Water and Carbon Cycles

Introduction

The water and carbon cycles are global systems that transfer energy and matter across the Earth’s surface and atmosphere. Understanding these cycles is fundamental to explaining climate patterns, ecosystem functioning, and the impacts of human activity on the environment. This topic examines the stores, flows, and feedback mechanisms within both cycles, with particular emphasis on how they interact and respond to change.

Key Concepts and Definitions

Term	Definition
Drainage basin	The area of land drained by a river and its tributaries, bounded by a watershed
Watershed	The boundary separating one drainage basin from another
Throughfall	Precipitation that passes directly through the canopy to the ground
Stemflow	Water that runs down plant stems and trunks to reach the ground
Infiltration	The downward movement of water from the surface into the soil
Percolation	The downward movement of water through soil and permeable rock into the groundwater store
Throughflow	The lateral movement of water through the soil towards a river
Baseflow	The sustained flow of a river fed by groundwater seepage
Stormflow	The rapid increase in river discharge following a rainfall event
Evapotranspiration	The combined processes of evaporation from surfaces and transpiration from plants
Potential evapotranspiration (PET)	The maximum possible evapotranspiration given unlimited water supply
Water budget	The balance between inputs (precipitation) and outputs (evapotranspiration, runoff) in a system
Carbon sink	A store that absorbs more carbon than it releases (e.g., oceans, forests)
Carbon source	A store that releases more carbon than it absorbs (e.g., volcanic eruptions, fossil fuel combustion)
Positive feedback	A process that amplifies change, moving the system further from equilibrium
Negative feedback	A process that counteracts change, returning the system towards equilibrium
Lithosphere	The rigid outer layer of the Earth, including the crust and upper mantle
Cryosphere	The portions of the Earth’s surface where water is in solid form (ice, snow)
Hydrosphere	All the water on or near the Earth’s surface
Atmosphere	The layer of gases surrounding the Earth

The Water Cycle

Global Stores and Flows

The global water cycle is a closed system — no water enters or leaves (in significant quantities). The major stores are:

Store	Volume (km³)	Percentage
Oceans	1,338,000,000	96.5%
Ice caps and glaciers	24,364,000	1.74%
Groundwater	10,530,000	0.76%
Surface water	189,000	0.014%
Atmosphere	12,900	0.001%
Biosphere	1,120	0.0001%

Key flows in order of magnitude: surface runoff, evaporation, precipitation, groundwater flow, transpiration.

The Drainage Basin as an Open System

A drainage basin is an open system — it receives inputs and produces outputs:

Inputs: Precipitation (rain, snow, hail, dew)
Stores: Interception (vegetation), surface storage (lakes, puddles), soil moisture, groundwater
Transfers/Flows: Throughfall, stemflow, infiltration, throughflow, baseflow, overland flow, channel flow
Outputs: Evapotranspiration, river discharge to the sea

Factors Affecting River Discharge

The storm hydrograph shows how river discharge responds to a rainfall event:

Factors increasing peak discharge and reducing lag time:

High-intensity or prolonged rainfall
Impermeable rock (e.g., granite, clay)
Steep gradients encouraging overland flow
Saturated soil (antecedent moisture)
Urbanisation (impermeable surfaces, drains)
Deforestation (reduced interception and infiltration)

Factors decreasing peak discharge and increasing lag time:

Gentle, prolonged rainfall
Permeable rock (e.g., limestone, chalk)
Gentle gradients
Dry soil (high infiltration capacity)
Rural land use with extensive vegetation
Dense forest cover (high interception)

The Water Budget

The water budget equation:

P = Q + E ± ΔS

Where P = precipitation, Q = runoff (discharge), E = evapotranspiration, ΔS = change in storage.

In the UK, the water budget varies seasonally:

Winter: Precipitation > evapotranspiration → soil moisture recharge → groundwater recharge → rising river levels
Spring: Transition period as temperatures rise and evapotranspiration increases
Summer: Evapotranspiration > precipitation → soil moisture deficit → declining river levels, baseflow dominates
Autumn: Transition back towards surplus as precipitation increases and evapotranspiration decreases

A soil moisture deficit occurs when PET exceeds precipitation and plants cannot draw enough water from the soil. A soil moisture surplus occurs when precipitation exceeds PET and the soil is at field capacity.

The Carbon Cycle

Global Stores and Flows

The carbon cycle is also a closed system at the global scale. Major stores:

Store	Carbon (Gt C)
Ocean (dissolved inorganic)	38,000
Fossil fuels	10,000
Marine sediments and sedimentary rocks	100,000,000
Soil	1,500
Atmosphere (as CO₂)	750
Vegetation (biomass)	560

Key flows: photosynthesis, respiration, decomposition, combustion, ocean-atmosphere exchange, weathering, volcanic outgassing.

The Terrestrial Carbon Cycle

Photosynthesis: Plants absorb atmospheric CO₂ and convert it to organic carbon (glucose). This is the primary pathway for carbon to enter the biotic system.
Respiration: Plants, animals, and decomposers release CO₂ back to the atmosphere by metabolising organic carbon.
Decomposition: Breakdown of dead organic matter by bacteria and fungi, releasing CO₂ and returning nutrients to the soil. Rate depends on temperature and moisture.
Combustion: Burning of biomass (natural fires, slash-and-burn) and fossil fuels releases stored carbon rapidly.
Weathering: Chemical weathering of rocks (especially calcium carbonate) can sequester CO₂ from the atmosphere over geological timescales.

The Oceanic Carbon Cycle

The ocean is the largest active carbon store:

Diffusion: CO₂ dissolves at the ocean-atmosphere interface. Solubility increases in colder water.
Biological pump: Marine organisms (phytoplankton) fix carbon through photosynthesis. When they die, organic carbon sinks to the deep ocean.
Carbonate pump: Marine organisms build shells from calcium carbonate. These form limestone over geological time.
Thermohaline circulation: Deep ocean currents transport dissolved carbon from the surface to the deep ocean, where it can be locked away for centuries.

The Geological Carbon Cycle

Over millions of years, carbon is cycled between the atmosphere, biosphere, hydrosphere, and lithosphere:

Sedimentation and burial of organic matter forms fossil fuels (coal, oil, natural gas)
Compression of marine carbonate shells forms limestone
Tectonic activity and volcanic eruptions release carbon back to the atmosphere via outgassing

Feedback Mechanisms

Positive Feedback Examples

Ice-albedo feedback: Rising temperatures melt ice → lower surface albedo → more solar radiation absorbed → further warming → more ice melts. This amplifies initial warming.
Permafrost thaw: Warming melts permafrost → releases trapped methane (CH₄) → enhanced greenhouse effect → further warming → more permafrost thaws.
Deforestation and CO₂: Trees removed → less CO₂ absorbed by photosynthesis → atmospheric CO₂ rises → enhanced greenhouse effect → warming → increased risk of forest fires → more tree loss.

Negative Feedback Examples

Increased photosynthesis: Rising CO₂ → increased plant growth (CO₂ fertilisation) → more CO₂ absorbed → atmospheric CO₂ stabilises or falls.
Cloud formation: Warming → increased evaporation → more cloud cover → higher albedo → less solar radiation reaching the surface → cooling effect.
Chemical weathering: Warming → increased chemical weathering of rocks → more CO₂ removed from atmosphere → cooling effect (operates over geological timescales).

Human Impacts on the Water and Carbon Cycles

Impacts on the Water Cycle

Urbanisation: Increases surface runoff, reduces infiltration and groundwater recharge, creates flashier hydrographs with higher peak discharge and shorter lag times. Impermeable surfaces (concrete, tarmac) can increase runoff by up to 90%.
Deforestation: Reduces interception and transpiration, increases overland flow and soil erosion. Can reduce local rainfall through decreased evapotranspiration and cloud formation (e.g., Amazon).
Agriculture: Drainage of wetlands reduces water storage. Irrigation can deplete groundwater stores (e.g., Ogallala Aquifer, USA). Soil compaction reduces infiltration.
Dam construction: Alters river flow regimes, increases evaporation from reservoir surfaces, traps sediment. Can enable water abstraction for irrigation and urban supply.

Impacts on the Carbon Cycle

Fossil fuel combustion: The single largest anthropogenic source of CO₂. Global emissions reached approximately 36.8 Gt CO₂ in 2023.
Deforestation and land-use change: Removes carbon sinks and releases stored carbon. Tropical deforestation accounts for roughly 10% of global CO₂ emissions.
Agriculture: Rice paddies and livestock produce methane (CH₄). Fertiliser use releases nitrous oxide (N₂O). Both are potent greenhouse gases.
Cement production: Decomposition of limestone (CaCO₃ → CaO + CO₂) releases CO₂, as does burning fossil fuels to heat kilns. Contributes approximately 8% of global CO₂ emissions.

Climate Change Impacts on Cycles

Rising global temperatures affect both cycles:

Accelerated hydrological cycle: Warmer atmosphere holds more moisture (Clausius-Clapeyron relation: ~7% more per 1°C) → more intense precipitation events and flooding in some regions, but also more intense droughts in others.
Melting cryosphere: Glacier retreat reduces long-term water storage. Loss of seasonal snowpack affects river regimes (e.g., Himalayan rivers supplying billions of people).
Permafrost thaw: Releases methane and CO₂, creating a positive feedback loop.
Ocean acidification: Increased dissolved CO₂ lowers ocean pH (has fallen by 0.1 pH units since pre-industrial times), threatening marine ecosystems and reducing the ocean’s capacity to absorb more CO₂.
Changes in vegetation distribution: Biomes shift polewards and to higher altitudes, altering carbon storage patterns.

Case Studies

Case Study 1: The Amazon Rainforest — Carbon Cycle Disruption

The Amazon Basin contains approximately 150–200 Gt of carbon in its biomass and soils. Under normal conditions, it acts as a net carbon sink, absorbing roughly 2 Gt CO₂ per year through photosynthesis.

However, deforestation in Brazil peaked at approximately 10,000 km² per year in 2004 (INPE data). Clearing and burning of forest releases stored carbon immediately and eliminates future carbon uptake. A 2021 study (Gatti et al.) suggested parts of the eastern Amazon have shifted from carbon sink to carbon source due to a combination of deforestation and climate change-induced drought.

Drought conditions, exacerbated by deforestation (reduced evapotranspiration and cloud formation), increase fire risk. The 2019 and 2020 fire seasons saw record numbers of hotspots detected by satellite. This represents a potential tipping point: if deforestation exceeds approximately 20–25% of the original forest area, large parts of the Amazon could transition to savannah (lovejoy and Nobre, 2018), releasing billions of tonnes of stored carbon.

Case Study 2: The River Eden, Cumbria — Flooding and the Water Cycle

The River Eden drains a catchment of approximately 2,300 km² in north-west England. In December 2015, Storm Desmond delivered record rainfall — 341 mm fell in 24 hours at Honister Pass. The resulting flood was among the most severe in recorded UK history.

Contributing factors included:

Geology: Impermeable slate and volcanic rocks in the upper catchment limited infiltration
Topography: Steep upland valleys funneled water rapidly into tributaries
Antecedent conditions: Soils were already saturated from preceding rainfall in November
Land use: Some upland areas had drained moorland and limited tree cover, reducing interception

Carlisle experienced severe flooding with over 1,700 homes affected. The flood peaked at approximately 1,720 m³/s at Eden Bushby gauge. Following the event, the flood defence scheme was upgraded (completed 2019) at a cost of £38 million, incorporating higher embankments, a pumped drainage system, and upstream natural flood management measures including tree planting and moorland restoration to slow runoff and increase infiltration.

Common Pitfalls

Confusing open and closed systems: The global water cycle is a closed system (fixed total volume); a drainage basin is an open system (exchanges matter and energy with its surroundings). Students often write about water “leaving” the global cycle — it does not.
Confusing positive and negative feedback: A positive feedback amplifies the initial change (e.g., ice-albedo effect accelerates warming); a negative feedback counteracts it (e.g., increased photosynthesis reduces atmospheric CO₂). The word “positive” does not mean “good.”
Ignoring temporal scale: The geological carbon cycle operates over millions of years (sedimentation, uplift, volcanic outgassing). The biological carbon cycle operates over days to decades. Do not conflate the timescales — for example, weathering sequesters CO₂ but far too slowly to offset current anthropogenic emissions.

Worked Examples

Example 1: 9-Mark Question

“Explain how human activities can modify the water cycle within a drainage basin.”

Answer:

Human activities can significantly modify stores and flows within the drainage basin system. Urbanisation is one of the most impactful changes. The construction of impermeable surfaces such as roads and buildings prevents infiltration, meaning a greater proportion of precipitation becomes overland flow. This reduces the soil moisture and groundwater stores, decreasing baseflow to the river in dry periods. Simultaneously, storm drains and sewers channel surface water rapidly into rivers, producing hydrographs with a higher peak discharge and shorter lag time, increasing flood risk.

Deforestation also modifies the cycle by removing the interception store. Without tree canopies, more precipitation reaches the ground as throughfall, increasing surface runoff and soil erosion. Transpiration is also reduced, which can lower local humidity and reduce downstream precipitation — a concern in regions such as the Amazon Basin.

Agricultural drainage converts wetland stores into farmland, reducing the capacity of the catchment to store water and releasing it more rapidly into river channels. Conversely, dam construction increases the surface water store within the reservoir, raising evaporation losses but also allowing regulated discharge downstream, which can reduce flood peaks in the short term.

These modifications alter the balance of the water budget equation (P = Q + E ± ΔS), commonly increasing Q (runoff) at the expense of soil moisture and groundwater stores.

Example 2: 6-Mark Question

“Outline the role of the ocean in the global carbon cycle.”

Answer:

The ocean is the largest active carbon store, holding approximately 38,000 Gt C as dissolved inorganic carbon. CO₂ from the atmosphere dissolves into seawater at the ocean surface, with solubility higher in colder polar waters. This physical diffusion is a major pathway transferring carbon from atmosphere to ocean.

The biological pump transfers carbon deeper: phytoplankton fix CO₂ through photosynthesis in the euphotic zone. When these organisms die, organic carbon sinks to the deep ocean as marine snow, sequestering it for centuries to millennia. Additionally, marine organisms such as coccolithophores build calcium carbonate shells, which eventually form limestone sediments on the ocean floor, locking carbon into the lithosphere for geological timescales.

The thermohaline circulation transports dissolved carbon from surface to deep waters, maintaining the vertical carbon gradient. However, ocean acidification from increased CO₂ absorption is reducing the ocean’s buffering capacity, potentially weakening its future role as a carbon sink.

Summary

The water cycle is driven by solar energy and gravity, moving water between atmosphere, hydrosphere, lithosphere, and biosphere.
The carbon cycle transfers carbon through photosynthesis, respiration, decomposition, combustion, weathering, and ocean processes.
Both cycles operate as closed systems globally but open systems at the drainage basin or ecosystem scale.
Positive feedback loops (e.g., ice-albedo, permafrost thaw) amplify environmental change; negative feedback loops (e.g., CO₂ fertilisation) counteract it.
Human activities — particularly fossil fuel combustion, deforestation, and urbanisation — are fundamentally altering both cycles, with consequences including enhanced greenhouse warming, altered flood regimes, and ocean acidification.
Understanding feedback mechanisms and temporal scales is essential for predicting future changes and evaluating management strategies.

Sources: AQA Geography (7037) specification; Witherick et al., AQA A-level Geography (Hodder, 2016); IPCC AR6 WG1 (2021); INPE deforestation data; Gatti et al. (2021), Nature; Environment Agency flood data for Cumbria 2015.

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