From the shade of a flourishing mesquite tree, Ron Stoltz peered up at the wild orchids that were climbing the south wall of a three-story, metal building. Below them, water drizzled out of a rectangular fountain that fed a man-made pond surrounded by lilies and desert grass.
“This garden used to be a parking lot,” Stoltz said with a slight smile.
In 2009, the designers of The Underwood Family Sonoran Landscape Laboratory won an award for their work from the Arizona Chapter of the American Society of Landscape Architects.
Credit: Joe Abraham, CLIMAS, The University of Arizona
The garden, the Underwood Family Sonoran Landscape Laboratory, sits at the base of The University of Arizona’s School of Landscape Architecture in Tucson and illustrates how landscape in the arid Southwest can serve a purpose in a future stressed for resources.
Supplying vegetation with water, combating urban flooding, and reducing the urban heat island (UHI) effect were among the challenges that Stoltz, the director of the School of Landscape Architecture, emphasizes.
The UHI effect results from the heat that is absorbed and radiated by concrete and stone in heavily developed areas. It can increase city temperatures by as much as 10 degrees Fahrenheit compared to surrounding rural land, according to the U.S. Environmental Protection Agency. This increase can be exacerbated by human-induced climate change, further raising the threat of heat-related illness in urban areas.
By preventing sunlight from being absorbed into the ground, the mesquite trees that surround the garden help reduce the impact of the UHI effect and create a cool environment, Stoltz said.
Perennial vegetation like trees can also help remove carbon dioxide from the atmosphere. The U.S. Department of Energy has shown that through photosynthesis 100 ten-year-old trees can remove one ton of carbon dioxide from the atmosphere each year, according to the American Society of Landscape Architects. This number is a rough estimate, however, because the type and size of the tree and quality of soil also play a role in its sequestering capability.
When deciding which trees to use, Christy Ten Eyck, the landscape architect who designed the garden, opted for those normally found in southern Arizona.
“I like being authentic to the region,” she said, adding that native species are better equipped to withstand the harsh climate.
The garden is generally comprised of mesquite, willow, and palo verde trees that are native to the area. However, some imported ash trees line the north border of the property, which abuts Speedway Boulevard, to keep continuity with previous plantings along the street.
Urban areas in the Sonoran Desert are known for flooding and the design of the garden helps these trees more sustainably reduce the UHI effect and remove carbon dioxide from the atmosphere by keeping more rainwater around the trees instead of it running off into urban drainage systems. During the monsoon season, rain often falls at an intense rate for short periods of time. With much of the ground covered with non-absorptive concrete and asphalt, the rain water has nowhere to go and runs down streets, causing flooding.
“The water in front [of the building] pools into micro basins,” Stoltz said. These basins are small sinks surrounding trees where water collects and absorbs into the ground rather than running off into drainage systems. Because water pools in basins expose less surface area than sheets of water in streets, there is also less evaporation.
Around back, there is more evidence of attempts to keep the water where it falls. Urbanite, a ground cover made of broken-up chunks of brick and concrete, is scattered across pockets of drainage. When blended into the surrounding environment it not only looks good, but it also serves to slow down water run off so the ground has more time to absorb it.
Rain only comes to this arid desert a few times a year. The garden looks to the building for the rest of the water it requires.
The garden will “live off of the waste of the building,” Ten Eyck said. Because of several reuse and harvesting methods, the amount of drinkable water that the garden requires has been reduced by roughly 83 percent. Once the garden is fully established, Stoltz plans to completely wean the plants off potable water.
Of the 280,310 gallons of water required to irrigate the garden in a year, approximately 84,000 gallons are harvested from the building’s roof run-off. This mixes with roughly 95,000 gallons of condensate that is collected from rooftop air conditioning units before it flows into a holding tank that is fed by a nearby well.
Water is harvested from the air conditioning units on top of the adjacent building to reduce potable water for the garden and pond.
| Enlarge This Figure |
Credit: Joe Abraham, CLIMAS, The University of Arizona
When the tank is full, excess water gushes out of a rectangular fountain and spills into the pond where it overflows and meanders its way through the garden. If there is little water pressure in the tank, potable water is supplemented.
Although water from the roof accounts for most of the garden’s needs, it is not the only source.
Every weekday, Louis Muschette, a senior engineer at the UA, backwashes the holding tank and chlorinates the well water to make it suitable for drinking.
The process used to waste nearly 200 gallons of water a day, but Stoltz and some graduate students found a new use for it. Now, Muschette sends that wastewater into a corrugated aluminum container before it is gravity-pumped underground and bubbles up into the garden’s pond.
Roof run-off, condensate, and well blow-off, combined with the building’s graywater—wastewater not derived from toilets or food waste—comprise approximately 230,000 gallons of irrigation water a year.
The garden landscape and the inner-workings of the adjacent building were intended to be a learning laboratory in which students and the public could look inside and see how things work. Future generations of students will be able to use it as a classroom as well as an oasis.
Like the vines climbing the south wall, this project will continue to grow. The future holds plans to place 10 kilowatts of solar power panels on the roof, and to cover the remaining roof space with vegetation to create a roof garden or green roof that can help cool the building and reduce the roof’s contribution to the UHI effect.
South of the garden, heat radiates off the black asphalt that still dominates the landscape. Stoltz has more plans for what is left of this parking lot. He plans to tear it up and build a public green space. It will serve as a cool, natural environment where people can take a break on a hot Tucson afternoon.
The American Society of Landscape Architecture Statement on Climate Change
| http://www.asla.org/pdf/climatechange.pdf |
American Planning Association paper on the use of city parks for climate change management
U.S. Environmental Protection Agency report on reducing the urban heat island effect with trees and vegetation
| http://www.americanforests.org/resources/climatechange/ |