Nature’s Swamp Cooler : Research: Two neighborhoods, one lush with greenery the other more paved and unshaded, are the focus of a new study on the cooling influence of trees and plants.
A canopy of towering deodars, oaks, eucalyptus and palms envelop the winding, residential streets in Arcadia around the Los Angeles County Arboretum. Beds of ivy, roses and lilies ring ever-green lawns, surrounded by thick clumps of oleander.
South, near the San Gabriel Mission, the trees are fewer and thinner, lawns are smaller, and shrubbery is sparse along the streets with tidy, single-family houses, apartments, condominiums, stores and office buildings.
An innovative study on the cooling influence of trees and plants is examining these two neighborhoods for evidence that urban greenery provides more than eye-pleasing surroundings and environmental correctness--that it can actually cool whole neighborhoods to the point where people could turn down their air conditioners a bit.
“It’s real obvious that if you sit in the shade, it’s better--and cooler--than if you sit in the sun,” said Paul R. Doose, who is heading a team of scientists for Rosemead-based Southern California Edison Co.
Less obvious, and less understood, he said, is the amount of moisture released from urban vegetation and the indirect cooling benefit it has--in other words, whether tree sweat can cool people off. In addition, he said, there is little hard data on how trees and plants influence the meteorology of a neighborhood and how they can make it cooler in warm months.
So from the utility’s standpoint, he said, it may be cheaper to plant trees or financially reward its consumers for planting trees, instead of constructing expensive power plants that would generate electricity to fuel air conditioners.
Edison is spending $200,000 to $300,000 this year to begin its study in the western San Gabriel Valley with the help of scientists from the U.S. Forest Service and Indiana University.
The utility, which supplies electricity to customers in a 50,000-square-mile area from Bakersfield to the Mexican border, plans to extend the research over the next four years to broader swaths of Southern California, from the mountains to the desert to the sea.
Next year, the climatologists plan to examine Long Beach and the San Fernando Valley to see how a coastal neighborhood contrasts with an inland valley. Other future sites may include Palm Desert and San Bernardino.
For Edison, Doose said, the results will help the utility to make its case before state regulators if it decides to start a tree planting program.
“The reason this project is so important is it’s the first look of its kind at the indirect benefits of vegetation in urban neighborhoods,” said Daniel G. Levitt, an Arcadia-based Forest Service research meteorologist on the project.
Studies have shown for years that cities tend to hold heat more than rural areas because asphalt and building materials absorb and retain warmth in an effect known as the “urban heat island.” Other studies have looked at the climate created by farm fields and forests.
But rather than measuring and contrasting large areas, this study will examine urban microclimates, the differences that can be created within the same urban area just by changing the greenery.
There have been no urban studies “as detailed as this will be,” said Sue Grimmond, an Indiana University climatologist who is directing part of the project and has studied the effects of urban vegetation in Vancouver, B.C., Sacramento and Chicago.
From a meteorological standpoint, “we know relatively little about how a whole neighborhood behaves,” Grimmond said.
As Levitt eyed a grand eucalyptus near the Arboretum, he explained in concrete terms the effect vegetation can have. A eucalyptus, he said, can give off 100 gallons a day in moisture.
A mature oak tree might give off as much as 50 gallons of water in a day, he said, and a thickly leaved orange tree, 10 gallons.
That much water being released into the air can have a considerable cooling effect on the climate around a house and can also contribute to the overall cooling of a neighborhood. It works much like the way perspiration cools off people: the water evaporates, taking some of the heat’s energy in the process.
“It’s like having a big swamp cooler in the neighborhood,” Levitt said of the effects of trees, shrubs and plant life.
Because Southern California tends to have dry, relatively low humidity days in the warmer months, he said, this “natural” swamp cooler can work quite effectively without generating so much moisture that the air becomes uncomfortably humid.
The study specifically is looking at the upward flow of moisture as it evaporates from lawns, shrubs, plants and trees.
The two neighborhoods in the first study each encompass about one square mile. One is centered on Arcadia and the part of unincorporated Los Angeles County sandwiched between Pasadena and Arcadia. The other is located in San Gabriel and Alhambra. The two were chosen because of the vast contrast in greenery--so striking that it shows up on photographs taken from space satellites.
In each neighborhood, researchers are setting up a series of weather station towers at two levels: one about 6 to 10 feet high in residential back yards to measure localized areas, and the other at 100 feet, above the treetops, to measure the overall neighborhood.
The two types of stations will measure moisture that comes from irrigation and swimming pools as well as from trees and other plants in the process known as transpiration.
The stations, linked by modems and telephone lines to the meteorologists’ computers, daily record detailed weather information on temperature, humidity, wind and solar radiation.
In addition, four university students this summer are conducting field surveys in neighborhoods throughout the western San Gabriel Valley, including the two with towers.
Leaf by leaf, house by house, block by block, neighborhood by neighborhood, the Indiana University team is studying 25 different communities to compare differences among the trees, plants and shrubs and to flesh out a more regional weather picture of an inland valley.
This broader study is looking at the contrast between the large, lush estates of San Marino and the shadeless, industrial streets of South El Monte. On the east, the study goes as far as Monrovia; on the north, Altadena; the south, Monterey Park, and the west, Pasadena.
To help understand how urban features might capture or reflect heat from sunlight, the surveys are assessing the percentage of streets, sidewalks and driveways in an area, the type of roofing and building materials, and the color of the houses and other buildings. Examination of the greenery might help determine which plants give off the most moisture with the least watering required.
All of this information will be used to help create an accurate portrait of neighborhood weather and can be fed into computer and mathematical models for Southern California Edison.
The scientists still are gathering their data and have not analyzed it. The assumption, Levitt said, is that the differences will be slight but essentially due to the contrast in vegetation.
Even so, he said, “if you have two neighborhoods where the temperature difference is say only 2 degrees apart, that still can make a big difference in the energy savings.”
