In summer 2010, Los Angeles was losing about 100 gallons of water per person per day to the atmosphere through the evaporation and plant uptake of lawns and trees.
Lawns accounted for 70 percent of the water loss, while trees accounted for 30 percent, according to a University of Utah study published in Water Resources Research.
The results, based on measurements taken before Los Angeles enacted mandatory watering restrictions in 2014, shows a pattern of systemic overwatering in the city’s lawns, and a surprising water efficiency in tree cover.
Evaporation + Transpiration = Evapotranspiration, or ET The water loss that Litvak and Diane Pataki, professor of biology, measured is best described as "evapotranspiration," a measurement that adds together the evaporation of water from soil and the release of water vapor, called transpiration, from plants.
Evapotranspiration (ET) rates depend on several factors, including plant type, temperature, humidity and the amount of water in the soil.
And according to Litvak’s measurements, LA’s soils were an abundant source of water.
Water loss from an over-irrigated lawn is that much and more, because transpiration from the grass pumps even more water from the soil to the atmosphere.
To measure ET from lawns, Litvak devised a shoebox-size chamber that could measure rapid changes of the temperature and humidity above the grass.
"I have been surprised that we can maintain the tree canopy of LA with relatively little water," Pataki says.
"Whether that changed people’s preferences for landscapes in a long-lasting way — that’s something we still need to study."