Peak Water

Via Big Picture Agriculture, Lester Brown looks at areas that have hit a peak in irrigation water:

In the United States, farmers are over-pumping in the Western Great Plains, including in several leading grain-producing states such as Texas, Oklahoma, Kansas and Nebraska. In these states, irrigation has not only raised wheat yields but it has also enabled a shift from wheat to corn, a much higher-yielding crop. Kansas, for example, long known as the leading wheat state, now produces more corn than wheat.
Irrigated agriculture has thrived in these states, but the water is drawn from the Ogallala aquifer, a huge underground water body that stretches from Nebraska southwards to the Texas Panhandle. It is, unfortunately, a fossil aquifer, one that does not recharge. Once it is depleted, the wells go dry and farmers either go back to dryland farming or abandon farming altogether, depending on local conditions.
In the states that draw their irrigation water from the Ogallala aquifer, wells are starting to go dry. In Texas, a large grain and cattle state, which is located on the shallow end of the aquifer, irrigated area peaked in 1975 and has dropped 37% since then. In Oklahoma, irrigation peaked in 1982 and has dropped by 25%. In Kansas the peak did not come until 2009, but during the three years since then it has dropped precipitously, falling nearly 30%. Nebraska, now also a leading corn-producing state, saw its irrigated area peak in 2007. Since then its grain harvest has shrunk by 15%. Even though aquifer depletion is reducing grain output in several key states, it is not yet sufficient to reduce the overall US grain harvest, the bulk of which is produced in the rain-fed midwestern corn belt.

The rain-fed midwestern corn belt is one of, if not the, most significant natural advantages the United States has.  My fear is that in the long term, climate change may hurt the corn belt, either by drying some areas out or making others too wet, or both.