The Water Cycle: A Complete Guide for PSLE

Water is the most precious substance on Earth. It covers about 71% of our planet's surface, makes up around 60% of the human body, and is essential for every known form of life. Yet the total amount of water on Earth never changes — the same water molecules that fell as rain millions of years ago are still cycling through our environment today. This continuous movement of water is called the water cycle (or hydrological cycle), and it is a core topic in Singapore Primary Science from P4 through PSLE.

Understanding the water cycle means understanding not just the four main stages, but also how they are connected, why they matter for Singapore's water supply, and how to answer the specific question types that examiners use at PSLE.

The Four Main Stages of the Water Cycle

The water cycle has no true starting or ending point — it is a continuous loop. However, for examination purposes, it is described in four main stages:

Stage 1: Evaporation

Evaporation is the process by which liquid water is converted into water vapour (gas) and enters the atmosphere. It happens from the surface of oceans, lakes, rivers, and puddles when the Sun heats the water. The Sun's energy gives water molecules enough energy to overcome the forces holding them together and escape into the air as invisible water vapour.

Evaporation is the main driver of the water cycle. The oceans account for about 86% of global evaporation — they are the primary source of atmospheric water vapour. The rate of evaporation increases with higher temperature, stronger wind, and lower humidity (drier air).

Stage 2: Transpiration

Transpiration is the evaporation of water from plants. Plants absorb water from the soil through their roots, transport it up through the stem, and release it as water vapour through tiny pores called stomata on the surfaces of their leaves. A large tree can transpire hundreds of litres of water per day.

The combined process of evaporation and transpiration is called evapotranspiration, and together they return enormous quantities of water vapour to the atmosphere. In tropical rainforests like those found near Singapore, transpiration by trees contributes significantly to local rainfall — the forests essentially create their own rain.

Stage 3: Condensation and Cloud Formation

As water vapour rises into the atmosphere, it encounters cooler temperatures at higher altitudes. When air cools below its dew point, the water vapour condenses — it changes back from a gas into tiny liquid water droplets. These tiny droplets cluster around microscopic particles of dust, pollen, and sea salt (called condensation nuclei), forming clouds and fog.

This is the same process you see when a cold drink can forms water droplets on its surface — the warm, humid air near the can is cooled below its dew point by the cold surface, causing the water vapour to condense onto the metal.

Stage 4: Precipitation

Precipitation is any form of water that falls from clouds to the Earth's surface. This includes rain, drizzle, snow, sleet, and hail. In Singapore's tropical climate, precipitation almost always occurs as rain. Precipitation happens when the water droplets in clouds combine and grow heavy enough that they can no longer remain suspended in the air.

Singapore receives an average of about 2,400 mm of rainfall per year — significantly more than most cities — due to its location near the equator. This abundant rainfall is one reason Singapore has relatively good water security compared to many countries, though it still imports significant quantities of water from Malaysia and treats water for reuse.

Stage 5: Collection and Runoff

After precipitation, water flows across the land surface (surface runoff) into rivers, lakes, and the sea. Some water soaks into the ground (infiltration), becoming groundwater stored in underground aquifers. Some water is absorbed by plant roots and re-enters the cycle through transpiration. Rivers eventually carry the water back to the ocean, where evaporation begins the cycle again.

Why the Water Cycle Matters for Singapore

Singapore faces a unique water challenge: it is a small island with no natural lakes, rivers large enough for major water supply, or underground aquifers. All fresh water must come from rainfall collected in reservoirs, imported from Malaysia through pipelines, desalinated from seawater, or recycled from used water (NEWater).

Understanding the water cycle helps explain why Singapore's 17 reservoirs are so important — they collect rainfall before it runs off into the sea. The reservoirs are fed by rain falling on two-thirds of Singapore's land area, which is one of the highest proportions of water catchment to land area in the world. Changes to the water cycle (due to climate change causing more intense but less frequent rainfall) pose real challenges for water planners.

The Role of the Sun and Gravity

The water cycle is driven by two forces: the Sun's energy and gravity. The Sun provides the heat energy that drives evaporation and transpiration — without the Sun, water would not rise into the atmosphere. Gravity pulls the water back down as precipitation and causes water to flow downhill as runoff back to the ocean. Together, these two forces keep the cycle moving continuously.

Changes of State in the Water Cycle

The water cycle involves three changes of state — solid, liquid, and gas — that are important to understand for exams:

Human Impact on the Water Cycle

Human activities significantly affect the water cycle in ways that are relevant for PSLE open-ended questions:

• Deforestation — removing trees reduces transpiration, decreasing local rainfall. It also reduces the ability of soil to absorb water, increasing flooding risk and soil erosion.
• Urbanisation — replacing natural ground with concrete and tarmac prevents infiltration (water soaking into the ground). Rain runs off rapidly instead of being absorbed, increasing flood risk.
• Climate change — global warming increases evaporation rates, leading to more intense rainfall events (heavier rain in shorter periods) and longer droughts between rain events.
• Reservoir construction — humans deliberately modify the water cycle by building dams and reservoirs to store water for drinking, irrigation, and power generation.