Page 416 - Environment: The Science Behind the Stories

For this reason, dams store water from wetter months that can to promote agricultural self-sufficiency, drastically lowering
be used in drier times of the year when river flow is reduced. water costs for farmers.
As if the existing mismatches between water availabil-
ity and human need were not enough, global climate change Excessive water withdrawals
(Chapter 18) will worsen conditions in many regions by
altering precipitation patterns, melting glaciers, causing can drain rivers and lakes
early-season runoff, and intensifying droughts and flooding. In many places we are withdrawing surface water at unsus-
A 2009 study found that one-third of the world’s 925 major tainable rates to meet our many demands for fresh water. As
rivers experienced reduced flow from 1948 to 2004, with a result, many of the world’s major rivers regularly run dry
the majority of the reduction attributed to effects of climate before reaching the sea. The Colorado River often fails to reach
change. The reduction in water flow into the Pacific Ocean the Gulf of California after the many withdrawals of its water
during this period was equivalent to the output of the mighty in the arid western United States and Mexico. This reduction in
Mississippi, demonstrating the scale of changes involved.
flow threatens the future of the cities and farms that depend on
the river. And it has drastically altered the ecology of the river
Water supplies households, industry, and its delta, changing plant communities, wiping out popula-
and especially agriculture tions of fish and invertebrates, and devastating fisheries.
The Colorado’s plight is not unique. Several hundred
We all use water at home for drinking, cooking, and clean- miles to the east, the Rio Grande also frequently runs dry, the
ing. Most mining, industrial, and manufacturing processes victim of overextraction by both Mexican and U.S. farmers
require water. Farmers and ranchers use water to irrigate in times of drought. China’s Yellow River also often fails to
crops and water livestock. Globally, we allot about 70% of reach the sea. Even the river that has nurtured human civiliza-
our annual fresh water use to agriculture. Industry accounts tion longer than any other, the Nile in Egypt, now peters out
for roughly 20%, and residential and municipal uses for only before reaching its mouth.
10%. Nations with arid climates tend to use a greater share Nowhere are the effects of surface water depletion so evi-
for agriculture, whereas heavily industrialized nations use a dent as in the Aral Sea. Once the fourth-largest lake on Earth,
greater share for industry. just larger than Lake Huron, it lost over four-fifths of its vol-
When we remove water from an aquifer or surface water ume in just 45 years (Figure 15.10). This dying inland sea, on
body and do not return it, this is called consumptive use. Our the border of present-day Uzbekistan and Kazakhstan, is the
primary consumptive use of water is for irrigation, which is the victim of irrigation practices. The former Soviet Union insti-
water applied to crops (p. 251). In contrast, nonconsumptive tuted industrial cotton farming in this dry region by flooding
use of water does not remove, or only temporarily removes,
water from an aquifer or surface water body. Using water to
generate electricity at hydroelectric dams is an example of Russia
nonconsumptive use; water is taken in, passed through dam Kazakhstan
machinery to turn turbines, and released downstream. Uzbekistan
Why do we allocate 70% of our water use to agriculture?
Our rapid population growth requires us to feed and clothe
more people each year, and the intensification of agriculture
during the Green Revolution (pp. 236, 265–266) required
significant increases in irrigation. As a result, we withdraw Black Sea Iran
Turkmenistan
70% more water for irrigation today than we did 50 years ago Caspian Sea
and have doubled the amount of land under irrigation. So far,
expansion of irrigated agriculture has kept pace with popula-
tion growth; irrigated area per person has remained for four
2
decades at around 460 m (4950 ft )—one-tenth of a football
2
field for each of us. CHAPTER 15 • Fr E shwat E r s yst E m s and rE sour CE s
Irrigation can more than double crop yields by allow-
ing farmers to apply water when and where it is needed. The
18% of world farmland that we irrigate yields fully 40% of
our produce, including 60% of the global grain crop. Still,
most irrigation remains highly inefficient, and crop plants
end up using only 40% of the water that we apply. Inefficient
“flood and furrow” irrigation, in which fields are liberally
flooded with water that may evaporate from standing pools,
accounts for 90% of irrigation worldwide. Overirrigation (a) Satellite image of Aral (b) Satellite image of Aral
leads to waterlogging and salinization (pp. 252–253), which Sea, 1987 Sea, 2009
affect one-fifth of farmland today and reduce farming income Figure 15.10 The Aral Sea in central Asia was once the
worldwide by $11 billion. Such inefficient irrigation is pos- world’s fourth largest lake. However, it has been shrinking (a, b)
sible because many national governments subsidize irrigation because so much water was withdrawn to irrigate cotton crops. 415

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