Water and carbon cycles

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infoWhy this? This topic introduces water and carbon cycles as key Earth systems operating at different scales. It develops understanding of systems concepts such as inputs, outputs, stores, flows and feedback. Students examine how natural processes and human activities affect these cycles and their role in climate, ecosystems and sustaining life. It also builds awareness of issues like climate change and water security, while developing core geographical and fieldwork skills.

scheduleWhy now? This topic is taught first to provide key knowledge, concepts and skills needed for A Level Geography, helping students understand how the Earth works as a system and preparing them for later topics.

A geographical model is a simplified representation of a complex real-world system, used to identify key components, processes and relationships.
A system has inputs, outputs, stores, flows or transfers, boundaries, and feedbacks, and many physical geography systems operate in dynamic equilibrium.
An open system exchanges both matter and energy with its surroundings, while a closed system exchanges energy but has little or no exchange of matter.
Positive feedback amplifies a change in a system, while negative feedback counteracts a change and helps return the system towards equilibrium.
The main Earth systems are the atmosphere, hydrosphere, cryosphere, lithosphere and biosphere, and the water and carbon cycles transfer matter between these interconnected spheres.
About 96.5% of Earth’s water is stored in the oceans, about 2.5% is freshwater, and most freshwater is locked in glaciers, ice caps and groundwater rather than being easily accessible surface water.
Water changes state between solid, liquid and gas through melting, freezing, evaporation, condensation, sublimation and deposition, and these changes involve gains or releases of latent heat energy.
Precipitation forms when moist air cools to its dew point, water vapour condenses into cloud droplets or ice crystals, and these particles grow large enough to fall as rain, snow, sleet or hail.
Cryospheric stores change over long timescales because glacial periods increase water stored as ice, while interglacial periods reduce ice storage and increase ocean storage.
A drainage basin is an open system with precipitation as the main input and evapotranspiration and runoff as major outputs.
Drainage basin stores include interception storage, vegetation storage, surface storage, soil water storage, groundwater storage and channel storage.
Drainage basin transfers include throughfall, stemflow, infiltration, percolation, overland flow, throughflow, groundwater flow and channel flow.
The soil moisture budget compares precipitation inputs with evapotranspiration losses and soil water storage, producing periods of soil moisture recharge, surplus, utilisation and deficit.
A flood hydrograph shows how river discharge changes during and after a storm event, including rising limb, peak discharge, lag time and receding limb.
Short lag times and high peak discharges are encouraged by steep relief, impermeable rock, saturated or compacted soils, sparse vegetation, urban surfaces and intense rainfall.
The largest carbon store is the lithosphere in sedimentary rocks and fossil fuels, while other major stores include the oceans, soils, living biomass, the atmosphere and the cryosphere.
The fast carbon cycle moves carbon over short timescales through photosynthesis, respiration, decomposition and combustion between the atmosphere, biosphere and soils.
Photosynthesis transfers carbon dioxide from the atmosphere into plant biomass, while respiration and decomposition return carbon from living organisms and dead organic matter to the atmosphere and soils.
The slow carbon cycle transfers carbon over thousands to millions of years through weathering, river transport, ocean sequestration, burial, compaction, fossil fuel formation, tectonic uplift and volcanic outgassing.
Ocean carbon pumps transfer carbon between the atmosphere and oceans: the biological pump uses marine organisms and sinking organic matter, while the physical pump uses dissolution, downwelling and ocean circulation, especially in cold waters.

rocket_launchYou must be able to

Construct an annotated systems diagram that accurately labels inputs, outputs, stores, transfers, boundaries and feedback loops.
Classify examples of water and carbon cycle processes as stores, transfers, inputs or outputs, using precise geographical terminology.
Interpret water store and carbon store data by calculating proportions and presenting them accurately as pie charts, divided bars or proportional circles.
Explain water state changes by linking evaporation, condensation, cloud formation and precipitation to temperature change and latent heat energy.
Identify drainage basin features on a map or diagram, including watershed, source, tributary, confluence, mouth and likely areas of infiltration, runoff and storage.
Construct and interpret a flood hydrograph by labelling rising limb, peak discharge, lag time and receding limb, then explaining how catchment characteristics affect its shape.
Analyse a soil moisture budget or climate graph by identifying periods of recharge, surplus, utilisation and deficit and linking them to precipitation and evapotranspiration.
Explain human impacts on the water cycle by tracing how deforestation, agriculture, soil drainage and abstraction alter stores, transfers and outputs at different scales.
Construct a carbon cycle flow diagram that distinguishes fast and slow transfers and shows how carbon moves between atmosphere, biosphere, hydrosphere and lithosphere.
Evaluate climate mitigation strategies by explaining which carbon transfers they alter, the scale at which they operate, and their likely benefits and limitations for people and environments.

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