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1 The Water Cycle

The water cycle, also known as the hydrological cycle, describes the continuous movement of water within the Earth and its atmosphere. It is a complex system that includes many different processes, and it is fundamental to understanding weather, climate, and life on our planet.

1.1 Evaporation and Transpiration

The cycle begins when energy from the sun heats the surface of oceans, lakes, and rivers, causing water to evaporate into the atmosphere as water vapor. Plants also contribute through transpiration, releasing water vapor through their leaves. Together, these processes are called evapotranspiration and account for roughly 90% of the moisture in the atmosphere.

1.2 Condensation and Precipitation

As water vapor rises, it cools and condenses around tiny particles (dust, pollen, sea salt) to form cloud droplets. When these droplets combine and grow heavy enough, they fall as precipitation. The type of precipitation depends on temperature:

• Rain — when air temperature is above 0 °C throughout the atmosphere
• Snow — when the air column is below freezing
• Sleet — rain that freezes as it falls through a cold layer
• Hail — ice pellets formed by strong updrafts in thunderstorms

2 Water Distribution on Earth

Only about 2.5% of all water on Earth is freshwater, and most of it is locked in ice caps and glaciers. The table below summarises the distribution:

3 Climate Impact and Feedback Loops

The water cycle is tightly coupled with Earth's energy balance. Changes in one component create feedback loops that amplify or dampen climate effects:

4 Human Influence on the Water Cycle

Human activities have significantly altered the natural water cycle through several mechanisms:

1. Urbanisation — impervious surfaces (roads, buildings) prevent infiltration and increase surface runoff by up to 55%.
2. Deforestation — removing trees reduces transpiration, decreasing local rainfall and increasing soil erosion.
3. Irrigation — accounts for 70% of global freshwater withdrawals, depleting aquifers faster than natural recharge.
4. Climate change — alters precipitation patterns, intensifies droughts and floods, and accelerates glacial melting.

The chart below illustrates how global average precipitation has shifted over the past decades, with notable increases in extreme events: