Gwynne Dyer: Food crisis looms as a result of cutbacks in irrigation A lot of those aquifers are “fossil”, meaning that they filled with water long ago and are now cut off from the surface. They will eventually be pumped dry. Others still recharge from surface water that filters down, but they are almost all being pumped at many times their recharge rate, so they will effectively go dry, too. Then the world will have to make do with the one-third of irrigated land that gets its water from the weather. It won’t be enough.
Obviously, the aquifers won’t all go dry at once. Some are bigger than others, and some have been pumped much longer or more heavily than others. But most of them are going to go dry at some point or other in the next 30 years.