Unit 8: Water Resources // Section 5: World Demand for Water
How much water do humans use? The answer depends on where they live and on their socioeconomic status. Under primitive conditions a person will consume three to five gallons per day for drinking and subsistence farming. In a city where water is also used for cleaning, manufacturing, and sanitation, per capita use is around 150 gallons per day. In the United States, which has among the highest water consumption rates in the world, each person uses an average of 1,340 gallons of water per day. Table 2 shows how much water is required to produce common goods and services.
|1 pound of cotton||2,000|
|1 pound of grain-fed beef||800|
|1 loaf of bread||150|
|1 kilowatt hour of electricity||25|
|1 pound of rubber||100|
|1 pound of steel||25|
|1 gallon of gasoline||10|
|1 load of laundry||60|
|1 ten-minute shower||25-50|
As discussed in Unit 2, "Atmosphere," and Unit 3, "Oceans," water resources are not distributed evenly in space or time around the world. Global circulation patterns create wet and dry climate zones, and in some regions seasonal or multi-annual climate cycles generate distinct wet and dry phases. As a result, some regions have larger freshwater endowments than others (Fig. 7).
Figure 7. World freshwater supplies
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Source: © United Nations Environment Programme.
Although developed nations generally have more water available than many countries in Africa and the Middle East, some areas with good water endowments still are subject to "water stress" because they are withdrawing water from available supplies at extremely high rates (Fig. 8). High-intensity water uses in industrialized nations include agricultural production and electric power generation, which requires large quantities of water for cooling. In the United States electric power production accounts for 39 percent of all freshwater withdrawals (footnote 4), although almost all of this water is immediately returned to the rivers from which it is withdrawn. Agriculture consumes much more water because irrigation increases transpiration to the atmosphere.
Figure 8. Current and projected freshwater stress areas
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Source: © Philippe Rekacewicz, UNEP/GRID-Arendal.
As of 2002, 1.1 billion people around the world (17 percent of global population) did not have access to safe drinking water and 2.6 billion people (42 percent of global population) lived without adequate sanitation. As a result, millions of people die each year of preventable water-related diseases. Most of the countries with inadequate supplies of safe drinking water are located in Africa, Asia, and the Pacific, but problems persist elsewhere as well. For example, many households lack adequate sewage treatment services in Eastern Europe. And inequity among water users is widespread: cities often receive better service than rural areas, and many poor communities in both rural and urban areas lack clean water and sanitation (footnote 5).
Although these challenges apply in many regions, it is hard to make broad generalizations about water resources at the global or national level; to paraphrase the famous saying about politics, all hydrology is local. The basic geologic unit that scientists focus on to characterize an area's water supply and water quality with precision is the watershed or catchment area—an area of land that drains all streams and rainfall to a common outlet such as a bay or river delta. Large watersheds, such as the Amazon, the Mississippi, and the Congo contain many smaller sub-basins (footnote 6).
To see why water issues are best studied at the watershed level, consider Washington State, which is divided centrally by the Cascade Mountains. West of the Cascades, Washington receives up to 160 inches of rainfall annually, and the mild, humid climate supports temperate rainforests near the Pacific coast. Across the Cascades, rainfall is as low as six inches per year in the state's semiarid interior where groundwater is pumped from deep within basalt formations to grow wheat (Fig. 9). Urban Seattle residents and ranchers in rural eastern Washington thus face very different water supply, runoff, and water quality issues.
Figure 9. Average annual precipitation, Washington, 1971–2000
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Source: © 2006 by the PRISM Group and Oregon Climate Service, Oregon State University.
Currently 10,000 to 12,000 cubic kilometers of freshwater are available for human consumption each year worldwide. In the year 2000 humans withdrew about 4,000 km3 from this supply. About half of the water withdrawn was consumed, meaning that it was evaporated, transpired by plants, or contaminated beyond use, and so became temporarily unavailable for other users. The other 50 percent was returned to use: for example, some water used for irrigation drains back into rivers or recharges groundwater, and most urban wastewater is treated and returned to service.
Of the water withdrawn for human use, 65 percent went to agriculture, 10 percent to domestic use (households, municipal water systems, commercial use, and public services), 20 percent to industry (mostly electric power production), and 5 percent evaporated from reservoirs (footnote 7). About 70 percent of the water used for agriculture was consumed, compared to 14 percent of water used for domestic consumption and 11 percent of water used for industry.
Both population levels and economic development are important drivers of world water use. If current patterns continue, the World Water Council estimates that total yearly withdrawals will rise to more than 5,000 km3 by 2050 as world population rises from 6.1 billion to 9.2 billion. During the 20th century, world population tripled but water use rose by a factor of six (footnote 8). The United Nations and the international community have set goals of halving the number of people without adequate safe drinking water and sanitation by 2015. Meeting this target will require providing an additional 260,000 people per day with clean drinking water and an additional 370,000 people per day with improved sanitation through the year 2014, even as overall world demand for water is rising (footnote 9).