World Supply and Sources of Water: Review

The sources of water in the world may appear limitless, what with the vast and deep seas, the many lakes and rivers that keep flowing, and the seemingly unlimited supply of water from underground reservoirs and from the atmosphere.

But it is not so.

The April 2010 special issue of National Geographic presents the following data on the world sources and volume of water:

The total volume of water on Earth is about 366 million trillion gallons (MTG) or, using the convertion equivalent 1 gallon = 3.78541 liters = 3.78541178 × 10-12 cubic kilometers (km3), about 1,385 million trillion liters or 1.385 billion km3.

Of this total volume, about 97.5% is saltwater including around 1% thereof as brackish groundwater.

Only around 2.5% (~9.25 MTG or 35 million km3) is freshwater.

Of the world’s freshwater, about 69.6% (6.44 MTG or 24.4 million km3; ~2% of total water) is locked up in permanent snow, ice, glaciers, and permafrost while 0.3% (31,341 trillion gallons or about 118,638 km3) occurs as surface water (lakes, rivers, wetlands), in plant and animal bodies, and in the atmosphere.

The remaining 30.1% (2.78 MTG or 10.5 million km3) is underground water in soil and aquifers which accumulate through surface seepage. 

The total volume of freshwater that is available for the growing of crops, cooling of power plants, and domestic human use is only less than 1% of the total world water.

The main sources of water for these uses are groundwater, surface water, and desalinated water.

According to UN-Water, groundwater consists of shallow and deep underground deposits up to 2,000 meters deep ( 2013).

Some 16 billion gallons of fresh water are produced worldwide from seawater and brackish groundwater through desalinization technology.

Most countries that obtain freshwater from seawater are those in the Persian Gulf. 

The UN-Water, a coordination mechanism of the United Nations for issues related to water ( 2013), also provides summaries of statistics on the supply and sources of water.

These water statistics are roughly the same as those provided above.

Additionally, it informs that the volume of water in the atmosphere amounts to about 13,000 km3 or 3,434.2 trillion gallons

Some Causes of Concern on the Supply and Sources of Water

The total volume of water on Earth remains as it was during prehistoric times.

National Geographic (2010) put it succinctly with a note of concern: “The amount of moisture on Earth has not changed. The water the dinosaurs drank millions of years ago is the same water that falls as rain today. But will there be enough for a more crowded world?” 

There is cause indeed for a waning optimism.

Water use has increased at the rate of twice that of population increase.

By 2025, 1.8 billion people will be living in parts of the world where water is scarce ( 2013).

By 2050, the human population is projected to increase by about 2.4 billion, a rate of increase that is equivalent to about 35% over the 2010 population (click on to read source page).

The case of the lake called the Aral Sea shows that even large natural sources of water are susceptible of drying up.

It used to be the fourth-largest lake in the world, with parts in southern Kazakhstan and northern Uzbekistan in Central Asia.

It is fed by two rivers, the Amo Darya and the Syr Darya.

But starting the 1960’s, according to the educational website (2013), the Russian government diverted the flow of these rivers to irrigate crops in the desert region surrounding the lake.

By 1998, it was apparent that the volume of water in the lake was severely reduced.

The water level went down by 20 meters compared to the original while its salinity increased ten-fold.

It contained a total of 210 cu. km of water, a mere 19.8 percent of the 1,060 cu. km volume in 1960.

Further, groundwater as a source of water may not be a stable source as well. This underground supply may not have been fully replenished.

In 2010, the American Geophysical Union reported that the rate of removal of groundwater more than doubled between 1960 and 2000.

The large volume of the water that is drawn out by man from this underground reservoir is ultimately added to the oceans mainly by evaporation followed by precipitation.

Its contribution to the annual rise in sea level has been quantified at around 25%.


American Geophysical Union. 2010. Groundwater depletion rate accelerating worldwide. ScienceDaily. Retrieved February 25, 2013, from

COLUMBIA.EDU. 2013. The Aral Sea crisis. Retrieved Feb. 25, 2013, from

[FAO] Food and Agriculture Organization of the United Nations. 2011. Fast facts: The state of the world’s land and water resources. Retrieved Mar. 2013 from

NATIONAL GEOGRAPHIC. 2010. Water Our Thirsty World: A Special Issue. Journal of the National Geographic Society (April 2010). Vol. 217, No. 4.

UNWATER.ORG. 2013. Accessed Mar. 28, 2013, via 

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Ben Bareja

Ben Bareja, the owner-founder-webmaster of This website was conceptualized primarily to serve as an e-library for reference purposes on the principles and practices in crop science, including basic botany. Read more here

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