About 40 trillion gallons of water hover above the United States on any given day. As the day passes, about a tenth of that will fall to earth, running off into lakes and rivers, and some will soak into the ground. The remainder evaporates into the atmosphere, completing what’s known as the water cycle. Nearly all the water on the earth has passed through the water cycle countless times. In fact, scientists note that very little water has been created or lost over the past one billion years. The water cycle has four stages: storage, evaporation, precipitation and runoff. (See the diagram to the right to understand how the earth’s water cycle works.)

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Where Does Rain Go?

Polluted Runoff

Water Quality Initiative
Milwaukee Metropolitan Sewerage District
July 26, 2008