Paved paradise? Impervious surfaces affect a region's hydrology, ecosystems—even its climate

Science News, Sept 4, 2004 by Sid Perkins

In four streams that flow into and through Anchorage, Frenzel and his colleagues found that sites within the boundaries of an undeveloped military reservation upstream of the city--and therefore not affected by any impervious surfaces--were home to a diversity of insect larvae and other invertebrates. Many of the species found in those waterways don't survive well if their ecosystem is disrupted. At downstream sites surrounded by areas with impervious surfaces, invertebrate biodiversity was diminished and species known to tolerate some degree of ecosystem disruption, such as sediment-dwelling worms, dominated, Frenzel and his colleagues reported in the July 2003 Hydrobiologia.

NO SWEAT When a stream's velocity increases as a result of urban runoff, its channel often carves deeper into the terrain, says Deborah Slawson, a consulting ecologist in Glenville, Pa. In such instances, the rushing water doesn't spill out among streamside vegetation that would otherwise provide havens for some aquatic life. Many such streams lack shallows where fish can spawn.

Diverting urban runoff through storm drains rather than into streams creates a different set of problems, Slawson adds. If the amount of water that soaks into permeable areas of a watershed and recharges its aquifers is greatly reduced, then the underground supply of water that nourishes streams during dry seasons is diminished as well. In that case, small streams become more susceptible to temperature fluctuations when they receive warm runoff and are less capable of diluting pollutants washed into the waterway.

Increasing the amount of runoff from an urban area influences the local climate. If precipitation doesn't soak into the urban landscape, it isn't available to absorb heat, evaporate, and thereby cool the city, explains David J. Sailor of Portland State University in Oregon. By losing its capacity to retain soil moisture, a city, in a sense, loses its capacity to sweat. Other factors contributing to this heat-island effect include heat emitted by industrial activity, gasoline combustion in the city's automobiles, and the use of utilities such as electricity and natural gas.

Portland creates only a moderate urban heat island, says Sailor. Temperatures in a 2-[km.sup.2] industrial district near the city's core, on average, are between 0.5[degrees]C and 1.0[degrees]C higher than those in areas outside the city. About 29 percent of that industrial district is; now composed of rooftops, and construction on vacant lots there may boost that figure to 57 percent by 2050.

To avoid greatly increasing the temperature, some environmentalists propose a solution consistent with urban development: Buildings could be topped with soil and vegetation, which act as a reservoir for precipitation that would otherwise flow out of the city.

If all the buildings in Portland's industrial district converted their impervious roofs to so-called ecoroofs, in which vegetation covers at least 75 percent of the rooftop, then daytime temperatures in the district could be as much as 0.5[degrees] (2 cooler than they would be otherwise. Those cooler temperatures could lower the demand for air conditioning in the summer and reduce the adverse health effects that accompany heat waves, Sailor notes.