Vertex LearnEarth's fresh water: ice, lakes, rivers and groundwater
What this lesson covers
- How small Earth's share of fresh water is
- Fresh water stored as ice, and in lakes and rivers
- Groundwater and aquifers
- Where our tap water comes from
- Showing amounts of water in grids, diagrams and graphs
Fresh water is rare
- Fresh water is water with very little salt in it. After cleaning, people, animals and plants can use it.
- The U.S. Geological Survey, a science agency that studies water, estimates that only 2.5% of Earth's water is fresh. All the rest is salty.
- That means for every 100 buckets of water on Earth, only about two and a half are fresh.
Where the fresh water is kept
- Most fresh water is frozen. A glacier is a huge, slow river of ice that creeps downhill, like the Franz Josef and Fox Glaciers in the Southern Alps. An ice cap is a thick sheet of ice covering land, like the one over Antarctica.
- The next biggest store is underground.
- Lakes and rivers, the fresh water we see most, hold only a tiny share. Lake Taupō is New Zealand's largest lake, and the Waikato River is its longest river.
| Fresh water store | Share of Earth's fresh water (%) |
|---|---|
| Ice caps, glaciers and permanent snow | 68.7 |
| Groundwater | 30.1 |
| Lakes | 0.26 |
| Air | 0.04 |
| Rivers | 0.006 |
Groundwater and aquifers
- Groundwater is fresh water stored underground, in the tiny gaps between grains of soil, sand and gravel, and in cracks in rock.
- An aquifer is a layer of rock or gravel that holds groundwater and lets it flow through, a bit like a giant sponge.
- Rain soaks down through the soil until it reaches ground that is already full of water. The top of this wet layer is called the water table.
- People reach groundwater by digging a well or drilling a narrow hole called a bore, then pumping the water up.
Where tap water comes from
- Rivers, lakes and groundwater supply nearly all the water people use. The ice is too far away and too hard to melt.
- Christchurch's tap water is pumped from aquifers deep under the Canterbury Plains.
- Auckland's comes mostly from dams in the Waitākere and Hūnua Ranges, which store rain and stream water in lakes, and from the Waikato River.
- Many farms and small towns have their own bores into the groundwater.
Try it: where is the fresh water?
Showing Earth's water in 100 squares
- Draw all of Earth's water as a grid of 100 squares, so each square stands for 1%. How many squares does each store get?
| Store | Share of all Earth's water (%) |
|---|---|
| Ice caps, glaciers and permanent snow | 1.74 |
| Fresh groundwater | 0.76 |
| Lakes, rivers and air together | less than 0.02 |
| Salty water | the rest |
- Ice: 1.74 rounds to 2 squaresround to whole squares
- Fresh groundwater: 0.76 rounds to 1 squareround to whole squares
- Lakes, rivers and air: less than 0.02, so 0 squarestoo small to show
- Salty water: 100 − 2 − 1 = 97 squaresthe rest
- 97 salty squares, 2 ice squares and 1 groundwater square
- Rounding gives fresh water 3 squares, a little more than its real 2.5%
Try it: Earth's water in 100 squares
Other ways to show the amounts
- A bar graph puts each store on its own bar, so you can compare the sizes quickly.
- A pictogram uses a small picture, like a drop, for a set amount of water.
- A scaled model shrinks the real amounts. If all Earth's water fitted in a 1 litre jug, the fresh water would be just 25 mL, about 5 teaspoons.
- A map can show where the water is, such as which regions draw on groundwater.
Common mistakes
- Most fresh water is in lakes and rivers.Most fresh water is frozen as ice, and most of the rest is groundwater; lakes and rivers hold well under 1% of it.
- Groundwater is an underground lake or river.Most groundwater fills the tiny gaps between grains of soil and gravel and the cracks in rock, like water in a sponge.
- With 2.5% fresh water, people can use 2.5% of all Earth's water.Most of that 2.5% is locked in ice far away, so the fresh water people can easily reach is far smaller.
Earth's water in a jug
You need: a 1 litre measuring jug, water, a teaspoon, 2 small cups, a saucer, an ice cube tray, salt
- Fill the jug to 1 litre. This stands for all the water on Earth.
- Take out 5 teaspoons (25 mL) into a cup. This is all the fresh water. Then stir a teaspoon of salt into the jug to make the rest salty, like the sea.
- Put 3 and a half teaspoons of the fresh water into the ice cube tray and freeze it. That is the ice caps and glaciers. The last 1 and a half teaspoons in the cup is groundwater.
- Dip a finger in the groundwater and let 2 or 3 drops fall onto the saucer. That is roughly all the lakes, and all the rivers would be less than one drop.
- Look for this: a full jug of water, but only a few teaspoons of fresh water, and just a few drops in the places people find easiest to reach.
Safety: Ask an adult to help, and do not drink the salty water, because seawater makes you thirstier; wipe up spills so nobody slips.
Your turn
1
If all of Earth's water was 1000 mL, how much would be fresh water?
2
Which store holds the most fresh water?
3
What is an aquifer?
4
Where does Christchurch get its tap water from?
5
In a 100-square grid of Earth's fresh water only, about how many squares would groundwater get?
Show answer
1. 2.5% of 1000 mL is 25 mL. 2. Ice caps, glaciers and permanent snow, with 68.7% of it. 3. A layer of rock or gravel that holds groundwater and lets it flow through. 4. From aquifers deep under the Canterbury Plains. 5. Groundwater is 30.1% of fresh water, so about 30 squares.
Recap
- Only about 2.5% of Earth's water is fresh.
- Most fresh water is frozen in ice caps and glaciers; most of the rest is groundwater.
- Lakes and rivers hold only a tiny share, but they are the easiest to reach.
- Groundwater sits in gaps in soil and rock; an aquifer holds it and lets it flow.
- Rivers, lakes and groundwater supply nearly all the water people use.
- Grids, bar graphs, pictograms and scaled models make the amounts easy to compare.
The maths behind it
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