But the plants on greenroofs can absorb some of this water - "like a sponge being saturated," says Olyssa Starry, a graduate student at the University of Maryland at Baltimore County.
studied a greenroof atop a Baltimore building in comparison to a similar building without a greenroof to determine how well the roof would absorb water from frequent storms.
By measuring water flowing out of building downspouts, she
found that the greenroof retained from 30 to 75 percent of water from storms, compared to a negligible amount retained by the building with no greenroof.
results are preliminary, Starry thinks that cities can reap benefits from making greenroofs a part of their building requirements, as cities like Toronto and Berlin have recently done.
Using GIS satellite imagery, she
estimated the number and area of buildings that could hold greenroofs within one watershed in the Baltimore area.
If all these roofs were greened, she
says, the city could save the watershed 8 million gallons of water per year, or about 10 percent of its yearly water loss.
"We need to understand what implementing these greenroofs at the whole watershed scale can do," she
Olyssa Starry: Latebreaking Urban Ecosystems Poster Session, Fri., Aug. 7, 8:30-10:30 a.m.