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AQA A Level Geography Water and Carbon cycle revision notes
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Revision notes for AQA A Level Geography - water and carbon cycles. Follows what is required by the specification.
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Water and Carbon Cycle notes3.1.1.1 Water and carbon cycles as natural systems
o Systems in physical geography: systems concepts and their application to the water and
carbon cycles inputs – outputs, energy, stores/components, flows/transfers,
positive/negative feedback, dynamic equilibrium.
3.1.1.2 The water cycle
o Global distribution and size of major stores of water – lithosphere, hydrosphere,
cryosphere and atmosphere.
o Processes driving change in the magnitude of these stores over time and space, including
flows and transfers: evaporation, condensation, cloud formation, causes of precipitation
and cryospheric processes at hill slope, drainage basin and global scales with reference to
varying timescales involved.
o Drainage basins as open systems – inputs and outputs, to include precipitation,
evapotranspiration and runoff; stores and flows, to include interception, surface, soil
water, groundwater and channel storage; stemflow, infiltration overland flow, and channel
flow.
o Concept of water balance.
o Runoff variation and the flood hydrograph.
o Changes in the water cycle over time to include natural variation including storm events,
seasonal changes and human impact including farming practices, land use change and
water abstraction.
3.1.1.3 The carbon cycle
o Global distribution, and size of major stores of carbon – lithosphere, hydrosphere,
cryosphere biosphere, atmosphere.
o Factors driving change in the magnitude of these stores over time and space, including
flows and transfers at plant, sere and continental scales.
o Photosynthesis, respiration, decomposition, combustion, carbon sequestration in oceans
and sediments, weathering.
o Changes in the carbon cycle over time, to include natural variation (including wild fires,
volcanic activity) and human impact (including hydrocarbon fuel extraction and burning,
farming practices, deforestation, land use changes).
o The carbon budget and the impact of the carbon cycle upon land, ocean and atmosphere,
including global climate.
3.1.1.4 Water, carbon, climate and life on Earth
o The key role of the carbon and water stores and cycles in supporting life on Earth with
particular reference to climate.
o The relationship between the water cycle and carbon cycle in the atmosphere.
o The role of feedbacks within and between cycles and their link to climate change and
implications for life on Earth.
o Human interventions in the carbon cycle designed to influence carbon transfers and
mitigate the impacts of climate change.
3.1.1.5 Quantitative and qualitative skills
o Students must engage with a range of quantitative and relevant qualitative skills, within
the theme water and carbon cycles.
o Students must specifically understand simple mass balance, unit conversions and the
analysis and presentation of field data.
3.1.1.6 Case studies
o Case study of a tropical rainforest setting to illustrate and analyse key themes in water
and carbon cycles and their relationship to environmental change and human activity.
, o Case study of a river catchment(s) at a local scale to illustrate and analyse the key themes
above, engage with field data and consider the impact of precipitation upon drainage
basin stores and transfers and implications for sustainable water supply and/or flooding.
Water and carbon cycles as natural systems
Systems are made up of stores, flows, boundaries, inputs and outputs.
o Inputs – when matter or energy (e.g. solar energy) is added to the system
o Outputs – when matter or energy leaves the system
o Stores (or components) – where matter or energy builds up
o Flows (or transfers) – when matter or energy moves from one store to another
o Boundaries – the limits of the system
In a drainage basin system, water enters as rain (input). The systems watershed is the boundary.
Some water is stored in the soil and in vegetation. Water travels from the drainage basin to the
river and then down the river (flows). It leaves the system where the river meets the sea
(output).
Systems can be open or closed. Systems can be isolated (neither matter nor energy can enter or
leave) but these aren’t found in nature.
Open systems:
o Both energy and matter can enter and leave an open system – there are inputs and
outputs of both.
o Example: drainage basins are open systems – energy from the Sun enters and leaves the
system. Water is input as precipitation, and output as river discharge into the sea.
Closed systems:
o Matter can’t enter or leave a closed system – it can only cycle between stores.
o Energy can enter and leave a closed system – it can be input or output.
o Example: the carbon cycle is a closed system – energy is input (e.g. from the sun by
photosynthesis) and output (e.g. by respiration), but the amount of carbon on Earth stays
the same because there are no inputs or outputs of matter.
Systems are affected by feedbacks
o If the inputs and outputs of a system are balanced, the system is in equilibrium – flows and
processes continue to happen, but in the same way at all times, so there are no overall
changes to the system.
o However, in reality there are lots of small variations in the inputs and outputs of a system
(e.g. the amount of precipitation entering a drainage basin constantly varies). These
variations are usually small, so the inputs and outputs remain about balanced on average.
The system is said to be in dynamic equilibrium.
o Large, long-term changes to the balance of inputs and outputs can cause a system to
change and establish a new dynamic equilibrium.
o Changes can trigger positive or negative feedback.
Positive feedback:
o Positive feedback mechanisms amplify the change in the inputs or outputs.
o This means the system responds by increasing the effects of the change, moving the
system even further from its previous state.
o Example: temperatures rise – ice covering cold parts of earth melts due to higher
temperatures – less ice cover means less of the suns energy is reflected – less of suns
energy being reflected means more is absorbed by the earth – temperatures rise…
Negative feedback:
o Negative feedback mechanisms counteract the change in the inputs or outputs.
o This means that the system responds by decreasing the effects of the change, keeping the
system closer to its previous state.
, o Example: large amounts of CO2 emitted - CO2 in atmosphere increases – extra CO2 cause
plants to increase growth – plants remove and store most CO2 from atmosphere – amount
of CO2 in atmosphere reduces.
Systems term Definition Drainage basin Woodland carbon
cycle
Input Material or energy Precipitation Precipitation with
moving into the dissolved carbon
system from outside dioxide
Output Material or energy Run off Dissolved carbon
moving from the within runoff
system to the outside
Energy Power or driving force Latent heat Production of glucose
associated with through the process
changes in the state of photosynthesis
of water
Stores/components The individual Trees, puddles, soil Trees, soil, rocks
elements or parts of a
system
Flows/transfers The links or Infiltration, Burning, absorption
relationships between groundwater flow,
the components evaporation
Positive feedback A cyclical sequence of Raising sea levels Increased
events that amplifies (due to thermal temperatures due to
or increases change. expansion and climate change cause
Positive feedback melting of freshwater melting of
loops exacerbate the ice) can destabilise permafrost. Trapped
outputs of a system, ice shelves, greenhouse gases are
driving it in one increasing the rate of released, enhancing
direction and calving. This leads to the greenhouse
promoting an increase in effect, raising
environmental melting, causing sea temperatures further.
instability levels to rise further.
Negative feedback A cyclical sequence of Increased surface Increased
events that damps temperatures lead to atmospheric CO2
down or neutralizes an increase in leads to increased
the effects of a evaporation from the temperatures,
system, promoting oceans. This leads to promoting plant
stability and dynamic more cloud cover. growth and rates of
equilibrium Clouds reflect photosynthesis. This,
radiation from the in turn, removes more
sun, resulting in a CO2 from the air,
slight cooling of counteracting the rise
surface temperatures. in temperature.
Dynamic equilibrium This represents a Remote and Remote and
state of balance unaffected basin unaffected woodland
within a constantly where there has been where there has been
changing system no significant natural no significant natural
or human impacts, or or human impacts, or
one that has had time one that has had time
to adjust to change. to adjust to change.
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