Removing all high-water-use plants is not the panacea in times of rising temperatures and droughts across the southwest, according to a new study.
Deadly flooding of the Indus River in Pakistan back in 2010 and a heatwave five years later are the two events that motivated Rubab Saher, who has a doctorate in civil and environmental engineering, to study urban climates.
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“You know, these things keep on happening every five or 10 years, but shame on us for not coming up with better ideas and better infrastructures,” Saher said. Unless it’s resolved in every city, I don’t think my motivation will dry out.”
Saher is a native of Pakistan from the Sindh district in Halas. She came to the United States in 2016 for a semester as a research fellow at the University of Utah’s U.S. Pakistan Center For Advanced Studies in Water in Salt Lake City.
Working with like-minded researchers at the center, she said she no longer felt like a “misfit” and went on to obtain her doctorate at the University of Nevada Las Vegas. Now, she’s working on solutions to climate impacts in urban areas, trying to balance wishes with reality as communities heat up.
Amid a multi-decade drought, cities across the American southwest are seeking ways to reduce water use. Las Vegas, a city known for excess, has created a new reputation as one of the most water-efficient cities in the world. The city is recycling most of its indoor water and its ban on nonfunctional grass is the first law of its kind in the nation.
The average rainfall in this Mojave desert is 4.2 inches per year. Water is scarce and lawns watered with sprinklers take too much of the city’s drinking water supply. One square foot of grass requires 78 gallons of water annually — that’s a 10-foot tall column of water. Drip irrigated landscaping on average uses much less, about 13 gallons per square foot per year. However, what kinds of plants the turf is replaced with matters. A lot.
Certain landscapes can contribute to the “urban heat island effect” in which afternoon temperatures in cities, big and small, tend to get 15 to 20 degrees warmer than surrounding rural landscapes due to the plethora of pavement and blacktop radiating heat. Cooling down cities could save lives. The National Institutes of Health reported this year that extreme heat is related to an increased number of deaths in the U.S. between 2008 to 2017.
As a postdoctoral research associate at the Desert Research Institute, Saher led a study this year published in the journal Hydrology looking at common arid landscapes and how they affect temperature. The scientists analyzed three types of sites in Phoenix consisting of low to high water consumption:
- Xeric, desert plants requiring minimal water
- Oasis, a mix of desert and high-water-use plants
- Mesic, a tree and turf grass site with water intensive plants
“And then we estimated the surface temperature, evaporation rates and irrigation water requirement for these three landscapes,” Saher said.
They also measured air temperature and wind speed. Saher said the results were unexpected. The oasis landscape provided the best long-term outcome for both water savings and cooling. It showed 35.6 degrees more daytime cooling than the mesic.
“So this dual personality of … saving water for oasis because oasis has less water than mesic, at the same time contributing to the daytime cooling, was kind of a pleasant surprise.”
So even though the mesic site had the coolest air and surface temperatures overall, it required the most water, and the xeric one was simply too hot. Drought resistant plants retain water, limiting the cooling effect on their surroundings. Air temperatures measured 5.4 degrees higher than in the other two landscapes.
Oasis was the Goldilocks. Saher said this landscape requires light drip irrigation and contributes to cooling through the evapotranspiration of the plants. She said an example of canopy trees in the oasis include Acacia, ghost gum, or shrubs like dwarf poinciana.
Saher suggests a middle ground in the garden — that sweet spot where a few luscious plants, a rain-fed tree and native shrubs can actually cool things down and save water in the long term.
Las Vegas does mitigate the urban heat island effect with some landscaping requirements. According to Bronson Mack, Southern Nevada Water Authority spokesperson, areas converted from grass must include at least 50% canopy coverage.
“That was a trend that was occurring as our community developed and long before we ever implemented a turf conversion program,” Mack said.
Since it is the desert, Mack said they don’t have much of a native plant palette so they look to Utah for some of the state’s native species.
“Especially those that are flowering plants, there’s benefit to the bees, the hummingbirds, the pollinators of the world. It actually is a more efficient and higher oxygen-producing plant choice than going with grass, especially when you consider the amount of fossil fuels that it takes for that grass. You have to consider all of the water the grass uses has to be pumped and delivered, that consumes energy and power.”
Utah is the second-driest state in the nation, just behind Nevada. And Utah has a lot of catching up to do if the state wants to match southern Nevada’s water-conservation feats. As efforts ramp up to conserve water in Utah by ditching the overabundance of lawns, Shaun Moser, Jordan Valley Water Conservancy District’s Conservation Park manager, urges caution.
“Some people think the solution is to rip out all the lawn, and put in nothing but rock and no plants,” Moser said. “I see that as an overreaction and overcorrection in my mind. If we do that, I think you will start to see issues with urban heat island effect, maybe our power bills start to go up a little bit just because rock is not transpiring like lawn is.”
Utah lawns and landscapes suck up much of the state’s drinking water, about 50% to 70%. To use less and still cool down, Moser suggests something like the district’s Localscapes that reduces water use by up to 66%.
“Most of the time if someone installs a Localscape they put lawn in their front and backyard but it’s only like 20% to 30% of the landscape area. And the other part is patios and vegetable gardens and planting beds with trees and shrubs so it’s more of a balanced landscape that conserves water at the same time.”
For those grass holdouts, he said he’s seeing a trend toward warm-season types such as Buffalo and Blue grama that tend to use less water than the common Kentucky bluegrass.
“They are dormant in the spring and fall so that’s the tradeoff.”
A couple of native plants he said to consider are the Bigtooth maple tree and Western Sundancer daisy, a small perennial flower. A non-native tree he recommends is a type of hybrid elm that includes the Frontier, Homestead and Accolade.
“These are plants that do really well in our environment here in Utah and grow fast and give you shade quickly and don’t use a lot of water,” Moser said.
The 100-acre Utah State University Botanical Center in Kaysville is doing its part. It cut its water use by 75% over two years by simply watering less and putting its plants in “survival mode,” said director Jerry Goodspeed.
“We’ve realized a lot of the plants do just fine, to our surprise. … Learning as we go.”
He said the center has a program called Sego Supreme in which it is trying to develop and promote perennials that don’t require a lot of water. In partnership with Colorado State University, it looks for these native flowers in the foothills.
“It’s kind of that win/win situation, where we don’t have to see that plant you grew up with, water plants from the English cottage gardens, the water-loving perennials.”
Goodspeed said he can see the benefit of an oasis type landscape that is water wise and cooling, and also reduces carbon dioxide. And he doesn’t mind a little grass in it — just not too much.
“We are way off on some weird tangent of grass and luscious plants,” Goodspeed said. “There is a balance, people, and we gotta swing this pendulum way back and remember we are living in a desert.”
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