Water distribution on Earth

• 1. Water Distribution on Earth

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  Water distribution on Earth offers a number that stops most people cold. Of the estimated 1.386 billion cubic kilometres of water on this planet, only 0.3% of the fresh portion exists as liquid on the surface. The oceans command 96.5% of all water, at an average salinity of 35 parts per thousand. About 34 grams of dissolved salts fill every kilogram of seawater. The ratio of salt water to fresh water across the whole planet is roughly 50 to 1. Where that remaining fresh water actually sits is a question that crosses glaciology, hydrology, and geology alike. Of the liquid water that does reach the surface, 87% is held in lakes, 11% in swamps, and only 2% flows through rivers.

• 2. The Frozen Majority

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  The Antarctic ice sheet holds 21.6 million cubic kilometres of fresh water, making it the single largest freshwater reservoir on Earth. It accounts for 61.7% of all fresh water on the planet. The Greenland ice sheet adds another 2.34 million cubic kilometres, or 6.68% of global fresh water. Together, all glaciers and permanent snow cover about 1.76% of all water on Earth. That may sound modest, but it represents more stored fresh water than all the world's groundwater, lakes, and rivers combined. Permafrost and ground ice contribute a further 300,000 km3. Glaciers on mountain ranges outside the polar regions hold an additional 40,600 km3. This glaciated world is a geologically recent condition. During warm periods such as the Mesozoic and Paleogene, no glaciers existed anywhere on Earth, and all fresh water ran through rivers and streams. The planet's next largest freshwater reservoir is invisible to the naked eye. It sits in rock and sediment beneath the surface, sometimes untouched for thousands of years.

• 3. Hidden and Ancient

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  Fresh groundwater totals 10.53 million cubic kilometres, a volume that exceeds all of the world's lakes, swamps, and rivers combined. Its distribution broadly mirrors that of surface river water. In hot, dry climates, groundwater stores are far more shielded from evaporation than open dams. In Yemen, groundwater captured during the rainy season from erratic rainfall is the primary source of irrigation. Fresh groundwater is particularly valued in arid countries such as China. Measuring how quickly these underground reserves recharge is notoriously difficult. Estimates for the same region can vary enormously depending on the methodology used. A deeper problem is fossil groundwater: water that accumulated thousands of years ago with no meaningful renewal since. This fossil water must not be treated as renewable. Cases where fossil groundwater is extracted faster than it recharges are widespread. The Ogallala Aquifer is one of the best-documented examples, and its depletion rate was underestimated in severity when it was first developed. Saline groundwater, a separate 12.87 million cubic kilometres, further strains water quality in arid regions. The fresh water that collects in visible lakes distributes itself with remarkable unevenness. A handful of basins hold the majority, and most sit far from where people live.

• 4. Where Lakes Gather

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  Earth's lakes collectively hold 199,000 km3 of water. Most cluster at high northern latitudes, far from the world's largest population centers. The North American Great Lakes are a striking exception. They contain 21% of all fresh water by volume and sit at the heart of a basin home to more than 35 million people. The Canadian cities of Thunder Bay, Toronto, Hamilton, and Kingston line the Great Lakes shore. So do the American cities of Detroit, Chicago, Cleveland, Buffalo, and Rochester. Lake Baikal in Siberia holds 23,615 km3, making it the single most voluminous freshwater lake on Earth. The African Great Lakes together contain 30,070 km3. Fresh water lakes as a category hold 91,000 km3 in total. Saline lakes add another 85,400 km3, with the Caspian Sea alone accounting for 78,200 km3 of that total. The Middle East and North Africa, by contrast, share just 140 km3 of river runoff per year across their combined territories.

• 5. The River Map

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  Total river water on Earth amounts to just 2,120 km3, which is 0.49% of all surface fresh water. Rivers and river basins are more often compared by annual flow than by static volume. The distribution of that runoff is strikingly uneven. Asia, excluding the Middle East, generates 13,300 km3 per year. South America produces 12,000 km3 annually. North America contributes 7,800 km3 per year. The Amazon and Orinoco basins together deliver 6,500 km3 per year, representing 15% of global runoff. South and Southeast Asia, taken together, generate 8,000 km3 per year, or 18% of global totals. Within that region, the Ganges contributes 900 km3 per year, the Irrawaddy 500, and the Mekong 450. In Siberia, the Yenisey holds over 5% of the world's fresh water in its basin, second only to the Amazon. The Ob River adds over 500 km3 annually, and the Lena over 450. Canada accounts for over 10% of the world's river water, with the Mackenzie contributing over 250 km3 per year and the Yukon over 150. The Fly and Sepik rivers in New Guinea together deliver over 300 km3 per year from a basin of only about 150,000 km2. That density of flow stands in sharp contrast to Australia. There, a quarter of the continent's limited renewable fresh water concentrates in the almost uninhabited Cape York Peninsula.

• 6. Where Soil Changes Everything

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  The San Joaquin Valley's agricultural productivity depends on glacier melt from high mountain regions, arriving reliably through summer when demand for water peaks. Most arid zones worldwide follow the same pattern, one that historically supported major ancient civilizations. Rivers in Australia and Southern Africa, however, behave differently from their equivalents on every other continent. Temperate and arid climate rivers there show as much as three times the coefficient of variation of runoff compared to similar climates elsewhere. The reason lies in ancient soils. Soils in Australia and Southern Africa have been largely unaltered since at least the early Cretaceous. In some areas, they have sat undisturbed since the Carboniferous ice age. On every other continent, Quaternary glaciation and mountain building reshaped soil composition. That process never reached Australia or Southern Africa. Available nutrient levels in those soils are orders of magnitude lower than in similar climates elsewhere. Native plants compensate by developing much higher rooting densities, including specialized structures such as proteoid roots, to absorb minimal phosphorus and other nutrients. These roots absorb so much water that runoff does not begin until about 300 mm of rainfall has already fallen. In most other regions, even light rainfall triggers runoff. In the tropical climates of Australia and Southern Africa, however, vegetation can use organic phosphorus or phosphate dissolved in rainwater, reducing the need for such dense root systems. Tropical rivers there do not show the same runoff shortfall as their temperate and arid counterparts. Many temperate and arid rivers in these two regions are theoretically impossible to regulate. Evaporation from any dam large enough to smooth out their variability would consume most of the water it was meant to store. The rivers of the Lake Eyre Basin are among those in this category. Even for more manageable Australian rivers, a storage three times as large is needed. It delivers only a third of the supply that similar climates in southeastern North America or southern China would provide. Other regions also show high runoff variability, including the Brazilian Nordeste, Southwest Asia, and the Great Plains of the United States. There, erratic rainfall drives the irregularity rather than soil differences. Researchers have hypothesized a still deeper reservoir: water cycling through Earth's mantle hundreds of kilometres below the surface.

• 7. Water Inside the Earth

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  In 2014, a hydrous ringwoodite sample enclosed in a diamond from Juína, Brazil provided direct evidence of water inside Earth's mantle. Ringwoodite and its closely related mineral wadsleyite are found at the transition zone between Earth's upper and lower mantle. Both can incorporate up to a few weight percent of water into their crystal structures. Water in the mantle is not liquid. It exists primarily as hydroxyl groups, or OH, dissolved in nominally anhydrous minerals. These OH impurities influence rock viscosity, lubricate tectonic plate movement, affect melting processes, and slow down seismic waves. Seismic observations have also found signs of water in dehydration melt at the top of the lower mantle under the continental United States. Even ice-VII, water's high-pressure form, has been discovered inside super-deep diamonds. Estimates suggest that 1.5 to 11 times the volume of all surface oceans may be stored hundreds of kilometres down. The precise total remains under active debate. Some researchers propose the entire mantle water budget could amount to tens of ocean masses. The lower mantle alone may hold as much as five times more water than all of Earth's surface water combined. If those estimates prove correct, the scale of Earth's water cycle extends far deeper than any glacier, lake, or aquifer.