CONTENTS
HIGHLIGHTSPopulation Reports is published by the Population Information Program, Center for Communication Programs, The Johns Hopkins School of Public Health, 111 Market Place, Suite 310, Baltimore, Maryland 21202-4012, USA
September, 1998 |
Water Availability and Use Some 70% of the earth's surface is water, but most of that is ocean. By volume, only 3% of all water on earth is fresh-water, and most of this is largely unavailable (39, 57). About three-quarters of all freshwater is locked away in the form of ice caps and glaciers located in polar areas far removed from most human habitation; only about 1% is easily accessible surface freshwater. This is primarily the water found in lakes, rivers, and the soil at underground levels shallow enough to be tapped at an affordable cost. Only this amount is regularly renewed by rain and snowfall and thus available on a sustainable basis (174) (see Figure 2). In all, only one one-hundredth of one percent of the world's total supply of water is considered easily accessible for human use (108). Globally, between 12.5 and 14 billion cubic meters of water (12,500 to 14,000 cubic kilometers) are considered available for human use on an annual basis. This amounts to about 9,000 cubic meters per person per year, as estimated in 1989 (30, 107, 145, 157). (Note: 1 cubic meter equals 1,000 liters.) By the year 2025 global per capita availability of freshwater is projected to drop to 5,100 cubic meters per person as another 2 billion people join the world's population (184). Even then, this amount would be enough to meet human needs if it were distributed equally among the world's population (157). Global per capita figures on water availability give a false picture, however. The world's available freshwater supply is not distributed evenly around the globe, throughout the seasons, or from year to year. In some cases water is not where we want it, nor in sufficient quantities. In other cases we have too much water, in the wrong place, at the wrong time. We "live under the tyranny of the water cycle," observes hydrologist Malin Falkenmark, referring to the earth's hydrological cycle (62). The earth's hydrological cycle acts like a giant water pump that continually transfers freshwater from the oceans to the land and back again (see Figure 3). In this solar-driven cycle, water evaporates from the earth's surface into the atmosphere and is returned as rain or snow. Part of this precipitation evaporates back into the atmosphere. Another part flows into streams, rivers, and lakes, commencing a journey back to the sea. Still another part sinks into the soil and becomes soil moisture or groundwater. Plants incorporate soil moisture into their tissues and release it into the atmosphere in the process of evapotranspiration (174). Much of the groundwater eventually works its way back into the flow of surface waters (176). |