The Water Cycle

Water on planet Earth

When looking at the Earth, it becomes quickly apparent that water covers over half of the Earth's surface. In fact, water - in one form or another - covers approximately 75% of Earth's surface.

It should be no surprise to you that water is a vital resource on our planet, integral to creating, sustaining and nourishing all forms of life.

Water is everywhere

Water is practically everywhere on Earth - even in the hottest and driest corners of the planet. It can exist in a range of forms, including as a gas, a liquid, and a solid. The bulk of this water is found in Earth's oceans, which makes up approximately 96.5% of all water on the planet. 1.7% of water is currently in solid-form, locked up in polar icecaps, glaciers and permanent snow. 1.7% of water is stored in groundwater, lakes, rivers, streams, and the soil beneath our feet.

Explore the 3D Scene below.

  • Spin the Earth around one full rotation. Observe the vast amounts of water readily observable in the planet's oceans.
  • Rotate the Earth to the north and south poles. Find the water that is locked up in icecaps, glaciers and permanent snow.
  • Use the search bar in the top-left corner of the 3D scene viewer to search for an Australian lake or river (e.g. Lake Argyle in Western Australia or Murray River) and zoom in until the surface water is visible.
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Earth from space

But what's missing?

You may have noticed that there were no clouds when you looked at the original view of the Earth in the above scene viewer. The reality is that clouds often cover views of the oceans and lands when seen from space. Consider the image below.

NASA satellite imagery capturing cloud cover

A very small percentage of the world's water exists as a gas or water vapour. This water vapour is sometimes imperceptible to human eyes and is simply in the air around us. At other times, we can see this water vapour collected in cloud formations. The percentage of the world's water vapour fluctuates as it rains but is considered to be less than 1% of the Earth's water.

The Water Cycle

Water doesn't stay in the same place. Rivers flow. Clouds blow. Ice melts. Ocean currents move. Environmental factors affect the movement of water and this is described as the water cycle. The entire system is powered by the energy of the Sun, which sees water exchanged between oceans, the atmosphere, and the land.

The Water Cycle

The entire water cycle can be considered a continuous process. There is no definitive beginning and there is no definitive end. The process is like a circle and as one process is happening somewhere, another process is occurring elsewhere.

Evaporation

The sun heats up the surface of the water, where some liquid evaporates.

The sun plays a central role in the water cycle. As the sun heats up bodies of water - like oceans, rivers and lakes - it causes water to change from its liquid form into water vapour (gas).

This can be easily visualised if you think of how rain puddles "disappear" once the sun comes out or when the wet clothes on the clothesline dry. The water doesn't vanish; it simply evaporates into a gas.

In the water cycle, sunlight warms the surface of water. As the surface of the water heats up, some of it escapes as a gas, evaporating.

Transpiration

The process of transpiration

Transpiration is the movement of water through the root systems of a plant or tree. The water gradually moves through the tree or plant and evaporates through the leaves. In some cases, transpiration is also referred to as evapotranspiration.

Explore the map below:

  • Click anywhere on the map to pan around the map and explore the rate of evapotranspiration in different countries

Global Evapotranspiration

  1. Observe evapotranspiration in Australia. Based on your knowledge of Australia's climate, explain why lower rates of evapotranspiration occur in Australia's remote and rural regions.
  2. Pan the map to South-East Asia or mid-Africa. Why might these regions have higher rates of evapotranspiration?

Condensation

When water vapour condenses into liquid form, the droplets form clouds

Water that has been transformed into water vapour - as a result of evaporation and transpiration - travels up into the atmosphere. As the water vapour reaches the atmosphere it begins to cool. As it cools, the water vapour changes back to liquid form and becomes water droplets, which form clouds. This process is referred to as condensation.

We can see condensation occur a little bit closer to home - we don't have to travel high into the sky to see it. Condensation can occur when water vapour in warm air at ground level has a close encounter with a cool surface, turning the vapour back into liquid form.

Can you think of any examples where you have seen condensation occur?

Precipitation

As water vapour continues to condense into liquid droplets of water, these droplets get bigger and bigger. Precipitation occurs when the drops are heavy enough and fall back to Earth. This can happen in a number of ways depending on what climate and location the clouds are in when the droplets become too heavy.

If the clouds are at higher altitudes, or in the polar regions, the water droplets may freeze to form ice. These ice crystals can then fall back to Earth in the form of snow, sleet or hail. Alternatively, in warmer environments, the water droplets fall in the form of rain.

The map below shows the average annual rainfall experienced by different regions of the Earth. Click on the legend in the bottom-left corner of the map to identify what each colour represents with regards to rainfall.

Global rainfall

Engaging with the map

    What factors may be contributing to higher quantities of rainfall?
    Pan the map to Antarctica and the Arctic. Why might these areas be receiving little to no annual rainfall?

Run-off

Water run-off

As precipitation reaches the ground, gravity and the path of least resistance transports some water along the surface of the ground until it reaches bodies of water like rivers, lakes, streams or the ocean. This is referred to as run-off.

Run-off occurs with both rain and snow, hail or sleet. Although snow might stay in one place during a winter, it may melt in the summer. As the water melts back into liquid form, it can then be transported across the surface of the ground to bodies of water.

Infiltration

Infiltration occurs when water filters through the Earth's surface into the ground. Although some water becomes surface run-off after precipitation, other water soaks into the ground. This water becomes known as groundwater.

Due to the cyclical nature of the water cycle, some groundwater is absorbed by plants and trees, which in turn begins the process of transpiration again.

Putting it all together

Let's revisit the image of the entire water cycle in action. This time however, red arrows have been added so that you can clearly see the continuous patterns of the water cycle.

Throughout the water cycle, water can change forms - liquid, gas, solid - numerous times and although it seems to often disappear from one place, it is appearing in another. Environmental factors like temperature, weather patterns and altitude, along with geographical location, can influence how the water cycle occurs in each location.

The Water Cycle

NASA satellite imagery capturing cloud cover

The Water Cycle

The sun heats up the surface of the water, where some liquid evaporates.

The process of transpiration

When water vapour condenses into liquid form, the droplets form clouds

The Water Cycle