Water + Carbon Cycles


Earth as a Natural System

Earth seen as a closed system - energy input from sun + output to space but matter not inputed or outputed 


  • cryosphere - includes all the parts of the earths system where it's cold enough for water to freeze e.g. glacial landscapes
  • lithosphere - outermost part of the earth that includes crust + upper part of the mantle
  • biosphere - part of the earths system where all living things found
  • hydrosphere - includes all water on earth in either liquid (lakes + rivers), gas (water vapour) or solid form (ice in cryosphere)
  • atmosphere - layer of gas between earths surface + space

These subsystems all interlinked by cycles + processes that keep earth running as a system 

Matter (e.g. water + carbon) + energy move between subsystems with this meaning earths system is a cascading system

Changes occuring in one subsystem affects what happens in others

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Water Stores

Stored in solid, liquid + gas forms

Hydrosphere contains 1.4 sextillion litres of water:

  • 69% is frozen in cryosphere
  • 30% is groundwater (water stored underground in lithosphere)
  • 0.3% liquid freshwater on earths surface in lakes + rivers
  • 0.04% stored as water vapour in atmosphere

Water must be physically + economically accessible for humans to be able to use it so very little water can be used by humans

Water can change between solid, liquid + gaseous forms:

  • for water to boil or melt, it has to gain energy e.g. from the sun
  • for water to condense or freeze, it has to lose energy

Water continuously cycled between different stores known as global hydrological cycle (closed system)

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Water Cycle

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Occurs when liquid water changes state into a gas, becoming water vapour + gaining energy usually from solar radiation

Increases amount of water stored in atmposphere

Magnitude of the evaporation flow varies by location and season

If lots of solar radiation, a large supply of water + warm, dry air, the amount of evaporation will be high

If not so much solar radiation, little avaliable liquid water + cool air that is already nearly saturated, the evaporation will be low

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Condensation occurs when water vapour changes state to become a liquid as it loses energy to surroundings and cools to its dew point (temperature gas changes to liquid)

Water droplets can stay in atmosphere or flow to other subsystems e.g. when water vapour condenses it can form dew on leaves + other surfaces - decreasing amount of water stored in atmosphere

Magnitude of condensation flow depends on amount of water vapour in atmosphere + temperature e.g. if there's lots of water vapour in air + rapid temperature drop, condensation will be high

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Cloud Formation + Precipitation

Cloud formation + precipitation ae essential parts of the water cycle - precipitation is main flow of water from atmosphere to ground

Clouds form when warm air cools down, causing the water vapour in it to condense into water droplets which gather as clouds, when droplets get big enough they fall as precipitation

Factors causing warm air to cool, leading to precipitation:

  • other air masses - warm air less dense than cool air + as a result, when warm air meets cool air, the warm air is forced up above the cool air which cools down as it rises + results in frontal precipitation
  • topography - when warm air meets mountains, it's forced to rise, causing it to cool + results in orographic precipitation
  • convection - when sun heats up ground, moisture on ground evaporates + rises up in column of warm air, as it gets higher it cools, resulting in convective precipitation

Water droplets caused by condensation are too small to form clouds on their own as for clouds to form there has to be tiny particles of other substances e.g. dust or soot to act as condensation nuclei to give water a surface to condense on

Cloud formation + precipitation can vary seasonally + by location 

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Cryospheric Processes

Cryospheric processes such as accumulation + ablation change the amount of water stored as ice in cryosphere

Balance of accumulation + ablation varies with temperature

During periods of global cold, inputs into cryosphere are greater than outputs e.g. water is transferred to it as snow + less water is transferred away due to melting

During periods of warmer global temperatures, the magnitude of the cryosphere store reduces as losses due to melting are larger than the inputs of snow

Earth still emerging from a glacial period that reached its maximum 21,000 years ago so are extensive stores of ice on land in antarctica + greenland, as well as alpine glaciers and sea ice

Variations if cryosphereric processes happen over different timescales e.g annual temperature fluctuations mean that more snow falls in the winter than in summer

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Drainage Basins as Natural Systems

Drainage basins are open, local hydrological cycles

A river's drainage basin is the area surrounding the river where rain falling on the land flows into that river (also called river's catchment)

Boundary of a drainage basin is the watershed + any precipitation falling beyond watershed enters a different drainage basin

Drainage basins are open systems with inputs + outputs

Water comes into the system as precipitation + leaves via evaporation, transpiration + river discharge

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Typical Drainage Basin System

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Drainage Basin Inputs + Outputs

Inputs - precipitation includes all ways moisture comes out of the atmosphere e.g. rain, snow, hail, dew + frost


  • evaporation - water turning into water vapour
  • transpiration - evaporation from within leaves as plants + trees take up water through their roots + transport it to their leaves where it evaporates into the atmosphere
  • evapotranspiration - the process of evaporation + transpiration together so total amount of output from drainage basin
  • potential evapotranspiration - the amount of water that could be lost by evapotranspiration
  • actual evapotranspiration - what actually happens e.g. in a desert potential evapotranspiration is high (because heat increases evaporation) but actual transpiration is low (because there isn't much moisture)
  • river discharge or river flow is another output
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Drainage Basin Stores

Interception - when some precipitation lands on vegetation or other structures like buildings + concrete + tarmac surfaces before it reaches the soil which creates a significant store of water in wooded areas but only temporary because collected water may evaporate quickly or fall from leaves as throughfall

Vegetation storage - water that's been taken up by plants

Surface storage - water in puddles (depression storage), ponds + lakes

Soil storage - moisture in soil

Groundwater storage:

  • water stored in ground either in soil or rocks
  • water table is top surface of the zone of saturation - the zone of soil or rock where all the pores in soil or rock are full of water
  • porous rocks (with lots of holes in them) that hold water are called aquifers

Channel storage - water held in river or stream channel

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Drainage Basin Flows

Infiltration - water soaking into the soil + infiltration rates are influenced by soil type, soil structure + how much water's already in soil

Overland flow (runoff) - water flowing over the land as a whole surface or little channels + occurs because rain is falling faster than infiltration

Throughfall - water dripping from one leaf to another

Stemflow - water running down plant stem or tree trunk

Throughflow - water moving slowly downhill through soil + faster through 'pipes' such as cracks in soil or animal burrows

Percolation - water seeping through soil into water table

Groundwater flow - water flowing slowly below the water table through permeable rock + highly permeable rocks e.g. limestone have faster flow

Baseflow - groundwater flow that feeds into rivers through river banks + river beds

Interflow - water flowing downhill through permeable rock above the water table

Channel flow (river's discharge) - water flowing in the river or stream itself

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Water Balance

Balance between inputs (precipitation) + outputs (channel discharge + evapotranspiration) 

Effects how much water is stored in the basin

Seasonal patterns can be seen:

  • in wet seasons - precipitation exceeds evapotranspiration, creating a water surplus + ground stores fill with water so theres more surface runoff + higher discharge so river levels rise
  • in drier seasons - precipitation is lower than evapotranspiration so ground stores are depleted as some water is used by plants + humans + some flows into river channels but isn't replaced by precipitation
  • at end of a dry season - there's a deficit of groundwater + ground stores are recharged in next wet season
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Show river discharge over time

River discharge - volume of water in cubic metres that flows in river per second

Measured in cumecs (cubic metres per second)

High runoff increases discharge as more water reaches river

Flood hydrographs - river discharge around storm event

Peak discharge - highest point on graph where discharge greatest

Lag time - delay between peak rainfall + peak discharge + delay form water flowing (quicker flow = short lag time + high peak discharge)

Rising limb - part of graph up to peak discharge as rainwater flows into river

Falling limb - part of graph after peak discharge as less water flowing into river (shallow falling limb = water flowing long time after stopped raining)

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Factors Affecting Runoff + Hydrographs

Size of drainage basin - larger drainage basins catch more precipitation so have higher peak discharge + smaller drainage basins have smaller lag times because precipitation has less distance to travel

Shape of drainage basin - circular basins have flashy hydrographs as all points on watershed are same distance from discharge measurement so water will reach these points at the same time

Ground steepness - water flows quicker downhill in steep-sided drainage basins, shortening lag times + increasing runoff as water has less time to infiltrate soil

Rock + soil type - impermeable rocks + soil don't let water infiltrate which increases surface runoff + therefore increases peak discharge

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Physical Factors Affecting Water Cycle

Storms + precipitation:

  • intense storms generate more precipitation + greater peak discharge
  • larger input of precipitation causes flows e.g. runoff + stores e.g. groundwater to increase
  • some flows e.g. infiltration won't occur at quick enough pace + increase runoff

Seasonal changes + vegetation:

  • size of inputs, flows + stores vary by season e.g. lower in UK summer
  • freezing in water can temporarily reduce flows + increase stores but heavily increase flows after melting
  • plants that intercept have seasonal variation e.g. less leaves in winter
  • more vegetation = more water lost from transpiration + evaporation so runoff + peak discharge lower
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Human Factors Affecting Water Cycle

Farming practices affecting infiltration:

  • ploughing - breaks up surface so more water can infiltrate + less runoff
  • crops - increase infiltration, interception + evapotranspiration which increases rainfall
  • livestock - cattle, trample + compact soil, decreasing infiltration + increase runoff
  • irrigation (artifical watering of land) - increases runoff as can't infiltrate but river levels fall if water extracted for irrigation

Land use changes:

  • deforestation reduces interception + reduces infiltration when dead plant material removed
  • construction of buildings + roads creates impermeable layer over land, preventing infiltration + risking flooding from increased runoff

Water abstraction - more water abstracted to meet demand from more densly populated area so stores depleted e.g. lakes

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Where Carbon Found

Lithosphere - 99.9% of carbon on earth stored in sedimentary rocks such as limestone + abut 0.004% stored in fossil fuels e.g. coal + oil

Hydrosphere - CO2 dissolved in water + oceasn second largest carbon store at about 0.04% + small amount on surface exchanged with atmosphere

Atmosphere - only about 0.001% stored in atmosphere + either carbon dioxide (CO2) or methane (CH4)

Cryosphere - less than 0.01% of earths carbon + mainly decomposing plants + animals in permafrost

Biosphere - carbon stored in tissues of living organisms + transferred to soil when organisms die + decay (0.004% of total carbon)

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Carbon Cycle

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Carbon Store Changes

Photosynthesis - transfers carbon stored in atmosphere to biomass (which is passed through food chain + released through respiration + decomposition) when plants + phytoplankton use energy from sun to change CO2 + water into glucose + oxygen for growth

Combustion - transfers carbon in living, dead or decomposed biomass into atmosphere by burning + wildfires accelerate this

Respiration - transfers carbon from living organisms to atmopshere when plants + animals break down glucose for energy, releasing CO2 + methane

Decomposition - transfers carbon from dead biomass into atmosphere from soil when bacteria break down organisms + release CO2 + methane

Ocean uptake + loss - CO2 directly transferred from atmosphere to ocean + sea organisms e.g. plankton take up CO2 but lost from ocean to atmosphere when rises up from deep

Weathering - chemical weathering tranfers carbon from atmosphere to biosphere + hydrosphere when atmospheric carbon reacts with water vapour to form acid rain, dissolving rocks which has molecules wahsing into sea to form calcium carbonate which is used by sea creatures to make shells

Sequestration - carbon from atmosphere sequested (captured + held) in sedimentary rocks over millions of years when dead animal + plant material compacted on ocean floor until burnt in combustion

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Carbon Flows

Fast carbon flows - transfer carbon between sources e.g. photosynthesis, respiration, combustion + decomposition

Sequestration - slow carbon flow e.g. millions of years for carbon to be sequestered in sedimentary rocks

Depends on spatial scale:

  • plant scale - respiration + photosynthesis are main flows
  • ecosystem scale - carbon flows such as combustion + decomposition also occur
  • continental scale - all carbon flows occur
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Natural Processes Affecting Carbon Cycle


  • rapidly transfer large quantities of carbon from biomass to atmosphere
  • loss of vegetation decreases photosynthesis so less carbon removed from atmosphere
  • in long term, fires can encourgae growth of new plants which takes in carbon from atmosphere for regrowth
  • fires can therefore have a neutral effect on amount of atmospheric carbon

Volcanic activity - carbon stored within magma released during volcanic eruptions which enters atmosphere as CO2 + one large eruption could heavily disrupt carbon cycle

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Human Affects on Carbon Cycle

Hydrocarbon use - extracting + burning of fossil fuels releases CO2 into atmosphere as would otherwise stay sequestered

Deforestation - forest clearance reduces size of carbon store + burning of forest causes rapid flow of carbon form biosphere to atmosphere

Farming practices:

  • animals release CO2 + methane when respire + digest food
  • ploughing can release CO2 stored in soil
  • growing rice releases methane
  • mechanisation of farming has increased CO2 emissions
  • population rise casues increased food production so increased carbon emissions

Land use changes:

  • vegetation removed to make way for buildings, reducing carbon storage in biosphere
  • concrete production releases lots of CO2
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Carbon Budget

Carbon budget - the difference between inputs of carbon into a subsystem + outputs of carbon from it

Balance of inputs + outputs of subsystem determine whether it acts as carbon store or sink

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Carbon Cycle Impact

Atmosphere + Climate

  • carbon cycle affects amount of gases containing carbon (CO2 + methane) in atmosphere
  • these are greenhouse gases that trap some of the suns energy (increasing heat of planet)
  • concentration of greenhouse gases increasing rapidly + increasing global warming


  • carbon cycle allows plants to grow as carbon allows photosynthesis + decomposition allows nutrients to be recycled
  • carbon cycle changes reduces amount of carbon stored in land e.g. global warming removes permafrost which releases carbon into atmosphere
  • increase in global temperature increases frequency of wildfires


  • marine organisms use CO2 for photosynthesis
  • CO2 increases acidity of oceans + this negatively affects marine life
  • global warming negatively affects marine life + if they die less CO2 will be taken from atmosphere for photosynthesis
  • warmer water can't absorb CO2 as easily
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Importance of Carbon Water Stores

Carbon fundamental building block of life as all organisms contain carbon + need water

Plants form base of most food chains + use photosynthesis to convert water + CO2 into biomass which is passed up food chain

Water is present in atmosphere as water vapour + carbon exist as carbon dioxide + methane which are greenhouse gases

Greenhouse gases causes greenhouse effect that prevents energy from escaping into space + reflecting back to earth

Human acitvities increasing the concentration of grenhouse gases in atmosphere enhance greenhouse effect + further increase climate change

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Feedback in Water Cycle

Positive feedback:

  • temperature rise
  • evaporation increases
  • amount of water vapour in atmosphere increases
  • greenhouse effect increases

Negative feedback:

  • temperatures rise
  • evaporation increases
  • amount of water vapour in atmosphere increases casuing more clouds to form
  • increase cloud cover reflects more of suns energy back to space
  • temperatures fall
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Feedback in Carbon Cycle

Positive feedback:

  • temperatures rise
  • plant respiration rate increases
  • amount of CO2 in atmosphere increases
  • greenhouse effect increases

Negative feedback:

  • CO2 in atmosphere increases
  • extra CO2 causes pants to increase growth
  • plants remove + store CO2 from atmosphere
  • amount of CO2 in atmosphere reduces
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Climate Change Affect on Life

Pattern of precipitation expected to change as wet areas will get wetter and dry areas will get drier which will cause water shortages

Extreme weather events will get more frequent + have worse effect on developing countries

Agricultural productivity will decrease + cause food shortages

Sea levels will rise further + flood coastal + low-lying areas

Species range will develop + some may become extinct

Plankton numbers will decline + marine food chains will be affected

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Human Influence on Carbon Cycle

Extracting + burning fossil fuels has caused 40% more CO2 in atmosphere than in 1750

IPCC - Intergovernmental Panel on Climate Change ran by UN to share knowledge + prevent temperature rise

Individuals - people can choose to use cars less + buy more fuel efficient cars as well as make homes + energy efficient e.g. insulation

Regional + national:

  • governments reduce reliance on fossil fuels with renewable energy
  • afforestation + restoring degraded forests can increase carbon uptake
  • developments increase sustainability with more green spaces + improving public transport
  • carbon capture + storage when CO2 emitted from burning fossil fuels is captured + stored underground

Global - Kyoto protocol (1997), Paris Agreement (2015) + carbon trading schemes control greenhouse gases used with set limits on emissions

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