Summary AQA A Level Geography: Water and Carbon Cycles Detailed Notes
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Water and Carbon Cycles
Instelling
AQA
Full, detailed, A* level notes about the Water and Carbon Cycles topic in the Geography A Level. Case studies included. Clear and concise notes to help you build knowledge on the topic and achieve higher grades. Send me a message if you have any questions about what is in the document!
Water and Carbon Cycles Revision
Natural Systems ● The Water Cycle ● Drainage Basins ● Variations in
Runoff and the Water Cycle ● The Ouse Drainage Basin ● The Carbon
Cycle ● Changes in the Carbon Cycle ● The Carbon Cycle: Impacts and
Mitigations ● The Amazon Rainforest ● Water, Carbon and Climate
Natural Systems
Systems
There are five key parts of systems:
● Inputs- matter or energy that is added to the system
● Outputs- matter or energy that leaves the system
● Stores/components- where matter or energy builds up
● Flows/transfers- when matter or energy moves from one store to another
● Boundaries- the limits of the system
In a drainage basin, water enters as rain (input) and leaves where the river meets
the sea (output). It is stored in soil and vegetation (stores) and it travels down
rivers to the mouth (flows). The system’s watershed is the boundary.
Systems can be open or closed:
Open Systems
● Energy and matter can enter and leave an open system- there are inputs
and outputs of both.
● For example, drainage basins are open systems as both energy (from the
sun) and matter (water) can enter and leave the system.
Closed Systems
● Matter cannot enter or leave a closed system- it can only cycle between
stores
● Energy can enter and leave- it can be input and output
● For example, the carbon cycle is a closed system because energy can
enter (sun causes photosynthesis) and leave (respiration) but the amount
of carbon on the earth stays the same because there are no inputs or
outputs of it.
If the inputs and outputs of a system are balanced, the system is in equilibrium-
flows and processes continue to occur, but in the same way at all times so there
are no overall changes to the system.
There may be small variations (the amount of precipitation entering a drainage
basin constantly varies) but on average, the inputs and outputs remain the same
putting the system in dynamic equilibrium.
Large, long-term changes to the balance of inputs and outputs can cause a
system to change and establish a new dynamic equilibrium.
Changes can trigger positive or negative feedback:
,Positive feedback
● Positive feedback mechanisms amplify the changes in inputs and outputs
● This means the system responds by increasing the effects of the change,
moving the system further away from its previous state
● For example, temperature rises 🠊 Ice covering cold parts of the earth melts
🠊Less ice cover means less of the sun’s energy is reflected 🠊 Less of the
sun’s energy being reflected back means more is being absorbed by the
earth 🠊 Temperature rises etc.
Negative Feedback
● Negative feedback mechanisms counteract the change in the inputs or
outputs
● This means that the system responds by decreasing the effects caused by
the changes, keeping the system closer to its previous state
● For example, Large amounts of CO₂ emitted 🠊 Co2 in atmosphere
increases 🠊 Extra CO2 causes plants to increase growth 🠊 Plants remove
and store more CO2 from atmosphere 🠊 Amount of CO2 in the
atmosphere reduces
The Earth as a system
The Earth is a closed system because energy is input by the Sun and output into
space, but no matter is input or output.
The whole Earth system can be broken down into smaller subsystems:
● The Cryosphere- all the parts of the Earth where it is cold enough for
water to freeze e.g. glacial landscapes.
● The Lithosphere- the outermost part of the Earth, including the crust and
upper parts of the mantle.
● The Biosphere- where living things are found, including all living things
like plants, animals, fungi, insects and bacteria.
● The Hydrosphere- all the water on earth in liquid form (rivers and oceans),
solid form (ice stored in the cryosphere) and gas form (water vapour
stored in the atmosphere). This can be saline or fresh.
● The Atmosphere- the layer of gas between the Earth’s surface and space,
held in place by gravity.
These subsystems are interlinked by cycles (water and carbon cycles) and
processes that keep the Earth running as normal.
Matter and energy move between the subsystems- the output of one is the
input of the next. Because of the way matter and energy move through
subsystems, the Earth system is said to be a cascading system. Changes that
occur in one subsystem can affect what happens in others.
The Water Cycle
The hydrosphere contains 1.4 x 10² litres of water, 97% of which is saline oceanic
and 3% is freshwater. Of the world’s freshwater:
● 69% is frozen in the cryosphere
● 30% is groundwater (water stored underground in the lithosphere)
, ● 0.3% is liquid freshwater on the Earth’s surface (lakes, rivers etc.)
● 0.04% is stored as water vapour in the atmosphere
Water must be physically and economically accessible to humans to be used
(groundwater is hard to access so may not be cost-effective to extract) so there is
only a small amount of water on the planet that humans can use.
Water can change between solid, liquid and gaseous forms.
For water to boil or melt, it has to gain energy.
For water to condense or freeze, it must lose energy.
The Global Hydrological Cycle
Water is continuously cycled between different stores in a closed system known
as the global hydrological cycle.
Water passes through four stores: the hydrosphere (as liquid water- oceans,
rivers and lakes), the lithosphere (groundwater), the cryosphere (glaciers and ice
caps) and the atmosphere (as water vapour and in clouds).
The amount of water present in each store varies over scales from local (an
individual hillslope) to global and by time before moving to another store:
● Soil Water- stores for 1-2 months
● Rivers- store for 2-6 months
● Snow- stores for 2-6 months
● Glaciers- store for 20-100 years
● Lakes- store for 50-100 years
● Shallow ground aquifers- store 100-200 years
● Deep ground aquifers- store 10,000 years
The magnitude of each store depends on the amount of water flowing between
them. Different flows occur at a range of spatial and temporal scales:
Evaporation
● When liquid water changes state into a gas (water vapour) by gaining
energy- usually from solar radiation.
● Evaporation increases the amount of water stored in the atmosphere.
● The magnitude of the flow of evaporation varies by location and season. In
a place or time where there is a lot of solar radiation, a large supply of
water and warm, dry air (e.g. a jungle, in the summer) the flow of
evaporation will be high.
● If there is little solar radiation, little available liquid water and cool air that
is nearly saturated, evaporation will be low.
Condensation
● Water vapour changes state to become a liquid by losing energy to the
surroundings. This happens when the air containing water vapour cools to
its dew point (the temperature at which relative humidity reaches 100% so
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