Water cycle

The water cycle (or hydrologic cycle) is a wonderful journey that water takes around our planet. It moves water between different places, such as from rivers to the ocean, or from the ocean to the sky. This cycle never really stops, and it helps keep our Earth healthy and full of life.

Water changes forms during its journey. It can be a liquid, like in rivers and lakes, or a solid like ice, and it can even become vapor up in the air. Important steps in the water cycle include evaporation, where water turns into vapor; condensation, where vapor forms clouds; and precipitation, where water falls back to Earth as rain or snow.

The ocean plays a big role because most evaporation happens there. The water cycle also helps move heat around the world, which keeps temperatures balanced in different places. It’s very important for plants, animals, and people because it provides fresh water and helps shape the land we live on.

People are changing the water cycle in many ways. Things like cutting down forests, building cities, and taking water from underground can upset the natural balance. Climate change is also affecting the water cycle, changing when and where rain happens. These changes can affect where water is available and how plants grow.

Description

Further information: Water distribution on Earth

The water cycle is driven by energy from the sun. The sun heats water in oceans, lakes, and rivers, turning it into water vapor through a process called evaporation. Water can also move into the air from plants and soil through evapotranspiration. This vapor rises and forms clouds when it cools and condenses into tiny water droplets.

Air moves water vapor around the world. When the water droplets in clouds become heavy enough, they fall as precipitation, such as rain, snow, or hail. Most of this water returns to the oceans, but some flows over land as surface runoff or soaks into the ground as infiltration. This groundwater can stay underground for a very long time or come back up to the surface in springs. Eventually, water returns to the oceans to begin the cycle again.

Important processes in the water cycle include:

• Advection: The movement of water through the air.
• Condensation: The change of water vapor to tiny water droplets in the air, making clouds and fog.
• Evaporation: The change of water from liquid to vapor as it moves from the ground or water bodies into the air.
• Infiltration: The flow of water from the ground surface into the soil.
• Percolation: Water moving down through soil and rocks due to gravity.
• Precipitation: Water vapor that condenses and falls to the Earth's surface.
• Runoff: The ways water moves across land.
• Subsurface flow: The movement of water underground.
• Transpiration: The release of water vapor from plants and soil into the air.

The residence time of a place in the water cycle is the average time a water molecule stays there. Groundwater can stay underground for over 10,000 years. Water in the air stays for about 9 days before falling as precipitation. Large ice sheets store ice for very long periods.

Further information: Water resources and Water distribution on Earth

Changes caused by humans

People can change the water cycle, especially in certain places. This happens when we change how we use and cover the land. For example, building cities, growing more farms, and cutting down forests can change how water moves and where it goes. These changes can cause more water to run off quickly and less water to soak into the ground.

Humans have also caused climate change, which affects the water cycle all over the world. Since the middle of the last century, we have seen big changes, and scientists think these changes will continue to grow. Reports from groups like the Intergovernmental Panel on Climate Change say that the water cycle will become more intense in the future.

Related processes

The water cycle is linked to many other important processes on Earth. When water runs off from land into rivers and lakes, it carries soil, phosphorus, and other materials. This runoff changes the salt levels in oceans and can cause problems in lakes. It also helps move nitrogen and carbon from the land to water bodies.

Over very long periods, some of the lightest gases, like hydrogen, can escape from Earth’s upper atmosphere into space. This slow loss happens because these gases move fast enough to leave the planet.

Historical interpretations

Long ago, people had many ideas about how water moves around the Earth. For example, the ancient Greek poet Hesiod wrote about water vapor rising into the air and turning into rain. Hebrew scholars noticed that rivers flow into the sea, but the sea never overflows, suggesting that water returns in some way.

Later thinkers like Aristotle and Chinese scientist Wang Chong also described parts of the water cycle, recognizing the sun's role in turning water into vapor. It wasn't until the 1500s that Bernard Palissy suggested that rain alone could supply enough water to rivers, a idea that was later tested and confirmed.