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Influences on the water cycle

Water is a renewable resource, cycling constantly through the environment.

The water cycle operates at different scales in different places. It is strongly influenced by location, temperature and topography. Australia is the driest inhabited continent on earth – and large parts of it are unsuitable for agriculture. The largest, most productive agricultural areas are in the south-east through a combination of suitable temperatures, rainfall, groundwater and soils. 

These activities guide students as they investigate and discover which parts of the world, and of Australia, receive the most reliable rainfall. They also learn how much of the water becomes accessible, and predict issues this causes (see the next unit, availability of water).

Outcomes

By the end of this resource, students will:

  • understand factors that influence the water cycle in nature;
  • understand that Australia has a limited and variable water supply.

Curriculum focus

  • ACSSU116 - Some of the Earth’s resources are renewable, including water that cycles through the environment.
  • ACHGK037 - Classification of environmental resources and the forms that water takes as a resource
  • ACHGK038 - The way that flows of water connects places as it moves through the environment and the way this affects places
  • ACHGK039 - The quantity and variability of Australia’s water resources compared with other continents.

Preparation

1: Options to engage with the topic

Watch: NASA’s Water Cycle video as a class.

This animation shows one molecule of water completing the hydrologic cycle. Heat from the sun causes the molecule to evaporate from the ocean's surface. Once it evaporates, it is transported high in the atmosphere and condenses to form clouds. Clouds can move great distances and eventually the water molecule will fall as rain or snow. Ultimately, the water molecule arrives back where it started: at the ocean.

Do:  the three water cycle activities:

2. Explore the topic

Do:  Small groups of students each investigate and each produce a five minute presentation to the class on one of the five factors influencing water availability globally and in Australia: location, topography, seasonality, variability and evaporation. To assist them, the following questions (per topic) are provided on their worksheet.

Factor:

Location

Worksheet questions Teaching notes

Students investigate:

NASA earth observatory’s cloud animation and global water vapour animation.

NASA’s 2000-2017 land surface temperature animations and this video showing 2017 in detail.

A world deserts map, such as this example from National Geographic.

  • Where is it most/least cloudy throughout the year?
  • Where is the most water in the atmosphere?  Do they line up?
  • Where is it consistently hottest? Coldest?
  • What conclusions can you draw from all this data?
The dry regions are roughly at 30 degrees north or south of the equator. At these latitudes, there are belts of high pressure in the atmosphere. These are caused as trade winds collide and storms around the equator cause the air to rise very high and cool, causing most of the rain to fall nearby in the tropics. As the remaining drier air moves away from the equator and sinks it becomes denser and warmer. In these latitudes, winds may also be less strong and don’t blow as much moisture from the oceans onto land. The warm dry air at these latitudes then increases evaporation, which leaves the land underneath even drier.
Factor:

Topography

Worksheet questions Teaching notes

Students investigate:

NASA’s Australian topography map

The Bureau of Meteorology’s Australian average annual rainfall map and climate classification map.

  • Where are the highest mountains? The lowest areas?  Why do you think Australia is so flat?
  • Compare Australia’s highest mountain to Mt Everest and some others in the world.
  • How do the topography and climate zones match up with the rainfall? (Is the rainfall higher in the tropics? Is it lower in the arid zones?)
  • What about rainfall on either side of a mountain range? Can you see areas where this is obvious?
  • Which zone do you live in?

Australia is a very flat country. Mt Everest is 8,846 metres. Europe’s ten highest mountains range between 4,545 and 5,642m. Our highest mountain is 2228 m. This is because our mountains are much older and have eroded away.

Why does this matter? Because as rain-carrying winds cross land, they rise when encountering a mountain. As air rises, it cools. Cooler air condenses more easily, forming droplets (or if even colder, crystals).  Note that most of this rain will fall on the ‘catching’ side of the mountain - the other side (rain shadow side) generally gets much less.  Because our mountains are low, the air doesn’t rise, or cool, as much.  There is more rainfall east of the Great Dividing Range (the west is in rain shadow).

In the south-east (where much of Australia’s population lives) west of the divide is the Murray–Darling Basin. 45% of all the water that runs into rivers comes from rain falling on just 11% of its land area)

Factor:

Seasonality

Worksheet questions Teaching notes

Students investigate:

NASA’s total rainfall map
(Observe the rain belt move onto and off Australia).

  • What times of year are wetter? Drier?
  • Compare this effect in the north and in the south.
  • How likely is it that the area west of the Great Dividing Range in south-east Australia will receive rainfall above 300 mm in May to July
  • What part of this area does get rainfall above 300mm at any time during the year?
  • When and where do you think most rain falls in south-east Australia?

In the north (tropics) most rain is in summer, while in the south it is winter–spring. The tropics are around the widest part of the earth, and as the earth tilts, as it rotates the sun warms the area more in summer and less in winter. Along with the heat shifting, so does the tropical rain belt.

In the north and south of the earth (narrower) bits, this effect is intensified. The bits of land we are on are either hot, less hot, colder, or cold. In these zones, most rain forms as air rises and cools. So as we are so flat, this cooling is mostly only happening in the colder months.

In the south-east it’s rare to get rainfall above 300mm. This is particularly so west of the Great Dividing Range, apart from the Alps (which is where most rain falls).
Factor:

Variability

Worksheet questions Teaching notes

Students investigate:

Bureau of Meteorology’s rainfall variability map

Climate Kelpie

  • What do you notice about where rainfall is most variable?
  • What are the four main factors leading to such high variability?
  • What do you think about Australia’s rainfall patterns in general?

The only places in Australia with low variability are in the far south edges, large areas of Australia are in the high to very high categories.

The variability in the tropics is not the same as in temperate zones. (Monsoonal versus four seasons).
Factor:

Evaporation

Worksheet questions Teaching notes

Students investigate:

The Bureau of Meteorology’s Annual average evaporation map.

The Australian landscape water balance map (showing how much water is in the soil)

  • Where is evaporation the highest/lowest?
  • Which places have moderate to high soil moisture? (roughly)
  • How do things change year to year?
  • Looking at 2007, and west of the Great Dividing Range in the south-east, what could you say about conditions for growing things?
  • Look at Actual evapotranspiration.  Click through the months of a year.  When is it highest and lowest?
  • Thinking about Australia’s places, what things do you think are adding to evaporation rates?

The combination of heat and flatness in Australia leads to very high evaporation.

Because our rivers are mostly travelling over fairly low topography, they move relatively slowly. This allows the heat of the sun to more easily suck up water.

The flat places west of the divide have lowest soil moisture, the tropics, coastal areas on the east and higher areas have more.

2007 was a drought year. In the Murray?Darling Basin (a major food growing area) there was very low soil moisture.

Evapotranspiration is highest in the hotter months.

In Australia on average, 94% of rainfall evaporates or is transpired by plants!

3. Elaborate

Do: Students analyse the following data (provided on their worksheet) and create a pie or bar chart.

Type of water % of Earth’s water
Water vapour 0.001
Fresh – on the surface (rivers, lakes, swamps) 0.025
Fresh – under the ground 0.76
Fresh – frozen in icecaps, glaciers, snow 1.76
Salty - under the ground 0.93
Salty - oceans 96.54

Students can use Excel if available, and may need to adjust the x-axis scale to be easily able to determine the huge difference between ocean sources and all others.

Write: Students write their analysis of the data.

4. Conclude

In Australia, only 4% of annual rainfall (on average) becomes a water resource for people and nature. Because Australia is drier than many other countries, we have relatively low surface water reserves. About 17% of our fresh water reserves are under the ground, and in an average year, about 30% of our water is drawn from this source.

Continue to availability of water

Updated: 30 Aug 2018