The water, or hydrological cycle is the Earth’s water recycling system. Since water rarely escapes the planet or arrives from space, the water cycle keeps rivers relentlessly flowing into the oceans and the atmosphere supplied with clouds and rain. Without it, life simply couldn’t exist.
The water cycle circulates water between the oceans and atmosphere, sometimes via the land. When ocean water is heated, it turns into water vapour, which rises into the atmosphere and is carried by winds. The vapour cools at some point and forms clouds. Around 78 per cent of the rain, snow and other forms of precipitation falling from these clouds goes straight back into the ocean. The rest falls over the Earth’s continents and islands.
Some of this water runs into rivers and lakes and is carried back to the sea. Water also seeps back to the oceans through deep soil and rocks, becoming the Earth’s groundwater. Water falling as snow over the polar ice sheets can be buried, sometimes for millions of years, until it reaches the sea via slow-moving glaciers.
Water that stays in shallow soil can be lifted back into the atmosphere when it warms. Alternatively, plants may suck up soil water through their roots and return it to the atmosphere through their leaves. When animals eat plants, they take the water into their bodies and expel it into the air in their breath.
Humans are increasingly altering the water cycle on land by building cities and flood controls, and capturing water for drinking, agriculture and industry.
When the water cycle lets us down?
Floods affect tens of thousands of people each year, as is evident from the devastating monsoon flooding across Pakistan in 2010. The flood, which affected some 20 million people, was the result of the heaviest monsoon rains in the area for generations. On 8 August 2010 the River Indus burst its banks, sweeping away entire communities. While it’s normal for Pakistan to receive half its annual rainfall (250-500mm) during the monsoon months of July and August, the country was reportedly bombarded with 300mm on 20 July alone. The Met Office suggests several possible reasons for the unusually heavy rains, including changes to upper atmosphere airflow, active monsoon systems, and La Nina (El Nino in reverse).
Water cycle facts
2. Drop to drink - Most people get water from rivers and lakes, which form just 0.014 per cent of the world’s water. The rest is mainly In the oceans (96.5 per cent), ice or underground.
3. Olympic deluge – A small thunderstorm can produce, on average, 2,000 tons of rain in just 30 minutes. That’s enough to fill an Olympic-sized swimming pool.
4. Earliest water – Liquid water may have existed on Earth for 4.4 billion years. The water in your glass is almost as old as our planet and significantly older than the dinosaurs.
5. Slow moving – Water can spend more than 10,000 years locked up in deep groundwater or the polar ice sheets, but just a few days in the atmosphere.