Thinking in systems

AQA wants you to treat the water and carbon cycles as systems — sets of components linked by flows of energy and matter. The vocabulary is examinable in its own right:

• Isolated system — no exchange of matter or energy with the surroundings. Effectively hypothetical on Earth.
• Closed system — energy crosses the boundary, matter does not. Planet Earth is closed for water and carbon.
• Open system — both energy and matter cross the boundary. A drainage basin is the classic example.
• Inputs, outputs, stores (reservoirs) and flows (transfers/fluxes) are the four building blocks.

Dynamic equilibrium is the state in which inputs and outputs are balanced over time, so stores stay roughly constant even though the system is constantly moving. Disturb it, and the system responds through feedback until a new equilibrium is reached.

Examiner's eye: the strongest scripts use system language precisely. "Water moves through the soil" is GCSE. "Throughflow transfers water laterally from the soil-moisture store to the channel store, so the flux increases and the store depletes" is A-level.

What kind of system?

Global water stores and fluxes

The overwhelming majority of the Earth's water sits in the oceans — of the order of 97% of the total. Of the small freshwater remainder, most is locked in the cryosphere (ice sheets and glaciers) or held underground as groundwater. Rivers, lakes, soil moisture and the atmosphere together hold a tiny fraction.

• Residence time is the key idea: atmospheric water turns over in days; soil moisture in weeks to months; deep groundwater and ice in thousands of years.
• A store can be huge and yet almost inert (deep groundwater), or tiny and yet critically important (the atmosphere, which drives the whole cycle).
• Fluxes between stores — evaporation, precipitation, run-off — are what actually move water, and they respond fast to change.

Store, flow, or human intervention?

Tap a term, then tap the column it belongs to. Getting this distinction automatic is worth marks in every 6-marker.

The drainage basin as an open system

A drainage basin is the area drained by a river and its tributaries, bounded by a watershed. Precipitation is the input; evapotranspiration and channel discharge are the outputs; everything in between is a store or a flow.

Two ways to get overland flow. Infiltration-excess (Hortonian) — rain falls faster than the soil can absorb it. Saturation-excess — the soil is already full, so any further rain runs off. Naming the mechanism, not just the outcome, is what separates a Band 3 answer from a Band 4.

The water balance

The water balance applies the systems idea to a catchment over a year. It is simply inputs = outputs ± change in storage:

In a temperate maritime climate the balance swings through the year. In winter, precipitation exceeds evapotranspiration, so there is a soil-moisture surplus, stores refill and run-off is high. In summer, evapotranspiration exceeds precipitation, creating a soil-moisture deficit that must be recharged in autumn before run-off recovers.

Why it matters: the water balance is the bridge between climate and river regime. A catchment with a large deficit going into a wet autumn will absorb the first storms; the same catchment already at field capacity will convert that rain straight into run-off — which is why antecedent conditions dominate flood forecasting.

Your turn — the water balance

The storm hydrograph

A storm hydrograph plots discharge (cumecs, m³ s⁻¹) against time after a rainfall event. Four terms carry the marks: lag time (peak rainfall → peak discharge), peak discharge, the rising limb and the recession limb, with baseflow beneath.

• Physical controls: basin size and shape, drainage density, relief, geology (permeable chalk vs impermeable clay), soil type.
• Variable controls: rainfall intensity and duration, antecedent moisture, snowmelt, vegetation season.
• Human controls: urbanisation (impermeable surfaces, storm drains), deforestation, agricultural drainage — all reduce lag time; afforestation and SuDS increase it.

Reading the hydrograph

How the water cycle changes

AQA asks for change at different timescales, and for natural and human causes:

• Diurnal: convectional rainfall peaks in the afternoon as surface heating drives uplift; evapotranspiration collapses at night.
• Seasonal: monsoon circulation, snowmelt-fed spring peaks in nival regimes, the summer soil-moisture deficit in temperate catchments.
• Long-term: at the last glacial maximum, vast volumes of water were held in ice sheets — the cryospheric store grew and sea level fell. Deglaciation reverses the transfer.
• Human, direct: water abstraction from aquifers (which can exceed recharge), reservoir impoundment, land drainage.
• Human, indirect: deforestation removes interception and reduces evapotranspiration; urbanisation replaces infiltration with rapid storm-drain flow; farming compacts soils and lowers infiltration capacity.

Coupling: because plants both transpire and photosynthesise, land-use change hits both cycles at once. Deforestation shifts the water cycle towards run-off and shifts the carbon cycle from a sink towards a source. That link is the whole reason AQA teaches these cycles together.

Where the carbon is

The carbon cycle has the same architecture — stores and fluxes — but its stores differ by orders of magnitude in size and by millions of years in residence time.

• Lithosphere — by far the largest store. Carbon locked in carbonate rocks (limestone, chalk) and in fossil fuels. Residence time: millions of years.
• Hydrosphere / oceans — the largest of the mobile stores, holding dissolved CO₂, bicarbonate and marine biomass.
• Biosphere — living vegetation, and (crucially) soils, which hold more carbon than vegetation and atmosphere combined.
• Cryosphere — carbon frozen in permafrost, currently immobile but vulnerable.
• Atmosphere — small, but the store whose concentration governs the greenhouse effect.

Split the cycle in two: the slow (geological) carbon cycle — chemical weathering of silicates and carbonates, carbon burial in ocean sediments, subduction, volcanic outgassing — turning over on a 10⁶-year timescale; and the fast (biological) carbon cycle — photosynthesis, respiration, decomposition, ocean exchange — turning over in years to decades.

The biggest store

Fluxes, sequestration and the carbon budget

Carbon is sequestered (moved from atmosphere to a longer-term store) and released again by an opposing flux:

• Photosynthesis fixes atmospheric CO₂ into biomass — gross primary productivity (GPP).
• Respiration by the plants themselves returns some of it. What remains is net primary productivity (NPP): the carbon actually available to build the ecosystem.
• Decomposition returns litter carbon to the atmosphere and soil; cold, waterlogged or acidic conditions slow it, which is why peat and permafrost accumulate carbon.
• Ocean uptake works through the physical (solubility) pump — cold, dense polar water dissolves CO₂ and sinks — and the biological pump — plankton fix carbon, die, and rain down as sediment.
• Combustion — natural (wildfire) and anthropogenic (fossil fuels) — is the great return flux.

The carbon budget is the balance of these fluxes. Long-run measurements at Mauna Loa (the Keeling curve) show a steadily rising atmospheric concentration with a sawtooth seasonal oscillation — northern-hemisphere vegetation draws CO₂ down each summer and releases it each winter. It is the clearest single piece of evidence that the fast cycle is now unbalanced.

Your turn — net primary productivity

Name that process

Tap a description on the left, then the process it defines. Precision with these terms is the difference between describing and explaining.

Positive and negative feedback

Negative feedback dampens a change and pushes the system back towards equilibrium. Positive feedback amplifies it and drives the system away from equilibrium. Neither word means "good" or "bad".

Evaluative point: negative feedbacks are finite. CO₂ fertilisation is limited by nitrogen, water and temperature, and ocean uptake acidifies the surface ocean and slows further absorption. A good essay argues that the damping loops are being outrun by the amplifying ones, then supports it.

Which loop is which?

Fossil fuels, land-use change and the energy mix

Human activity affects the carbon cycle chiefly by transferring carbon out of long-term stores into the atmosphere:

• Fossil fuel combustion takes carbon that the slow cycle buried over millions of years and returns it in decades. This is the dominant anthropogenic flux.
• Land-use change — deforestation, drainage of peatlands and wetlands, ploughing — releases biomass and soil carbon and destroys future sink capacity.
• Cement production releases CO₂ chemically when limestone is calcined, on top of the fuel burned.
• Mitigation runs the other way: afforestation, peatland restoration, wetland conservation, agricultural practices that build soil carbon, and the shift in the energy mix towards renewables and nuclear. Carbon capture and storage aims to return carbon to the geological store, but remains costly and small-scale relative to emissions.

Consequences to argue in an essay: an enhanced greenhouse effect warms the lower atmosphere; that intensifies the hydrological cycle (higher evaporation, more atmospheric moisture, heavier rainfall events and, elsewhere, drought); the oceans warm, expand and acidify; and cryospheric loss feeds back into both cycles. Notice how the two cycles are locked together at every step.

Case study — water and carbon in the Amazon

AQA requires a case study of a tropical rainforest environment showing water and carbon cycling and the impact of human activity. The Amazon basin is the standard choice.

• Water: dense canopy gives very high interception; transpiration plus evaporation returns a large share of rainfall to the atmosphere, generating convectional rainfall and effectively recycling moisture across the basin. Deep root systems and litter maintain high infiltration, so overland flow on undisturbed forest floor is limited.
• Carbon: high year-round temperature and rainfall drive very high NPP. Rapid decomposition means most carbon is held in the biomass rather than the soil — the opposite of a boreal forest or a peatland.
• Deforestation (cattle ranching, soy, logging, roads, hydropower, mining): removes interception, raises overland flow and soil erosion, cuts evapotranspiration and therefore local rainfall recycling, and converts a carbon sink into a source through burning and soil oxidation.
• Feedback risk: researchers argue that beyond a threshold of forest loss, reduced moisture recycling could cause parts of the basin to shift towards a drier, savanna-like state — a positive feedback in which drying begets further forest loss. The exact threshold is contested; treat it as a debated projection, not a fact.

Honesty in case studies: quote figures only when you are sure of them. An examiner rewards a precise mechanism far more than a half-remembered statistic — and penalises confident invention. "A large and increasing share of clearance is driven by pasture expansion" beats a wrong percentage every time.

Case study — a river catchment at local scale

The second required case study is a river catchment, illustrating the water cycle's role in flooding and in water supply, with the human impacts on it. Whichever catchment your school uses, structure it the same way:

• Physical context: relief, geology (permeable vs impermeable), soils, land cover — these set the baseline hydrograph shape.
• Human modification: urban expansion and impermeable surfaces; channel straightening and embankments (which move the problem downstream); land drainage; upland grazing and moorland burning that reduce infiltration; abstraction for supply.
• Flood risk: explain events through antecedent conditions + rainfall intensity + catchment characteristics, not just "heavy rain".
• Management: contrast hard engineering (flood walls, storage reservoirs) with catchment-based / natural flood management — afforestation, re-meandering, leaky dams, peat restoration and SuDS — which slow the flow, raise interception and infiltration, and lengthen lag time while also storing carbon.
• Evaluation: hard engineering delivers rapid, quantifiable protection but is expensive, ages, and can encourage floodplain development; natural approaches are cheaper and co-deliver carbon and biodiversity benefits, but their effect on extreme events is smaller and harder to guarantee.

Writing the evaluative essay

Paper 1 essays are marked on AO1 (knowledge), AO2 (application/analysis) and, crucially, a conclusion that answers the question. A reliable structure for "To what extent…" / "Assess…":

• Define and frame: unpack the command and the key term (e.g. what counts as "the water cycle" — global, catchment, or both?). State your line of argument in the introduction.
• Argue in blocks, not lists: each paragraph = a claim + a mechanism + evidence + a "but".
• Use system language: stores, fluxes, residence time, equilibrium, feedback, thresholds.
• Weigh scale and timescale: an effect that is huge locally and negligible globally is a marked distinction — say so explicitly.
• Conclude with judgement: not a summary. Say to what extent, and on what condition your judgement depends.

Try it: "Assess the extent to which human activity has disrupted the natural balance of the carbon cycle." Plan three blocks — fossil fuel combustion (largest flux, global scale), land-use change (large, regionally concentrated, also hits the water cycle), and natural counterweights (ocean and biosphere sinks — but finite and weakening). Judgement: disruption is profound in the fast cycle and negligible in the slow cycle, and it is the mismatch between those timescales that creates the problem.

Sharpening the judgement

The big ideas to know

That is the whole of AQA 3.1.1 — and the systems vocabulary you will now reuse in coasts, glaciers, hazards and ecosystems. Press Finish to see your score.