and permafrost, or else lies deep underground, of little 

 use to those of us living on the land above. 



This special single-topic issue of Natural History examines 

 that last 1 percent, that precious supply that keeps us alive. 

 The articles range from technical to fanciful. Christopher 

 J. Mundy, Shawn M. Kathmann, and Gregory K. Schenter 

 look at the properties of water, and how it plays its funda- 

 mental role in the planet's heat budget and weather systems. 

 Dolly Setton's graphic depicts the many remarkable ways 

 animals have adapted to limited water resources. Merry D. 

 Camhi, who has assisted me in my role as curator of the 

 exhibition "Water: H2O = Life," which opens this month 

 at the American Museum of Natural History, joins me 

 in a look at why freshwater ecosystems are so vulnerable 

 to declines in biodiversity. Sharon P. Nappier, Robert S. 

 Lawrence, and Kellogg J. Schwab detail the worldwide 



the bare minimum gallon-plus per day of safe drinking 

 water, and 2.6 billion lack access to basic sanitation. In 

 contrast, those of us who live in the United States and 

 Canada each consume, on average, more than 150 gallons 

 a day for domestic and municipal purposes (not including 

 agricultural and industrial usage). In the United Kingdom 

 people do fine with about a fifth as much. 



People appropriate more than half the world's available 

 surface freshwater. Globally, 70 percent of withdrawals 

 from rivers and groundwater are used for agriculture. 22 

 percent for industry, and the remaining 8 percent for homes 

 and municipal use. As demand increases, driven by both 

 population growth and soaring consumption rates, water 

 appropriation is projected to rise to 70 percent by 2025. 

 In many ways, we are already damaging the systems that 

 provide us with this critical natural resource. 



One-sixth of the world's population 

 cannot access even a gallon of safe drinking water a day. 



human health concerns of contamination, pollution, and 

 waterborne diseases. Shea Penland presciently outlined, in 

 the February 2005 issue of Natural History, the threat to 

 New Orleans from hurricanes and catastrophic flooding. 

 Now, he gives a sobering appraisal of current "reconstruc- 

 tion" efforts. Chuck Carter's illustration depicts a range 

 of creative high- and low-tech solutions to the challenges 

 of limited or contaminated water supplies. Azzam Alwash 

 describes the heartening recovery of marshlands in post- 

 Saddam Iraq. And Sandra L. Postel explores the politics 

 of water in the Middle East. 



Freshwater is not evenly distributed across the globe. 

 The Americas have the largest amount and Oceania 

 (Australia, New Zealand, and the Pacific islands) 

 the smallest. Thinly inhabited Oceania, however, has the 

 greatest per capita supply, more than 9.5 million gallons per 

 person per year. Asia has the lowest. By country, Brazil. 

 Canada, China, Colombia, Indonesia, and Russia together 

 have half the world's supply of freshwater; northern Africa 

 and the Middle East are the water-poorest. The United 

 Nations defines water scarcity as less than 500 cubic meters 

 (132,000 gallons) per person per year. Kuwait has a natural 

 supply only one-fiftieth that amount, but given its huge 

 supply of oil, it can afford to run desalination plants. 



At the individual level, further inequities emerge. Al- 

 though a person can manage for a few' days on a gallon 

 or two a day, an adequate supply of clean water is about 

 thirteen gallons per person per day. Ten percent of it is 

 needed for drinking, the rest for sanitation and hygiene 

 (40 percent), bathing (30 percent), and cooking (20 per- 

 cent). In 2006 the UN estimated that more than a billion 

 people — one-sixth of the world's population — lack even 



Groundwater is one of the major systems being stressed. 

 Overpumping, or extracting water faster than the under- 

 ground systems recharge, has led to plummeting water 

 tables, not only in the Middle East and northern Africa, 

 but also in China, India, Iran, Mexico, and the U.S. The 

 Ogallala aquifer, one of the world's largest, stretches under 

 parts of eight states in the High Plains of the central U.S.. 

 from South Dakota to Texas. Water began collecting in 

 porous sediments there some 5 million years ago; a geo- 

 logically slow rate of recharge means that deep wells still 

 bring up water from the end of the last Ice Age, more 

 than 10,000 years ago, making it truly "fossil water." But 

 the aquifer is being pumped out many times faster than it 

 can be replenished. Between the early 1900s, when the 

 Ogallala was first tapped for irrigation, and 2005, the 

 water table dropped by more than 150 feet in some parts 

 of Texas, Oklahoma, and Kansas. The raising of crops 

 has become uneconomical for some Great Plains farmers, 

 and further depletions could have substantial ripple effects 

 on billions of people around the world who depend on 

 American farm products. 



As more land is paved over, rainwater can no longer 

 soak into the ground or evaporate slowly to recharge the 

 system. In coastal areas, a falling water table may open 

 an aquifer to an influx of saltwater, impairing or even 

 ruining it as a freshwater source. 



Human activities are affecting other aquatic systems 

 as well. Canals, dams, and levees that impede the 

 natural flow of water can change not only the ab- 

 solute quantity but the quality of water downstream: its 

 concentration of pollutants, its sediment load, its tem- 

 perature, and so on. People on both sides ot the barrier 



NATURAL history November 2007 



