CogKnoSys

Knowledge you can walk. Read the idea below, then step up to the general, across to peers or down to the specific.

Physical

Water Cycle

The water cycle is the constant movement of water on, above and below Earth's surface as it changes between liquid, ice and vapor.

Learn, discuss, create, buy or invest

The idea

The water cycle, also called the hydrologic cycle, is the constant movement of water on, above and below Earth's surface. Water moves between reservoirs such as ice, fresh water, salt water and water vapor in the air. Water flows from one reservoir to another, for example from a river to the ocean or from the ocean to the air. Some water stays put for a long time in ice sheets or deep underground, while rain that runs off into a stream moves on quickly. How much water sits in each reservoir shifts with the climate.

Several processes move water from place to place: evaporation, transpiration from plants, condensation, precipitation, sublimation, infiltration into the ground, surface runoff and underground flow. Along the way, water changes between liquid, solid ice and vapor. The ocean is the source of 86% of the world's evaporation.

Heat drives the whole cycle. Water absorbs heat when it evaporates and gives heat off when it condenses or freezes. Because evaporation leaves salts and other particles behind, rain and snow bring fresh water back to the land. Flowing water also carries minerals and slowly reshapes the land through weathering, erosion and deposition. Most life and ecosystems on the planet depend on this cycle.

Check yourself

Question 1 of 2

What share of the world's evaporation comes from the ocean?

Pick one. The first pick counts.

Composition

what it's made of

  • Reservoirs — Places that hold water: oceans, ice, lakes, rivers, groundwater and water vapor in the air.
  • Evaporation and Transpiration — Liquid water turns into vapor from open water and from plant leaves.
  • Condensation — Water vapor cools into tiny droplets that gather as clouds.
  • Precipitation — Rain, snow, sleet and hail fall back to the surface.
  • Runoff and Infiltration — Water flows over land into streams or soaks into the ground to become groundwater.
  • Heat — Energy, mostly from the Sun, powers the changes between liquid, ice and vapor.

Connection

how it relates

Under Earth Systems

Cause and effect

  1. Thermal Energystrength 70 of 100

    Evaporation absorbs heat and condensation releases it, so the water cycle constantly moves thermal energy around.

  2. Biodiversitystrength 60 of 100

    The fresh water the cycle delivers keeps most life and ecosystems going.

Related

  1. Ocean Currentsstrength 60 of 100

    Ocean circulation helps water carry heat from the tropics toward the poles.

  2. Convection Currentsstrength 50 of 100

    Warm, moist air rises in convection currents, cools and lets water vapor condense into clouds.

Change

how and why it evolves

Past
In 1580 Bernard Palissy became the first published thinker to argue that rainfall alone was enough to keep rivers flowing. His idea was not tested scientifically until 1674, in a study commonly credited to Pierre Perrault. Over Earth's long history the total mass of water has stayed fairly constant, while ice ages and warm periods have shifted how much is locked up as ice.
Now
Human activities such as cutting down forests, building cities and pumping groundwater are changing the water cycle. Climate change is also intensifying it, shifting rain patterns and making extreme weather more frequent.
Next
These changes will likely keep affecting water supplies, farming and ecosystems, so communities may need new ways to store and share water.

Knowledge you can walk. Read the idea, then step up, across or down.

Water Cycle

The water cycle is the constant movement of water on, above and below Earth's surface as it changes between liquid, ice and vapor.

Learn, discuss, create, buy or invest

The idea

The water cycle, also called the hydrologic cycle, is the constant movement of water on, above and below Earth's surface. Water moves between reservoirs such as ice, fresh water, salt water and water vapor in the air. Water flows from one reservoir to another, for example from a river to the ocean or from the ocean to the air. Some water stays put for a long time in ice sheets or deep underground, while rain that runs off into a stream moves on quickly. How much water sits in each reservoir shifts with the climate.

Several processes move water from place to place: evaporation, transpiration from plants, condensation, precipitation, sublimation, infiltration into the ground, surface runoff and underground flow. Along the way, water changes between liquid, solid ice and vapor. The ocean is the source of 86% of the world's evaporation.

Heat drives the whole cycle. Water absorbs heat when it evaporates and gives heat off when it condenses or freezes. Because evaporation leaves salts and other particles behind, rain and snow bring fresh water back to the land. Flowing water also carries minerals and slowly reshapes the land through weathering, erosion and deposition. Most life and ecosystems on the planet depend on this cycle.

Check yourself

2 quick questions, instant feedback.