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The virtual water trade is a little-discussed secret not publicized by political leaders. Most people don’t enjoy hearing that their country is food-dependent, or that it uses its water to support others. North America is the world’s biggest exporter of virtual water. Many countries—including much of Europe, the Middle East, North Africa, Japan, and Mexico—are net importers. Unbelievably, about 40% of all human water consumption is moved around in this way, embedded in global trade flows of agricultural and industrial products.227 Without these flows the world would look very different than it does today. Dry places would support far fewer people. Lacking distant markets, large areas of terrific farmland would either surge in population or become abandoned. Global trade may be bad for local economies, bad for energy consumption, bad for resource exploitation, bad for other things . . . but it’s also spreading the wealth—of water—around.

Despite its endless recirculation, there are parts of the hydrologic cycle that smell suspiciously like depletion of a finite natural resource. This is especially true for underground sources, collectively called groundwater.

Groundwater is a very attractive water source. Unlike rainfall and rivers, which have tiny holding capacity and variable throughput, aquifers hold large volumes and are relatively stable. Humans have dug wells for thousands of years—the Egyptians, Chinese, and Persians had them as early as 2000 B.C. However, wells more than seventy to eighty feet deep are a modern invention, brought about by centrifugal pumps and the internal combustion engine.228 In water-scarce areas this new technology quickly triggered a water-drilling boom, much like the oil-drilling boom described in the previous chapter. We became a horde of mosquitoes, piercing and probing the planet with steel proboscises in search of fluids.

Tapping subterranean water meant that farmers could convert drylands and deserts into lush, productive fields virtually overnight. Here’s a dirty little secret about the agricultural “green revolution” of the latter half of the twentieth century. The green revolution was brought about not only by new petrochemicals, hybrid seeds, and mechanized agriculture, but also by a massive ballooning in the pumping of groundwater to irrigate crops. In just fifty years the world’s irrigated land area doubled from 60 million acres in 1960 to 120 million and growing by 2007.229 Much of that irrigation water came from underground. Today, many farmers in California, Texas, Nebraska, and elsewhere are utterly dependent upon groundwater for their livelihoods.230

A common misconception about groundwater arises from photographs of headlamp-wearing spelunkers wading through mysterious dark pools in underground caverns. Actually an “aquifer” is rarely a subterranean river or pool but instead just a geological layer of saturated sediment or bedrock, the best material being porous sand.231 Water is removed from the aquifer by drilling a hole into the layer and installing a pump to raise water to the surface. This creates a cone of depression in the water table, causing surrounding groundwater to ooze through the porous matrix toward the borehole, providing a continuous water supply. Water raised from deep aquifers is normally reliable, clear, cold, and delicious. Deep aquifers don’t flood or go into drought. In some of our driest, most water-stressed civilizations, it is the discovery and tapping of giant aquifers—ancient relicts that took many thousands of years to form—that has watered cities and exploded lawns across deserts from Texas to Saudi Arabia.

The problem is that no one knew or cared where the groundwater came from. In the early days many drillers thought it was infinite, or replenished somehow by mysterious underground rivers. But because aquifers are ultimately recharged by whatever rainfall manages to percolate down from the surface, they refill slowly. If water is pumped out faster than new water can ooze