The Water Cycle

The water cycle (also called the hydrological cycle) describes the continuous movement of water between the oceans, atmosphere, land, and living things. It is driven by solar energy and gravity and provides the fresh water that all life depends on.

What Is the Water Cycle?

The water cycle (hydrological cycle) is the continuous circulation of water on, above, and below the surface of the Earth. It has no beginning or end — the same water molecules have been cycling through oceans, clouds, rivers, living things, and back again for billions of years.

The water cycle is driven by two main forces:

Solar energy (from the Sun): provides the heat to evaporate water.
Gravity: pulls precipitation down and keeps water flowing in rivers toward the sea.

Main Stages of the Water Cycle

1. Evaporation

Heat from the Sun causes water from oceans, lakes, and rivers to turn to water vapour and rise into the atmosphere. About 86% of water vapour comes from the oceans.

2. Transpiration

Plants absorb water through their roots and release it as water vapour through their leaves. Evapotranspiration combines evaporation and transpiration.

3. Condensation

As water vapour rises, it cools. Cool air holds less water vapour. The vapour condenses around tiny dust particles to form tiny water droplets — these droplets gather to form clouds.

4. Precipitation

When water droplets in a cloud combine and become heavy enough, they fall from the cloud as precipitation — rain, snow, sleet, or hail, depending on temperature.

5. Collection and Run-off

Water collects in oceans, lakes, and rivers. Water soaks into the ground (infiltration) and moves through rock and soil as groundwater. Surface run-off flows over the land into streams and rivers, eventually returning to the sea.

Note

The total amount of water on Earth stays roughly the same — it just changes form and location. Earth holds about 1.386 billion km³ of water. About 97.5% is saltwater in oceans. Only 2.5% is fresh water, and most of that (about 68.7%) is frozen in glaciers and ice caps. Less than 1% of Earth's total water is available as accessible fresh water for humans and ecosystems.

Tip

Deforestation disrupts the water cycle. Trees absorb huge amounts of water from the soil and return it to the atmosphere through transpiration. When forests are cleared, this transpiration stops, the local air becomes drier, rainfall patterns change, and less water infiltrates the soil — leading to drier conditions, increased surface run-off, flooding, and even desertification in some areas.

The Water Cycle and River Systems

Rivers are a crucial part of the water cycle. After precipitation:

Some water runs off the surface into streams (surface run-off).
Some infiltrates into the soil and slowly moves to rivers as throughflow.
Some reaches deeper rock layers as groundwater, slowly moving to the sea.

All these pathways eventually return water to the oceans, from where evaporation starts the cycle again.

Real world: The water cycle and your morning cup of tea

The water you drink today has cycled through the water cycle millions of times. The same water molecules may have once been part of the Mediterranean Sea, a Himalayan glacier, or even a dinosaur. When you boil a kettle, you are causing evaporation — the steam condenses on the cold window as condensation. Your whole house is a miniature version of the water cycle happening every day.

Real world: The Amazon rainforest as a 'flying river'

The Amazon rainforest is so vast that its trees release 20 billion tonnes of water vapour per day through transpiration. This forms what scientists call "flying rivers" — invisible streams of water vapour that drift south and east, eventually condensing as rain over Brazil, Paraguay, and Argentina. Deforestation of the Amazon is not just a local issue — it reduces transpiration, disrupts rainfall patterns, and threatens farming thousands of kilometres away.

Real world: Droughts and the broken water cycle

When the water cycle is disrupted — by drought, deforestation, or climate change — the effects are severe. The Cape Town water crisis of 2018 ("Day Zero") nearly saw a major city completely run out of water. The city's reservoirs, fed by winter rainfall, shrank to below 13% capacity after three consecutive dry years. Agriculture was rationed and residents limited to 50 litres per day. A combination of good rains and strict conservation eventually averted complete shutdown — a reminder that the water cycle cannot be taken for granted.

Tip

Where do you see this in real life? Fog on a cold window, dew on grass in the morning, puddles evaporating after rain — these are all stages of the water cycle happening around you every day.

Practice Problems

Why does rainfall occur more frequently over mountains than over flat plains?

Show answer

When moist air moves from the sea over flat land, it can continue moving without cooling much. When it hits a mountain range, it is forced to rise. As it rises, it cools, and the water vapour condenses to form clouds and rain (relief/orographic rainfall). The sheltered side of the mountain receives less rain — this is called a rain shadow. Mountains 'wring out' moisture from passing air masses.

Trace the journey of a water molecule from the ocean to a river, naming the processes involved.

Show answer

1. Evaporation: Heat from the Sun evaporates the water molecule from the ocean surface, turning it into water vapour. 2. Water vapour rises and is carried inland by wind. 3. Condensation: The vapour cools at altitude and condenses around dust particles to form a water droplet in a cloud. 4. Precipitation: The droplet joins others, becomes heavy, and falls as rain. 5. Run-off/infiltration: The rain either runs over the surface or soaks into the ground, eventually reaching a stream or river.