Droughts make matters worse. But the real problem isn't shrinking water levels - it's population growth. Over the next four decades, America will have 120 million more people, the equivalent of one new person every 11 seconds. More people will put a huge strain on our water resources.

The fear is that as populations grow and development spreads, vicious battles will erupt between water-rich and water-poor states and countries, particularly in major river basins where upstream populations control the flow of downstream water.

In the US, we've traditionally engineered our way out of water shortages by diverting more from rivers, building dams or drilling groundwater wells. But many rivers, including the Colorado and the Rio Grande, already dry up each year. The dam-building era from the 1930s to the 1960s tamed so many rivers that only 60 in the country remain free-flowing. Meanwhile, we're pumping so much water from wells that the levels in aquifers are plummeting. We're running out of technological fixes.

Viable solutions include desalination of ocean water, reuse of municipal waste and aggressive conservation strategies. But none of these is a cure-all. Desalination is expensive, burns energy and generates waste. Reclaiming water has a major "yuck" factor, but it's also quite expensive, requiring a set of pipes that is completely separate from the drinking-water system. Conservation does work, but it's not enough.

If the current rates of growth in supply and demand continue, then water will become a very scarce resource. When I was in India recently, there were BIG strikes between neighboring states over new laws that changed water rights. As water becomes more and more scarce in populated areas, conflicts will inevitably be the appropriate response to water shortages.

Are we heading for an era in which rivers and lakes and aquifers become national security assets that are fought over? With water availability shrinking across the Middle East, Asia and sub-Saharan Africa, violent conflict between states is increasingly likely. Major government agencies, including the CIA, have already raised the specter of future "water wars".

As the world's population grows, competition for food, water and energy will increase, brewing a "perfect storm". But, that's another story which we'll save for another eNews.

• Our Water Supply, Down the Drain
• Water wars on the horizon
• Water and Conflict