What about supply? The three environmental trends I mentioned earlier—the shortage of freshwater, the loss of topsoil and the rising temperatures (and other effects) of global warming— are making it increasingly hard to expand the world’s grain supply fast enough to keep up with demand. Of all those trends, however, the spread of water shortages poses the most immediate threat. The biggest challenge here is irrigation, which consumes 70 percent of the world’s freshwater. Millions of irrigation wells in many countries are now pumping water out of underground sources faster than rainfall can recharge them. The result is falling water tables in countries populated by half the world’s people, including the three big grain producers—China, India and the U.S.
Usually aquifers are replenishable, but some of the most important ones are not: the “fossil” aquifers, so called because they store ancient water and are not recharged by precipitation. For these—including the vast Ogallala Aquifer that underlies the U.S. Great Plains, the Saudi aquifer and the deep aquifer under the North China Plain—depletion would spell the end of pumping. In arid regions such a loss could also bring an end to agriculture altogether.
In China the water table under the North China Plain, an area that produces more than half of the country’s wheat and a third of its corn, is falling fast. Overpumping has used up most of the water in a shallow aquifer there, forcing well drillers to turn to the region’s deep aquifer, which is not replenishable. A report by the World Bank foresees “catastrophic consequences for future generations” unless water use and supply can quickly be brought back into balance.
As water tables have fallen and irrigation wells have gone dry, China’s wheat crop, the world’s largest, has declined by 8 percent since it peaked at 123 million tons in 1997. In that same period China’s rice production dropped 4 percent. The world’s most populous nation may soon be importing massive quantities of grain.
But water shortages are even more worrying in India. There the margin between food consumption and survival is more precarious. Millions of irrigation wells have dropped water tables in almost every state. As Fred Pearce reported in New Scientist: Half of India’s traditional hand-dug wells and millions of shallower tube wells have already dried up, bringing a spate of suicides among those who rely on them. Electricity blackouts are reaching epidemic proportions in states where half of the electricity is used to pump water from depths of up to a kilometer [3,300 feet].
A World Bank study reports that 15 percent of India’s food supply is produced by mining groundwater. Stated otherwise, 175 million Indians consume grain produced with water from irrigation wells that will soon be exhausted. The continued shrinking of water supplies could lead to unmanageable food shortages and social conflict.
The greatest drain on supplies of freshwater is irrigation, which accounts for 70 percent of freshwater usage. Irrigation is essential to most high-yield farming, but many aquifers that supply irrigated crops are being drawn down faster than rain can recharge them. Furthermore, when farmers tap “fossil” aquifers, which store ancient water in rock impermeable to rain, they are mining a nonrenewable resource. Pumping from ever deeper wells is problematic in another way as well: it takes a lot of energy. In some states of India, half of the available electricity is used to pump water.
Source of Information : Scientific American(2009-05)