Year 7 Science · Unit 2 · Lesson 12
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Learning Goals
1. Evapotranspiration is two water cycle processes added together. Name both processes, and say where the water vapour comes from in each.
Order the steps: from the ocean to the Murray mouth
Number the steps from 1 (first) to 6 (last).
| Step (1–6) | What happens |
|---|---|
| Winds carry the water vapour inland towards the Great Dividing Range. | |
| The Darling joins the Murray, which carries the water to the sea at Encounter Bay in South Australia. | |
| The droplets grow heavy and fall as rain on the Great Dividing Range, including its western slopes. | |
| The Sun’s energy evaporates the water from the surface of the ocean off the east coast. | |
| Rain that does not soak in flows downhill as runoff, into creeks that feed the Darling River. | |
| The air rises over the mountains, and the water vapour cools and condenses into cloud droplets. |
Cause and effect: a warmer climate
Fill each empty box. Each box is caused by the one before it, and each row carries on from the end of the row above.
Compare three water strategies
Complete the table using what you learned about Australian water security.
| Strategy | Where the water comes from | Does it still work in a long drought? Why? |
|---|---|---|
| Dams | ||
| Desalination | ||
| Recycled water |
Read the data: Australia’s water numbers
Scenario: the driest inhabited continent
Table 1 gives the lesson’s figures for rain and evaporation. “Evaporation potential” is how much water could evaporate in a year if there were always water there to evaporate.
| Measurement | Average per year (mm) |
|---|---|
| Rainfall, whole world | about 800 |
| Rainfall, Australia | about 470 |
| Rainfall, Australia’s interior | under 200 |
| Evaporation potential, Australia | about 2000 |
Source: figures from Lesson 12, rounded.
2. How many times bigger is Australia’s evaporation potential than its rainfall? Show your working and round to one decimal place.
Answer times
3. Liam looks at Table 1 and says: “Four times more water evaporates than falls as rain, so Australia is losing its water and will run dry one day.” Use the water cycle and the law of conservation of matter to explain why Liam is wrong.
4. Adelaide takes a large share of its drinking water from the Murray River, and it also has a desalination plant. In a drought, the city could run the plant harder and take less from the river. Evaluate the environmental impact of relying more on desalination.
Question 1
Evaporation: from the surface of oceans, lakes, rivers and soil (1). Transpiration: released by plants through the stomata in their leaves (1). [2 marks]
Order the steps: from the ocean to the Murray mouth
Reading down the table as printed, the correct step numbers are 2, 6, 4, 1, 5, 3. In order: evaporation; winds carry vapour inland; condensation over the range; rain on the range, including its western slopes; runoff to the Darling; the Murray carries it to Encounter Bay.
Cause and effect: a warmer climate
Box 1: more water evaporates from soil, rivers and dams (1). Box 2: rain falls less often, in heavier downpours (1). Box 3: soils dry out, so less rain runs off into rivers (1). [3 marks]
Compare three water strategies
1 mark per correct row. Dams: stored rain and river water; no, little rain refills them and water evaporates. Desalination: seawater with the salt removed; yes, it does not need rain. Recycled water: treated sewage; mostly yes, a city keeps making used water in a drought. [3 marks]
Question 2
2000 ÷ 470 (1) = 4.3 times (1). [2 marks]
Question 3
Matter cannot be created or destroyed, so evaporated water is not lost from Earth (1). The vapour condenses and falls as precipitation somewhere else (1). The figure is only what could evaporate: where there is no water, none evaporates (1). [3 marks]
Question 4
Marking guide: 1 how it works (salt removed from seawater); 1 benefit: less taken from the Murray, more left for the river; 1 benefit: drought-proof, needs no rain; 1 cost: high energy use, so greenhouse gas emissions if the power is from fossil fuels; 1 cost: salty brine returned to the sea can harm marine life; 1 judgement with a reason. [6 marks]
Model answer: Desalination removes salt from seawater. It does not need rain, so it works in a drought, and every litre it makes is a litre left in the Murray for wetlands. But it uses a lot of energy, so it causes greenhouse gas emissions if the power comes from fossil fuels, and its salty brine can harm marine life. On balance it is worth using in droughts, if it runs on renewable power.
Common error: treating evaporated water as “lost”; it falls again somewhere else.