Introduction - Hydro-Environmental Analysis: Freshwater Environments - page 5

1.2.1 S o , H ow M ucH w ater I S t Here and w Here I S I t ?
The total water supply of the world is 326 million cubic miles (about 360 trillion gallons or 1.36
trillion cubic kilometers) of which about 0.02 mi 3 (0.02 trillion gallons or 0.08 km 3 ) is in motion at
any one time. Most of the water is stored in the oceans, frozen in glaciers, held in lakes, or detained
underground (Table 1.1). The 317 million cubic miles of water held by the oceans constitutes 97.3%
of the earth's supply.
According to the U.S. Geological Survey (USGS 1984a):

• Only about 3,100 mi 3 of water (12,921 km 3 ), chiely in the form of invisible vapor, is con-

tained in the atmosphere at any given time.

• If the water were to fall all at once, the earth would be covered with only about 1 in. of

water.

• Of the 102,000 mi 3 (425,000 km 3 ) of water that passes into the atmosphere annually,

78,000 mi 3 (325,000 km 3 ) falls directly back into the oceans.

• Streams and rivers collect and return to the oceans some 9,000 mi 3 (37,000 km 3 ) of water,

including a large quantity of water that, as “groundwater,” has moved slowly to natural
outlets in the beds and banks of streams.

While water is abundant, only about 0.3% is in a form that is usable to us, and most of that
water is inaccessible. So, of the 326 million cubic miles or 1.36 trillion cubic kilometers of water,
only about 15,000 mi 3 (62,500 km 3 ) maintains life processes, principally as soil moisture, which
provides water necessary for vegetation. This water reaches the atmosphere again by the process of
evapotranspiration.

1.2.2 w Here d oeS I t G o ?
In comparison with the earth's total water supply, the total amount of water available is small.
However, both the amount and the distribution of this available water have large ecological and
human use impacts.
The quantity of water available and the water demands also vary with time and location around
the United States, and the world. In 2000, approximately 408 billion gallons per day (1.5 trillion
L day -1 ) was withdrawn in the United States for all uses (Hutson et al. 2000), of which about
220 billion gallons per day (833 billion L day -1 ) was returned to streams after use. As illustrated

TABLE 1.1
Global Water Source and Volume

Water Source Water Volume (km 3 ) Percentage of Total

Oceans 1,230,000,000 97.20

Icecaps and glaciers 28,600,000 2.15

Groundwater 8,300,000 0.61

Freshwater lakes 123,000 0.01

Inland seas 104,000 0.01

Soil moisture 67,000 0.01

Atmosphere 12,700 0.001

Rivers 1,200 0.0001

Total water volume 1,360,000,000 100

Source: USGS, The hydrologic cycle (pamphlet), U.S. Government Printing
Ofice, Washington, DC, 1984a.