Environmental Engineering Reference
In-Depth Information
TAble 4.1
World Water Distribution
Percent (%)
of Total
Location
Land areas
2.8
Freshwater lakes
0.009
Saline lakes and inland seas
0.008
Rivers (average instantaneous volume)
0.0001
Soil moisture
0.005
Groundwater (above depth of 4000 meters)
0.61
Ice caps and glaciers
2.14
Atmosphere (water vapor)
0.001
Oceans
97.3
Total all locations (rounded)
100
Source:
Adapted from Peavy, H.S. et al.,
Environmental Engineering
,
McGraw-Hill, New York, 1985, p. 12.
DID YoU KNoW?
Of all the freshwater on Earth, only about 0.3% is contained in riv-
ers and lakes. Not only are rivers and lakes the water we are most
familiar with, but they are also where most of the water we use in
our everyday lives exists.
amount, about 66% returns to the atmosphere through evaporation directly
from the surface of lakes and rivers and transpiration from plants. This leaves
about 1250 billion gallons per day to flow across or through the ground to
return to the sea (see
Table 4.2
). Although municipal usage of water is only a
small fraction of this great volume, the per capita consumption of water in the
United States is rather high—as much as 150 gallons per person per day, prob-
ably because public water is relatively inexpensive here. In areas where water
supplies are less readily available and thus more costly, per capita consump-
tion is much lower, due to both financial and conservation concerns.
The 1250 billion gallons per day of surface runoff water, exposed and open
to the atmosphere, results from the movement of water on and just beneath
the surface of the Earth, referred to as
overland flow.
. Simply put, overland
flow and surface runoff are the same—water flow that has not yet reached a
definite stream channel. This occurs when the rate of precipitation exceeds
either the rate of interception and evapotranspiration or the amount of rain-
fall readily absorbed by the surface of the Earth. The total land area that
contributes runoff to a stream or river is called a
watershed
,
drainage basin
, or
catchment area
.
