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Water is widely distributed on Earth as freshwater and salt water in the oceans. The Earth is often referred to as the "blue planet" because when viewed from space it appears blue. This blue color is caused by reflection from the oceans which cover roughly 70% of the area of the Earth.

The oceanic crust is young, thin and dense, with none of the rocks within it dating from any older than the breakup of Pangaea. 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.

As a result, the vast bulk of the water on Earth is regarded as saline or salt water, with an average salinity of 3.5‰, though this varies slightly according to the amount of runoff received from surrounding land. In all, oceanic water, saline water from marginal seas, and water from saline closed lakes amounts to over 98% of the water on Earth, though no closed lake stores a globally significant amount of water. Renewable saline groundwater is believed to total at least 100km³ globally, but is seldom considered except when evaluating water quality in arid regions.

The remainder of the Earth's water constitutes the planet's fresh water resource. Typically, fresh water is defined as water with a salinity of less than 1 percent that of the oceans - ie. below around 0.035‰. Water with a salinity between this level and 1‰ is typically referred to as marginal water because it is marginal for many uses by humans and animals.

The planet's fresh water is also very unevenly distributed. Although in warm periods such as the Mesozoic and Paleogene when there were no glaciers anywhere on the planet all fresh water was found in rivers and streams, today the distribution is approximately as follows:

  • Ice caps and glaciers - 68.7%, of which
  • Groundwater - 30.1%, 1400 year renewal interval
  • Surface water - 0.3%, of which
    • Freshwater lakes - 87%, 17 years renewal interval
    • Swamps - 11%
    • Rivers - 2%, 16 days renewal interval
  • Ground ice and permafrost - 0.86%
  • Atmosphere 0.04%

Of these sources, only river water is generally valuable. Most water in lakes is in very inhospitable regions such as glacial lakes of Canada. Lake Baikal and Lake Khövsgöl, both protected from Quaternary glaciation by aridity, have equivalent amounts of water, and the latter has been used in Mongolia as a source of drinking water.. Although the total volume of groundwater is known to be much greater than that of river runoff, a large proportion of this groundwater is saline and should therefore be classified with the saline water above. There is also a lot of fossil groundwater in arid regions that has never been renewed for thousands of years; this must not be seen as renewable water.

However, fresh groundwater is of great value, especially in arid countries such as India. Its distribution is broadly similar to that of surface river water, but it is easier to store in hot and dry climates because groundwater storages are much more shielded from evaporation than are dams. In countries such as Yemen, groundwater from erratic rainfall during the rainy season is the major source of irrigation water.

Because groundwater recharge is much more difficult to accurately measure than surface runoff, groundwater is not generally used in areas where even fairly limited levels of surface water are available. Even today, estimates of total groundwater recharge vary greatly for the same region depending on what source is used, and cases where fossil groundwater is exploited beyond the recharge rate (including the Ogallala Aquifer[1]) are very frequent and almost always not seriously considered when they were first developed.

Distribution of river water

The distribution of renewable river water across the Earth's surface is very uneven.

Continent or region Renewable river water (km³) Percent of world total
Sub-Saharan Africa 4,000 9.20
Middle East and North Africa 140 0.32
Europe 2,900 6.70
Asia (excluding Middle East) 13,300 30.6
Australia 440 1.01
Oceania 6,500 14.9
North America 7,800 17.9
South America 12,000 27.6

Even within these regions, there can be huge variations. For example, as much as a quarter of Australia's limited renewable fresh water supply is found in almost uninhabited Cape York Peninsula[2]. 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 26km³ in an area of 695,622km², or South Africa, with only 44km³ in 1,221,037 km²[3]. The areas of greatest concentration of renewable water are:

And also the ratio of salt water to fresh is 40 salt to 1 fresh.

Variability of water availability

Variability of water availability is of major importance 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[4]. 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[5]. 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 (eg. 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 300mm (12 inches) or more of rainfall has occurred. In other continents, runoff will occur after quite light rainfall due to the low rooting densities.

Climate type (Köppen[6]) Mean annual rainfall Typical runoff ratio
for Australia and Southern Africa
Typical runoff ratio
for rest of the world
BWh 250mm (10 inches) 1 percent (2.5mm) 10 percent (25mm)
BSh (on Mediterranean fringe) 350mm (14 inches) 3 percent (12mm) 20 percent (80mm)
Csa 500mm (20 inches) 5 percent (25mm) 35 percent (175mm)
Caf 900mm (36 inches) 15 percent (150mm) 45 percent (400mm)
Cb 1100mm (43 inches) 25 percent (275mm) 70 percent (770mm)

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 effects 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 specialised 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[7]:

See also

References

  1. ^ Reisner, Marc; Cadillac Desert: The American West and its Disappearing Water; pp. 438-442. ISBN 0-14-017824-4
  2. ^ Brown, J. A. H.; Australia’s surface water resources. ISBN 064402567X.
  3. ^ Ibid.
  4. ^ McMahon, T.A. and Finlayson, B.L.; Global Runoff: Continental Comparisons of Annual Flows and Peak Discharges. ISBN 3-923381-27-1.
  5. ^ Peel, Murray C.; McMahon, Thomas A. and Finlayson, Brian L.; "Continental differences in the variability of annual runoff: update and reassessment"; in Journal of Hydrology, 295; pp. 185-197.
  6. ^ This section uses a slightly modified version of the Köppen system found in The Times Atlas of the World, 7th edition. ISBN 0723002657
  7. ^ Peel, McMahon, and Finlayson; "Continental differences in the variability of annual runoff: update and reassessment"
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