The water cycle

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Lesson notes

Overview of the Water Cycle

The water cycle is a nonstop recycling process with no starting or ending point.
Water constantly moves between living organisms (e.g., plants) and non-living things (e.g., clouds, rivers, oceans).
It involves oceans, lakes, land surfaces, and the atmosphere.
Water molecules can cycle through almost anywhere on Earth due to water's properties.

A hand-drawn water cycle diagram showing evaporation from the ocean and from mountain streams, precipitation falling as rain over the hills, and freezing, melting and runoff on the mountainside.

Key Processes in the Water Cycle

Evaporation: Liquid water changes to gas (water vapor) when it gains energy from the Sun.
Condensation: Water vapor loses energy, becomes denser, and changes to tiny liquid droplets, forming clouds.
Precipitation: Cloud droplets collide and grow; when heavy enough, they fall as rain, snow, sleet, or hail.
Runoff: Precipitation that flows over land into streams and rivers, eventually returning to oceans.
Infiltration: Water seeping into the ground.
Transpiration (or evapotranspiration): Water evaporates from plant leaves, returning water to the atmosphere.

The USGS water cycle diagram, showing evaporation and evapotranspiration rising from the ocean and land, condensation and precipitation over the mountains, and runoff, infiltration and groundwater flow returning water underground and across the surface to the ocean.

Forces Driving the Water Cycle

Solar Energy: The Sun provides the energy that drives evaporation and the water cycle.
Density: Heated air expands, becomes less dense, and rises; cooler, denser air sinks.
Gravity: Pulls denser air downward, causing rising air; also causes water to flow downhill as runoff.
Energy transfer: Condensation releases energy to surrounding air, heating it and causing updrafts that keep cloud droplets aloft.

Earth's Water Reservoirs

Oceans: Store about 97% of Earth's water.
Atmosphere: Holds water vapor, clouds, and precipitation.
Streams and Lakes: Freshwater bodies fed by runoff and melting snow/ice.
Soil: Holds soil moisture important for plant growth.
Groundwater: Water stored in aquifers (porous rock layers) for centuries.
Biosphere: Water stored in plants and animals; plants release water vapor via transpiration.

The USGS water cycle diagram, showing the ocean, groundwater storage, freshwater and plant-uptake reservoirs where water is held as it moves through the cycle.

Water Cycle and States of Matter

Water exists in solid (ice, snow), liquid (rain, lakes), and gas (water vapor) states in the cycle.
The hydrosphere includes all water in all states of matter.
Condensation can form liquid droplets; if cold enough, water vapor can freeze into ice crystals.

Water Cycle and Living Things

Plants absorb water through roots and release it via transpiration.
Animals depend on water for survival and are part of the biosphere reservoir.
Aquatic habitats include freshwater (streams, lakes) and saltwater (oceans, seas).
A small percentage of Earth's water is freshwater; most is saltwater.

Slides

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Practice questions

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1.What is the primary source of energy that drives the water cycle?
Easy
- AThe Sun
- BThe Moon
- CEarth's internal heat
- DWind
2.Which process involves the change of water from liquid to gas?
Easy
- ACondensation
- BEvaporation
- CPrecipitation
- DSublimation
3.What is the term for the movement of water through the atmosphere?
Easy
- AAdvection
- BConvection
- CInfiltration
- DRunoff
4.What percentage of global evaporation occurs over the oceans?
Medium
- A78%
- B86%
- C97%
- D50%
5.Which of the following is NOT a process in the water cycle?
Medium
- ASublimation
- BTranspiration
- CPhotosynthesis
- DInfiltration
6.What happens to water vapor as altitude increases in the atmosphere?
Medium
- AIt expands and heats up
- BIt condenses due to lower temperature
- CIt evaporates more quickly
- DIt remains unchanged
7.Which human activity is most likely to decrease infiltration of water into the ground?
Hard
- APlanting trees
- BBuilding parking lots
- CCreating reservoirs
- DIrrigating crops
8.What is the term for water that flows over the ground surface and enters rivers?
Easy
- AGroundwater
- BInfiltration
- CSurface runoff
- DPrecipitation

Revision notes

## The Water Cycle (Hydrologic Cycle) The water cycle is the continuous movement of water on, above, and below Earth's surface, driven by **energy from the sun** and **gravity**. Water changes between liquid, solid (ice), and vapor phases as it moves through reservoirs (oceans, atmosphere, ice, groundwater). ### Key Processes - **Evaporation**: Liquid water → water vapor (gas). Absorbs heat (latent heat of vaporization). 86% of global evaporation comes from oceans. - **Transpiration**: Water vapor released from plants. - **Evapotranspiration**: Combined evaporation from soil and transpiration from plants. - **Condensation**: Water vapor → liquid water droplets (clouds, fog). Releases heat. - **Precipitation**: Water falls as rain, snow, hail, or sleet. - **Sublimation**: Ice/snow → water vapor directly. - **Infiltration**: Water soaks into the ground. - **Surface runoff**: Water flows over land into rivers, lakes, oceans. - **Groundwater flow**: Water moves underground; some emerges as springs or discharges into surface water. ### Reservoirs & Distribution - Oceans hold **97%** of Earth's water. - 78% of precipitation falls over oceans. - Freshwater is stored in ice caps, glaciers, lakes, rivers, and aquifers. ### Energy & Purification - Phase changes transfer heat: evaporation absorbs heat; condensation releases heat. - The cycle purifies water: evaporation leaves salts behind; condensation produces freshwater. - Water cycle transports heat from tropics to poles via ocean currents. ### Human Impacts - Deforestation, urbanization, groundwater extraction alter natural water flow. - Climate change intensifies the cycle, causing shifts in precipitation patterns, more extreme weather, and changes in rainfall timing. ### Key Formula/Concept - **Latent heat of vaporization**: Heat absorbed when water evaporates. - **Density**: Water vapor is less dense than air (lower molecular mass), so it rises. > Remember: The sun provides energy; gravity pulls water downhill and causes precipitation.

Evaporation: The process by which liquid water changes into water vapor (gas) due to heat from the sun; absorbs latent heat of vaporization.
Condensation: The transformation of water vapor into liquid water droplets, forming clouds or fog; releases heat.
Precipitation: Water falling from the atmosphere as rain, snow, hail, or sleet.
Transpiration: The release of water vapor from plants into the atmosphere.
Evapotranspiration: The combined process of evaporation from soil and transpiration from plants.
Sublimation: The direct change of ice or snow into water vapor without melting.
Infiltration: The process by which water on the ground surface enters the soil.
Surface runoff: Water flow over land surfaces, often into rivers and streams, eventually reaching oceans.
Groundwater: Water stored underground in aquifers; can discharge into surface water or be taken up by plants.
Latent heat of vaporization: The heat energy absorbed when water evaporates from liquid to vapor.
Advection: The movement of water vapor through the atmosphere, enabling precipitation over land.

Flashcards

What drives the water cycle?

Energy from the sun and gravity drive the water cycle.

What are the main processes in the water cycle?

Evaporation, condensation, precipitation, and collection (including runoff and infiltration).

What is evaporation?

Evaporation is the process where liquid water changes into water vapor (gas) due to heat from the sun.

What is condensation?

Condensation is the process where water vapor cools and changes back into liquid water droplets, forming clouds or fog.

What is precipitation?

Precipitation is water falling from clouds as rain, snow, sleet, or hail.

What is transpiration?

Transpiration is the release of water vapor from plants into the atmosphere.

What is evapotranspiration?

Evapotranspiration is the combined process of evaporation from surfaces and transpiration from plants.

What is sublimation in the water cycle?

Sublimation is the direct change of ice or snow into water vapor without melting into liquid first.

What is infiltration?

Infiltration is the process where water on the ground surface soaks into the soil.

What is surface runoff?

Surface runoff is water that flows over land surfaces into streams, rivers, and oceans without infiltrating.

What is groundwater?

Groundwater is water stored underground in porous rock layers called aquifers.

What role does the ocean play in the water cycle?

The ocean holds 97% of Earth's water, provides 86% of global evaporation, and receives 78% of global precipitation.

How does the water cycle purify water?

Evaporation separates water molecules from salts and impurities, and condensation forms fresh water droplets.

What is advection?

Advection is the movement of water vapor through the atmosphere, allowing evaporated water to travel to other regions.

How does the water cycle transfer heat?

Evaporation absorbs heat (latent heat of vaporization), and condensation releases heat, helping transfer energy from tropics to poles.

What are aquifers?

Aquifers are underground layers of rock or sediment that store and transmit groundwater.

What is the hyporheic zone?

The hyporheic zone is the area where surface water and groundwater mix in riverbeds and floodplains.

How do human activities affect the water cycle?

Deforestation, urbanization, groundwater extraction, and climate change alter natural water cycle processes and patterns.

What is the latent heat of vaporization?

The latent heat of vaporization is the heat absorbed when water changes from liquid to vapor, cooling the environment.

Why do clouds form?

Water vapor rises, cools, and condenses into tiny droplets around particles, forming clouds.

What is fog?

Fog is condensation of water vapor close to the ground, forming a cloud at ground level.

How does the water cycle affect Earth's geology?

Flowing water causes weathering, erosion, and deposition, reshaping landforms and transporting minerals.

What percentage of Earth's water is in the ocean?

About 97% of Earth's water is in the ocean.

What is an atmospheric river?

An atmospheric river is a narrow corridor of concentrated water vapor in the atmosphere that transports large amounts of moisture.

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