Botkin Environmental Science Earth as Living Planet 8th txtbk

376 CHAPTER 18 Water Supply, Use, and Management
North Platte R
South
WYOMING
P
NEW
MEXICO
i
ver
latte River
Arkansas River
COLORADO
Canadian
River
0 50 100 kilometers
Approximate scale E-W
SOUTH DAKOTA
NEBRASKA
Platte River
KANSAS
OKLAHOMA
TEXAS
FIGURE 18.6 Groundwater-level changes as a result of
pumping in the Texas–Oklahoma–High Plains region. (Source: U.S.
Geological Survey.)
the Santa Fe Trail, dried up decades ago due to pumping
groundwater. It was a symptom of what was coming. Both
Ulysses and Lubbock are now facing water shortages and will
need to spend millions of dollars to find alternative sources.
Desalination as a Water Source
IOWA
Seawater is about 3.5% salt; that means each cubic
meter of seawater contains about 40 kg (88 lb) of salt.
Desalination, a technology for removing salt from water,
is being used at several hundred plants around the world
to produce water with reduced salt. To be used as a freshwater
resource, the salt content must be reduced to about
0.05%. Large desalination plants produce 20,000–
30,000m 3 (about 5–8 million gal) of water per day. Today,
about 15,000 desalination plants in over 100 countries are
in operation, and improving technology is significantly
lowering the cost of desalination.
Even so, desalinated water costs several times as much
as traditional water supplies in the United States. Desalinated
water has a place value, which means that the price rises
quickly with the transport distance and the cost of moving
water from the plant. Because the various processes that remove
the salt require large amounts of energy, the cost of the
water is also tied to ever-increasing energy costs. For these
Missouri R
iver
Water level changes from
predevelopment since 1940
Declines of over 15 m
(50 ft)
Changes between –15 m
& +3 m (–50 ft and +10 ft)
Rises over 3 m (10 ft)
reasons, desalination will remain an expensive process, used
only when alternative water sources are not available.
Desalination also has environmental impacts.
Discharge of very salty water from a desalination plant
into another body of water, such as a bay, may locally increase
salinity and kill some plants and animals. The discharge
from desalination plants may also cause wide fluctuations
in the salt content of local environments, which
may damage ecosystems.
18.3 Water Use
In discussing water use, it is important to distinguish
between off-stream and in-stream uses. Off-stream use
refers to water removed from its source (such as a river
or reservoir) for use. Much of this water is returned to
the source after use; for example, the water used to cool
industrial processes may go to cooling ponds and then be
discharged to a river, lake, or reservoir. Consumptive use
is an off-stream use in which water is consumed by plants
and animals or used in industrial processes. The water enters
human tissue or products or evaporates during use
and is not returned to its source. 4
In-stream use includes the use of rivers for navigation,
hydroelectric power generation, fish and wildlife
habitats, and recreation. These multiple uses usually create
controversy because each requires different conditions.
For example, fish and wildlife require certain water levels
and flow rates for maximum biological productivity.
These levels and rates will differ from those needed for
hydroelectric power generation, which requires large fluctuations
in discharges to match power needs. Similarly, instream
uses of water for fish and wildlife will likely conflict
with requirements for shipping and boating. Figure 18.7
demonstrates some of these conflicting demands on
a graph that shows optimal discharge for various uses
River flow (Discharge)
Fish and wildlife
Winter Spring Summer Fall
Time of Year
Hydroelectric
Navigation
Recreation
FIGURE 18.7 In-stream water uses and optimal discharge
(volume of water flowing per second) for each use. Discharge is the
amount of water passing by a particular location and is measured in
cubic meters per second. Obviously, all these needs cannot be met
simultaneously.