Botkin Environmental Science Earth as Living Planet 8th txtbk

Groundwater
Ocean storage
18.1 Water 371
Soil
Rain clouds
Precipitation
Infiltration
Groundwater to soil
Precipitation
Evaporation from vegetation
Evaporation from lakes
Surface
runoff
Evaporation
Transpiration from trees
Evaporation from streams
Cloud formation
Evaporation from soil
Transpiration from crops
Evaporation from ocean
Groundwater to vegetation
Groundwater to streams
Groundwater to ocean
Percolation
Rock
FIGURE 18.2 The hydrologic cycle, showing important processes and transfer of water.
(Source: Modified from Council on Environment Quality and Department of State, The Global 2000
Report to the President, vol. 2 [Washington, DC].)
Water can be found in either liquid, solid, or gaseous
form at a number of locations at or near Earth’s surface.
Depending on the specific location, the water’s residence
time may vary from a few days to many thousands of years
(see Table 18.1). However, as mentioned, more than 99%
of Earth’s water in its natural state is unavailable or unsuitable
for beneficial human use. Thus, the amount of water
for which all the people, plants, and animals on Earth
compete is much less than 1% of the total.
As the world’s population and industrial production
of goods increase, the use of water will also accelerate. The
global per capita use of water in 1975 was about 700 m 3 /year,
or 2,000 gallons/day (185,000 gal/yr), and the total human
use of water was about 3,850 km 3 /year (about 10 15
gal/yr). Today, world use of water is about 6,000 km 3 /yr
(about 1.58 10 15 gal/yr), which is a significant fraction
of the naturally available freshwater.
Compared with other resources, water is used in very
large quantities. In recent years, the total mass (or weight)
of water used on Earth per year has been approximately
1,000 times the world’s total production of minerals,
including petroleum, coal, metal ores, and nonmetals.
Where it is abundant and readily available, water is generally
a very inexpensive resource. In places where it is not
abundant, such as the southwestern United States, the
cost of water has been kept artificially low by government
subsidies and programs.
Because the quantity and quality of water available
at any particular time are highly variable, water shortages
have occurred, and they will probably occur with
increasing frequency, sometimes causing serious economic
disruption and human suffering. 4 In the Middle
East and northern Africa, scarce water has led to harsh
exchanges and threats between countries and could even
lead to war. The U.S. Water Resources Council estimates
that water use in the United States by the year 2020 may
exceed surface-water resources by 13%. 4 Therefore, an
important question is, How can we best manage our water
resources, use, and treatment to maintain adequate
supplies?