By now most people have heard of the concept peak oil (I hope). This is the notion and probable fact that at some point the reserves of oil can no longer provide enough to meet demand and the supply of oil will inexorably begin to decline. Predictions for peak oil run from ‘it’s already happened’ to about thirty years away with a current concentration of the argument in the period 2015 to 2025. Obviously for the modern economy, oil is a fundamental source of energy and materials; therefore it’s very important (not to mention the source of great wealth).

Even more fundamental, not only for the economy but for life itself is water, more specifically fresh water. Now in some obvious ways water is not like oil (including the bit about not mixing well). For one thing, there’s a lot more water than oil; for another, water is at least partly a renewable resource, oil is not. Both of these points make water less expensive than oil, but not less crucial. They are both finite resources. That’s why in a new study in the May 24, 2010 Proceedings of the National Academy of Sciences (PNAS), researchers from the Pacific Institute have highlighted the concept of peak water and more importantly have brought attention the idea that peak water comes in three forms:

1. Peak renewable water
2. Peak non-renewable water
3. Peak ecological water

[Source: PNAS: Peak water limits to freshwater withdrawal and use]

The idea behind the study is not to jump up and down about ‘running out of water.’ The authors, Dr. Peter Glieck and Meena Palaniappan, don’t believe the Earth will run out of water – after all there are those oceans. What they are concerned about and want the relevant managers, businesspeople, and policy makers to understand, is that in various regions of the world the supply of renewable fresh water is near or already past the point where the environmental damage of human water use exceeds the benefit of using the water.

To frame this point, they describe peak water in the three ways mentioned above.

Peak renewable water occurs particularly in areas with limited flowing or underground stocks of fresh water (non seawater), for example the Colorado River basin of the American Southwest. Here the demand for water has exceeded the long-term renewal of the supply for some time. Reaching a peak of renewable water doesn’t mean there is already a water crisis, but it does mean that the demand outstrips supply in the long run.

Peak non-renewable water occurs where the use of water exceeds what are essentially non-renewable stocks, for example underground aquifers with low recharge rates. In cases like this the combination of over-pumping and water contamination leads to a very noticeable and usually economically significant decline in the supply of water relative to the demand. This situation is actually very similar to an oil producing region. Examples include the Ogallala Aquifer of the American Great Plains and numerous basins in India.

Peak ecological water is perhaps the most urgent point, where even beyond the immediate impact on economics and human consumption, the total costs of ecological disruption and damage caused by the lack of fresh water exceeds the total value provided by the use of that water. Half of the world’s wetlands have already disappeared, almost entirely through human intervention. Correspondingly the number of freshwater species has declined by 50% since 1970. The costs of such ecological damage, which economist like to abstract behind the term ‘externalities’, should be included in the concept of peak ecological water.

While estimating the supply of water and measuring water use is difficult (at least as difficult as it is for oil), the point the authors are making is that it’s time to consider ‘peak water’ as a real problem for a number of important regions of the world. Many have already surpassed peak ecological water – the human use of water is causing serious ecological damage. Many more are at or approaching the point where serious economic repercussions are about to happen. As they put it:

The realization that there are limits to peak renewable water use are forcing new discussions about improving water-use efficiency and developing innovative technologies for water treatment and reuse as alternatives to expanding traditional supply projects to further mine overtapped renewable water sources. The bad news is that we are increasingly reaching peak water limits. The good news is that recognizing and understanding these limits can stimulate innovations and behaviors that can reduce water use and increase the productivity of water, shifting water policy toward a more sustainable water future.

[Source: PNAS: Peak water limits to freshwater withdrawal and use]