Page 424 - Environment: The Science Behind the Stories

These actions add to the extensive draining of wetlands for The process converts saline water with up to 35,000 parts per
agriculture (p. 256). As wetlands disappear, we lose the many million (ppm) of dissolved salts to fresh water with less than
ecosystem services (pp. 21, 134–135, 170, 208) they provide 1000 ppm dissolved salts.
us, such as filtering pollutants, harboring wildlife, control- Over 20,000 desalination facilities are operating world-
ling floods, and helping to maintain drinking water supplies. wide. However, desalination is expensive, requires large
A 2010 report estimated the ecosystem services provided by inputs of fossil fuel energy, kills aquatic life at water intakes,
Louisiana’s coastal wetlands alone at $12–47 billion a year, and generates concentrated salty waste. As a result, large-scale
showing the economic value in conserving these natural desalination is pursued mostly in wealthy oil-rich nations
resources. where water is extremely scarce (Figure 15.19). In Saudi Ara-
Fortunately, many people today see the value in wetlands bia, desalination produces half the nation’s drinking water.
and are trying to protect and restore them. In 1971 an interna- The largest facility in the United States is in Tampa, Florida,
tional agreement was reached in Ramsar, Iran, to document and whose groundwater suffers from saltwater intrusion.
protect wetlands around the world. The Ramsar Convention
on Wetlands of International Importance seeks the “conserva-
their ecological character . . . within the context of sustainable FaQ Can’t we just use desalination to fulfill
tion and wise use of all wetlands” through the “maintenance of
our demand for water?
development.” Today the Chesapeake Bay estuary (p. 123), Given the seemingly endless supply of water in Earth’s oceans,
Azraq oasis in Jordan, and nearly 1900 other sites covering many people assume that desalination is the answer to our
2
185 million ha (714,000 mi ) across the globe are granted a world’s water crises. So why aren’t we eagerly utilizing this
degree of protection as Ramsar Wetlands—wetlands noted for technology everywhere?
their ecological, social, and economic importance. Simply put, we lack the abundant, clean energy sources
needed to make the widespread use of desalination economi-
Solutions to Depletion cally viable and environmentally sustainable. For example,
the United States withdraws over 700 billion liters (185 billion
of Fresh Water gallons) of fresh water every day for use in food production,
industry, and public supplies. Diverting the energy necessary
Population growth, expansion of irrigated agriculture, and to supply even a tiny fraction of this quantity from desalination
industrial development doubled our annual fresh water use in would cause prices for electricity, gasoline, and other fuels to
the last 50 years. We now use an amount equal to 10% of skyrocket. Using fossil fuels as an energy source for desalina-
total global runoff. The hydrologic cycle makes fresh water tion would also drastically increase U.S. emissions of air pol-
a renewable resource, but if we take more than a lake, river, lutants and greenhouse gases. Due to these constraints, it is
or aquifer can provide, we must either reduce our use, find unlikely that desalination will be widely embraced in the United
another water source, or be prepared to run out of water. States unless we are able to find abundant, environmentally
friendly energy sources.
Solutions can address supply or demand

To address shortages of fresh water, we can aim either to Agricultural demand can be reduced
increase supply or to reduce demand. We can increase supply
temporarily through more intensive extraction, but this is gen- Because most water is used for agriculture, it makes sense
erally not sustainable. Diversions may solve supply problems to look first to agriculture for ways to decrease demand.
in one area while causing shortages in others. In contrast, strat- Farmers can improve efficiency by lining irrigation canals to
egies for reducing demand include conservation and efficiency prevent leaks, leveling fields to minimize runoff, and adopt-
measures. Lowering demand is more difficult politically in ing efficient irrigation methods. Low-pressure spray irriga-
the short term but may be necessary in the long term. In the tion squirts water downward toward plants, and drip irrigation
developing world, international aid agencies are increasingly systems target individual plants and introduce water directly
funding demand-based solutions over supply-based solutions, onto the soil (see Figure 9.22b, p. 251). Both methods reduce CHAPTER 15 • Fr E shwat E r s yst E m s and rE sour CE s
because demand-based solutions offer better economic returns water lost to evaporation and runoff. Experts estimate that
and cause less ecological and social damage. drip irrigation (in which as little as 10% of water is wasted)
could reduce water withdrawals while raising yields by
Desalination “makes” more fresh water 20–90% and producing $3 billion in extra annual income for
farmers of the developing world.
A supply strategy with some potential for sustainability is Choosing crops to match the land and climate in which
to generate fresh water by desalination, or desalinization, they are farmed can save huge amounts of water. Currently,
the removal of salt from seawater or other water of marginal crops that require a great deal of water, such as cotton, rice,
quality. One method of desalination mimics the hydrologic and alfalfa, are often planted in arid areas with government-
cycle by evaporating allotments of ocean water with heat and subsidized irrigation. As a result of the subsidies, the true cost
then condensing the vapor—essentially distilling fresh water. of water is not part of the costs of growing the crop. Elimi-
Another method forces water through membranes to filter nating subsidies and growing crops in climates with adequate
out salts; the most common such process is reverse osmosis. rainfall could greatly reduce water use. 423

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