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.

However, people are changing the water cycle in big ways. Things like cutting down forests, building cities, and taking water from underground can upset the natural balance. Climate change is also making the water cycle stronger and changing when and where rain happens. These changes can affect where water is available, how plants grow, and even how safe our weather is.

Description

Further information: Water distribution on Earth

The water cycle is powered 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 get 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 get 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 thousands of years or come back up to the surface in springs. Eventually, water makes its way back to the oceans to start the cycle again.

Important physical processes within the water cycle include:

• Advection: The movement of water through the atmosphere.
• Condensation: The transformation of water vapor to liquid water droplets in the air, creating clouds and fog.
• Evaporation: The transformation of water from liquid to gas phases as it moves from the ground or bodies of water into the atmosphere.
• Infiltration: The flow of water from the ground surface into the ground.
• Percolation: Water flows vertically through the soil and rocks under the influence of gravity.
• Precipitation: Condensed water vapor that falls to the Earth's surface.
• Runoff: The variety of ways by which water moves across the land.
• Subsurface flow: The flow of water underground.
• Transpiration: The release of water vapor from plants and soil into the air.

The residence time of a reservoir within the hydrologic cycle is the average time a water molecule will spend in that reservoir. Groundwater can stay underground for over 10,000 years. Water in the atmosphere stays for about 9 days before falling as precipitation. The major ice sheets store ice for very long periods.

Further information: Water resources and Water distribution on Earth

Changes caused by humans

Human activities can change the water cycle, especially in specific areas. This happens through changes in how land is used and covered. For example, building cities, expanding farms, and cutting down forests can affect how water moves and is stored. These changes can lead to more runoff and less water soaking into the ground.

Climate change, caused by humans, has also affected the water cycle worldwide. Since the middle of the last century, we have seen big changes, and scientists predict these changes will keep growing. Reports from groups like the Intergovernmental Panel on Climate Change show 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 with it soil, phosphorus, and other materials. This runoff affects the salt levels in oceans and can cause problems in lakes by adding too much phosphorus from fertilizers. 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 can reach speeds high 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.