Drought conditions have persisted in California since 2012, prompting Los Angeles and other cities throughout the state to encourage the replacement of thirsty lawns with plants that require less water. This switch could significantly affect local urban temperatures, according to a new study by Vahmani and Ban-Weiss.
In Los Angeles, programs such as Cash for Grass offer incentives to residents willing to pull up their lawns and lay down native plants, which thrive with less irrigation. Despite the success of similar programs already in place in dry cities in other states—including Phoenix, Ariz., and Las Vegas, Nev.—little is known about how such efforts affect the urban climate.
The researchers used a climate model described in a previous study that accounts for the region’s complex mix of varying urban land cover, coastal sea breezes, and surrounding mountains to investigate the effects of replacing lawns in the megacity of Los Angeles. Using the model, the team simulated scenarios in which existing vegetation was replaced with native plants and irrigation was stopped completely.
The scientists found that replacing lawns with drought-tolerant vegetation raised summer daytime temperatures by up to 1.9°C in their models. This rise was mostly due to decreases in irrigation and subsequent reductions in evaporation of water, which acts as an air conditioner to keep temperatures down.
However, the model also showed that hotter daytime temperatures were balanced out by cooler summer nights. A switch from lawns to drought-resistant vegetation lowered nighttime temperatures by an average of about 3°C because of changes in soil moisture that altered ground heat fluxes. The nighttime cooling effects of decreased irrigation could help the Los Angeles region’s 13 million residents recover from the heat of the day during extreme heat events—an important public health consideration.
The authors note that their study reflects an extreme scenario in which irrigation stops completely. Nonetheless, the findings could help scientists better predict the effects of lawn replacement efforts in Los Angeles and ensure that such programs will actually help, and not harm, the region’s climate. (Geophysical Research Letters, doi:10.1002/2016GL069658, 2016)
—Sarah Stanley, Freelance Writer