Water distribution

In this video, Dave Campbell and Keith Hunter discuss how the Earth’s water is distributed.

The Earth has a finite amount of water, with most of the water being found in the oceans. There is only a small amount of freshwater. This is locked up in glaciers with only a small amount on the land surfaces (lakes, rivers and streams). There is also water in the atmosphere, but this is a tiny amount. While our oceans contain salt in the water, when it evaporates into the atmosphere and then precipitates out, the water has been purified, leaving the salt in the sea.

Transcript

NARRATOR
The total amount of water on Earth is estimated to be between 1–2 billion cubic kilometres. This amount is finite.

DR DAVE CAMPBELL
There is more or less a fixed amount of water on planet Earth, and it moves through a cycle that we called the hydrological cycle. It’s not absolutely fixed because some water is lost to space, but only a very small amount. Some water is locked up in rocks and minerals and so it becomes part of the geological cycle, and that water will be cycled through the Earth's tectonic plates and rocks, and eventually will emerge in volcanoes. But that is over very long periods of time, so we tend to discount that water as being active in the hydrological cycle. So mostly what we see is what we get. Most of the water is in the oceans, and most of the rest is in glaciers and ice shelves in places like Antarctica and Greenland. Only a very small amount of water is on the land surfaces, and even a smaller amount of land water – approximately 0.001% – is present in the atmosphere at any one time.

NARRATOR
It appears that there is an abundance of water on Earth yet only a fraction of this is available as freshwater. Most of our water is in the sea and is transported through evaporation to the atmosphere and reaches land again through precipitation. This process purifies the available water, so it can be used by animals and plants.

PROF KEITH HUNTER
All the water that we encounter on the land, most of it has originally come from the ocean, and the ocean recharges the rest of the water around the Earth through evaporation, and then of course the water falls down as rain, and that rain will either fill up a lake, or it will make up a river or a river flood, or it will recharge a subterranean aquifer. But it’s that process of evaporating water from the ocean that purifies it all the time. Otherwise, all the water on the Earth would become rank and full of chemicals and all sorts of things, so it dominates the cycle by acting as, if you like, a reservoir that is constantly supplying pure water to the land.

Acknowledgements:
Dr David Campbell, Waikato University
Prof. Keith Hunter, Otago University
Globe image, courtesy of NASA
Ice shelf image, Dr Mike Williams, NIWA
Lake Wakatipu image by Chris Jewiss, licensed through 123RF Ltd
Iceberg image, Dr Mike Williams, NIWA
Flyover NZ/Rotorua Lakes model by Mathew Allan