Water is precious for many reasons. It is an essential resource for sustaining human, animal, and vegetative life. A living cell is mostly water. An adult human's body is about 65 percent water; blood is 90 percent water. Agriculture is absolutely dependent on water to produce food crops and livestock. Water is crucial to tourism, navigation, and industry. Enormous amounts are used to generate power, mine materials, and produce goods. Water is an ingredient, a medium, and a means of conveyance or cooling in most industrial processes. Water supplies a vital habitat for many of Earth's creatures, from the whale to the tadpole. There are entire ecosystems that are water-based.

All of these competing uses put an enormous strain on Earth's water supply. Overall, the amount of water on Earth remains constant, simply passing from one stage to another in a circular pattern known as the hydrologic cycle...