18 countries, together containing half the world's people, are now overpumping their underground water tables to the point – known as "peak water" – where they are not replenishing and where harvests are getting smaller each year.

The situation is most serious in the Middle East. According to Brown: "Among the countries whose water supply has peaked and begun to decline are Saudi Arabia, Syria, Iraq and Yemen. By 2016 Saudi Arabia projects it will be importing some 15m tonnes of wheat, rice, corn and barley to feed its population of 30 million people. It is the first country to publicly project how aquifer depletion will shrink its grain harvest.

"The world is seeing the collision between population growth and water supply at the regional level. For the first time in history, grain production is dropping in a geographic region with nothing in sight to arrest the decline. Because of the failure of governments in the region to mesh population and water policies, each day now brings 10,000 more people to feed and less irrigation water with which to feed them."

There is also concern about falling water tables in China, India and the US, the world's three largest food-producing countries. "In India, 175 million people are being fed with grain produced by overpumping, in China 130 million. In the United States the irrigated area is shrinking in leading farm states with rapid population growth, such as California and Texas, as aquifers are depleted and irrigation water is diverted to cities."

Brown warned that many other countries may be on the verge of declining harvests. "With less water for irrigation, Mexico may be on the verge of a downturn in its grain harvest. Pakistan may also have reached peak water. If so, peak grain may not be far behind."

Via: http://www.theguardian.com/global-development/2013/jul/06/food-supply-threat-water-wells-dry-up