By Deborah Netburn
2 August 2016
(Los Angeles Times) – Last summer, a revolution occurred in Los Angeles landscaping: Across the city, tens of thousands of homeowners tore up their water-thirsty lawns and replaced them with gravel, turf, decomposed granite and a wide range of drought-tolerant plants at a rate never seen before.
The water-saving benefits of this massive landscape overhaul are undisputed — and that’s important in a region that was facing what researchers called the worst drought in 1,200 years. Still, some experts warned that Angelenos’ zeal to use less water in the backyard would ultimately make the city warmer.
“Gardens and lawns act as air conditioning for L.A., which is only getting hotter with climate change,” landscape designers Mia Lehrer, Claire Latané and Margot Jacobs wrote in a 2015 op-ed in the Los Angeles Times. “Plants and trees provide shade and transpire moisture to cool the air; gravel and artificial turf don't.”
The trio advocated replacing water-hungry landscapes with living landscapes that require almost no irrigation at all. But an environmentally sensitive Angeleno might still find herself confused: Could even savvy drought-tolerant landscaping cause the city to heat up?
Now there is a scientific answer to that question.
In a paper published Monday in Geophysical Research Letters, University of Southern California post-doctoral research associate Pouya Vahmani and USC civil and environmental engineering professor George Ban-Weiss analyze what would happen to the city’s overall temperature during the month of July if every lawn were replaced with drought-tolerant plants.
After running different land-cover scenarios through a climate model tailored to Los Angeles’ unique mix of coastal breezes, mountains and expansive urban cover, the authors report that a lawn-less Los Angeles would be up to 3.4 degrees warmer during the day than it is now. As expected, this warming would mostly be due to decreases in irrigation, the researchers said.
But there’s a flip side. The authors also report that transforming lawns to drought-tolerant vegetation has an average nighttime cooling effect of about 5.4 degrees that more than makes up for the daytime warming.
“We hypothesized that our model would predict daytime warming, but we did not anticipate the nighttime cooling signal,” Ban-Weiss said. “In retrospect, it makes sense that reducing soil moisture would change the thermal properties of the soil and surface-atmosphere coupling in this way.” [more]
ABSTRACT: During 2012-2014, drought in California resulted in policies to reduce water consumption. One measure pursued was replacing lawns with landscapes that minimize water consumption, such as drought tolerant vegetation. If implemented at broad scale, this strategy would result in reductions in irrigation, and changes in land surface characteristics. In this study, we employ a modified regional climate model to assess the climatic consequences of adopting drought tolerant vegetation over the Los Angeles metropolitan area. Transforming lawns to drought tolerant vegetation resulted in daytime warming of up to 1.9 ° C, largely due to decreases in irrigation that shifted surface energy partitioning toward higher sensible and lower latent heat flux. During nighttime, however, adopting drought tolerant vegetation caused mean cooling of about 3 ° C, due to changes in soil thermodynamic properties and heat exchange dynamics between the surface and ground. Our results show that nocturnal cooling effects, which are larger in magnitude and of great importance for public health during heat events, could counterbalance the daytime warming attributed to the studied water conservation strategy. A more aggressive implementation, assuming all urban vegetation was replaced with drought tolerant vegetation, resulted in an average daytime cooling of 0.2 °C, largely due to weakened sea-breeze patterns, highlighting the important role of land surface roughness in this coastal megacity.