By Robert Hazen, Ph.D., George Mason University
Water plays a unique role in planet Earth’s geological and biological history. Earth’s water cycle—or the hydrological cycle, as it’s often called—deals with the movement of water molecules, whether they’re in their liquid, solid or gaseous state, and the Earth’s water reservoirs. While some of the reservoirs are accessible, others are not.
Earth’s Water: A Closed Loop
Earth’s water budget is almost completely closed. A small amount of water escapes each year through the atmosphere into space, a trivial amount, just through gravitational escape, but a similarly small amount of water probably comes into the Earth’s atmosphere through the inclusion of comets.
We’re talking about very small amounts: maybe the size of an Olympic swimming pool each year, in terms of the total amount of water that’s exchanged with space. That’s a trivial fraction compared to the Earth’s total water budget.
Accessible Water Reservoirs
Depending on how you count reservoirs, there are perhaps six or seven significant water repositories of water on Earth.
The accessible reservoirs—the ones where we can get water, if we wanted to, from technological means—include the oceans, the ice caps and glaciers, and freshwater lakes and rivers. Then there’s groundwater: water locked underground in vast, interconnected reservoirs and channel ways. There is also the atmosphere, which contains some water. And finally, there are living organisms, which also are a stock of water.
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A repository that is not accessible to us is hydrous minerals. These are minerals in which water molecules, or else just hydrogen protons, are locked into the crystal structure. These are potentially vast but relatively inaccessible reservoirs of water.
Indeed, these types of water repositories may be cycled down into the mantle, and so the mantle itself may hold vast amounts of water. Subduction carries the lithosphere, the wet lithosphere of the ocean, down deep underneath a plate and then down into the mantle. Wet rock is carried down into the mantle; therefore, water molecules are carried down into the mantle.
If even a small fraction of subducted water becomes locked into high-pressure minerals deep in the mantle, it’s very possible that the mantle—which is a huge volume—contains many times the total amount of surface water, deep within the Earth, locked in.
It’s also possible that this water cycles into the mantle and back out on the scale of a few hundred million years. Although we don’t know yet, it is even conceivable that the entire oceans are replaced once every few hundred million years through mantle cycles. That’s part of the water cycle we just don’t understand completely yet, but it’s a fascinating one.
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Studying the Oceans
By far, the largest reservoir of accessible water is the oceans. It holds about 98 percent of all the accessible water at the Earth’s surface. Between two-thirds and three-quarters of the Earth’s surface is covered by oceans. By the way, the average depth of the ocean is now about two kilometers. If you look at the surface area and think about two kilometers deep, on average, that is a vast amount of water.
Also, water is an excellent solvent. You can dissolve all sorts of ions into the water. This is why, over billions of years, the oceans have become salty. The principal elements in solution include sodium and chlorine, as well as calcium and magnesium; and of course, the ocean then becomes a very important part of the geochemical cycle of those elements.
Glaciers and Fresh Water
The second repository, glaciers and ice sheets, now contain an estimated 2 percent of Earth’s accessible water. Most of the ice covers the Antarctic, the significant landmasses, and also Greenland. The rest occurs as very thin coverings, either of the polar oceans, the Arctic Ocean, or widely scattered in mountain glaciers and high altitudes.
The third reservoir is fresh water on land; and that accounts for, at most, only about four-tenths of a percent of all the accessible water. Of this amount, more than 90 percent—by some estimates, as much as 98 percent—is stored beneath the surface as groundwater.
There are huge underground reservoirs that are formed by porous rock layers, sandwiched between impervious layers. This is not a big volume of continuous water, but rather it’s water that has penetrated, seeped through the pores of various porous rocks.
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Near-surface Water and Atmosphere
We also have lakes, rivers and streams as accessible water reservoirs. Though a major water source for humans, they represent less than 0.04 percent of Earth’s near-surface water.
Then there is the atmosphere. The atmosphere holds only about 0.001 percent of all the water on Earth, and it’s mostly in clouds. Even though this amount seems trivial, it’s very significant because water cycles through the atmosphere so quickly. The atmosphere can move very large quantities of water efficiently from one place to another.
Common Questions about the Earth’s Major Water Reservoirs
Out of the many water reservoirs on Earth, some are accessible to us. For example, the oceans, glaciers, freshwater lakes, rivers, groundwater, and living organisms. These are reservoirs from where we can get water, if we want to.
As water is an excellent solvent, you can dissolve all sorts of ions into the water. This is why, over billions of years, the oceans have become salty. The principal elements in solution include sodium and chlorine, as well as calcium and magnesium.
Although the atmosphere only contains 0.001 percent of the water on the planet, which is small compared to other water reservoirs like the ocean, it’s very significant because water cycles through the atmosphere very quickly. The atmosphere can move very large quantities of water efficiently from one place to another.