tree and grass landscape. Still, the lowest surface temperature in the mixed oasis landscape was 5 degrees celsius higher than that of the “mesic” tree and turf-grass landscape.

The lowest surface temperature for the desert xeric landscape was 7 degrees celsius higher than the “mesic” landscape and 3 degrees celsius higher than the “oasis” landscape.

On average, the desert xeric landscape was 3 degrees celsius higher in temperature than the “mesic” and “xeric” landscapes, due to less water being released into the atmosphere and an increase in air temperatures.

“For every acre of turf grass removed, we also need to plant native or rainfed trees to make arid cities livable in the long run,” said Saher.

Trees that require lighter irrigation but also contribute to cooling through evaporation and a dense canopy, including Acacia, ghost gum, or shrubs like dwarf poinciana, are the best solution, said Saher.

A recent study by American Forests found that more than 560,000 trees will need to be planted in Southern Nevada to reach “tree equity,” or the number of trees needed so that all residents can benefit from shade, cooler temperatures and reductions in carbon by trapping it in the soil.

“What we wish, ideally, is different from our reality. We need to make sure not to threaten our sustainability in the long run,” Saher said.