What's Left to Eat?

Irrigation: Is Our Water Use All Wet?

Irrigation gulps two-thirds of the freshwater we use each year. Good news: Crops thrive where they'd never grow naturally. Bad news: We're depleting water supplies and damaging soil.

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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:

• chicken (115)
• wheat bread (21.2)
• cheese (56)
• tomato (1.9)
• lettuce (0.7)
• BBQ sauce (20.3)

Those ingredients add up to 215.1 gallons, and there are two more meals in an ordinary day! You can see the whole menu at a University of California website.

 

Need to perk up that sandwich in your lunch bag? If you add 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 <span>Resources</span> Conser<span>vation</span> Service
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

French aqueduct. Source: Art Images for College Teaching
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, Nevada. Source: USDA Natural Resources Conservation Service
Hoover Dam, in Nevada, slows the flow of the once-wild Colorado River.
Source: USDA Natural Resources 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.

click to enlarge image
Satellite images. Source: NASA Earth Observatory: Aral Sea
Satellite images show the shrinking Aral Sea in 1973 (top), 1987 (middle), and 2000 (bottom).
Source: NASA Earth Observatory: Aral Sea

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
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.")

 

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What happened to Old MacDonald's farm? back_eatnext_eat What happens when we mess around in nature's lab?

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