Water moves between the stores by thirteen flows. Learn each one by what happens to the water.
| Flow | What happens | You can see it in |
|---|---|---|
| Evaporation | Liquid water turns to vapour from seas, lakes, soil | Puddles drying in the sun |
| Transpiration | Plants release water vapour through their leaves | A forest on a hot day |
| Sublimation | Ice turns straight into vapour, skipping liquid | Snow vanishing in dry wind |
| Condensation | Vapour cools and turns into droplets | Clouds, dew, mist |
| Advection | Wind carries vapour or clouds sideways | Sea clouds blown inland |
| Precipitation | Water falls from clouds | Rain, snow, hail |
| Melting / freezing | Ice turns to water / water turns to ice | Spring snowmelt; a frozen lake |
| Surface run-off | Water flows over the ground | Rain running down a road |
| Infiltration | Water enters the soil | Rain soaking into a lawn |
| Percolation | Water moves down through soil and rock | Water reaching an aquifer |
| Streamflow | Water flows in a river channel | The Thames flowing to the sea |
| Groundwater flow | Water moves slowly through rock | A spring feeding a stream |
The four the guide defines, and their neighbours
- Sublimation: ice to vapour, with no liquid stage.
- Advection: wind carrying vapour or clouds, for example from the sea over the land.
- Infiltration: water entering the soil. Percolation: water moving down through it.
- Surface run-off flows over the ground; streamflow flows in rivers.
- Groundwater flow is slow movement through rock towards rivers and the sea.
Real example: near the top of Mount Everest, strong dry winds make snow on the summit ridge sublimate: it turns straight into vapour, so the snow vanishes without ever melting.
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Every flow is either a transformation, which changes the state of the water, or a transfer, which moves it to a new place.
The points to remember
- A transformation changes the state of water: liquid, gas or solid.
- Transformations: evaporation, transpiration, condensation, sublimation, melting, freezing.
- A transfer moves water to a new place without changing its state.
- Transfers: advection, precipitation, run-off, infiltration, percolation, streamflow, groundwater flow, plant uptake.
- If the figure says evapotranspiration, write that word, not 'evaporation' or 'transpiration' alone.
Remember it as: Transformation changes its form; transfer changes its place.
Transformations (state changes)
- Evaporation, transpiration
- Condensation
- Sublimation
- Melting, freezing
Transfers (place changes)
- Advection, precipitation
- Run-off, streamflow
- Infiltration, percolation
- Groundwater flow, plant uptake
Real example: on the River Thames, rain soaking into the Cotswold hills is a transfer (infiltration), and the mist rising off the river on a cold morning comes from a transformation (evaporation then condensation).
Only the first answer counts: Give one transfer and one transformation, both from the figure. A wrong first answer is not rescued by a right second one.
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Evaporation and transpiration both put water vapour into the air, so they are often measured together as evapotranspiration.
The points to remember
- Evaporation is from non-living surfaces: seas, lakes, wet soil.
- Transpiration is from living plants, through stomata.
- Evapotranspiration is the two together: the total loss of water vapour from an area.
- It rises with heat, wind, dry air, more plants and more water available.
- Animals lose water by sweating and breathing, but only plants transpire.
More evapotranspiration
- Hot: puddles dry faster
- Windy: washing dries faster
- Dry air
- Lots of plants
- Plenty of water
Less evapotranspiration
- Cold
- Still air
- Humid air: washing dries slowly
- Few plants
- Dry ground: nothing to lose
Real example: a single large oak tree can transpire over 400 litres of water on a hot summer day. In the Sahara, by contrast, evapotranspiration is tiny: the sun is strong, but there is almost no water to lose.
Another favourite question shows a figure with one way water enters the air, and asks for one more.
Ways water vapour enters the atmosphere
- Evaporation from oceans, lakes, rivers and wet soil.
- Transpiration (or evapotranspiration) from plants.
- Sublimation from snow and ice.
- Small amounts from respiration by animals and from burning fuels.
- Pick one the figure does not already show.
Real example: on this figure, evaporation from Sumatra's rivers or from the Indian Ocean would be a correct extra input: both put vapour into the air but neither is drawn.
Check it is not already there: If the figure already shows evapotranspiration, 'transpiration' repeats it. Choose evaporation from water or soil, or sublimation.
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The guide expects you to create a systems diagram of the water cycle and use it. Build it in five steps.
Five steps to draw it
- Draw a box for each store: oceans, atmosphere, ice, lakes and rivers, soil, groundwater, plants.
- Join them with arrows in the direction the water moves.
- Write the process on every arrow, using the guide's names.
- Mark which arrows are transformations (state changes) and which are transfers.
- Check every box has at least one arrow in and one out: nothing is a dead end.
Real example: to draw the River Severn's water cycle, start with boxes for the clouds over Wales, the soil, the groundwater and the Severn itself, then add arrows: precipitation, infiltration, percolation, groundwater flow and streamflow to the Bristol Channel.
Use it, too: Once drawn, the diagram answers questions: which flow did paving change? Which store is filling? Point to the arrow or box.
How this comes up: Paper 2, Section A: identify one transfer and one transformation on a water-cycle figure [2].
Figure 1 shows the water cycle as a systems diagram.
Identify one transfer and one transformation shown in Figure 1.
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