June 15, 2004 (ENS): The combined size of all
highways, streets, buildings, parking lots and other
solid structures within the lower 48 states and the
District of Columbia is some 43,480 square miles, roughly
the size of the state of Ohio.
The finding comes from a study by Christopher Elvidge
of the National Oceanic and Atmospheric Administration's
National Geophysical Data Center in Boulder, Colorado,
who along with colleagues from several universities
and agencies produced the first national map and inventory
of impervious surface areas in the United States.
The study appears in the June 15 issue of "Eos,"
which is published by the American Geophysical Union.
The researchers note the new map is important because
impervious surface areas affect the environment.
The qualities of impervious materials that make them
ideal for construction also reduce heat transfer from
Earth's surface to the atmosphere, creating urban heat
In addition, the replacement of heavily vegetated areas
by impervious surface areas reduces sequestration of
carbon, which plants absorb from the atmosphere.
Both effects can play a role in climate change.
Within watersheds, impervious surface areas alter the
shape of stream channels, raise the water temperature,
and sweep urban debris and pollutants into aquatic environments.
These effects are measurable once 10 percent of a watershed's
surface area is covered by impervious surface areas,
An increase in impervious surfaces means fewer fish
and fewer species of fish and aquatic insects, as well
as a general degradation of wetlands and river valleys.
The researchers found the impervious surface area of
the Lower 48 states is already slightly larger than
that of its wetlands, which cover 38,020 square miles.
Elvidge notes that few areas have impervious surface
area maps, because they are difficult and expensive
He used a variety of data sources to produce the map
accompanying his article, including nighttime lights
observed by satellite, Landsat images, and data on roads
from the U.S. Census Bureau, along with aerial photography.
The map should provide a useful benchmark to track
the growth of impervious surface areas, in particular
because the U.S. population is increasing by some three
million people each year. In addition, roughly one million
new single family homes and 10,000 miles of new roads
are added annually.