by Doug Prince
Vörösmarty, 46, is part of the Water Systems Analysis Group at UNH. He has been studying and reporting on water shortages around the world for the past several years. He and his research team have developed a computer model that analyzes the problem on a much finer scale than any previous study. His research, which began as a yearlong project in a graduate course that he taught, shows how the availability of water is likely to be affected by population growth, economic development and climate change. For many parts of the world, the picture looks bleak indeed.
Each day, each person on Earth withdraws, directly and indirectly, an average of 450 gallons of water (see sidebar). Two-thirds of this amount goes to irrigate the crops they eat (or crops used to feed the animals they eat), and the remainder is used for domestic or industrial purposes. Societies begin to experience moderate levels of "water stress" when they use 20 percent of the water supply available to them. Those needing 40 percent or more are at serious risk, with health repercussions and severe restrictions on economic activity.
Vörösmarty points out that those figures represent percentages of mean annual flow in the water supply. There are large variations in flow from season to season, and many parts of the world don't have reservoirs to store water for dry times of the year. "Whenever water use is 20 percent or greater, there is an increasing preoccupation with getting water," he notes.
A 1997 study by the United Nations tried to gauge the amount of water stress around the world by examining countries one at a time, comparing the amount of water available to each country with its overall domestic, industrial and agricultural needs. The results indicated that more than two billion people live under conditions of moderate water stress, and almost half a billion are already experiencing extreme stress.
Dividing the world picture into 208 countries is a good start, but it's bound to overlook water problems in areas that are smaller than an entire country. The United States, for example, has an adequate water supply overall, but the outlook in Tucson is quite different from that in Seattle or Durham.
Vörösmarty's group divided the globe into a grid of 60,000 cells (at the equator each cell is a square, about 30 miles on a side). They then built a computer model that combined a wealth of information: population distributions, industrial consumption statistics, climate-change models and "digital rivers"--detailed models of rivers and their water supplies compiled with data from more than 600 stream gauges and thousands of rainfall monitoring sites around the world. The researchers used the computer model to calculate water-stress levels for each of the 60,000 cells. They found that the United Nations study had vastly underestimated the number of people who are living with water shortages, and the number experiencing extreme shortages is about four times as high as the U.N. estimate. "By our reckoning, it's probably about a billion more people," Vörösmarty says.
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