The over watering of trees and lawns caused Los Angeles to lose 100 gallons of water per person per day to the atmosphere through evaporation in the summer of 2010, a new study found. Moreover, over-irrigation resulted in the loss of 70 billion gallons of water per year.
Lawns accounted for 70 percent of the water loss, while trees accounted for 30 percent, according to the study funded by the National Science Foundation and published in the journal Water Resources Research.
The study, based on measurements before Los Angeles mandated watering restrictions in 2014, show a “pattern of systemic overwatering of the city’s lawns, and a surprising water efficiency of its tree cover.” The researchers found a correlation between water loss and household income.
“California’s recent drought highlights the need for urban water conservation,” says Tom Torgersen, program director in the Division of Earth Sciences in NSF’s Geosciences Directorate.
Researchers measured evapotranspiration (ET), the evaporation of water from the soil and the transpiration, or release of water vapor, from plants
In Los Angeles, the greatest ET was due to turf grass and seed-producing trees.
“Both provide an alleviation of the urban heat island effect and reduce the need for air conditioning,” Torgersen says. “However, the benefit is not evenly shared. The higher the median income, the greater the local ET, with cooler temperatures in wealthier areas and higher temperatures in poorer sections of the city.”
ET rates in the wealthiest neighborhoods, they found, were roughly twice those of poorer neighborhoods.
Trees use far less water than grassy lawns, the study said. Under dry conditions, trees will rein in transpiration so they can retain water.
“It’s surprising that we can maintain the tree canopy of L.A. with relatively little water,” said Diane Pataki of the University of Utah, one of the researchers in the study. “There’s this assumption that we need abundant irrigation to support trees. But we can drastically reduce water use and still have trees.”
Los Angeles’ watering restrictions were lifted this spring after a very wet winter. At present, it’s too early to tell whether Los Angeles residents’ watering patterns and landscaping choices will return to pre-drought excesses, Pataki said.
“Whether the drought changed people’s landscape preferences in a lasting way, that’s something we still need to find out,” she said.
Source: National Science Foundation