This story is a supplement to the feature "Facing the Freshwater Crisis" which was printed in the August 2008 issue of Scientific American.

Lots of Water, but Not Always Where It Is Needed
One hundred and ten thousand cubic kilometers of precipitation, nearly 10 times the volume of Lake Superior, falls from the sky onto the earth’s land surface every year. This huge quantity would be enough to easily fulfill the requirements of everyone on the planet if the water arrived where and when people needed it. But much of it cannot be captured (top), and the rest is distributed unevenly (bottom). 

Where does the rain go?
More than half of the precipitation that falls on land is never available for capture or storage because it evaporates from the ground or transpires from plants; this fraction is called green water. The remainder channels into so-called blue-water sources—rivers, lakes, wetlands and aquifers—that people can tap directly. Farm irrigation from these free-flowing bodies is the biggest single human use of freshwater. Cities and industries consume only tiny amounts of total freshwater resources, but the intense local demand they create often drains the surroundings of ready supplies.

Water supplies today
Much of the Americas and northern Eurasia enjoy abundant water supplies. But several regions are beset by greater or lesser degrees of “physical” scarcity—whereby demand exceeds local availability. Other areas, among them Central Africa, parts of the Indian subcontinent and Southeast Asia, contend with “economic” water scarcity, where lack of technical training, bad governments or weak finances limit access even though sufficient supplies are available.