A major effort to remove grass lawns across the Las Vegas Valley has saved 203 billion gallons of water, but environmental research shows that the large-scale change has also brought a serious rise in local surface temperatures. According to data from the Southern Nevada Water Authority’s Water Smart Landscapes Rebate programme, property owners across the metropolitan area have removed 250 million square feet of non-functional grass since 1999. The programme offers financial incentives of up to $5 per square foot to help residential and commercial property owners replace water-heavy lawns with desert landscaping and drip irrigation. The push gained legal backing after the Nevada Legislature passed Assembly Bill 356, alongside earlier laws including the Nevada Revised Statutes Water Conservation and Colorado River Act.
The law requires decorative, non-functional grass to be removed from commercial properties, multi-family buildings, government sites and homeowners association properties across the region. However, scientific research into the effects of large-scale grass removal has found that getting rid of lawns can also raise local surface temperatures, particularly during the hottest months of summer.
A study published in the academic journal Urban Climate found that removing turf grass in the Las Vegas Valley increased summer land-surface temperatures by 20 ± 0.3 Kelvin, or about 20°C.
“Turf grass removal increased temperatures by 20 ± 0.3 K, while urbanization, in general, increased LST by 5 ± 0.3 K,” the researchers wrote in the Urban Climate study. Researchers examined surface energy, sunlight reflection and evapotranspiration across urban, rural and barren parts of the dry valley. Grass lawns naturally cool their surroundings by releasing water through evapotranspiration. Replacing them with bare gravel, rocks, artificial grass or hard surfaces removes much of that cooling effect. They also noted that commercial buildings and asphalt roads remain the main sources of the wider urban heat island effect across the valley. The researchers found that grass keeps land surfaces cooler, but it requires large amounts of water to do so. This creates a difficult choice for Las Vegas between protecting its limited water supply and reducing heat in neighbourhoods.