The concept of peak oil, where the inaccessibility of remaining deposits ensures that extraction rates start an irreversible decline, has been the subject of regular debate for decades. Although that argument still hasn't been settled—estimates range from the peak already having passed us to its arrival being 30 years in the future—having a better sense of when we're likely to hit it could prove invaluable when it comes to planning our energy economy. The general concept of peaking has also 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.

There are really two ways to do this. The first is to simply make more of the water accessible that's currently off limits. So, for example, there are a lot of areas where we could change our habits of dumping industrial and municipal waste into the environment, and clean up the existing watershed in order to make that supply available for other uses.

The authors consider this analogous to what economists have termed a "backstop" technology for other finite resources. For example, renewable energy acts as a backstop for fossil fuels. Although some forms of renewable energy are currently expensive in comparison with fossil fuels, the latter's scarcity will ultimately cause its price to rise until some the renewable backstops become competitive; further scarcity will ultimately bring more backstops into play until use of the nonrenewable resource becomes negligible.

In an analogous manner, cleaning up existing renewable water sources acts as one backstop for the post-peak water scenario. But, as the authors note, "the ultimate water backstop is still water." Long distance transport is still possible, as is large-scale desalinization, which currently is largely confined to island nations and the Mideast.

Is large-scale desalinization inevitable? The authors make the case that it's not, based on the US. Although they caution that water-use figures, which are notoriously fragmented, aren't entirely reliable, they use them to suggest that US water use roughly paralleled GDP growth for most of the 20th century. The two separated around 1970, as water use tailed off, peaking around 1975. After a short period of decline, water use has remained stable even as both GDP and population have continued to climb.

This pattern, the authors suggest, is a lot like what they expect peak water to look like. And, if that really was the peak, the experience of the US might provide valuable lessons for economic planning.

PNAS, 2010. DOI: 10.1073/pnas.1004812107 (About DOIs).