Notes · GeographyUK · A-Levels

Water and Carbon Cycles

The compulsory physical core treats the Earth's surface as a set of interconnected systems through which water and carbon are stored and transferred. It develops the systems vocabulary of stores, flows, feedback and dynamic equilibrium, applies it to the drainage-basin water cycle and the global carbon cycle, and examines how these cycles are linked to the climate and disturbed by human activity.

6 sections~23 min reading time3 competenciesLevel Foundation 1 · Standard 3 · Advanced 2

T·0111 / 12

Exam profile

AO1 · Understand systems concepts and the stores and fluxes of the water and carbon cycles at global and drainage-basin scalesAO2 · Apply stores, flows and feedback to explain change over time and space and evaluate human impact on the cycles and climateAO3 · Interpret and analyse hydrographs, water-balance and carbon-budget data and construct evidenced arguments

Operators:explainanalyseassessevaluateto what extentinterpretcalculatedescribe the distribution

basic level

At AS-Level the emphasis is on describing the systems, stores and flows of the water and carbon cycles and interpreting straightforward hydrographs and data.

higher level

The full A-Level requires the operation of feedback loops, the links between the two cycles and the climate, and evaluative case-study judgement about human impact.

Depth

Text

Contents · 6 sections

Water and Carbon Cycles

Systems in physical geography#

FoundationAQA 7037 3.1.1DfE GCE Geography - water and carbon cycles

The structure of an open system

Fig. 1An open system exchanges both matter and energy across its boundary; energy drives the internal flows between stores.

A system is a set of interrelated components working together, and thinking in systems is the organising idea of the whole physical-geography specification. Every system has inputs (matter and energy entering), outputs (matter and energy leaving), stores or components (where matter is held) and flows or transfers (the movement of matter and energy between the stores). Energy - ultimately from the Sun and from the Earth's interior - drives the flows. Setting a landscape out as a system forces you to be precise about where water or carbon is held and how it moves, rather than describing features in isolation.

Systems are classified by what crosses their boundary. An isolated system exchanges neither matter nor energy with its surroundings; these effectively exist only in the laboratory, and the whole universe is the only true example. A closed system exchanges energy but not matter across its boundary - the planet Earth as a whole is close to a closed system for water and carbon, because energy (sunlight, heat) crosses the boundary but the amount of water and carbon is essentially fixed. An open system exchanges both matter and energy with its surroundings, and almost every system a geographer studies - a drainage basin, a glacier, a stretch of coast - is open, gaining and losing both water and energy.

The state of a system is described by its balance of inputs and outputs. When inputs equal outputs the amount held in the stores stays constant and the system is in a steady state, or dynamic equilibrium: individual components keep changing (water keeps flowing through) but the overall system is balanced. If an input rises above output, the stores grow (a positive water balance); if output exceeds input, the stores shrink (a negative water balance). A change that pushes a system out of balance can trigger self-adjustment through feedback.

Feedback is the mechanism by which a change in one part of a system affects other parts and loops back to the original change. Negative feedback dampens the original change and restores equilibrium - it is self-regulating and stabilising, so that a system returns towards its balanced state. Positive feedback amplifies the original change, driving the system further from its starting point and often accelerating change. Recognising and explaining feedback loops - especially the positive loops in the climate system - is one of the most demanding and highly rewarded skills in this topic.

Negative feedback: self-regulation

Fig. 2In negative feedback a change triggers an opposite response that dampens it and restores equilibrium.

Worked example

Classifying a drainage basin as a system

A drainage basin receives rain, loses water by evaporation and by river discharge, and holds water in the soil and groundwater. Classify it as an isolated, closed or open system and justify your answer using systems vocabulary.

Identify what crosses the boundary
Water enters as precipitation (an input of matter) and leaves as evaporation and channel discharge (outputs of matter); solar energy also enters and leaves. Both matter and energy cross the boundary.
Match to a category
A system that exchanges both matter and energy with its surroundings is an open system - this rules out closed (energy only) and isolated (neither).
Note the equilibrium
Over a year, if precipitation roughly balances evaporation plus discharge, the stores stay steady and the basin is in dynamic equilibrium even though water is constantly moving through it.

Result: It is an open system in dynamic equilibrium: both water and energy cross its boundary, and inputs balance outputs over time.

Typical mistakes

Calling the Earth an open system for carbon - across the whole planet the boundary passes energy but essentially not matter, so it is closed.
Confusing positive feedback (amplifies change) with something 'good' - positive here means self-reinforcing, and it is often destabilising.

Active revision

Explain, using the terms input, output, store and flow, why a drainage basin is described as an open system in dynamic equilibrium.

Active recall

Recall the key points — then reveal.

Sources: GCE AS and A level subject content for geography (Department for Education) · AQA A-level Geography 7037 specification (AQA)

The global and drainage-basin water cycle#

StandardAQA 7037 3.1.1DfE GCE Geography - the water cycle

The drainage-basin water cycle as a system

Fig. 3One input (precipitation), two output routes (evapotranspiration and channel discharge), and a set of stores linked by named flows.

Globally, water is held in four great stores: the oceans (by far the largest, holding roughly 97% of all water), the cryosphere (ice and snow), the terrestrial store (groundwater, soil water, lakes and rivers) and the atmosphere (a very small but rapidly cycling store of water vapour). At the global scale water is a closed system - the total amount is fixed - so the cycle is really the transfer of water between these stores by evaporation, condensation, precipitation and runoff, powered by solar energy and gravity. The sizes of the stores change over long timescales: during a glacial period more water is locked in the cryosphere and sea levels fall.

The drainage basin is the fundamental open sub-system of the water cycle: the area drained by a river and its tributaries, bounded by the watershed. Its single input is precipitation. Its outputs are evaporation and transpiration (together evapotranspiration) and the discharge of water out of the basin through the river channel. In between, water moves through a sequence of flows and is held in a sequence of stores, and it is this internal routing that determines how a basin responds to rainfall.

The flows must be named precisely. Rain is first caught by vegetation (interception) before reaching the ground by stemflow and drip. At the surface, water either soaks in (infiltration) or, if the ground is saturated or impermeable, runs across it (overland flow, or surface runoff). Water in the soil moves downslope as throughflow or drains deeper (percolation) into the groundwater store, from which it moves slowly as groundwater flow and feeds the river as baseflow. Each flow has a different speed, and the mix of fast and slow flows in a basin controls the shape of its hydrograph.

The balance of these inputs, outputs and stores over a period is captured by the water balance (or water budget), usually written

P = Q + E \pm \Delta S

, where

P

is precipitation,

Q

is runoff (channel discharge),

E

is evapotranspiration and

\Delta S

is the change in storage. In a temperate climate the balance has a seasonal rhythm: in winter, precipitation exceeds evapotranspiration so there is a water surplus and the stores recharge; in summer, high evapotranspiration creates a deficit and the stores are used up (soil-moisture utilisation and then deficit). Understanding this budget is essential for water resource management.

P = Q + E \pm \Delta S

The water balance

Precipitation (P) is partitioned into runoff (Q), evapotranspiration (E) and a change in storage (delta S). A positive delta S means the stores are recharging; a negative delta S means they are being drawn down.

Worked example

Using the water-balance equation

Over one year a drainage basin receives 1000 mm of precipitation and loses 400 mm to evapotranspiration, and its soil and groundwater stores increase by 100 mm. Calculate the runoff for the year.

Write the equation
The water balance is $P = Q + E \pm \Delta S$ ; rearrange for runoff: $Q = P - E - \Delta S$ .
Substitute the values
The stores increased, so delta S is +100 mm: $Q = 1000 - 400 - 100$ .
$Q = 1000 - 400 - 100 = 500\ \text{mm}$
Interpret
Half the precipitation left the basin as runoff; the rest was lost to the atmosphere or stored. If the stores had fallen, runoff would have been larger.

Result: Annual runoff is 500 mm.

Typical mistakes

Treating the water cycle as open at the global scale - globally it is a closed system with a fixed total volume.
Muddling throughflow (water moving through the soil) with groundwater flow (water moving through the saturated rock below the water table).

Active revision

A basin receives 1200 mm of precipitation, loses 500 mm to evapotranspiration, and its stores gain 150 mm over the year. Calculate the annual runoff.

Active recall

Recall the key points — then reveal.

Sources: GCE AS and A level subject content for geography (Department for Education) · AQA A-level Geography 7037 specification (AQA)

The storm hydrograph and runoff variation#

StandardAQA 7037 3.1.1DfE GCE Geography - runoff and hydrographs

A storm hydrograph

Fig. 4Illustrative values: peak rainfall at hour 4, peak discharge at hour 10, giving a lag time of about 6 hours.

A storm (or flood) hydrograph plots the discharge of a river - the volume of water passing a point per second, measured in cubic metres per second or cumecs - against time following a rainfall event. It is the single most important data figure in the water-cycle topic, because its shape reveals how a basin routes water from cloud to channel. Reading it fluently, and explaining why it takes the shape it does, is examined repeatedly.

The anatomy of a hydrograph must be learned. Discharge before the storm is the baseflow, sustained by groundwater. After rain falls, discharge rises along the rising limb to the peak discharge, then falls along the more gradual falling (recession) limb back to baseflow. The two key measurements are the lag time - the delay between the peak rainfall and the peak discharge - and the peak discharge itself. A short lag time and a high, sharp peak describe a flashy hydrograph; a long lag time and a low, rounded peak describe a subdued one.

The shape depends on how much water reaches the channel quickly (as overland flow) versus slowly (as throughflow and baseflow). Physical basin factors that make a hydrograph flashier include steep slopes, impermeable geology or soils, high drainage density, a small or circular basin, and already-saturated ground. Storm factors matter too: intense or prolonged rainfall, and rain falling as snowmelt, produce more rapid runoff. Vegetation slows the response by intercepting rain and encouraging infiltration.

Human activity strongly alters the hydrograph, which is why the topic links directly to flood risk. Urbanisation replaces permeable ground with tarmac and roofs and adds drains and gutters, so far more water reaches the channel as rapid overland flow: lag time falls, peak discharge rises, and the flood risk increases. Deforestation has the same effect by removing interception and reducing infiltration. Conversely, afforestation and sustainable drainage restore storage and lengthen the lag time - the reasoning that underpins catchment-based flood management.

Worked example

Reading lag time and flashiness

Using the illustrative hydrograph (peak rainfall at hour 4, peak discharge of 28 cumecs at hour 10, baseflow about 5 cumecs), calculate the lag time and state whether the basin is flashy or subdued, justifying your answer.

Find the two peaks
Peak rainfall occurs at hour 4; peak discharge (28 cumecs) occurs at hour 10.
Calculate lag time
Lag time = time of peak discharge minus time of peak rainfall = $10 - 4 = 6$ hours.
Judge the shape
A lag time of only 6 hours with a peak more than five times baseflow (28 against 5 cumecs) and a steep rising limb indicates a relatively flashy response, suggesting steep, impermeable or partly urbanised ground.

Result: Lag time is 6 hours; the steep rising limb and high peak relative to baseflow indicate a flashy hydrograph.

Typical mistakes

Confusing lag time (peak rainfall to peak discharge) with the time from the start of the storm to the peak.
Asserting that urbanisation 'causes more rain' - it does not change precipitation but speeds runoff, so peak discharge rises and lag time falls.

Active revision

Using the hydrograph, compare the likely shape of the graph for the same storm if the basin were urbanised, and explain your reasoning.

Active recall

Recall the key points — then reveal.

Sources: GCE AS and A level subject content for geography (Department for Education) · AQA A-level Geography 7037 specification (AQA)

The carbon cycle: stores and fluxes#

StandardAQA 7037 3.1.1DfE GCE Geography - the carbon cycle

The carbon cycle: stores and fluxes

Fig. 5Fast fluxes (photosynthesis, respiration) link the atmosphere, biosphere and ocean surface; slow fluxes (burial, volcanism, weathering) involve the lithosphere.

Carbon, like water, is held in stores and moved between them by fluxes, and globally it too is a closed system with a fixed total. The great stores are the lithosphere (sedimentary rocks and fossil fuels - overwhelmingly the largest), the oceans (dissolved carbon dioxide, marine organisms and deep-ocean carbon - the largest of the mobile stores), the biosphere (living and dead organic matter, including soils) and the atmosphere (carbon dioxide and methane - a small store, but the one whose changes drive the climate). Because the atmospheric store is small, even modest transfers into or out of it change its concentration markedly.

It is useful to divide the cycle into a fast and a slow component. The fast (biological) carbon cycle moves carbon between the atmosphere, biosphere and surface ocean over years to decades, mainly through photosynthesis (which removes carbon dioxide from the air) and respiration and decomposition (which return it). The slow (geological) carbon cycle moves carbon between the lithosphere, oceans and atmosphere over thousands to millions of years, through weathering, sedimentation, burial and volcanic outgassing. Human burning of fossil fuels is significant precisely because it transfers carbon from the slow store into the fast one far faster than nature would.

The main fluxes should be understood as processes with a direction. Photosynthesis fixes atmospheric carbon dioxide into plant tissue; respiration and decomposition release it again; combustion (natural wildfires and, above all, the burning of fossil fuels) releases it rapidly; and carbon is transferred to the oceans by diffusion and dissolving at the surface. In the ocean, the biological pump and the carbonate pump move carbon to depth, where over long periods it is buried in sediments and locked into the lithosphere - carbon sequestration. Chemical weathering of rock, especially the carbonation of limestone, draws carbon dioxide out of the atmosphere over geological time.

Over the very long term these fluxes have kept atmospheric carbon dioxide roughly in balance, but the balance is dynamic and has varied naturally - for example with the glacial-interglacial cycles, when ocean uptake and release shifted atmospheric concentrations. What matters for the modern climate is that the fluxes into and out of the atmosphere are no longer balanced: the human addition of carbon from the slow store now exceeds the rate at which the oceans and biosphere can absorb it, so the atmospheric store is growing. This imbalance is the subject of the carbon budget.

Worked example

Fast or slow carbon?

Classify each of the following as part of the fast (biological) or the slow (geological) carbon cycle, and state its timescale: (a) photosynthesis in a forest, (b) the weathering of limestone, (c) the burning of coal, (d) the burial of dead plankton in ocean sediment.

Photosynthesis
Moves carbon from atmosphere to biosphere over years - the fast cycle.
Weathering of limestone
Draws carbon dioxide from the air into dissolved bicarbonate over thousands to millions of years - the slow cycle.
Burning coal and burying plankton
Burning coal releases slow-store carbon rapidly into the fast cycle (human-accelerated); burial of plankton locks carbon into the lithosphere over geological time - the slow cycle.

Result: (a) fast; (b) slow; (c) transfers slow-store carbon into the fast cycle; (d) slow. Burning fossil fuels is significant because it short-circuits the slow cycle into the fast one.

Typical mistakes

Listing the fluxes without a direction - photosynthesis removes carbon from the atmosphere, respiration and combustion return it.
Assuming the oceans are a minor store - they hold far more carbon than the atmosphere and biosphere combined.

Active revision

Explain how carbon is transferred from the atmosphere into long-term geological storage, naming each flux involved.

Active recall

Recall the key points — then reveal.

Sources: GCE AS and A level subject content for geography (Department for Education) · AQA A-level Geography 7037 specification (AQA)

Feedback, the carbon budget and the climate#

AdvancedAQA 7037 3.1.1DfE GCE Geography - carbon budget and climate

Positive feedback: the ice-albedo loop

Fig. 6A self-reinforcing loop: each step drives the next in the same direction, amplifying the original warming.

The carbon budget is the balance of the fluxes into and out of the atmospheric store. Natural fluxes - photosynthesis, respiration, ocean exchange, volcanism, weathering - were broadly balanced over recent geological time. Human activity has added a new, one-directional flux: the combustion of fossil fuels, deforestation and cement production transfer carbon from the slow store into the atmosphere faster than the oceans and biosphere can remove it. The result is a growing atmospheric store, measurable as the rising concentration of carbon dioxide, and this is the driver of enhanced global warming.

The atmospheric carbon store matters for climate because carbon dioxide and methane are greenhouse gases: they are transparent to incoming short-wave solar radiation but absorb and re-emit the long-wave radiation the Earth emits, warming the lower atmosphere. Adding carbon to the atmosphere strengthens this natural greenhouse effect. Because the water and carbon cycles share the same energy budget, changes in one affect the other - a warmer atmosphere holds more water vapour (itself a greenhouse gas), intensifying the water cycle and further amplifying warming.

The most dangerous aspect is positive feedback, which amplifies warming. As the planet warms, reflective sea ice and snow melt, exposing darker ocean and land that absorb more solar energy and warm further (the ice-albedo feedback). Warming thaws permafrost, releasing stored carbon dioxide and methane that add to the greenhouse effect. Warmer oceans hold less dissolved carbon dioxide, so they release some back to the air. Each loop reinforces the original warming and can accelerate change beyond the initial forcing.

There are also negative (stabilising) feedbacks, though they act more slowly. Higher atmospheric carbon dioxide can increase the rate of photosynthesis (carbon fertilisation), drawing some carbon back into the biosphere; warming and higher carbon dioxide can accelerate chemical weathering, removing carbon over long periods; and increased cloud cover can raise albedo and reflect more sunlight. Evaluating whether the positive or negative feedbacks dominate - and over what timescale - is the essence of the highest-level analysis in this topic, and the honest answer is that the amplifying feedbacks currently outweigh the dampening ones on human timescales.

Negative feedback: carbon fertilisation

Fig. 7A dampening loop: the response opposes the original change and pulls the store back towards balance.

Worked example

Explaining the permafrost feedback

Explain how the thawing of permafrost acts as a positive feedback that amplifies global warming.

Start with the forcing
Rising atmospheric carbon dioxide enhances the greenhouse effect, warming high-latitude regions where permafrost occurs.
The loop
Warming thaws the permafrost, allowing microbes to decompose the previously frozen organic carbon, which releases carbon dioxide and methane to the atmosphere.
The amplification
These are greenhouse gases, so they enhance the greenhouse effect further, causing more warming and more thaw - the change is reinforced, not dampened.

Result: Warming thaws permafrost, releasing greenhouse gases that cause further warming - a self-reinforcing positive feedback loop.

Typical mistakes

Describing the greenhouse effect as trapping incoming sunlight - greenhouse gases absorb outgoing long-wave radiation, not incoming short-wave.
Treating carbon fertilisation as a full solution - it is a slow, limited negative feedback that does not offset the human carbon flux.

Active revision

'Positive feedback makes global warming a self-accelerating process.' Assess this statement with reference to at least two feedback mechanisms.

Active recall

Recall the key points — then reveal.

Sources: GCE AS and A level subject content for geography (Department for Education) · AQA A-level Geography 7037 specification (AQA)

Case studies: the Amazon and a river catchment#

AdvancedAQA 7037 3.1.1DfE GCE Geography - water and carbon case studies

How deforestation disrupts the water and carbon cycles

Fig. 8One change - the loss of forest cover - propagates through both the water and the carbon system.

The specification requires a tropical-rainforest case study to show water and carbon cycling and the impact of change. The Amazon rainforest is the standard example. Its water cycle is intense and largely internal: high solar energy drives enormous evapotranspiration, dense vegetation intercepts rain, and much of the moisture is recycled locally, so the forest effectively generates a large share of its own rainfall. Its carbon cycle is equally active: rapid photosynthesis makes the forest a vast carbon store in the biomass and soils, with fast turnover through respiration and decomposition. The rainforest is therefore both a major store of carbon and a powerful engine of the regional water cycle.

Deforestation disrupts both cycles, and explaining the mechanism honestly is what earns marks. Removing the trees cuts interception and evapotranspiration, so less moisture is recycled and the regional climate tends to become drier; bare soil gives more overland flow, faster runoff and greater soil erosion, altering the local hydrograph. For the carbon cycle, felling and burning transfers the stored carbon to the atmosphere as carbon dioxide and removes the photosynthetic sink, so a carbon store becomes a carbon source. These changes illustrate the links between the water and carbon cycles and the sensitivity of both to human activity.

The specification also requires a river-catchment case study to show the water cycle operating and human interaction with it. Any well-understood catchment can serve, and the key is to apply the systems framework precisely rather than to recite a place. A catchment shows the water balance in action - the seasonal rhythm of surplus and deficit - and human interactions such as reservoir construction, water abstraction, land-use and channel change, and afforestation all alter the stores and flows and hence the discharge regime and flood risk. Reservoirs, for example, shift storage and even out discharge; urban growth flashens the hydrograph.

The evaluative skill is to weigh the significance of these human interactions against natural variability, and across different timescales and scales. A drought is a natural depletion of the stores, but abstraction can deepen it; a flood is a natural peak, but land-use change can intensify it. The strongest answers use the case study as evidence within a systems argument - showing how a change to one store or flow propagates through the whole system - rather than as a list of facts, and they reach a supported judgement about how far human activity has altered the cycle.

Worked example

Evaluating the impact of deforestation

Assess how the deforestation of a tropical rainforest changes both the water and the carbon cycle, and reach a judgement about which cycle is more affected.

Water cycle effects
Removing the canopy cuts interception and evapotranspiration, reducing moisture recycling and rainfall; bare soil increases overland flow and erosion and flashens the local hydrograph.
Carbon cycle effects
Burning and decay release the large biomass and soil carbon store to the atmosphere, and the loss of the trees removes a major photosynthetic sink, so the forest switches from carbon store to carbon source.
Reach a judgement
Both cycles are significantly disturbed and they are linked; a defensible judgement is that the carbon impact is globally significant (contributing to warming) while the water impact is most acute regionally - so the 'more affected' cycle depends on the scale considered.

Result: Deforestation disrupts both cycles and their linkage; the carbon impact is global while the water impact is strongest regionally, so the judgement is scale-dependent.

Typical mistakes

Describing the rainforest as only a carbon store - it is also a powerful driver of the water cycle through evapotranspiration and moisture recycling.
Writing a place description instead of a systems argument - examiners reward tracing a change through stores and flows to a supported judgement.

Active revision

'Human activity has a greater impact on the carbon cycle than on the water cycle.' To what extent do you agree, using a tropical rainforest case study?

Active recall

Recall the key points — then reveal.

Sources: GCE AS and A level subject content for geography (Department for Education) · AQA A-level Geography 7037 specification (AQA)

References & sources

Sources

Department for Education

GCE AS and A level subject content for geography

AQA

AQA A-level Geography 7037 specification

Water and Carbon Cycles

6 sections~23 min reading time3 competenciesLevel Foundation 1 · Standard 3 · Advanced 2

Water and Carbon Cycles

Key points

Classifying a drainage basin as a system

Key points

Using the water-balance equation

Key points

Reading lag time and flashiness

Key points

Fast or slow carbon?

Key points

Explaining the permafrost feedback

Key points

Evaluating the impact of deforestation

Sources

Water and Carbon Cycles

Key points

Classifying a drainage basin as a system

Key points

Using the water-balance equation

Key points

Reading lag time and flashiness

Key points

Fast or slow carbon?

Key points

Explaining the permafrost feedback

Key points

Evaluating the impact of deforestation

Sources