Where Earth's water is

From space, Earth looks very blue. That is because water covers most of it.

Water is in the sea, in rivers and lakes, in ice and in clouds. Water is under the ground too. It is even inside you!

Look at a globe and you will see a lot of blue. The U.S. Geological Survey (USGS) says about 71 percent of Earth's surface is covered with water. Water is almost everywhere: in the oceans, in rivers and lakes, in ice and snow, in the air as water vapor, in the ground, and inside plants, animals and people.

How much is there? If all of Earth's water were gathered into one ball, the USGS says it would be about 1,385 kilometers (about 860 miles) across. That sounds huge, but next to the whole Earth, the water ball looks small. And most of that water is not water we can drink.

About 71 percent of Earth's surface is water-covered, and water exists in the oceans, the air, rivers and lakes, ice caps and glaciers, soil, aquifers and living things. The USGS, using Igor Shiklomanov's 1993 estimates, gives the total as about 332.5 million cubic miles, or 1,386 million cubic kilometers. Gathered into one sphere, it would be about 1,385 km (860 miles) in diameter, which looks small next to the whole planet.

Two cautions apply to every number on this page. First, the figures are one estimate, and the USGS notes that percentages are rounded so do not add exactly to 100. Second, they are a snapshot: water never sits still, and the water cycle is constantly moving it from one store to another and one form to another, so the amounts in each store change over time.

Most of Earth's water is in the oceans.

Ocean water is salty. People need fresh water to drink. Out of every 100 cups of Earth's water, about 96 are salty sea water.

The oceans are by far Earth's biggest store of water. They hold about 96.5 percent of all the water on, in and above the planet. That means if you could pour all of Earth's water into 100 cups, about 96 and a half cups would be salty seawater.

There is salty water in other places too. Some lakes are salty, and some water deep underground is salty. The USGS points out that only fresh water is the kind life on land needs to survive. So even though Earth is a watery planet, the water that people, land plants and land animals depend on is only a small part of it.

In the USGS table, oceans, seas and bays hold about 1,338 million km³, 96.54 percent of all water. Saline water is not confined to the sea: about 12.9 million km³ of groundwater (0.93 percent of the total) is saline, as is about 85,400 km³ of lake water, so saline water altogether is about 97.5 percent of the total.

The oceans matter far beyond their volume. The USGS describes them as the main source of the evaporated water that feeds the water cycle; evaporation from the sea returns water to the air, and precipitation returns it to land as fresh water. Ocean currents also move water all around the globe. Because so little of the total is fresh, the small fresh stores, and how fast the water cycle refills them, are what matter most for people.

Only a little of Earth's water is fresh, not salty.

Most fresh water is frozen! It is ice and snow near the North and South Poles and on high mountains.

Only about 2.5 percent of Earth's water is fresh. And most of that fresh water is not runny at all: about 68.7 percent of it is frozen in ice caps, glaciers and snow that never melts away.

Most of this ice is in the polar regions and in Greenland. The USGS says glaciers cover almost 10 percent of the world's land, mostly in places like Greenland and Antarctica. Glaciers are a bit like frozen rivers: they flow downhill very slowly and are part of the water cycle too.

Fresh water makes up only about 2.5 percent of the total, and the largest fresh store is solid. Ice caps, glaciers and permanent snow hold about 24.1 million km³, 68.7 percent of all fresh water and 1.74 percent of all water; the USGS says this ice lies mainly in the polar regions and in Greenland. Ground ice and permafrost add about 300,000 km³ (0.86 percent of fresh water).

Glaciers cover almost 10 percent of the world's land today. The USGS compares a glacier to a frozen river: it flows downhill, erodes the landscape and moves water along in the water cycle, only much more slowly. It also notes that large ice sheets and sea ice influence weather and climate, because white ice reflects much more sunlight than darker land or water.

There is water under the ground!

It fills tiny spaces in soil and rock. People dig wells to reach it. This water also helps keep rivers flowing when it does not rain.

After ice, the next biggest store of fresh water is hidden under our feet. About 30 percent of Earth's fresh water is groundwater, soaked into the spaces in soil and rock. Places underground that hold lots of usable water are called aquifers, and people drill wells to reach them.

There is much more fresh water in the ground than in all the lakes and rivers on the surface. Groundwater also seeps slowly into riverbeds. That is why many rivers keep flowing even after weeks without rain. In desert towns, where there is little water on the surface, groundwater gives people the water they need.

The USGS table lists about 23.4 million km³ of groundwater, of which about 10.5 million km³ is fresh: 30.1 percent of all fresh water and more than 100 times the fresh water in lakes. Groundwater exists to some degree almost everywhere, but only in certain places, aquifers, can it be pumped in useful quantities.

Groundwater and surface water are one connected system. Precipitation seeps down to recharge aquifers, while groundwater seeps into riverbeds and keeps rivers flowing between rains. People rely on both: in 2010 the United States used about 275 billion gallons (1,041 billion liters) of surface water and about 79.3 billion gallons (300.2 billion liters) of groundwater each day. Surface water supplies more drinking water and irrigation overall, but groundwater supports people where visible water is scarce, such as desert towns in the western United States.

Lakes and rivers hold only a little of Earth's water.

But it is the water people use the most! There is water in the air and clouds too. And there is water in every plant and animal.

Lakes, rivers, wetlands and the water in the air hold only a tiny share of Earth's water. Fresh lakes hold about a quarter of 1 percent of the fresh water. Rivers hold even less: only about 1/10,000th of 1 percent of all Earth's water. Yet rivers and lakes are where most of the water people use every day comes from.

The air holds water too, mostly as invisible water vapor. If all the water in the air fell as rain at once, the USGS says it would cover Earth with only about 2.5 centimeters (1 inch) of water. And every living thing, from trees to you, holds a little water too.

The remaining stores are tiny but vital. In the USGS table, fresh lakes hold about 91,000 km³ (0.26 percent of fresh water), soil moisture 16,500 km³ (0.05 percent), the atmosphere 12,900 km³ (0.04 percent), swamp water 11,470 km³ (0.03 percent), rivers 2,120 km³ (0.006 percent) and living things 1,120 km³ (0.003 percent). Rivers amount to about 1/10,000th of one percent of all water, yet rivers and lakes supply most of the water people use every day.

These small stores are also the fastest-moving. The atmosphere holds only about 2.5 cm (1 inch) of rain-equivalent at any moment, but each day about 1,170 km³ of water evaporates or transpires into it, so the water in the air is replaced many times over each year. Because water keeps moving through the cycle, the small stores are refilled again and again.

Imagine all of Earth's water in a big bucket.

Almost the whole bucket is salty sea water. The fresh water would only fill about one cup. The lakes and rivers would be just a few drops!

Here is a way to picture the numbers. Imagine all of Earth's water shrunk down to fit in a 10-liter bucket. About 9.65 liters would be salty ocean water. The fresh water would fill only about 250 milliliters, about one cup. Of that cup, about two-thirds would be ice and almost a third would be underground.

The fresh water in all the world's lakes would be less than a single milliliter, just a few drops. And all the world's rivers would be smaller than one drop. That is one reason many people work to keep fresh water clean and not waste it.

Scaling the USGS percentages to a 10-liter bucket makes them easier to grasp (this is our arithmetic, not a USGS figure). Oceans: about 9.65 L. All fresh water (2.5 percent): about 250 mL. Within it, ice caps and glaciers about 170 mL (1.74 percent of the total) and fresh groundwater about 76 mL (0.76 percent). Fresh lakes, at 0.007 percent of all water, would be about 0.7 mL; the atmosphere about 0.1 mL; rivers, at 0.0002 percent, about 0.02 mL, less than a typical drop.

The model shows why fresh water is a resource to manage carefully. The usable fraction is small, and most of it is locked in ice or held underground; the USGS stresses that only a relatively small portion of fresh water is available to sustain human, plant and animal life.