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Water Cycle

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The water cycle (also known as the hydrological cycle) is the constant movement of the water on, above, and below the surface of the earth. It is made up of a series of processes: the water evaporates from the oceans and eventually condensates and precipitates on the land before returning again to the oceans through various pathways such as river runoff and direct groundwater discharge.

The movement of water in the global hydrological cycle

The amount of water on earth is finite and that is why it is a closed system. This means water cannot leave or enter the earth and its atmosphere. The three components for the cycle are stores, flows, and processes.

Water Cycle Global Water Cycle StudySmarterGlobal water cycle diagram. Image: LangeLeslie & Anna Wright. Source: Wikimedia, CC BY-SA 4.0

As you can see from the diagram above, water's nature and form change all the time. Global water stores include lakes, oceans, aquifers (underground lakes), and the cryosphere (glaciers, ice sheets). Global water can take shape as water, vapour, ice, saline, or freshwater. There are two processes that drive the cycle.

  • Solar energy: Energy from the sun that heats water, which causes evaporation (water turning into vapour or gas) or transpiration (movement of water through plants).
  • Gravitational potential energy: The way in which water accelerates under gravity, which transports it to the river and then to the sea. The water also infiltrates through the soil and then travels through the cracks within the rocks as groundwater.

As there is more evaporation due to global warming, there is more moisture in the atmosphere. This can lead to increased condensation as the air cools which turns into greater precipitation. In some places, it leads to increased cloud cover and precipitation as climate changes.

Gravitational potential energy in the water cycle

Gravitational potential energy is what drives the water through the system in a sequence of inputs, outputs, stores, and flows.

  • The system is continuous with outputs governing inputs as nothing is lost or gained.
  • There are some shifts in the world's climatic zones affected by climate change. This means that some stores are depleting. For example, the ice in the Polar regions and in mountain glaciers is melting without being replenished.
  • In the areas that are being warmed, the ground surface dries out as evaporation increases. Global air circulation takes this extra vapour to cooler areas, where it condenses into clouds and precipitation.

Storage in the water cycle

Water cycle Water stores StudySmarterWhere the water is in the world water stores, usgs.gov

Although the amount of water that is in the stores fluctuates, each store has a relative size. As seen in the graph above 96.5% of water is in the oceans, 2.5% is freshwater, and 0.9% is in other saline water sources. Most of the freshwater is in the cryosphere (glaciers and ice sheets) and most of the surface water is in ground ice and permafrost.

Flows

The transfer of water globally by flows from one store to another is known as fluxes. These fluxes vary with the temperature and the season. The variation is known as annual fluxes.

The global water budget

The ocean loses more water to the atmosphere from evaporation than it receives from precipitation, whereas it is the opposite for landmasses. Surface runoff makes up the difference known as the balance. If the balance was disturbed, the oceans would receive more water and the continents would dry. This balance is called the global water budget and stops this from happening. Water does not stay in the atmosphere for long. Its residence time is short there, whereas it resides in the ocean for longer periods.

The importance of the tropics

The importance of the Tropics comes from the fact that most of the world's rainfall is created there, in the Intertropical Convergence Zone (ITCZ). This means that it is the biggest flux transferring water from oceans to land. The steep angles of the sun over the tropical oceans cause high evaporation. Trade winds carry the water vapour towards the ITCZ where strong convectional currents lift the air that leads to cooling and condensing into clouds. These atmospheric flows of moisture are called tropospheric rivers.

Depending on the place of the ITCZs, there can be drastic changes in the rainfall in equatorial nations. This can lead to droughts or floods, such as the catastrophic drought in Brazil during 2014-2015, which will be touched upon in the article about droughts.

The importance of the polar regions

The importance of the polar regions comes from the fact that two thirds of the earth's freshwater are locked up there, in the cryosphere, in the form of ice sheets and glaciers. These regions contribute to the circulation of the water and transfer of heat around the world, driving the global hydrological cycle. This triggers ocean circulation known as thermohaline circulation, also known as the global conveyor belt.

Water Cycle Thermohaline Circulation StudySmarterThermohaline circulation. Source: Robert Simmon, NASA (Public Domain)

Fossil water

Beneath Greenland's ice sheet and under the Kenyan desert are water stores known as aquifers. These are untapped ancient stores of freshwater that exist in the polar regions and beneath many deserts.

What is water insecurity and what is its cause?

Water insecurity is the lack of a reliable source of water that has the appropriate quality and quantity to meet the local human population and environment. The cause is often linked to water scarcity, which is the imbalance between demand and supply. This could be due to physical scarcity or economic scarcity. One of the ways to tackle water insecurity is with water supply management using hard engineering. This will be explored further in the article about water supply management.

Climate change and the water cycle

Climate change is affecting rainfall and the risk for flooding in the UK is rising, as can be seen in the article about floods. Globally there are areas that will have surpluses within the water cycle and others will have a deficit. Case studies such as the Sahel receiving wetter years could develop to re-greening the desert, whereas the drought that is happening in California could lead to land that cannot provide agriculture. There has been research to try to predict climate change and map the uncertainties, comparing past years of weather patterns and pressure systems. This will be explored further in the climate change article.

Key takeaways

  • The global hydrological cycle is a closed system: water cannot leave or enter the earth and its atmosphere.
  • The three components for the cycle are stores, flows, and processes.
  • Gravitational potential energy is what drives the water through the system in a sequence of inputs, outputs, stores, and flows. If the balance was disturbed, the oceans would receive more water and the continents would dry. This balance is called the global water budget and stops this from happening.
  • The tropics and the polar regions play an important part in the global hydrological cycle. The former is where the atmospheric flows of moisture called tropospheric rivers are, and the latter is where the ocean circulation known as thermohaline circulation starts.

Frequently Asked Questions about Water Cycle

The water cycle is driven by solar energy and gravitational potential energy. Energy from the sun heats the water, causing evaporation or transpiration. Gravitational potential energy accelerates the water and transports it to rivers and then to the sea. 

Condensation in the water cycle is the process of air cooling and turning into precipitation.

Infiltration in the water cycle is the process of water travelling through the soil and then the cracks within the rocks. 

Evaporation in the water cycle is the process of water being heated by the sun and turning into vapour or gas. 

The water cycle (also known as the hydrological cycle) is the constant movement of the water on, above, and below the surface of the earth. It is made up of a series of processes where the water evaporates from the oceans and eventually condensates and precipitates on the land before returning again back to the oceans through various pathways such as river runoff and direct groundwater discharge.

Final Water Cycle Quiz

Question

What is a closed system?

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Answer

A closed system is when water cannot leave or enter the earth and its atmosphere.

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Question

What are the three components of the global hydrological cycle?


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The three components of the cycle are stores, flows, and processes.

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Question

Describe how solar energy drives the water cycle.


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The energy from the sun heats the water, which causes evaporation or transpiration.

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Question

What is gravitational potential energy?


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Answer

Gravitational potential energy is what drives the water through the system in a sequence of inputs, outputs, stores, and flows.

Show question

Question

Give an example of gravitational potential energy working within the system of continuous outputs governing inputs as nothing is lost or gained.


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In the areas that are being warmed, ground surface dries out as evaporation increases. Global air circulation takes this extra vapour to cooler areas, where it condenses into clouds and precipitation.

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Question

What is the biggest water store?


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The ocean.

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Where is most of the freshwater on the earth?


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Most of the freshwater is in the cryosphere (glaciers and ice sheets).

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What form does most of the surface water take?


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Most of the surface water is ground ice and permafrost.

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What are fluxes?


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The transfer of water globally by flows from one store to another is known as fluxes.

Show question

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What are annual fluxes?


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Fluxes vary depending on the temperature and the season. This variation is known as annual fluxes.

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Question

What happens when the ocean loses more water to the atmosphere from evaporation than from precipitation?


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Surface runoff makes up the difference. This is known as the balance.

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What happens when the balance is disturbed?


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If the balance was disturbed, the oceans would receive more water and the continents would dry. The global water budget stops this from happening.

Show question

Question

What is the importance of the tropics for the global water budget?


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Answer

Most of the world's rainfall is created there, in the Intertropical Convergence Zone (ITCZ). This means that it is the biggest flux transferring water from oceans to land.

Show question

Question

What are tropospheric rivers?


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Answer

Trade winds carry the water vapour towards the ITCZ where strong convectional currents lift the air that leads to cooling and condensing into clouds. These atmospheric flows of moisture are called tropospheric rivers.

Show question

Question

What is the importance of the polar regions for the global water budget?

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Answer

Two thirds of the earth's freshwater is locked up there, in the cryosphere in the form of ice sheets and glaciers. These regions contribute to the circulation of the water and transfer of heat around the world, driving the global hydrological cycle.

Show question

Question

What is the cause of water insecurity?


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Answer

The cause is often linked to water scarcity which is the imbalance between demand and supply. This can be physical scarcity or economic scarcity.

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Question

What is a drainage basin?

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Drainage basins are hydrological processes that operate within areas drained by rivers and their tributaries.

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What is an open system?

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Open systems mean that their inputs are not governed by outputs and more water can be lost than received.

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How is water lost in an open system?

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Water is lost by:

-evaporation and evapotranspiration to the atmosphere

-surface runoff to the sea

-percolation into groundwater stores

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What is a catchment area?


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Catchment area is another name for drainage system, as they catch the precipitation falling within the watershed.

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What are the three pathways that water follows when precipitation occurs in the drainage basin?

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When precipitation occurs, the water follows three pathways.

-reaching the land surface and then infiltrating the topsoil.

-running off the surface as overland flow, also known as surface runoff.

-be evaporated back into the atmosphere.

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What can delay the water on a pathway?

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Any pathway taken can be delayed by:

-Water intercepted by plants and buildings, before evaporating or infiltrating the surface.

-Surface water infiltrating through the surface and eventually percolating through the rocks underneath to become groundwater, to then be stored in aquifers.

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What does interception mean as a hydrological process?

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Interception means temporary storage of water before reaching soil. It is water captured by plants, buildings and hard surfaces.

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What does infiltration mean as a hydrological process?

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Infiltration means water that enters topsoil. Common when there is slow or steady rainfall.

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What does percolation mean?

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Percolation is the vertical flow of water between soil and rock layers.

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What is channel flow?

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Channel flow is the water flowing in the river channel, also known as discharge and runoff.

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What are the four outputs of the hydrological cycle within a drainage basin?

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-Evaporation = Water into vapour.

-Transpiration = Water that has been taken up by plants and transpired onto the leaf surface.

-Evapotranpiration  = Both the effect of evaporation and transpiration.

-River discharge = A volume of water passing a certain point in the channel over a certain amount of time.

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What kind of precipitation is the western side of the UK prone to?

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The western side of the UK is particularly prone to orographic and frontal rainfall.

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What kind of precipitation is the eastern side of the UK prone to?

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The eastern side of the UK is much drier yet the summer months can experience heavy bursts of convectional rainfall.

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What human impact affects the hydrological processes?


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Over Abstraction, deforestation, urbanisation and reservoirs.

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What is over abstraction?

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Over abstraction is when abstracting too much water from groundwater reserves leads to rivers drying up in times of low rainfall.

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What is drought?

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Drought is when there is a water deficit in a particular place over a period of time, compared to the average rainfall for that same period.

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What are the three types of drought?

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Meteorological drought, agricultural drought, and hydrological drought.

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What is meteorological drought?

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Meteorological drought occurs when the degree of dryness is compared to average precipitation. It occurs when there are dry weather patterns dominating an area.

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What is agricultural drought?

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Agricultural drought is when there is insufficient water for crops which can lead to crop failure.

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What is hydrological drought?

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Hydrological drought is where the drainage basins suffer shortfalls due to a rainfall deficit.

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What were some of the consequences of the drought in Brazil in 2014-2015?

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Answer

In 2014-2015, there was the worst case of drought in Brazil for 80 years. This suspended power supplies because HEP schemes were so low, agriculture fell into crisis and urban water supplies stopped.

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Where does the moist air come from and travel in Brazil?

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Usually rainfall in Brazil comes from the moist air that moves in a westerly direction from the South Atlantic across the Amazon Basin. This air encounters the high Andes mountain range to the west of the continent which then is forced to turn southwards and maintains the moisture around the Basin.

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What caused the drought in Brazil?

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A series of high pressure systems diverted the rain-bearing winds further north away from the Amazon, and also prevented them from moving southwards towards the Andes. The heavy rains occurred in Bolivia and Paraguay and dry air remained in Brazil instead.

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Give an example of the impact of the drought in Brazil?

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The impact of the drought was devastating. More than four million people were affected by rationing and rolling power cuts. Arabica coffee, a commodity that Brazil supplies in bulk more than any other country, fell by 15% which raised the global price by half. Seventeen of the largest reservoirs are at their lowest, and increased groundwater abstraction led to aquifers becoming dangerously low.

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How is deforestation affecting the hydrological and climatic cycles?

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Answer

Climate scientists who studied this case reported that the deforestation in the Amazon may have passed the tipping point, changing the hydrological and climatic cycles permanently. Usually a rainforest is able to recycle half of their rainfall, however the positive feedback loop caused by deforestation and less rainfall is reducing the ability of the rainforest to regenerate. The fragile rainforest ecosystems are less resilient and have reduced soil water storage and evapotranspiration. This leads to lower precipitation, and has a domino effect of feedback loops which reinforce the drying of the Tropics.

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What does the combination of global climate change and deforestation mean?

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Answer

This could mean:

  • The decline in the Amazon's capacity to absorb carbon

  • Changes in regional water cycles and increases in soil temperatures.

  • Amazon rainforest replaced with savannah-like grasslands. 

  • Increase in wildfires which would lead to increase in carbon in the atmosphere.

  • Reduced rainfall affecting HEP which generates 70% of Brazil's electricity.

  • The loss of a major carbon sink and source of moisture.

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What role does a rainforest play in the hydrological cycle, in particular the Amazon rainforest?

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The Amazon rainforest is seen as a giant water pump as it lifts up vast amounts of moisture into the air which then circulates through to the South and West falling as rain in central and southern Brazil. 20 billion tons of water daily are released from the rainforest and are transported by these ‘flying rivers’.

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How does drought affect rainforest ecosystems?

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Drought can have an impact on the forest with a series of chain reactions. Due to drought the younger trees die and reduce the canopy cover which then leads to humidity, water vapour and eventually rainfall. Without the canopy cover, surface tree litter and dying vegetation creates a potential tinderbox that can easily catch fire with lightning storms and high winds. Long term drought means thinner canopies and shorter trees.

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What is a wetland ecosystem?

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Answer

 A wetland is a distinct ecosystem that is flooded by water, either permanently or seasonally.

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Question

What effects did the 2014-2015 drought have on the Pantanal?

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Answer

The 2014-2015 drought had a major effect on the Pantanal and tested its resilience.

  • Tree mortality increased which reduced habitats for wild animals.

  • During the drought, wildfire became a major threat as cattle farmers would set old grass on fire during the dry season to clear vegetation left ungrazed by their cattle. 
    This would spread out of control into the wetlands and forests. 

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Question

What is the global water budget?

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Answer

The balance of inputs, outputs and stores of water in the global system.

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What are the main inputs to the water budget?

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Answer

Precipitation and water introduced by humans

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What are the main outputs in the water budget?

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Evapotranspiration, river discharge and water abstraction

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Question

What is it called when more water enters than leaves a system?

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Answer

Water surplus

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