AFRICA'S LAKE CHAD, ONCE a landmark for astronauts circling Earth, is now difficult for them to locate. Surrounded by Cameroon, Chad, Niger, and Nigeria--all countries with fast-growing populations--the lake has shrunk 96 percent in forty years. The region's soaring demand for irrigation water coupled with declining rainfall is draining dry the rivers and streams that feed the lake. As a result, Lake Chad may soon disappear entirely, its whereabouts a mystery to future generations.
The shrinkage of Lake Chad isn't unique. The world is incurring a vast water deficit--one that is largely invisible, historically recent, and growing fast. Because the deficit comes largely from aquifer overpumping, it is often discovered only when wells go dry.
This global water deficit is the result of demand tripling over the last half-century. The drilling of millions of irrigation wells has pushed water withdrawals beyond recharge rates, in effect leading to groundwater mining. The failure of governments to limit pumping to the sustainable yield of aquifers means that water tables are now falling in countries that contain more than half the world's population, including the big three grain producers--China, India, and the United States.
Beyond these traditional sources of water insecurity, climate change is now affecting water supplies. Rising temperatures are boosting evaporation rates, altering rainfall patterns, and melting the glaciers that feed rivers during the dry season. As the glaciers melt, they are threatening to convert perennial rivers such as the Ganges in India and the Yellow in China into seasonal rivers, increasing both water and food insecurity. With the earth's climate system and its hydrological cycle so intertwined, any changes in climate will alter the hydrological cycle.
The link between water and food is strong. We each drink on average nearly 4 liters of water per day in one form or another, while the water required to produce our daily food totals at least 2,000 liters--500 times as much. This helps explain why 70 percent of all water use is for irrigation. Another 20 percent is used by industry, and 10 percent goes for residential purposes. With the demand for water growing in all three categories, competition among sectors is intensifying, with agriculture almost always losing. Though most people recognize that the world is facing a future of water shortages, not everyone has connected the dots to see that this also means a future of food shortages.
WATER TABLES FALLING
Scores of countries are overpumping aquifers as they struggle to satisfy their growing water needs. Most aquifers are replenishable, but not all are. When most of the aquifers in India and the shallow aquifer under the North China Plain are depleted, the maximum rate of pumping will be automatically reduced to the rate of recharge.
Fossil aquifers, however, aren't replenishable. For these--the vast U.S. Ogallala aquifer, the deep aquifer under the North China Plain, or the Saudi aquifer, for example--depletion brings pumping to an end. Farmers who lose their irrigation water have the option of returning to lower-yield dryland farming if rainfall permits. But in more arid regions, such as in the southwestern United States or the Middle East, the loss of irrigation water means the end of agriculture.
Falling water tables are already adversely affecting harvests in some countries, including China, which rivals the United States as the world's largest grain producer. A groundwater survey released in Beijing in August 2001 revealed that the water table under the North China Plain, an area that produces over half of the country's wheat and a third of its corn, is falling fast. Overpumping has largely depleted the shallow aquifer, forcing well drillers to turn to the region's deep aquifer, which isn't replenishable.
Falling water tables, the conversion of cropland …