How is water on earth distributed?

Answer 1

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 oceanswhich cover roughly 75% 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 ofPangaea. 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 likeVenus, 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 andsnow. As a result, the vast bulk of the water on Earth is regarded as saline or salt water, with an average salinity of 35‰ (or 3.5%, roughly equivalent to 35 grams of salts in 1kg of seawater), 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 100 km³ 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 - i.e. below around 0.35‰. 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
  • Antarctic ice cap - 90%, 9700 years renewal interval
  • Greenland ice cap - 9%
  • Other glaciers - <1%, 1600 years renewal interval
• 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 ofirrigation 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) are very frequent and almost always not seriously considered when they were first developed.

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Answer 2

Water is also one of the most common molecules circulating in our atmosphere; the volume of the global oceans equals about 330 million cubic miles (1,400 million cubic kilometers) of water. It is estimated that if all the water on Earth were spread evenly over the planet, the resulting layer of water would be almost 2 miles (3 kilometers) thick.

In reality, the distribution of water is not even. It is stored mainly in the oceans, which hold about 97.2 percent of all the water in the world. Of the remaining 2.8 percent, 2.15 percent is found in ice sheets and ice shelves (or polar ice), and in glaciers; 0.62 percent is located in groundwater; 0.011 percent is in saltwater lakes and inland seas; rivers, freshwater lakes, and wetlands hold about 0.013 percent; and soils hold about 0.005 percent of the water. The final 0.001 percent is found in the atmosphere. Humans use less than 1 percent of all the water on the planet---mostly from groundwater, lakes, and rivers---some of which returns to the atmosphere and falls as rainwater, but much of which is not renewed. how-is-water-distributed-on-earth