Water distribution on Earth
Earth holds 1.386 billion cubic kilometers of water, yet only 2.5% of that total is fresh. The remaining 97.5% exists as saline seawater with an average salinity of 35 parts per thousand. This ratio means salt water to fresh water stands at roughly 50 to 1 across the entire globe. Most oceans and marginal seas contain over 97% of all planetary water. No single closed lake stores a globally significant amount of this liquid. Freshwater accounts for nearly 1% of the total volume when including ice and snow. Liquid freshwater on the surface makes up just 0.3% of all fresh water reserves. The planet appears blue from space because oceans cover roughly 70.8% of Earth's area.
Collectively Earth's lakes hold 199,000 cubic kilometers of water in their basins. Eighty-seven percent of liquid surface fresh water resides within these inland bodies. Only 11% flows through swamps while merely 2% occupies rivers themselves. The North American Great Lakes contain 21% of the world's fresh water by volume. More than 35 million people live within the Great Lakes Basin today. Cities like Detroit Chicago Buffalo and Toronto sit along its shores. Small quantities of water exist in the atmosphere and inside living beings. Atmospheric moisture holds only 0.037% of the total global water inventory. Biological water comprises a tiny fraction at 0.0032% of the entire system.
The Pacific Ocean covers 165.2 million square kilometers with a mean depth of 4,282 meters. Its volume reaches 707.6 million cubic kilometers making it the largest basin on Earth. The Atlantic Ocean spans 82.4 million square kilometers and averages 3,926 meters deep. Water flows into depressions formed by dense oceanic crust over billions of years. The Indian Ocean contains 291.0 million cubic kilometers across 73.4 million square kilometers. All oceans combined hold 1,370 million cubic kilometers of saline water. The Southern Ocean measures 5.18 million cubic kilometers while the Arctic Ocean holds 1.35 million cubic kilometers. These basins reflect blue light to create the planet's signature color from space.
Fresh groundwater is vital for arid countries like China where surface water is scarce. Groundwater storage is more shielded from evaporation than dams built in hot climates. Yemen relies heavily on erratic rainfall during the rainy season to fill its aquifers. Estimates of total groundwater recharge vary greatly depending on which source researchers use. Fossil groundwater in arid regions has never been renewed for thousands of years. Cases where fossil groundwater is exploited beyond recharge rates occur frequently without serious consideration. The Ogallala Aquifer represents one such example of depletion exceeding natural replenishment. Fresh groundwater accounts for 0.76% of Earth's total water volume.
The Amazon and Orinoco Basins provide a total of 6,500 cubic kilometers per year or 15 percent of global runoff. East Asia contains the Yangtze Basin with over 1,000 cubic kilometers annually. South and Southeast Asia contribute 8,000 cubic kilometers representing 18 percent of global flow. Australia and Southern Africa exhibit much higher runoff variability than other continental regions. Temperate rivers there have as much as three times the coefficient of variation compared to others. Soils in these areas remained unaltered since at least the early Cretaceous period. Native flora compensate through higher rooting densities like proteoid roots to absorb minimal phosphorus. Rivers in the Lake Eyre Basin are theoretically impossible to regulate due to evaporation rates from dams.
Scientists hypothesize that water exists within the Earth's crust mantle and even core. An estimated 1.5 to 11 times the amount of ocean water may be found hundreds of kilometers deep. The lower mantle might hold as much as five times more water than all surface water combined. Direct evidence appeared in 2014 based on a hydrous ringwoodite sample inside a diamond from Juína Brazil. Ringwoodite is the major phase between approximately 520 and 660 kilometers depth. These minerals could incorporate up to a few weight percent of water into their crystal structure. Molecular water forms high-pressure ice-VII found in super-deep diamonds rather than liquid form.
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Common questions
What percentage of Earth's total water is fresh?
Only 2.5% of the 1.386 billion cubic kilometers of water on Earth is fresh. Freshwater accounts for nearly 1% of the total volume when including ice and snow.
How much water do Earth's lakes hold in their basins?
Collectively Earth's lakes hold 199,000 cubic kilometers of water in their basins. Eighty-seven percent of liquid surface fresh water resides within these inland bodies.
Which ocean covers the largest area and holds the most water?
The Pacific Ocean covers 165.2 million square kilometers with a mean depth of 4,282 meters. Its volume reaches 707.6 million cubic kilometers making it the largest basin on Earth.
Where does groundwater account for 0.76% of Earth's total water volume?
Fresh groundwater accounts for 0.76% of Earth's total water volume. Groundwater storage is more shielded from evaporation than dams built in hot climates.
When did scientists find direct evidence of water deep inside Earth?
Direct evidence appeared in 2014 based on a hydrous ringwoodite sample inside a diamond from Juína Brazil. An estimated 1.5 to 11 times the amount of ocean water may be found hundreds of kilometers deep.