Water distribution on Earth

Distribution of river water

The total volume of water in rivers is estimated at 2,120 km³ (510 mi³), or 0.49% of the surface fresh water on Earth. Rivers and basins are often compared not according to their static volume, but to their flow of water, or surface run off. The distribution of river runoff across the Earth's surface is very uneven.

There can be huge variations within these regions. For example, as much as a quarter of Australia's limited renewable fresh water supply is found in almost uninhabited Cape York Peninsula. Also, even in well-watered continents, there are areas that are extremely short of water, such as Texas in North America, whose renewable water supply totals only 26 km³/year in an area of 695,622 km², or South Africa, with only 44 km³/year in 1,221,037 km². The areas of greatest concentration of renewable water are:

• The Amazon and Orinoco Basins (a total of 6,500 km³/year or 15 percent of global runoff)
• East Asia
  • Yangtze Basin – 1,000 km³/year
• South and Southeast Asia, with a total of 8,000 km³/year or 18 percent of global runoff
  • Ganges Basin – 900 km³/year
  • Irrawaddy Basin – 500 km³/year
  • Mekong Basin – 450 km³/year
• Canada, with over 10 percent of world's river water and large numbers in lakes
  • Mackenzie River – over 250 km³/year
  • Yukon River – over 150 km³/year
• Siberia
  • Yenisey – over 5% of world's fresh water in basin – second largest after the Amazon
  • Ob River – over 500 km³/year
  • Lena River – over 450 km³/year
• New Guinea
  • Fly and Sepik Rivers – total over 300 km³/year in only about 150,000 km² of basin area.

Area, volume, and depth of oceans

The oceanic crust is young, thin and dense because oceanic crust is continuously created at mid-ocean ridges and recycled into the mantle at subduction zones with none of the rocks within it dating from any older than the breakup of Pangaea and is less than about 200 million years old . Because water is much denser than any gas, this means that water will flow into the "depressions" formed as a result of the high density of oceanic crust (on a planet like Venus, with no water, the depressions appear to form a vast plain above which rise plateaux). Since the low density rocks of the continental crust contain large quantities of easily eroded salts of the alkali and alkaline earth metals, salt has, over billions of years, accumulated in the oceans as a result of evaporation returning the fresh water to land as rain and snow.

Variability of water availability

Variability of water availability is important both for the functioning of aquatic species and also for the availability of water for human use: water that is only available in a few wet years must not be considered renewable. Because most global runoff comes from areas of very low climatic variability, the total global runoff is generally of low variability.

Indeed, even in most arid zones, there tends to be few problems with variability of runoff because most usable sources of water come from high mountain regions which provide highly reliable glacier melt as the chief source of water, which also comes in the summer peak period of high demand for water. This historically aided the development of many of the great civilizations of ancient history, and even today allows for agriculture in such productive areas as the San Joaquin Valley.

However, in Australia and Southern Africa, the story is different. Here, runoff variability is much higher than in other continental regions of the world with similar climates. Typically temperate (Köppen climate classification C) and arid (Köppen climate classification B) climate rivers in Australia and Southern Africa have as much as three times the coefficient of variation of runoff of those in other continental regions. The reason for this is that, whereas all other continents have had their soils largely shaped by Quaternary glaciation and mountain building, soils of Australia and Southern Africa have been largely unaltered since at least the early Cretaceous and generally since the previous ice age in the Carboniferous. Consequently, available nutrient levels in Australian and Southern African soils tend to be orders of magnitude lower than those of similar climates in other continents, and native flora compensate for this through much higher rooting densities (e.g. proteoid roots) to absorb minimal phosphorus and other nutrients. Because these roots absorb so much water, runoff in typical Australian and Southern African rivers does not occur until about 300 mm (12 in) or more of rainfall has occurred. In other continents, runoff will occur after quite light rainfall due to the low rooting densities.

The consequence of this is that many rivers in Australia and Southern Africa (as compared to extremely few in other continents) are theoretically impossible to regulate because rates of evaporation from dams mean a storage sufficiently large to theoretically regulate the river to a given level would actually allow very little draft to be used. Examples of such rivers include those in the Lake Eyre Basin. Even for other Australian rivers, a storage three times as large is needed to provide a third the supply of a comparable climate in southeastern North America or southern China. It also affects aquatic life, favouring strongly those species able to reproduce rapidly after high floods so that some will survive the next drought.

Tropical (Köppen climate classification A) climate rivers in Australia and Southern Africa do not, in contrast, have markedly lower runoff ratios than those of similar climates in other regions of the world. Although soils in tropical Australia and southern Africa are even poorer than those of the arid and temperate parts of these continents, vegetation can use organic phosphorus or phosphate dissolved in rainwater as a source of the nutrient. In cooler and drier climates these two related sources tend to be virtually useless, which is why such specialized means are needed to extract the most minimal phosphorus.

There are other isolated areas of high runoff variability, though these are basically due to erratic rainfall rather than different hydrology. These include:

• Southwest Asia
• The Brazilian Nordeste
• The Great Plains of the United States

Possible water reservoirs inside Earth

It has been hypothesized that the water is present in the Earth's crust, mantle and even the core and interacts with the surface ocean through the "whole-Earth water cycle". However, the actual amount of water stored in the Earth's interior still remains under debate. An estimated 1.5 to 11 times the amount of water in the oceans may be found hundreds of kilometers deep within the Earth's interior, although not in liquid form.

Water in Earth's mantle

The lower mantle of inner earth may hold as much as 5 times more water than all surface water combined (all oceans, all lakes, all rivers).

The amount of water stored in the Earth's interior may equal or exceed that in all of the surface oceans. Some researchers proposed the total mantle water budget may amount to tens of ocean masses. The water in the Earth's mantle is primarily dissolved in nominally anhydrous minerals as hydroxyls (OH). These OH impurities in rocks and minerals can lubricate tectonic plate, influence rock viscosity and melting processes, and slow down seismic waves. The two mantle phases at the transition zone between Earth's upper and lower mantle, wadsleyite and ringwoodite, could potentially incorporate up to a few weight percent of water into their crystal structure. Direct evidence of the presence of water in the Earth's mantle was found in 2014 based on a hydrous ringwoodite sample included in a diamond from Juína, Brazil. Seismic observations suggest the presence of water in dehydration melt at the top of the lower mantle under the continental US. Molecular water (H₂O) is not the primary water-bearing phase(s) in the mantle, but its high-pressure form, ice-VII, also has been found in super-deep diamonds.