Water Resources

The USGS Water Science School estimates that the oceans, seas and bays hold about 96.5% of Earth's water. Another 0.9% is saline groundwater and salt lakes. That leaves only about 2.5% as freshwater.

Even that 2.5% is mostly out of easy reach. About 68.7% of freshwater is locked in ice caps, glaciers and permanent snow, and about 30.1% is groundwater. Surface water and other freshwater make up only about 1.2%. Lakes hold 20.9% of that small slice and rivers only 0.49%. The rest is in ground ice and permafrost, soil moisture, the air, swamps and living things.

The EPA's WaterSense program sums it up: less than 1% of Earth's water is available for human use. The rest is salt water, frozen in the polar ice caps, or too hard to reach for practical use.

Rain and snowmelt that do not soak in or evaporate run downhill into streams. The USGS defines a watershed (also called a drainage basin) as an area of land that drains all the streams and rainfall to a common outlet, such as the outflow of a reservoir, the mouth of a bay, or any point along a stream channel. Watersheds can be as small as a footprint or as large as all the land that drains into Chesapeake Bay.

The edge of a watershed is a drainage divide, usually a ridge. Rain that falls on one side flows to one stream; rain on the other side flows somewhere else. Small watersheds nest inside bigger ones. National Geographic Education points out that the Mississippi and Missouri Rivers form the backbone of the much larger Mississippi watershed.

Because everything in a watershed drains to the same place, what happens upstream, on farms, in towns and on roads, affects the water downstream.

How long water stays put

The water cycle never stops, but water moves through some parts much faster than others. The time water typically spends in one place is its residence time. NOAA's JetStream gives these rough estimates:

Water in rivers is replaced within weeks, so rivers respond quickly to both rain and drought. Groundwater stays put for centuries on average, which is why water pumped out of an aquifer can take a very long time to come back. For more on the water cycle itself, see the Ecosystems lesson.

Some rain and snowmelt soak into the ground. Near the surface, the ground still holds some water, but its spaces are not completely filled; this is the unsaturated zone. Deeper down, every space is filled with water. The top of this saturated zone is the water table. Precipitation that soaks down and adds water to an aquifer is called recharge.

The USGS calls water-bearing rock or sediment that readily passes water to wells and springs an aquifer. National Geographic Education describes two general types:

• An unconfined aquifer lies below a permeable layer of soil, and its top is the water table.
• A confined aquifer has a layer of impenetrable rock or clay above it. The water in it can be under pressure. In an artesian well, that pressure can push water up the well, and sometimes completely out of it, without a pump.

When a well pumps water, the water table around it drops in a funnel shape called a cone of depression. The USGS warns that pumping too much water too fast draws down the aquifer, so a well yields less and less water and can even run dry, along with a neighbor's well. Pumping faster than recharge across a wide area is groundwater depletion. Its effects include dry wells, less water in streams and lakes, poorer water quality, higher pumping costs and land subsidence.

The USGS sorts U.S. water use into categories: thermoelectric power, irrigation, public supply, self-supplied industry, aquaculture, mining, self-supplied domestic use and livestock. Its most recent report covering all eight, USGS Circular 1441, estimated that the United States withdrew about 322 billion gallons a day in 2015. That was 9% less than in 2010 and the lowest total since before 1970. About 87% was freshwater.

Withdrawal is not the same as consumption

A withdrawal is water taken from a river, lake or aquifer. The USGS notes that much of it returns to the environment. Consumptive use is the part that does not return to local water bodies or groundwater, because it evaporates, is consumed by people or livestock, or goes into products or crops. Thermoelectric power plants withdrew the most water in 2015, about 133 billion gallons a day, but consumed only about 4.31 billion, roughly 3%. Irrigation withdrew about 118 billion gallons a day and had the largest consumptive use, about 73.2 billion.

At home, WaterSense cites the USGS estimate that each American uses about 82 gallons a day. The average American family uses more than 300 gallons a day at home. Nationally about 30% of household use is outdoors, and it can be much higher in dry places.

Around the world

Globally, the Food and Agriculture Organization of the United Nations (FAO) reports that agriculture accounted for 72% of freshwater withdrawals in 2023, followed by industry (15%) and services (13%).

Virtual water

Every product takes water to make. The water needed to grow, process and transport food is called virtual water (or embedded water). When you eat food grown in another region, you are, in a sense, eating that region's water. Adding up the virtual water in everything a person uses and eats gives their water footprint.

The FAO defines water scarcity as demand for freshwater exceeding the available supply in a given area and time period. It can grow as demand rises or as the quantity or quality of water declines. UNICEF describes two kinds:

• Physical water scarcity: the water resources themselves are overused and no longer meet people's needs. Pollution and seasonal shortages can add to it.
• Economic water scarcity: enough water exists, but it is not fully accessible, for example because of poor governance, limited investment or not enough infrastructure.

The two can happen in the same place. The World Health Organization (WHO) reports that in 2022, 73% of the world's people (6 billion) used a safely managed drinking-water service: one on the premises, available when needed and free from contamination. That left 2.2 billion people without one. At least 1.7 billion people used a drinking-water source contaminated with feces, and over 2 billion people lived in water-stressed countries in 2021.

Desalination

Desalination removes dissolved salts from seawater or salty groundwater. The USGS classes water with less than 1,000 parts per million (ppm) of dissolved salts as fresh; ocean water has about 35,000 ppm. The leading method is reverse osmosis (RO), which uses pressure to push water through a membrane that holds back the salts. RO typically uses less energy than thermal methods, such as distillation.

The USGS also lists the tradeoffs. To supply large populations, today's processes have to run at a large scale, and they are expensive, energy-intensive and need large facilities. Plants also have to deal with leftover salty water, called brine: the USGS describes one proposed plant that would have desalted half the seawater it took in and discharged the other half back as saltier brine.

In the United States, Congress passed the Safe Drinking Water Act (SDWA) in 1974 to protect public health by regulating public drinking-water supplies. Under it, the EPA's National Primary Drinking Water Regulations are legally enforceable standards and treatment techniques for public water systems. They use three key terms:

• Maximum contaminant level goal (MCLG): the level at which there is no known or expected risk to health, with a margin of safety. Goals are not enforceable.
• Maximum contaminant level (MCL): the highest level of a contaminant allowed in drinking water. It is set as close to the goal as feasible, using the best available treatment and taking cost into account, and it is enforceable.
• Treatment technique (TT): a required process meant to reduce the level of a contaminant in drinking water.

Source water is the rivers, streams, lakes, reservoirs, springs and groundwater that supply public drinking-water systems and private wells. The EPA explains that protecting it can lower health risks by preventing exposure to contaminated water, and can reduce the need for complex treatment. Protection can also help wildlife, recreation and the amount of water available.

How water is treated before you drink it, and how wastewater is cleaned afterward, is covered in Pollution and Solutions, section 7: Groundwater, drinking water and wastewater.

Using less water for the same crop, product or shower leaves more in rivers and aquifers for other users and for dry years. The main approaches work at every scale, from farms to homes:

WaterSense by the numbers

The EPA's WaterSense label goes on products that are 20% more water-efficient than standard models and perform as well or better. According to WaterSense:

• A labeled showerhead uses no more than 2.0 gallons per minute; standard showerheads use 2.5.
• A labeled toilet uses 1.28 gallons per flush or less; the federal standard is 1.6, and older toilets can use as much as 6.
• Replacing all old, inefficient toilets can save the average family 13,000 gallons a year.
• Household leaks can waste 9,400 gallons a year for the average family, and nearly 900 billion gallons nationwide.
• A WaterSense labeled irrigation controller can cut an average home's irrigation water use by up to 30%.