Stores and flows: The water cycle is a system. Each store holds water for a while; each flow moves it on.
The points to remember
- The water cycle is a system: stores that hold water, joined by flows that move it.
- Globally it is closed for matter: no water enters or leaves the Earth.
- The amount of water stays about the same over millions of years; it is recycled.
- It is open for energy: sunlight comes in, heat goes out to space.
- A lake or a river basin is an open system: water flows in and out.
Remember it as: Same water, endlessly recycled; new energy, every day.
Matter: closed
- No water in or out of the Earth
- Tiny exceptions: hydrogen lost to space, icy comets
- Recycled between air, land, sea and living things
Energy: open
- Sunlight in: evaporation, melting
- Latent heat given out in condensation
- Heat lost to space
Real example: Biosphere 2 in Arizona is a huge sealed glass building. From 1991 to 1993 eight people lived inside, and its water was recycled through its 'ocean', air and plants. Like the Earth, it was closed for water but open for energy: sunlight came in through the glass.
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A system diagram shows the cycle as boxes and arrows. It is the drawing the exam asks for, so draw it the same way every time.
How to draw it
- Every store is a box with a noun in it: oceans, groundwater, ice.
- Every flow is an arrow with a process on it: evaporation, infiltration.
- The arrow points the way the water moves: from the ocean up to the air for evaporation.
- A system diagram has no pictures of clouds or trees: only boxes and labelled arrows.
- Each mark for a diagram needs three correct labels, so label every box and arrow.
Real example: to draw Lake Titicaca, on the border of Peru and Bolivia, as a system, the lake is a box. Rain and the 25 rivers that feed it are arrows in; evaporation and the Desaguadero River, its one outlet, are arrows out.
Store or flow?: 'Rain' and 'evaporation' are flows, never boxes. 'Groundwater' and 'glaciers' are stores, never arrows.
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The same diagram can show how people change the cycle: mark each changed arrow and say what changes it.
What people change, flow by flow
- Urbanisation: paving means less infiltration and more run-off.
- Deforestation: less transpiration and plant uptake, more run-off.
- Global warming: more melting, less freezing, more evaporation.
- Burning fossil fuels: NOx and SO2 make acid precipitation.
- Pumping wells: less groundwater and less groundwater flow.
- Industry: warmer or polluted streams; cloud seeding adds precipitation.
Real example: in London, roofs, roads and pavements now cover much of the city. Rain that once soaked into the soil runs into drains and the Thames, so infiltration falls and run-off rises.
Words and diagram both score: Up to four marks come from the impacts, each naming the flow, the change and its cause. Up to three come from the diagram: one mark for every three correct labels.
Why does the water cycle matter? Every ecosystem depends on the water it delivers. Here is what water does for living things.
What water does in ecosystems
- Plants use water in photosynthesis to make glucose and oxygen.
- Water dissolves minerals in the soil so roots can take them up.
- It is a habitat for aquatic life and a solvent for the reactions in every cell.
- It holds plants upright by turgor pressure, and about 70% of an organism is water.
- It moves heat and cools by evaporation; rivers carry nutrients, seeds and sediment.
Remember it as: Food, transport, home, support, cooling.
Real example: each year floodwater from Angola's highlands spreads across the Okavango Delta in Botswana, a wetland in the Kalahari desert. It brings nutrients and sediment, fills the channels where fish and hippos live, and lets grasses grow for elephants and antelope.
Ecosystems, not people: 'Drinking water for towns' or 'irrigation for farms' does not answer a question about ecosystems.
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How this comes up: Paper 2, Section B (b): explain, with a system diagram, how human activities affect flows [7]. Four impacts plus a labelled diagram.
Lagos in Nigeria grew from about 1.4 million people in 1970 to over 15 million today, while forest around it was cleared for farms.
Explain, with the use of a system diagram, how human activities affect flows in the water cycle.
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