Need to perk up that sandwich in your lunch bag? If you add
a piece of iceberg lettuce you'll get a fresh, crisp crunch—and
a taste of the complexities of modern farming. That's
because your lettuce probably grew in a desert. (Iceberg doesn't
offer nearly as much nutritional value as other lettuces. Color
is key.
Produce from a desert? Yuma County in southwestern Arizona
gets just three inches of rain in an average year. Yet the
county boasts nearly a quarter-million acres of cropland.
Much of that land grows lettuce, a notoriously thirsty plant.
In fact, only one U.S. county produces more lettuce than Yuma
does.
Lettuce crops in Yuma County, Arizona. Source:
USDA Natural Resources Conservation Service |
The secret to Yuma's success is irrigation—and
lots of it. Each year, Yuma farmers rely on 300 billion gallons
of water pumped from the Colorado River. That's the
once mighty waterway that carved out the Grand Canyon. Why
would anyone go to such trouble to grow lettuce in the desert?
There are three main reasons. Land there is fairly cheap.
Water is subsidized. And migrant farmworkers from nearby Mexico
don't demand high wages.
If you're mathematically minded, you might think: I + Y =
AP. Translation: Irrigation plus Yuma farmland equals affordable
produce—from a place, moreover, where crops would never
grown naturally! This appears to make sense, but read on.
ANCIENT ROOTS, MODERN SHOOTS
The Roman Empire, famed for its engineering
feats, built this French aqueduct about 2,000 years ago. Source:
Art Images for College Teaching |
As you likely know, irrigation is nothing new. Chinese farmers
built irrigation canals back in the third century B.C. The
Min River Valley has—and still uses—a thousand
kilometers of ancient canals. What is new is the huge amount
of irrigation that agriculture now uses.
Take the U.S., for example. In 1900, the nation had 7.8 million
acres of irrigated land. By 2000, that number had grown to
55.3 million acres. All that watering doesn't just allow
lettuce to thrive in Arizona. Irrigation can also make farmland
more productive. In some places, irrigation has doubled crop
yields. That's a big help to farmers trying to feed
a hungry planet. Forty percent of the world's food,
in fact, now comes from irrigated farmland, and more than
fifteen percent of the world's existing cropland is
irrigated.
SHALLOW WATER, DEEP TROUBLE
Hoover Dam, in Nevada, slows the flow of the
once-wild Colorado River. Source: USDA Natural Reseources
Conservation Service |
Clearly, irrigation is here to stay. We probably couldn't
feed ourselves without it. That makes it crucial to identify—and
find ways to address—the negative effects of large-scale
irrigation. The most visible of these problems is seen in
some of Earth's great rivers. The Colorado, for instance,
once roared through the Southwest on its way to the Gulf of
California. Not anymore. Dams and irrigation projects have
tamed the river, and barely a trickle reaches the sea. Go
to this University of California site for more about the Colorado
River.
Earth's longest river, the legendary Nile, has the
same problem, especially during the dry season. On maps, the
Nile empties into the Mediterranean Sea, but less and less
water reaches the Nile delta these days. Ditto for India's
beloved, and even revered, Ganges. Farther east, the Yellow
River in China irrigates 18 million acres of farmland. As
a result, the river's mouth has been dry during portions
of 10 of the past 12 years.
Then there's the Aral Sea, which straddles the border
of Kazakhstan and Turkmenistan in Central Asia. The Aral is
a prime example of overzealous irrigation. Starting in 1954,
the Soviet Union (which then ruled Central Asia) began building
massive irrigation canals in the region. They drained virtually
all the water from the Amu Darya and Syr Darya rivers, which
used to feed the Aral.
By 1990, the Aral Sea had lost 40 percent of its surface
area. And it's still shrinking. The region is becoming a desert,
and that's not all. Robbed of freshwater, the Aral gets more
and more salty. The change has devastated fish populations.
The Aral
Sea once yielded 100 million pounds of fish a year. Today
fishing crews would be lucky to get a fraction of that amount.
In addition to drying up rivers, irrigation can also deplete
groundwater that nature has stored in underground lakes called
aquifers. Rain and other natural processes replenish them
to a certain degree, but many farmers are now taking water
out of aquifers for irrigation faster than it can be replenished.
One day, the aquifers might run dry.
HOLD THE SALT
Wimpy rivers and a shrinking sea are pretty dramatic. Far
less dramatic—but potentially catastrophic—is
the issue of damaged or unusable farmland. Irrigation harms
farmland by increasing salinization.
All surface water contains dissolved mineral salts. Irrigating
crops with this water saturates the ground with water that
contains salts, and leaves the water nowhere to drain. Instead
the water sits on the land for too long. (Rain water is evaporated
water and does not contain these salts.) When the water finally
evaporates, the salt remains behind. You may get a mini-view
of this process by checking out the salt buildup in a kettle.
Salt buildup is a problem on 25 percent of the irrigated
land in the U.S. California, an agricultural powerhouse, has
salt buildup on 35 percent of its irrigated land. As land
grows saltier, a farmer's crop options grow narrower.
Lettuce, for instance, is finicky. It won't tolerate
salinity above 1,600 parts per million (ppm). Barley, on the
other hand, can grow in nearly 5,000 ppm. Eventually, though,
a field may grow so salty that nothing can survive there.
At that point, the land is dead.
Coping with salt buildup is a balancing act. To minimize
salinization, the farmer needs to drain the water—but
not too quickly. Hasty draining can lead to soil erosion.
Irrigated land is particularly vulnerable to erosion because
arid soil usually lacks organic matter that might ordinarily
hold dirt in place.
Soil erosion washes away valuable topsoil, leaving the land
less productive. Southern Idaho offers a sobering example.
Over the past 80 years, irrigation has caused serious erosion,
and farmland there is now 25 percent less productive.
WHAT CAN WE DO?
Ending irrigation everywhere is not an option because lack
of rainfall in some areas makes growing crops impossible.
Without irrigation, experts say, we'd need a billion new acres
of good farmland. We could only get that acreage by clearing
forests. That "solution" would cause myriad
environmental and health problems.
Drip irrigation waters California grapes. Source:
USDA Natural Resources Conservation Service |
The mostly likely answer will be a mix of tradition and technology.
In northwestern India, for example, villagers have built some
4,500 earthen dams and reservoirs. These handmade structures
trap rainwater, lessening the need for massive, expensive
irrigation projects. At the other end of the technological
scale, computer-guided drip
irrigation has transformed fruit growing in South Africa.
The country's largest family-owned citrus farm now uses only
a third of the water it used to need. Yet it produces four
times as much fruit.
We can also calculate the true costs of irrigated crops by
considering hard-to-see government subsidies such as providing
farmers with "cheap water" that is paid for by
taxpayers.
But conservation is key. "Water conservation,"
says one expert, "is where the big gains are to be made."
You can get tips on saving water from National
Geographic magazine. (Scroll down to "More to Explore.")
|
Researchers in California estimated that feeding
one American takes nearly 900 gallons of water
a day. Find that hard to swallow? Check out
the water (in gallons) needed just to make
a barbecued chicken sandwich:
