Natural sciences Year 5: Water on Earth: where it is, how it moves, how to manage it
Natural sciences, Year 5 (Portugal): Most of the planet's water is salty or frozen; it circulates in the water cycle, and using it sustainably means not using it up or spoiling it.
Where the planet's water is
Imagine all the water on Earth in 100 bottles. About 97 hold salt water from the oceans and only about 3 hold fresh water. Of those 3, about two thirds (2 bottles) are locked up as ice in glaciers and polar caps. That leaves roughly 1 bottle of liquid fresh water, in rivers, lakes and underground (rounded values). The fresh water we can use is a small part of the total.
Water never stops moving
Water circulates among the subsystems in the water cycle. The Sun warms the sea, rivers and plants, and water evaporates. The vapour rises, cools and condenses into clouds. It falls as rain, snow or hail (precipitation). On the ground it runs off over the surface or infiltrates the soil and becomes groundwater, and most of it ends up in the sea, where the cycle starts again.
Reading a table on water
A simplified table (rounded, per 100 parts): salt water 97, fresh water 3, of which ice 2. Step 1: check the total: 97 + 3 = 100. Step 2: the ice share of fresh water: 2 out of 3, about two thirds. Step 3: liquid fresh water: 3 − 2 = 1 part. Step 4: conclude: only about 1 % of the water on Earth is liquid fresh water. Maps in Geography show where the rivers and reservoirs that hold it are.
«It circulates, so it never runs out»
The water cycle renews water, but it does not solve everything. The fresh water we can use is only a small part of all water. An underground reservoir (aquifer) that is pumped faster than it refills runs low, and water that is polluted cannot be used until it is treated. So a shortage can happen in a place even though the cycle keeps working. Water has to be managed, not just expected.
Managing water sustainably
Sustainable management has three parts. Use: at home, in farming and in industry. Exploitation: taking water from rivers, dams (such as Alqueva) and boreholes. Protection: treating used water and not polluting. A dripping tap of one drop per second, with about 20 drops to the mL (approximate), wastes 86 400 × 0,05 mL ≈ 4,3 L a day. What could your school or street do to save water?
Test what you learned
Roughly how much of the water on Earth is salt water?
- a) About 3 %
- b) About half
- c) About 97 %
Correct answer: c) About 97 % — Right: about 97 % is salt water (rounded) and only about 3 % is fresh.
A simplified table, per 100 parts: salt water 97, fresh water 3, of which ice 2. How many parts are liquid fresh water?
- a) 3 parts
- b) 1 part
- c) 2 parts
Correct answer: b) 1 part — Right: 3 − 2 = 1 part, about 1 % of the water on Earth.
Why can a place run short of water even though the water cycle never stops?
- a) Usable fresh water is a small part, and it can be over-pumped or polluted
- b) Because the cycle destroys some water every year
- c) Because rain only falls on the sea
Correct answer: a) Usable fresh water is a small part, and it can be over-pumped or polluted — Right: the cycle renews water, but overuse and pollution can still leave a place short.
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