Water and Carbon Cycles Overview

Overview

The lecture covers the water and carbon cycles for AQA Geography A-Level, focusing on system concepts, local and global scale processes, human and natural impacts, and the interrelationships between cycles, including climate change mitigation.

Systems and Types

• Systems have inputs, outputs, stores, flows, and boundaries.
• Open systems exchange matter/energy with surroundings; closed systems do not.
• Dynamic equilibrium occurs when system inputs equal outputs.
• Positive feedback amplifies changes; negative feedback counteracts them.
• The water and carbon cycles are open locally, closed globally.

The Water Cycle: Local Scale

• Inputs: Precipitation (rain, snow, hail; convectional, relief, frontal types).
• Outputs: Evapotranspiration (evaporation + transpiration), streamflow.
• Flows: Infiltration, percolation, throughflow, surface runoff, groundwater flow, stemflow.
• Stores: Soil water, groundwater, river channel, interception, surface storage.
• The water table marks saturated ground.
• The water balance equation: Precipitation = Runoff + Evapotranspiration ± Change in Storage.

Local Water Cycle Modifiers

• Deforestation increases runoff, decreases soil water storage and transpiration.
• Storms and seasonal changes affect infiltration, runoff, and storage.
• Agriculture affects infiltration and runoff, depending on farming type and practices.
• Urbanisation increases runoff and flood risk due to impermeable surfaces.
• Soil water budget varies seasonally, with water surplus in colder months and deficit in summer.

The Water Cycle: Global Scale

• Oceans store 97% of global water, only 2.5% is freshwater (mostly in ice and groundwater).
• Water is stored in the hydrosphere, lithosphere, cryosphere, and atmosphere.
• Aquifers and glaciers vary in water retention time.
• The ITCZ creates high rainfall at the equator due to atmospheric circulation.
• Seasonal changes, droughts, and storm events impact flows and stores.

Human Impacts on Water Cycle

• Farming alters infiltration, runoff, and river flows.
• Land use changes like deforestation and urbanisation alter interception, runoff, and storage.
• Water abstraction reduces surface and groundwater stores, especially during dry seasons.

Flood Hydrographs

• Shows river discharge responding to rainfall over time.
• Key terms: discharge, rising/falling limbs, lag time, baseflow, stormflow, bankfull discharge.
• Flashy hydrographs (short lag, high peak) vs subdued hydrographs (long lag, low peak).
• Factors: rainfall intensity, geology, drainage density, vegetation, urbanisation.

The Carbon Cycle: Local Scale

• Key transfers: photosynthesis, respiration, combustion, decomposition, diffusion, weathering, burial, sequestration.
• Photosynthesis absorbs CO₂, respiration and combustion release it.
• Carbon can be sequestered naturally or artificially (e.g., CCS).
• Sere stages (lithosere, halosere, etc.) describe plant succession and carbon dynamics.

The Carbon Cycle: Global Scale

• Major stores: marine sediments/rocks, oceans, fossil fuels, soil organic matter, atmosphere, terrestrial plants.
• Carbon sinks absorb more carbon than they emit; carbon sources do the opposite.
• Distribution and changes in forested areas affect carbon storage worldwide.

Carbon Cycle: Changes Over Time

• Natural: wildfires, volcanic activity, and climate cycles (Milankovitch) alter carbon stores.
• Human: fossil fuel use, deforestation, and farming cause rapid changes (fluxes) in atmospheric CO₂.
• The enhanced greenhouse effect from excess GHGs drives global warming.

Impact on Regional Climates

• Tropical rainforests: deforestation reduces humidity, cloud, and rainfall.
• Oceans: warming reduces carbon storage, increasing CO₂ release and leading to positive feedback loops.

Feedback Loops

• Positive feedback amplifies warming (wildfires, ice melt, permafrost thaw).
• Negative feedback moderates changes (plant growth absorbing CO₂, increased cloud cover reducing temperatures).

Moorlands and Peatlands

• Moorlands store carbon; drainage lowers water tables and increases CO₂ release.
• Restoring peatlands improves water and carbon storage, reduces flood risk, and benefits wildlife.

Interrelationship Between Water and Carbon Cycles

• Deforestation disrupts both cycles, increases flooding, reduces rainfall, and changes carbon dynamics.
• Peatland drainage enhances carbon loss and accelerates decomposition.

Climate Change Mitigation

• Global: Paris Agreement aims to limit warming; regular reviews.
• Regional: EU targets for emissions, renewables, and efficiency.
• National: UK Climate Change Act sets legal emissions targets and carbon budgets.
• Local: actions include insulation, recycling, efficient energy use, smart meters, and public transport.

Key Terms & Definitions

• Open System — exchanges matter or energy with its surroundings.
• Closed System — no matter exchange, only energy.
• Dynamic Equilibrium — balanced inputs and outputs.
• Evapotranspiration — sum of evaporation and plant transpiration.
• Infiltration — movement of water into soil.
• Percolation — downward movement of water through soil and rock.
• Flashy Hydrograph — short lag, high flood peak.
• Carbon Sequestration — capturing and storing atmospheric carbon.
• Carbon Sink — stores more carbon than it emits.
• Carbon Source — emits more carbon than it stores.
• Enhanced Greenhouse Effect — human-driven increase in heat-trapping gases.
• ITCZ — zone of high rainfall at the equator.

Action Items / Next Steps

• Revise the water and carbon cycle processes and their modifiers.
• Review the differences between feedback types and their examples.
• Practice drawing and interpreting flood hydrographs.
• Complete homework or reading assignments as given by your teacher.