🌧️ P5/P6 · PSLE Topic

The Water Cycle

The water cycle explained for PSLE Science. Evaporation, condensation, precipitation, transpiration and runoff with Singapore examples, diagrams, and exam tips for P5/P6.

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Syllabus

P5/P6 · PSLE

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Reading time

8 minutes

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Exam weight

High — often tested

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Key skill

Apply + explain

💡 Part 1 — Simple explanation

Water's Never-Ending Journey

The water on Earth today is the same water that dinosaurs drank millions of years ago. It has been recycling continuously through the environment in what we call the water cycle. The Sun provides the energy that drives the entire cycle, and gravity pulls water back down to Earth after it has risen into the atmosphere.

The five key processes are: evaporation (liquid water becomes water vapour), transpiration (plants release water vapour), condensation (water vapour becomes liquid droplets in clouds), precipitation (water falls as rain or other forms), and runoff (water flows across the land back to the sea).

🇸🇬 Part 2 — Real-life Singapore examples

The Water Cycle in Singapore

Singapore's dramatic afternoon thunderstorms are the water cycle in action — accelerated by the equatorial climate. In the morning, Singapore's intense sun heats the sea surface and land, causing enormous amounts of water to evaporate from the Straits of Singapore, reservoirs like MacRitchie, and wet vegetation across the island. This warm, moist air rises rapidly.

By late morning, you can see massive cumulus clouds building vertically above Singapore — the water vapour is condensing as it rises and cools. By early afternoon, these clouds have grown into cumulonimbus thunderclouds, and heavy tropical rain falls — often the very same water that was in the Johor Straits that morning.

The rainwater runs off into Singapore's extensive network of drains and canals (like the Kallang River basin), flows into reservoirs, and is treated at waterworks before becoming tap water. Eventually it finds its way back to the sea, completing the cycle.

The trees at Bukit Timah Nature Reserve contribute significantly to Singapore's water cycle through transpiration — on a warm day, a large tree can release hundreds of litres of water vapour through its leaves, contributing to cloud formation and the afternoon rain that cools the island.

🔬 Part 3 — The 5 stages

Each Stage of the Water Cycle in Detail

Evaporation: The sun's heat energy converts liquid water from oceans, lakes, rivers, puddles, and wet soil into water vapour (an invisible gas). This is the main input of water into the atmosphere. Rate increases with higher temperature, lower humidity, and stronger wind.
Transpiration: Plants absorb water through their roots and release water vapour through their stomata (tiny leaf pores). In a dense forest, transpiration can contribute as much water vapour to the atmosphere as evaporation from bodies of water. Together with evaporation, this is sometimes called evapotranspiration.
Condensation: As water vapour rises, the surrounding air pressure decreases and the vapour cools. Cool air cannot hold as much water vapour as warm air, so the vapour condenses into tiny liquid water droplets around tiny dust particles in the air — forming clouds and fog.
Precipitation: As water droplets in clouds collide and combine, they grow heavier until they fall as precipitation — rain (in Singapore's case), or snow, hail, or sleet in colder climates. Singapore receives about 2,340 mm of rain per year, one of the highest rainfall totals in the world.
Runoff: Rainwater that is not absorbed by the soil flows over the land surface into streams, rivers, and eventually the sea. In Singapore, much of the runoff is channelled into reservoirs for water supply, making rainwater a key part of Singapore's water security strategy.

🔬 Part 4 — Transpiration

Plants Are Active Participants in the Water Cycle

Transpiration is not just a side effect — it is an important process in its own right. As water evaporates from leaf stomata, it creates a pulling force (called transpiration pull) that draws water upward through the xylem from the roots. This is the main mechanism by which tall trees get water from roots to leaves — there is no pump; the evaporation itself creates the suction.

Stomata open during the day to allow CO₂ in for photosynthesis — but this also allows water vapour out. At night, stomata close, so transpiration nearly stops. In dry conditions, plants can partially close their stomata to reduce water loss, but this also slows photosynthesis.

🔍 Part 5 — The "Why"

Why Does Water Vapour Rise into the Atmosphere?

When water evaporates, the resulting water vapour mixes with air near the ground. Warm, moist air is less dense than cool, dry air (water vapour molecules are lighter than nitrogen and oxygen molecules, and warm air has lower density than cool air). Less dense air rises — this is the same principle as convection.

As the warm moist air rises, the pressure around it decreases (there is less air above pressing down). Lower pressure means the air parcel expands, and expansion causes cooling. As the air cools, it can hold less water vapour, and the excess condenses into liquid droplets — forming clouds. The altitude at which this condensation begins is called the cloud base, and you can often see it as a flat bottom on cumulus clouds.

🚨 Part 6 — Exam traps

Common Mistakes

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Trap 1 — Clouds are water vapour

Clouds are made of tiny LIQUID water droplets (or ice crystals at high altitude), NOT water vapour. Water vapour is invisible. Clouds are visible because of the liquid droplets that form when vapour condenses.

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Trap 2 — Only oceans contribute to the water cycle

Evaporation occurs from any liquid water surface: lakes, rivers, reservoirs, puddles, wet soil, and even wet leaves. Transpiration from plants is also a significant contributor — especially in forested areas like Singapore.

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Trap 3 — Rain creates new water

The water cycle does not create new water — it simply moves the same water molecules around. The total amount of water on Earth remains roughly constant; it just changes state and location continuously.

📌 Part 7 — Mini summary

Key Points at a Glance

Water cycle: evaporation → transpiration → condensation → precipitation → runoff → sea → repeat
Energy source: the Sun (drives evaporation). Gravity pulls water back down
Transpiration: plants release water vapour through stomata — important contributor to cycle
Clouds = tiny liquid water droplets (not water vapour)
Condensation happens because rising warm air cools and can hold less water vapour
The same water molecules cycle continuously — no new water is ever created

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