The Earth's entire storage of water.  From land, sea, and even sky, the planet holds approximately 1,386,000,000 cubic kilometers of water across 3 forms*. This is a truly staggering amount of water.  I thought maybe I'd be able to compare this with a handy juxtaposition with Olympic-size swimming pools, but that would still be over 550 TRILLION pools!  Suffice to say, there's quite of a bit of water storage on Earth.  If we wanted to consider that volume at a planetary scale, though, if we wanted to, let's say, fill our Moon with all this water, it would actually only fill the Moon a little over 6% of its total volume.  So, a lot from a human perspective, but not so much on a worldly scale.

That's all of our planet's water, though.  Of that full amount, 97% is saline ocean water, leaving freshwater-needy organisms with the other 3% of freshwater*.  If only it were so easy, though.

Of that 3%, almost 70% (68.9) is frozen in ice either at the Earth's poles or encased in mountain glaciers. Over 30% (30.8) is groundwater.  The last fraction of a percent is readily accessible lakes, rivers, streams, and other well-known sources for freshwater*.  Now that's a small amount to share among not only all of humanity, but all organisms that require freshwater to survive.

Storing all the water in the hydrosphere would fill over 550 trillion Olympic-size swimming pools!

With how much freshwater humans alone use, how is it that we haven't seemed to run out yet?  Water is, by all means, a finite resource if it continuously leaves this system.  But is it really leaving the system?  Well, yes and no.  Enter the hydrologic cycle.

 Humans, too, are made of the Earth's water, making us a small contributor to the planet's Hydrosphere

The hydrosphere interacts with the other major reservoirs on the planet: the lithosphere, atmosphere, and biosphere.  While the biosphere does consumer water, for the most part, this water is continuously cycled out.  An example is that while humans are comprised mostly of water, we are not still made of the same water we were born with.  It doesn't just disappear either.  We expel it in a number of ways, giving it back to the planet.  In this sense, we are a part of the hydrosphere, and contribute to the cycle of water throughout the planet.

We make up an insignificant fraction of this system, however.  Primarily, water is constantly moving via the hydrologic cycle.  It's thanks to this cycle that we have freshwater reservoirs at all.  It's not terribly important where we start in the cycle, but I do like to go in and out on a high note.  Let's start in the sky!

When I say sky, I really mean the Earth's atmosphere.  This layer of the planet houses the Earth's reservoir of water in the form of water vapor.  To us, well, we know these collections of water vapor as clouds.  Water vapor that forms clouds is moved around the planet by wind.  It's this part of the cycle that makes life outside of the ocean possible.  By using clouds as transportation, water vapor can move over the terrestrial earth, via a process called advection, before succumbing to the next part of the cycle.

Precipitation: the method by which the water vapor locked in the atmospheric reservoirs is released back to Earth's lithosphere, the ground as we know it.  Water vapor generally falls to the ground as liquid water by way of condensation, the act of changing from a gaseous state to a liquid state.  It is possible to be so suddenly hit with a temperature (or pressure) change that the vapor transitions directly to a solid, which is called deposition (the reverse is called sublimation, by the way).  Fun fact about precipitation, though, is that only about 10% of it actually lands on the terrestrial earth.  The other whopping 90% falls right back into the ocean.

​And that 10% that does strike land starts its journey back to the ocean.  Depending on where the water fell, this can change its path pretty drastically.  Water landing at the poles may be encased in ice.  Water landing in lakes contributes to our accessible freshwater reserves.  Water landing on the ground may be taken into the soil as groundwater or absorbed by plants that will use it to aid in photosynthesis.  In the plants case, some of this water will be emitted back into the atmosphere through a process known as transpiration.  Basically a plant breathing water vapor back into the atmosphere.  For the groundwater case, this has a chance to be locked away for quite some time.  Some of our oldest underground freshwater reserves date back tens of thousands of years, earning them the title of fossil water.

Advection, the movement of clouds from the oceans to land, is a key part to why life is possible on Earth 

​Without it, rain would only fall onto the oceans

If nothing takes in the water, either locking it or releasing it back to the atmosphere, it will make its way into rivers and streams by way of runoff.  It's this process that begins to introduce salinity back into the water as it makes its way back to the ocean.  All of the soil and mineral deposits it travels through contributes to its salt content.  Ultimately, this is what has made the oceans so salty!

The oldest groundwater stores earn the nickname Fossil Water

Finally, the water makes it back to the ocean.  From here, the water will evaporate, leaving the salt content of the water in the ocean, but taking the water content into the atmosphere as water vapor.  This is why I kept using the phrase "back to" the ocean.  In a way water is always returning to the oceans, leaving the things it found along the way before evaporating.  Not to harsh on marine ecosystems, but to us freshwater-needy species, it's like the dog or cat that brings home dirty sticks and baubles after a walk.

There is one other vector by which water can move, though it's not a part of the cycle most people know about.  At the ocean floor, tectonic activity can shift the earth into the upper mantle, resulting in water being "lost" to the mantle in an event called subduction.  This phenomena is worth a discussion in and of itself, but for our purposes, water exchange via subduction only finds its way back by way of volcanic activity.

And there we have it!  Thanks to this wonderful cycle working in harmony with the Earth's systematic wind and that happy balance of water's ability to change states regularly enough based on our planet's location in the solar system, we (and every other freshwater-needy organism) are able to live.  Know that the next time you see, swim, or drink your way through some water, know that its been here for a very, very, VERY long time, and you're interacting with a component of our planet that has likely journeyed further than any of us can possibly imagine.

~ And, as always, don't forget to keep wondering ~

* Water statistics gathered from noaa.gov

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