To build on Tarek's Earth Day thoughts about a summer of water shortages, see this thought-provoking article in Wired magazine, "Peak Water: Aquifers and Rivers Are Running Dry. How Three Regions Are Coping." A tidbit:

Freshwater is the ultimate renewable resource, but humanity is extracting and polluting it faster than it can be replenished. Rampant economic growth -- more homes, more businesses, more water-intensive products and processes, a rising standard of living -- has simply outstripped the ready supply, especially in historically dry regions. Compounding the problem, the hydrologic cycle is growing less predictable as climate change alters established temperature patterns around the globe.

Water being renewable, the idea of "peak water" is different than "peak oil." We won't just run out. But we can and do overuse. And, unlike oil, there are no substitutes.