The Water Cycle

01
of 09

Why Should I Care About the Water Cycle?

Person's finger touching surface of mountain lake
Ascent Xmedia / Getty Images

You've likely heard of the hydrologic (water) cycle before and know that it describes how Earth's water journeys from the land to the sky, and back again. But what you may not know is why this process is so essential.

Of the world's total water supply, 97% is salt water found in our oceans. That means that less than 3% of available water is freshwater and acceptable for our use. Think that's a small amount? Consider that of that three percent, over 68% is frozen in ice and glaciers and 30% is underground. This means that under 2% of freshwater is readily available to quench the needs of everyone on Earth! Are you beginning to see why the water cycle is so essential? Let's explore its 5 main steps...     

02
of 09

All Water is Recycled Water

water-cycle-diagram
The water cycle is a never-ending process. NOAA NWS

Here's some food (or drink) for thought: every drop of rain that falls from the sky isn't brand new, nor is every glass of water you drink. They have always been here on Earth, they've just been recycled and re-purposed, thanks to the water cycle which includes 5 main processes:

  • Evaporation (including sublimation, transpiration)
  • Condensation 
  • Precipitation
  • Surface runoff (including snowmelt and streamflow)
  • Infiltration (groundwater storage and eventual discharge)
03
of 09

Evaporation, Transpiration, Sublimation Move Water Into the Air

Steam on hot surface - Bolivia
Werner Büchel / Getty Images

Evaporation is considered to be the first step of the water cycle. In it, water that's stored in our oceans, lakes, rivers, and streams absorbs heat energy from the sun which turns it from a liquid into a gas called water vapor (or steam).

Of course, evaporation doesn't just happen over bodies of water -- it happens on land too. When the sun heats the ground, water is evaporated from the top layer of soil -- a process known as evapotranspiration. Likewise, any extra water that isn't used by plants and trees during photosynthesis is evaporated from its leaves in a process called transpiration.

A similar process happens when water that's frozen in glaciers, ice, and snow converts directly into water vapor (without first turning into a liquid). Called sublimation, this happens when the air temperature is extremely low or when high pressure is applied.

04
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Condensation Makes Clouds

raindrops-2
Nick Pound/Moment/Getty Images

Now that water has vaporized, it is free to rise up into the atmosphere. The higher it rises, the more heat it loses and the more it cools off. Eventually, the water vapor particles cool so much that they condense and turn back into liquid water droplets. When enough of these droplets collect, they form clouds.

(For a more in-depth explanation of how clouds are created, read How Do Clouds Form?)

05
of 09

Precipitation Moves Water from the Air to Land

Pouring rain
Cristina Corduneanu / Getty Images

As winds move clouds around, clouds collide with other clouds and grow. Once they grow big enough, they fall out of the sky as precipitation (rain if the atmosphere's temperatures are warm, or snow if its temperatures are 32° F or colder).

From here, precipitating water can take one of several paths:

  • If it falls into the oceans and other bodies of water, its cycle has ended and it is ready to begin again by evaporating yet again.
  • On the other hand, if it falls on land, it continues on the water cycle journey and must find its way back to the oceans.

So that we can continue exploring the complete water cycle, let's assume option #2 -- that the water has fallen over land areas.

06
of 09

Ice and Snow Move Water Very Slowly Along In the Water Cycle

Close up of melting snow on tree branch over Crater Lake, Oregon, United States
Eric Raptosh Photography / Getty Images

The precipitation that falls as snow over land accumulates, forming seasonal snowpack (layers upon layers of snow that continually accumulates and becomes packed down). As spring arrives and temperatures warm, these large amounts of snow thaw and melt, leading to runoff and streamflow.

(Water also stays frozen and stored in ice caps and glaciers for thousands of years!)

07
of 09

Runoff and Streamflow Moves Water Downhill, Towards Oceans

Sandy plain with the glacial runoff of the Joekulsarlon glacier, aerial view, Iceland, Europe
Michael Fischer / Getty Images

Both the water that melts from snow and that which falls on land as rain flows over the surface of the earth and downhill, due to gravity's pull. This process is known as runoff. (Runoff is hard to visualize, but you've probably noticed it during a heavy rain or flash flood, as water flows hurriedly down your driveway and into storm drains.)

Runoff works like this: As water runs over the landscape, it displaces the ground's top-most layer of soil. This displaced soil forms channels which the water then follows and feeds into the nearest creeks, streams and rivers. Because this water flows directly into rivers and streams it is sometimes referred to as streamflow.

The runoff and streamflow steps of the water cycle play a key part in making sure water gets back into the oceans to keep the water cycle going. How so? Well, unless rivers are diverted or dammed up, all of them eventually empty into the ocean! 

08
of 09

Infiltration

Low section of a boy standing in a puddle
Elizabethsalleebauer / Getty Images

Not all of the water that precipitates ends up as runoff. Some of it soaks into the ground -- a water cycle process known as infiltration. At this stage, the water is pure and drinkable.

Some of the water that infiltrates the ground fills aquifers and other underground stores. Some of this groundwater finds openings in the land surface and re-emerges as freshwater springs. And still, some of it is absorbed by plant roots and ends up evapostranspiring from leaves. Those amounts that stay close to the land surface, seep back into surface bodies of water (lakes, oceans) where the cycle starts all over again

09
of 09

Additional Water Cycle Resources for Kids and Students

A young girl drawing the water evaporation cycle on a clear see through surface with a marker pen.
Mint Images - David Arky / Getty Images

Thirsty for more water cycle visualizations? Check out this student-friendly water cycle diagram, courtesy of the U.S. Geological Survey.

And don't miss this USGS interactive diagram available in three versions: beginner, intermediate, and advanced.

Activities for each of the water cycle's main processes can be found at the National Weather Service's Jetstream School for Weather Hydrologic Cycle page

 

Resources & Links:

The Water Cycle Summary, USGS Water Science School

Where is Earth's Water? USGS Water Science School