The water cycle in detail
Describe water moving through evaporation, condensation, precipitation, surface runoff and plant transpiration, explaining the roles of solar energy and gravity.
What a learner can do afterwards
- Identifies evaporation, condensation and precipitation on a water-cycle pathway diagram.
- Explains the roles of solar energy, cooling and gravity in evaporation, condensation, precipitation and runoff.
- Explains the role of transpiration (plants releasing water vapour) in the water cycle
- Discusses how human activities (deforestation, paving) affect the water cycle
1 · Read
Evaporation changes liquid water into invisible water vapour. Cooling can make vapour condense into liquid droplets. This is condensation. Clouds contain droplets, ice crystals or both. Precipitation is rain, snow or hail falling from clouds. Solar energy supplies heat for evaporation. Gravity pulls precipitation down and runoff downhill. Stored heat allows some evaporation after sunset; rain can fall at night too.
Surface runoff is water flowing over the ground towards streams, lakes or the sea. Infiltration is water soaking into soil. Plants take up soil water through roots. During transpiration, water evaporates inside leaves and escapes as vapour through tiny pores. These are different pathways, not steps that every drop must follow.
Rain reaches a forest hillside. Some water soaks into soil by infiltration. Some flows downhill to a stream as runoff. A tree takes up some soil water, and some later escapes from its leaves as vapour through transpiration. Water can return to the air from leaves or by evaporating from wet surfaces without first flowing into a stream.
Soil and vegetation can store rainwater and slow its movement. Sealed roads limit infiltration, so replacing a forest with sealed roads can send more runoff to streams sooner and increase flood risk. Tree removal can also reduce transpiration nearby. Reduced transpiration is a separate change, not the main immediate cause of fast storm runoff. Some paving lets water through, so the surface type matters.
Solar energy supports evaporation. Cooling allows condensation, and gravity moves water down. Runoff, infiltration and transpiration are different water-cycle pathways.
Lesson, questions and flashcards written for this dataset.
Four readings written for this dataset. The linked water-cycle and urbanization videos are included in Khan Academy courses.
More to read
Water can take different routes
A puddle can shrink as liquid water evaporates into invisible water vapour. Vapour can later condense into liquid droplets. Clouds contain droplets, ice crystals or both. Rain, snow and hail falling from clouds are forms of precipitation.
After precipitation reaches land, water has several possible routes. It may soak into soil, collect on a surface or flow downhill. Some soil water enters plants and returns to the air through their leaves. A water-cycle diagram connects these routes. It does not mean every drop visits every place or follows every arrow.
What moves the water?
Different arrows on a water-cycle diagram need different explanations. Solar energy supplies heat for evaporation. Cooling can allow vapour to condense into droplets. Gravity pulls precipitation towards the ground and water downhill. A place can receive rain while it is dark.
Plants provide another route between soil and air. Roots take up water, and some travels to leaves. Water evaporates inside leaves and escapes through tiny pores. This is transpiration. Calling it plant sweat can be misleading: the process is not the same as an animal producing sweat.
Compare the ground surfaces
Compare rain falling on planted soil with rain falling on sealed asphalt. Water can enter gaps in soil. Much less can enter through a sealed surface, so more may flow over it. This surface flow is runoff.
There is no fixed runoff amount for every garden or road. Rainfall, slope and how wet the soil already is all matter. Some paving is designed to let water through. Plants can also store water and return some to the air. To explain a change, identify the surface and the pathway rather than assuming all built areas behave alike.
Watch rainwater from a safe place
Watch rain through a window. You may see water collect on a path, flow towards a drain or enter a planted patch. Draw the routes you can see. Mark routes you cannot confirm with a question mark, because some water travels below the surface.
Stay away from floodwater and flowing drains. Do not climb to inspect gutters or change roof pipes. Adults need to check building safety and local requirements before altering drainage. Observing water does not require touching drains or changing where the water goes.
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2 · Watch
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