4.2 The Water Cycle

2026 Syllabus Objectives

By the end of this subtopic, you should be able to:

  1. Describe and interpret the water cycle
  2. Explain the following processes within the water cycle: precipitation, surface run-off, interception, infiltration, through-flow, groundwater flow, transpiration, evaporation, and condensation

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What is the Water Cycle?

The water cycle (also called the hydrological cycle) is the continuous movement of water through the Earth's systems — from the oceans, to the atmosphere, to the land, and back again. It has no beginning and no end; it is a never-ending loop.

Water does not disappear. It simply changes form (liquid, gas, or solid) and moves from one place to another. The same water that fell as rain millions of years ago is still being recycled today.

The Key Processes of the Water Cycle

Each process below is a step in the water cycle. Understanding each one will allow you to describe and interpret the full cycle.

1. Evaporation

Evaporation is the process by which liquid water is heated by the sun and turns into water vapour (an invisible gas), which then rises into the atmosphere.

  • This happens mainly from the surface of oceans, lakes, rivers, and puddles.
  • The sun provides the energy needed to convert liquid water into vapour.
  • Evaporation is one of the main ways water enters the atmosphere.

Simple example: When a puddle "disappears" after a sunny day, the water has evaporated — it hasn't gone, it has just turned into vapour in the air.

2. Transpiration

Transpiration is the process by which plants release water vapour into the atmosphere through tiny pores (holes) in their leaves called stomata.

  • Plants absorb water from the soil through their roots.
  • That water travels up through the plant's stem.
  • The plant releases excess water as vapour through its leaves.
  • This water vapour enters the atmosphere, just like evaporation does.

Tip: Evaporation and transpiration together are sometimes called evapotranspiration because both processes move water from the land surface into the atmosphere.

3. Condensation

Condensation is the process by which water vapour in the atmosphere cools down and turns back into tiny liquid water droplets, forming clouds.

  • As water vapour rises into the atmosphere, the air gets colder at higher altitudes (higher up in the sky).
  • When the vapour cools enough, it condenses into millions of tiny droplets around tiny particles of dust in the air.
  • These droplets group together to form clouds.

Simple example: When you breathe on a cold window, the warm moist air from your breath hits the cold glass and condenses into tiny water droplets — that's condensation.

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