Water distribution on Earth

Water covers most of our planet, but not all of it is the same. Most of the water on Earth is in the oceans and is salty. This salty water makes up over 97% of all the water on Earth. Only about 1% of Earth’s water is fresh water, which means it has very little salt and can be used by plants, animals, and people.

Fresh water is not spread evenly around the world. Much of it is frozen in ice caps and glaciers, or hidden underground. Only a small part of fresh water is in rivers and lakes that we can see. Most of the fresh water that is liquid is stored in lakes, with a smaller amount in swamps and rivers.

Imagine tiny cubes, each representing about 1,000 cubic kilometers of water. If we put together one million of these tiny cubes, they would form a huge block about 1,102 kilometers on each side! This helps us understand how much water there is, and how most of it is salty ocean water, with only a tiny fraction being the fresh water we rely on. The fresh water we use every day is a very important, but also very limited, resource on our planet.

Distribution of saline and fresh water

The Earth has about 1.386 billion km³ of water. Most of this—97.5%—is salt water. Only 2.5% is freshwater. Of that freshwater, just 0.3% is liquid and found on the Earth's surface.

Oceans cover about 70.8% of our planet. That is why Earth looks blue from space and is called the blue planet and the Pale Blue Dot. Liquid freshwater, like in lakes and rivers, covers about 1% of Earth's surface. When you add the ice on Earth, about 75% of the whole surface is water in some form.

Logarithm graph of fresh water's source (including saline lakes and saline groundwater)

Lakes

All the lakes on Earth together hold 199,000 km³ of water. Most lakes are far north, away from many people. One big exception is the North American Great Lakes. They hold 21% of the world's freshwater. The Great Lakes Basin is home to over 35 million people. Cities such as Thunder Bay, Toronto, and Chicago are near these lakes.

Groundwater

Fresh groundwater is very important, especially in dry places like China. It stores water that helps in hot and dry climates because it doesn't evaporate as quickly as water in dams. In places such as Yemen, groundwater from rainy seasons is a main source for irrigation.

It is harder to measure how much groundwater is refilled compared to water that runs over the land. Because of this, groundwater is usually not used where there is even a small amount of surface water available.

Source of water	Volume of water in km³ (cu mi)	% total water	% salt water	% fresh water
Oceans	1,338,000,000 (321,000,000)	96.5	99.0
Pacific Ocean	669,880,000 (160,710,000)	48.3	49.6
Atlantic Ocean	310,410,900 (74,471,500)	22.4	23.0
Indian Ocean	264,000,000 (63,000,000)	19.0	19.5
Southern Ocean	71,800,000 (17,200,000)	5.18	5.31
Arctic Ocean	18,750,000 (4,500,000)	1.35	1.39
Ice and snow	24,364,000 (5,845,000)	1.76		69.6
Glaciers	24,064,000 (5,773,000)	1.74		68.7
Antarctic ice sheet	21,600,000 (5,200,000)	1.56		61.7
Greenland ice sheet	2,340,000 (560,000)	0.17		6.68
Arctic islands	83,500 (20,000)	0.006		0.24
Mountain ranges	40,600 (9,700)	0.003		0.12
Ground ice and permafrost	300,000 (72,000)	0.022		0.86
Groundwater	23,400,000 (5,600,000)	1.69
Saline groundwater	12,870,000 (3,090,000)	0.93	0.95
Fresh groundwater	10,530,000 (2,530,000)	0.76		30.1
Soil moisture	16,500 (4,000)	0.0012		0.047
Lakes	176,400 (42,300)	0.013
Saline lakes	85,400 (20,500)	0.0062	0.0063
Caspian Sea	78,200 (18,800)	0.0056	0.0058
Other saline lakes	7,200 (1,700)	0.00052	0.00053
Fresh water lakes	91,000 (22,000)	0.0066		0.26
African Great Lakes	30,070 (7,210)	0.0022		0.086
Lake Baikal	23,615 (5,666)	0.0017		0.067
North American Great Lakes	22,115 (5,306)	0.0016		0.063
Other fresh water lakes	15,200 (3,600)	0.0011		0.043
Atmosphere	12,900 (3,100)	0.00093		0.037
Swamps	11,470 (2,750)	0.00083		0.033
Rivers	2,120 (510)	0.00015		0.0061
Biological water	1,120 (270)	0.000081		0.0032

Distribution of river water

There is about 2,120 cubic kilometers of water in rivers. This is a small part of all the fresh water on Earth.

The amount of water that flows in rivers is different in different places. Some places have a lot of water, while others have very little. For example, much of Australia’s fresh water is in the Cape York Peninsula, where few people live. In places like Texas in North America or South Africa, there is very little fresh water each year.

The places with the most fresh water in rivers include the Amazon and Orinoco Basins in South America, the Yangtze Basin in East Asia, and several river basins in South and Southeast Asia, Canada, Siberia, and New Guinea.

Continent or region	River runoff (km³/year)	Percent of world total
Asia (excluding Middle East)	13,300	30.6
South America	12,000	27.6
North America	7,800	17.9
Oceania	6,500	14.9
Sub-Saharan Africa	4,000	9.2
Europe	2,900	6.7
Australia	440	1.0
Middle East and North Africa	140	0.3

Area, volume, and depth of oceans

The ocean floor is made of thin, dense rock. These rocks are not very old because a very old landmass split apart long ago. Water is heavier than air, so it fills in low spots on Earth. On planets without water, like Venus, these low spots look like flat plains.

The rocks on the continents are lighter and contain salts. These salts wash into the oceans over billions of years. When water evaporates from the oceans, it falls back to land as rain and snow, but the salts stay behind, making the oceans salty.

Body of Water	Area (10⁶ km²)	Volume (10⁶ km³)	Mean Depth (m)
Pacific Ocean	165.2	707.6	4,282
Atlantic Ocean	82.4	323.6	3,926
Indian Ocean	73.4	291.0	3,963
All oceans and seas	361	1,370	3,796

Variability of water availability

Water availability changes a lot, and this is important for animals and people who need water. Most of the world's water comes from places where the climate stays fairly steady, so the amount of water doesn't change much.

In some places like Australia and Southern Africa, the amount of water in rivers changes a lot more than in other places. This is because the soil in these areas has not changed much for millions of years. The plants there have deep roots to reach nutrients and water, so it takes a lot of rain before water flows into rivers. This makes it hard to save enough water for everyone, especially when it is dry.

Climate type (Köppen)	Mean annual rainfall	Typical runoff ratio for Australia and Southern Africa	Typical runoff ratio for rest of the world
BWh	250 mm (10 in)	1 percent (2.5 mm)	10 percent (25 mm)
BSh (on Mediterranean fringe)	350 mm (14 in)	3 percent (12 mm)	20 percent (80 mm)
Csa	500 mm (20 in)	5 percent (25 mm)	35 percent (175 mm)
Caf	900 mm (35 in)	15 percent (150 mm)	45 percent (400 mm)
Cb	1,100 mm (43 in)	25 percent (275 mm)	70 percent (770 mm)

Possible water reservoirs inside Earth

Ringwoodite is the major phase at the Earth's mantle between ~520 and ~660 km depth, possibly containing several weight percent of water in its crystal structure.

Scientists think there might be water deep inside Earth, in its crust, mantle, and even its core. This water could connect to the oceans through a cycle that includes the whole Earth. But we are not sure exactly how much water is stored deep down.

Some believe the lower mantle might hold as much water as all the oceans, lakes, and rivers on Earth put together. This water is not in liquid form but is mixed into rocks and minerals. It can affect how these rocks move and melt.