The definition of freshwater is water containing less than 1000 milligrams per liter of dissolved solids, most often salt. The global distribution of freshwater resources varies greatly from region to region (see Figure 1). An 'inventory' of Earth's waters shows that approximately 97% of the global water supply is found in the oceans, which are saline. A very small amount of salty water is also located in saline lakes (e.g., the Caspian Sea). The remaining water inventory (3%) is 'freshwater'. Permanent ice (e.g., continental and mountain glaciers) is the largest freshwater storage on Earth, accounting for about 2% of the total global supply - or nearly 69% of the total freshwater supply. Freshwater is also found beneath the Earth's surface as groundwater (approximately 30% of the total freshwater supply) and in surface water storages such as lakes, streams, swamps and marshes. Minute amounts of freshwater are also stored in the soil, the atmosphere and in biological organisms.
Freshwater Use in the U.S.
Freshwater is of major importance to all living things; in some organisms, up to 90 percent of their body weight comes from water. Up to 60 percent of the human body is water, the brain is composed of 70 percent water, and the lungs are nearly 90 percent water. About 83 percent of our blood is water, which helps digest our food, transport waste, and control body temperature. Each day humans must replace 2.4 liters of water, some through drinking and the rest taken by the body from the foods eaten.
A report by the U. S. Geological Survey (USGS), U.S. Geological Survey Circular 1268, "Estimated Use of Water in the United States in 2000.", shows that about 408 billion gallons of water per day were withdrawn for use in the United States during 2000. Withdrawals in 1990 averaged nearly 1,620 gallons per day per person; in 2000, the per capita average had declined to about 1,430 gallons per day. During the same decade, the United States experienced a population increase of about 33 million. Total withdrawals increased steadily from 1950 to 1980 but have varied less than 3 percent since 1985.
Approximately 76 percent of the 2000 withdrawals were from surface water, and the remainder was from groundwater. About 85 percent of total withdrawals were freshwater because it is required for many uses. The saline withdrawals were nearly all from surface water. California, Texas, and Florida withdrew the most water; together they accounted for one-fourth of all water withdrawals in 2000.
Thermoelectric-power plants accounted for 40 percent of total withdrawals (136,000 million gallons per day [Mgal/d]) in 2000. Surface water was the source for more than 99 percent of total thermoelectric-power withdrawals, one third of which were saline. Historically, large supplies of water, mainly for cooling, had to be available to operate thermoelectric-power plants. For this reason, large power plants have been sited near the oceans, the Great Lakes, the Gulf of Mexico, and large rivers.
Withdrawals for irrigation were about 137,000 Mgal/d, second only to thermoelectric power nationwide. Irrigation represented 39 percent of total withdrawals and 31 percent of total freshwater withdrawals. Eighty-six percent of irrigation withdrawals, and 75 percent of the total land irrigated in 2000 were in the 17 conterminous Western States. Withdrawals for irrigation have remained nearly stable since 1985 despite an 8-percent increase in total acres irrigated.
During 2000, public suppliers withdrew about 43,300 Mgal/d of freshwater. Public-supply withdrawals constituted about 13 percent of total water withdrawals and about 10 percent of total freshwater withdrawals. About 63 percent of these withdrawals was from surface water sources. Public-water suppliers deliver water to users for household (domestic), industrial, commercial, and other purposes. If a home is not connected to a public-supply system, the household needs its own water supply. Typically homes in urban settings are served by public-supply systems, rural homes have their own wells, and suburban homes may have either source of water. During 2000, about 43.5 million people had their own water supply, using about 3,590 Mgal/d, most of which was groundwater. During 1950, 62 percent of the population was served by public supplies; by 2000 this had risen to 85 percent.
For 2000, self-supplied industrial withdrawals were an estimated 18,500 Mgal/d or about 5 percent of total withdrawals. Industrial water use includes water used for the manufacture and production of commodities such as food, paper, chemicals, refined petroleum, and refined metals. Water for industrial uses may be delivered from a public supplier or be self-supplied. Surface water was the source for 81 percent of total industrial withdrawals. Less than 7 percent of total industrial withdrawals were from saline water. Since 1985, withdrawals for industrial uses have declined steadily from about 25,800 Mgal/d to 18,500 Mgal/d in 2000. This decline may be due to a combination of factors. The number of production workers in manufacturing establishments has dropped about 5 percent since 1989; some intensive water-using industries, such as petroleum and coal products, have undergone large reductions. Environmental laws and regulations also have encouraged more efficient use of water by industrial facilities.
Combined withdrawals for livestock, aquaculture, and mining were less than 2 percent of total withdrawals in 2000. Aquaculture includes fish farms and fish hatcheries. Only freshwater was considered for the livestock and aquaculture categories. During 2000, groundwater accounted for 29 percent of the withdrawals for aquaculture. Livestock withdrawals were an estimated 1,760 Mgal/d, 57 percent from groundwater. Mining water use is the use of water for extracting solid minerals (such as copper), liquids (such as petroleum), and gases (such as natural gas). Mining water use in 2000 was 60 percent groundwater and 58 percent freshwater.
The following diagram uses a "cylinder" and "pipe" layout to show where our nation's water comes from, how it is used, and where it goes after use. A detailed explanation is given below the diagram. The data for this chart is from U.S. Geological Survey Circular 1268, "Estimated Use of Water in the United States in 2000." (Note: Figures in the diagram are rounded.)
The top row of cylinders represents where America's water came from (source) in 2000, either from surface water or from groundwater. You can see most of the water we use (262,000 million gallons per day (Mgal/d)) came from surface water sources, such as rivers and lakes. We used about 83,400 Mgal/d of groundwater (from wells).
The pipes leading out of the blue and brown cylinders on the top row show where the water was sent after being withdrawn from a river, well, etc. For example, the blue pipe coming out of the surface water cylinder and entering the Public supply grey cylinder shows that 27,300 Mgal/d of water was withdrawn from surface water sources for public-supply uses. Likewise, the brown pipe shows that public-suppliers withdrew another 16,000 Mgal/d of water from groundwater sources.
Each green cylinder represents a category of water use. The Industrial cylinder, for instance, shows how much water the United States used, each day, by industries. In 2000, about 18,500 Mgal/d of water was used for industrial purposes, with about 14,900 Mgal/d coming from surface water and about 3,570 Mgal/d coming from groundwater.
Fresh Groundwater Use
The main uses of groundwater include irrigation uses, drinking water and other public uses, and for supplying domestic water to people who do not receive public-supply water. The majority of water used for self-supplied domestic and livestock purposes came from groundwater sources. Of all the water used in the United States in 2000 (about 408 billion gallons per day (Bgal/d) of fresh and saline water), about 21 percent (69.8 Bgal/d) came from groundwater sources. Water from surface water sources accounted for the remaining 79 percent. Very little saline groundwater was used in 2000. Almost 99 percent of groundwater came from freshwater aquifers. A very small amount was used for industrial purposes, but most, over 60 percent, of the groundwater used in mining was saline.
Fresh Surface Water Use
The water in the nation's rivers, streams, creeks, lakes, and reservoirs are vitally important to our everyday life. The main uses of surface water include drinking water and other public uses, irrigation uses, and for use by the thermoelectric-power industry to cool electricity-generating equipment. The majority of water used for thermoelectric-power, public supply, irrigation, mining, and industrial purposes came from surface water sources. Of all the water used in the United States in 2000 (about 408 billion gallons per day (Bgal/d), fresh and saline), over 79 percent (387 Bgal/d) came from surface water sources. Water from groundwater sources accounted for the remaining 21 percent. Over 80 percent of all water used in 2000 was freshwater, although saline water was heavily used in the thermoelectric-power industry, and, to a lesser extent, for industrial and mining purposes.
Global Freshwater Use (2000)
Because much of the world's surface water is far from concentrations of human settlements, not all of it is readily usable. It is estimated that the freshwater available for human consumption varies between 12,500 km3 and 14,000 km3 each year
The availability of freshwater in 2000 varied from country to country (see below). The countries with the most freshwater resources were Suriname and Iceland; Egypt and the United Arab Emirates had the least freshwater resources.
Although the absolute quantities of freshwater on Earth have remained approximately the same, the uneven distribution of water and human settlement continues to create growing problems of freshwater availability and accessibility.
According to Population Action International, based upon the United Nations Medium Population Projections of 1998, more than 2.8 billion people in 48 countries will face water stress or scarcity conditions by 2025. Of these countries, 40 are in West Asia, North Africa or Sub-Saharan Africa. Over the next two decades, population increases and growing demands are projected to push all the West Asian countries into water scarcity conditions. By 2050, the number of countries facing water stress or scarcity could rise to 54, with their combined population being 4 billion people—about 40% of the projected global population of 9.4 billion. Many African countries, with a population of nearly 200 million people, are facing serious water shortages. By the year 2025, it is estimated that nearly 230 million Africans will be facing water scarcity, and 460 million will live in water-stressed countries. Today 31 countries, accounting for less than 8% of the world's population, face chronic freshwater shortages. Among the countries likely to run short of water in the next 25 years are Ethiopia, India, Kenya, Nigeria and Peru. Parts of other large countries (e.g. China) already face chronic water problems.
Globally, the agricultural sector is by far the biggest user of freshwater, followed by the industrial sector, and domestic water use.
- U.S. Geological Survery - Water Science for Schools
- U.N. Environment Programme - Freshwater
- U.N. Environment Programme - Vital Water Graphics
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