Environmental Engineering Reference
In-Depth Information
Water Sources*
In the real estate business, location is everything. We say the same when it
comes to sources of water. In fact, the presence of water defines “location” for
communities. Although communities differ widely in character and size, all
have the common concerns of finding water for industrial, commercial, and
residential use. Freshwater sources that can provide stable and plentiful sup-
plies for a community do not always occur where we wish. Simply put, on
land, the availability of a regular supply of potable water is the most impor-
tant factor affecting the presence—or absence—of many life forms. A map
of the world immediately shows us that surface waters are not uniformly
distributed over the surface of the Earth. American lands hold rivers, lakes,
and streams on only about 4% of their surface. The heaviest populations of
any life forms, including humans, are found in regions of the United States
(and the rest of the world) where potable water is readily available, because
lands barren of water simply cannot support large populations.
Note:
The volume of freshwater sources depends on geographic, land-
scape, and temporal variations, as well as on the impact of human
activities.
Just How readily Available is Potable Water?
Approximately 326 million cubic miles of water comprise Earth's entire water
supply. Although it provides us indirectly with freshwater through evapora-
tion from the oceans, only about 3% of this massive amount of water is fresh,
and most of that minute percentage of freshwater is locked up in polar ice
caps and glaciers. The rest is held in lakes, in flows through soil, and in river
and stream systems. Only 0.027% of Earth's freshwater is available for human
consumption (see
Table 4.1
for the distribution percentages of Earth's water
supply). We see from
Table 4.1
that the major sources of drinking water are
from surface water, groundwater, and from groundwater under the direct
influence of surface water (i.e., springs or shallow wells).
Surface Water Supplies
Most surface water originates directly from precipitation—rainfall or snow.
To gain an appreciation for the impact of this runoff on surface water sup-
plies, let's take a look at the water balance in the United States. Over the U.S.
mainland, rainfall averages about 4250 billion gallons per day. Of this massive
* Much of this section is adapted from Spellman, F.R.,
The Science of Water
, 2nd ed., CRC Press,
Boca Raton, FL, 2008.
