[QUOTE=Shagnasty]
The supply of water isn’t really shrinking because we don’t “use” water in the same way we use other things. It merely gets moved, maybe polluted some, cycled, and then treated again to be used tomorrow.
What if everyone started using 20% more water in my town that doesn’t have water shortages? The water treatment plant would have to work more but that is about it. There would be about the same amount of water that their always was because we have enough to cycle more than we do now.
[/QUOTE]
This is absolutely not true. Google ‘water depletion’ and check out the first couple dozen of entries. While water doesn’t disappear by using it, it is removed from the fresh water supply (i.e. underground aquifers), and is thus not available for irrigation, cleaning, and drinking. Municiple water purification/treatment typically reuses only a small percentage of overall waste water; most of what is done is taking environmental water–rivers, streams, underground supplies–and treating it to remove natural bacteria and parasites.
This may become a critical ecological issue–far more so than air pollution, soil contamination, or deforestation–in the 21st Century. Aquifers can only be replunished at a certain rate via filtration of atmospheric precipitation (rain, snow), and if we extract water from underground sources faster than it can be replaced, groundwater supplies will become depleted, which is exactly what is happening with aquifers in the American West and Eurasia where intensive agriculture is common. We assume that because water has always been available that it will continue to appear, in the same way that consumers were once told that oil supplies would never be depleted. The truth is (as residents of the Southwest, save for those serviced by Mulholland’s Great Water Buyoff, are well-aware) potable water is a precious commodity. Read Cadillac Desert for a more in-depth discussion on this.
As for what can be done: demanding that consumers use less water is nearly as futile as insiting that they use less energy. Unless people are penalized financially for overuse of water (as happens in high altitute areas or ‘desert’ islands where water has to be shipped in), they won’t reduce the usage. However, it is possible–albeit with massive infrastructure modifications–to use non-potable “grey water”, i.e. water that has already been used for for light cleaning or some such–for wastewater purposes like sewage, industrial waste removal, et cetera. There’s no reason to use potable water in your toliet, save that there is no system for doing otherwise. The only other alternative is to tap into the mass of water that covers nearly 80% of the Earth’s surface; that water, however, is non-potable and regretably corrosive owing to its high alkilinity and resulting corrosiveness. In order to make use of it, we’d have to have a cost-effective way of desalinating the water, then distributing it into inland municiple water supplies.
Well, I have a warm and fuzzy feeling now that I’ve been conserving water all along without even realizing it. 
Stranger