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

Science News, Sept 4, 2004 by Sid Perkins

When raindrops fall on uninhabited terrain, many things can happen. Precipitation that lands on craggy mountainsides flows downhill to streams. Drops that hit soil often soak in; some of that water later evaporates, while much of the rest seeps through the earth to replenish aquifers and nourish springs. Raindrops that splash into rivers immediately join a headlong tumble toward the sea. Consider, however, raindrops that fall on populated areas. Development brings homes, shopping centers, streets, sidewalks, driveways, parking lots, tennis courts--all of which typically shed water more effectively than the proverbial duck's back. On the green space that's left, even small amounts of traffic--tractors, golf carts, lawn mowers, mere human footfalls--can compress the soil and reduce the rate at which it absorbs precipitation.

As an environment becomes inhabited, therefore, less precipitation soaks into the ground and runoff increases. As a result, floods occur more often and develop more rapidly. These hydrological changes, along with the pollutants picked up by water as it pours across the urban landscape, can wreak havoc on aquatic ecosystems and damage habitats alongside waterways.

The relative dearth of water left behind in populated landscapes contributes to the phenomenon of urban heat islands, in which densely populated areas are warmer than the surrounding rural zones (SN: 7/10/04, p. 21). However, scientists suggest that innovative landscaping and architecture could alleviate some of the detrimental effects of this sprawl.

DENSITY'S CHILD Each year, the population of the United States grows by about 3 million. Construction costs to support that growth exceed $480 billion annually, a substantial portion of which builds about 1 million new single-family homes and more than 10,000 new miles of roads. These impervious surfaces aren't scattered evenly, however. In suburban areas where homes are built on one-third-acre lots, about 35 percent of the ground is impermeable to precipitation. In densely packed cities such as New York and San Francisco, impervious surfaces may cover more than 90 percent of the ground.

A new analysis of the lower 48 states captures details of the distribution of the nation's impervious surfaces. The information will be useful to a wide variety of scientists, including hydrologists, ecologists, and water-resource managers, says Christopher D. Elvidge of the National Oceanic and Atmospheric Administration in Boulder, Colo. He and his colleagues used government highway data and satellite images to assemble a first-of-its-kind map on a 1-square-kilometer grid.

Then, the researchers used detailed analyses of S0 high-resolution aerial photos taken in and around 13 urban areas to assign specific values to squares of the map. The results suggest that nationwide, almost 113,000 [km.sup.2]--an area a bit smaller than the state of Ohio--are covered with impervious surfaces. Elvidge and his colleagues describe their findings in the June 15 Eos.

The largest contributors to a region's impervious-surface area vary with the extent of local development, says Janet S. Tilley, a physical scientist at the U.S. Geological Survey in Reston, Va. In sparsely populated locales, highways account for the largest proportion of impermeable surfaces, she notes. Although many researchers consider pavement to be the biggest source of impervious area in more-developed regions as well, two other factors make major contributions there.

In analyses of satellite and aerial photos of six urban and suburban watersheds scattered across the lower 48 states, Tilley and E. Terrenee Slonecker, an environmental scientist at the Environmental Protection Agency in Reston, Va., found that roads accounted for about 28 percent of the impervious areas. Buildings were responsible for around 29 percent of the impervious surfaces, and parking lots made up nearly 25 percent.

Surprisingly, Tilley notes, the research showed that driveways, sidewalks, patios, and pools--features not typically considered in previous research--together made up about 14 percent of those areas' impermeable surfaces. She and Slonecker presented their findings in the April 1 GI Science and Remote Sensing.

SPILLOVER EFFECTS AS the amount of impervious-surface area increases, the environmental consequences become more serious. Previous research has suggested that when the percentage of impermeable surfaces in a watershed is 10 percent or less, streams typically retain good water quality and stable channels. When the proportion is between 10 and 25 percent, storm-fed flows cause noticeable erosion. Major floods tend to widen a stream's channel, spill over its banks, and scour away waterside vegetation.

Streams in watersheds where more than 25 percent of the surface area is impermeable usually show severe physical and ecological damage.

Because impervious surfaces typically absorb more solar radiation than natural ones do, the water entering streams in urbanized areas can be as much as 6[degrees]C warmer than it would be if the area were undeveloped. The stress of that temperature increase, as well as the insult of pollutants swept up from tainted pavement and soil, can kill some aquatic organisms or drive them downstream. Such noxious chemicals can include pesticides, nitrates and phosphates from fertilizers, and salt used to deice roadways.