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Earth's water cycle. Solar energy causes water vapor to rise into the atmosphere through evaporation. There, it condenses into droplets and forms clouds. Water returns to the surface through precipitation. It then evaporates, flows into rivers, lakes, or the ocean, or sinks through the ground, infiltrating underground aquifers. Image courtesy of the Marian Koshland Science Museum.

Precipitation levels vary widely across the United States. Image courtesy of the U.S. National Atlas.

Severe drought in 2007 left what is normally a pond outside Nicholasville, KY, nothing but dry, cracked ground. Image from the Lane Report; photo by Andy Olsen.

Residential uses of water in the united States (typically 200 gallons per day per household). (Click to enlarge) Data from Mayer, et al. Residential End Uses of Water, 1999.

Total desalination capacity by country, 2006. Copyright International Mapping Associates.

Deaths from infectious disease have declined sharply since utilities began using chlorine to disinfect drinking water. Image courtesy of the Marian Koshland Science Museum.

Projected annual replacement needs for water distribution infrastructure, 2000-2075. Many water delivery pipes in the United States will soon reach the end of their life expectancy. Image courtesy of the EPA.