OF ALL THE resources on the planet, none is more precious than water. Humans depend on water for drinking, bathing, cooking, and sanitation. Without water to irrigate crops that are grown on farms, there would be no food for the world's people. Power plants use water to create steam, which in turn makes electricity, and factories use water to clean industrial machinery and keep it cool. All forms of wildlife need water in order to survive, as do forests, flowers, and everything else that grows. In fact, freshwater is such an essential resource that without it, there could be no life at all on Earth.

Plentiful—yet scarce

Earth is a place where there is abundant water—approximately 326 million cubic miles of it. That is a tremendous amount, especially considering 1 cubic mile of water equals more than 1 trillion gallons. The numbers are...