When I started in futurology, one of the common beliefs was that future wars would be fought mainly over water supply. There are certainly some areas of the world where water-based wars could occur, but the main conflicts today are nothing to do with water at all.
Desalination used to be very expensive but new technology will reduce costs to not much more than standard fresh water sources. The discovery of graphene is a particularly important breakthrough because it allows water to go through easily but holds back impurities, even salt. Since graphene offers so many other benefits, research is proceeding enthusiastically to learn how to manufacture it in large quantities. Hot off the press today, http://pubs.acs.org/doi/abs/10.1021/nl502399y shows that it is easily strong enough for high pressure reverse osmosis desalination. That will allow not just faster and cheaper desalination, but also cheaper and safer recycling, taking load off the system, allowing less water to go further and making it easier to get that water in the first place. Together, desalination and recycling will reduce load and improve supply sufficiently to remove the stress and potential conflicts – desalination and water purification plants will be a lot cheaper than wars. There will certainly be squabbles and political pressures applied sometimes, but I don’t see full-scale water wars as a significant threat. Technology has effectively solved this problem.
In humanitarian disasters, lack of availability of clean water is often a major problem, and many people die from diseases picked up by drinking very polluted water. http://nvireuk.wordpress.com/2012/02/14/graphene-drinking-straw/ was my own water purification idea a few years ago. (nvireuk doesn’t exist any more but the article is still visible). It isn’t designed to be an everyday replacement for a proper supply, but should work well in emergency situations.
The absorbent material provides a smooth surface onto which to apply the graphene coating. The graphene coating filters out everything except the clean drinking water. The sponge then provides a reservoir from which to suck safe drinking water. When we get to the point that graphene can be produced cheaply and easily, this could save many lives in developing countries, in disaster zones, and even be useful to save carried weight for hikers, sailors and the military.
In the UK, we have lots of green types trying to make everyone use less water. Wasting is never a good idea, but really, we have no shortage of water here and the pressure to reduce usage is misdirected, there are plenty of real problems that need solved. We get abundant rainfall in the UK, and the only issue is cost of capture and storage against water-saving measures. It is a simple commercial trade-off, not a shortage of fresh water, most of which is allowed to go out to sea unused. There is no evidence that water companies make less profit as we save water, though they need less reservoir capacity and have lower treatment costs than otherwise, and in any case, leaks in their own system account for up to a third of the use of drinking water. The evidence is that they simply increase charges to maintain profits.
Water use for food production is likely to increase, but production will still tend to concentrate where resources are more readily available, such as prime agricultural land. Some hydroponics and vertical farms in cities will provide a small proportion of food. Meanwhile, whether there are fundamental shortages or not, better engineering will still mean lower requirements for resources than before right across industry. Where local shortages do exist, industry can simply recycle more. It is therefore hard to see any cause for concern for future water supply. There will always be local problems arising, but they can generally be solved.
In summary, there is too much panic about water in the future. We will face a lot of big problems, but water isn’t one of them.