http://arstechnica.com/science/news/2010/05/not-just-oil-us-hit-peak-…

The general concept of peaking has  been valuable as it applies to just about any finite resource. A new analysis suggests that it may be valuable to consider applying it to a renewable resource as well: the planet’s water supply.

The analysis performed by staff at the Pacific Institute recognizes that there are some significant differences between petroleum and water. For oil using it involves a chemical transformation that won’t be reversed except on geological time scales. Using water often leaves it in its native state with a cycle that returns it to the environment in a geologic blink of an eye. Still the authors make a compelling argument that not only can there be a peak water but the US passed this point around 1970 apparently without anyone noticing.

They make their case based on three ways in which water can run up against limits on its use. The first is peak renewable water for sources that rapidly replenish like river basins or snow melt. The classic example here is the Colorado River where for most years since 1960 essentially no water has reached the ocean. Although actual water use is governed by a series of interstate and international agreements these simply serve to allocate every drop of water. Similar situations are taking place in other river basins such as the Jordan.

The second is what they term peak nonrenewable water as exemplified by the use of aquifers that replenish on time scales that make them closer to a finite resource. (This issue is so well recognized that it has a Wikipedia entry.) At the moment the Ogallala and Central Valley Aquifers in the US along with a number in China and India are being drained at a rate that far exceeds their recharge. Ultimately usage will necessarily peak and start dropping as it gets harder to get access to the remainder. Eventually these water supplies will tail off to something in the neighborhood of their recharge rate.

The final issue the authors consider is peak ecological water. The gist here is that we’ve accepted the elimination of the Colorado near its terminus. For a wide variety of other water sources—think the Hudson or the Rhine—we’ll never tolerate the equivalent. This is because of the potential economic impact of eliminating the use of the waterway and because we’re no longer likely to accept wiping out species that rely on the habitats created by the rivers.

Combined these three peaks set a hard limit on the sustainable water use. We can exceed them for a while but we will eventually have to drop down to something near the limit unless we’re willing to start paying substantially more for our water supply.