Overpumping progressively lowers the water table and forces the use of deeper wells and bigger pumps. Throughout the world, Postel calculates, farmers are overdrawing water by 160 billion cubic meters per year. Thus, the roughly 10 percent of world grain production that depends on this water is unsustainable. Some areas, such as the Middle East, must import food because the lack of water limits agricultural production. As the University of London's Tony Allen points out, this amounts to importing water.
Irrigated agriculture is also in competition with urban water use.
This is perhaps less problematic than it may seem, because although cities are growing and reallocating water to them must be done with skill, urban needs can be met with a small fraction of the water consumed by farming. In Last Oasis , Postel reported that reducing irrigation needs by a tenth would free up enough water to roughly double domestic water use worldwide. City dwellers can afford to pay much more for a cubic meter of water than irrigators do, and water is many times more economically productive and job-generating in industry than in agriculture. Also in competition with irrigation are the claims of recreational water users and the pressing need to retain enough water in streams and wetlands to preserve riverine ecosystems. Postel discusses these water conflicts with expertise.