The photographs of earth taken by astronauts show a strikingly blue planet, seemingly a world of water spinning in space. Yet this impression of water wealth may be as deceptive as a desert mirage. Only about 2.5 percent of all the water on earth is fresh, and two-thirds of that is locked in glaciers and ice caps. The renewable freshwater supply on land-that made available year after year by the solar-powered hydrologic cycle in the form of precipitation-totals some 110,300 cubic kilometers (1 cubic kilometer equals 1 billion cubic meters), a mere 0.008 percent of all the water on earth.Each year, nearly two-thirds of this renewable supply returns to the atmosphere through evaporation or transpiration, the uptake and release of moisture by plants. This process supplies the water needed for forests, grasslands, rain-fed croplands, and all other nonirrigated vegetation. The remainder, just over one-third of the renewable supply-about 40,700 cubic kilometers per year-is runoff, the flow of freshwater from land to sea through rivers, streams, and underground aquifers. This is the source for all human diversions or withdrawals of water-for irrigated agriculture, industry, and households as well as a variety of “instream” water services, including the dilution of pollutants, navigation, and the generation of hydroelectric power. Rivers also carry nutrients from the land to the seas and in this way help support the highly productive fisheries of coastal bays and estuaries. Thus, by virtue of the hydrologic cycle, the oceans water the continents, and the continents nourish the oceans.
Though the volume of runoff seems huge, nature’s delivery of this freshwater supply does not correlate well with the distribution of world population. Asia, for example, receives 36 percent of global runoff but is home to 60 percent of the world’s people; South America, on the other hand, supports 6 percent of the population yet has 26 percent of the world’s runoff. The Amazon River alone carries 15 percent of the earth’s runoff but is accessible to only 0.4 percent of the world’s population. Much of the river flow in the tropics and high latitudes is virtually inaccessible to people and economic activity and is likely to remain so for the foreseeable future, since water is difficult and expensive to transport long distances. In fact, 55 rivers in northern North America, Europe, and Asia, with combined annual flows equal to about 5 percent of global runoff, are so remote that no dams have been constructed on them, even for hydropower.
According to a 1996 study conducted by this author and Gretchen Daily and Paul Ehrlich of Stanford University, the total amount of runoff within reach geographically totals some 32,900 cubic kilometers or about 81 percent of the total runoff. But that’s not the end of the story. About three-fourths of this amount is flood water and therefore not accessible on demand when it is needed most. To add to the remaining one-fourth that is accessible, engineers have built large dams and reservoirs, raising the stable supply of water provided by underground aquifers and year-round river flows by about half. This brings the total stable renewable supply to 12,500 cubic kilometers.
Globally, people now use about 35 percent of this accessible supply, or some 4,430 cubic kilometers per year. At least an additional 19 percent is used “instream” to dilute pollution, sustain fisheries, and transport goods. Thus humanity is already appropriating, directly or indirectly, more than half of the water supply that is now accessible. The problem is that water use tripled between 1950 and 1990 as world population soared by some 2.7 billion. Given that the population is projected to climb by nearly the same amount over the next 30 years, this is a troubling prospect. Worldwide demand for water cannot triple again without causing severe shortages for crop irrigation, industrial use, basic household needs, and critical life-supporting ecosystems.