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Summary Earths life support systems Geography OCR A level notes
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A* standard Earth's life support systems Geography OCR A level notes. Follows specification fully in detail. Case studies included.
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ELSSWater
- Water comes in liquid form due to the earth lying a distance from the sun known as
‘’the goldilocks zone’’
The importance of water in supporting life on earth
- Provides a medium that allows organic molecules to mix and form more complex
structures
- Helps create benign thermal conditions, eg; oceans covering 71% of the land store
97% of heat, clouds also reflect around 1/5th of incoming solar radiation lowering
surface temperatures, water vapour however is also a potent greenhouse gas and is
estimated to keep surface temperatures 15 degrees higher than they would be
otherwise
- Used in photosynthesis, transpiration, by plants to maintain their rigidity,
respiration, chemical reactions in animals, sweating and panting
- Used is economic activities; hydropower, irrigation, swimming pools, paper, steel
brewing industries etc
,The water cycle
- At a global scale the atmosphere is a closed system however on a local scale it is an
open system
- At a macro-scale the global water cycle consists of three main stores; the
atmosphere, oceans and land, and flows (Precipitation, transpiration,
evapotranspiration and run off/ groundwater flow)
Atmosphere
- Water enters as Evaporation from the oceans and evapotranspiration from the land
- Water leaves as precipitation
Oceans
- Water enters as precipitation and run off/ ground water flow
- Water leaves as evaporation
Land
- Water enters as precipitation
- Water leaves as evapotranspiration and run off/ ground water flow (water is
released from glaciers as ablation (sublimation or melting)
Water balance equation;
- Precipitation= evapotranspiration + streamflow +/- storage
Stores in the water cycle;
Oceans= 97%
Polar ice and glaciers (cryosphere)= 2%
Ground water= 0.7%
Lakes= 0.01%
Soils=0.005%
Atmosphere= 0.001%
Rivers= 0.0001%
Biosphere= 0.00004%
The US Geological Survey (USGS) estimates the global water cycle budget circulates around
505,000 km cubed a year of water
,Processes in the water cycle:
Precipitation
- Water/ ice that falls from clouds
- Forms when water vapour in the atmosphere reaches its dew point (the
temperature in which air becomes saturated) and condenses into tiny water droplets
or ice particles to form clouds, they clump together and eventually become so heavy
it falls as precipitation
Transpiration
- The diffusion of water vapour to the atmosphere from leaf pores (stomata) of plants,
it is responsible for around 10% of moisture in the atmosphere
- Influenced by temperature, wind speeds, water availability to plants, deciduous/
coniferous trees (deciduous plants purposefully shed their leaves in climates with
dry or cold seasons to reduce moisture loss through transpiration)
Condensation
- Change of water state from vapour to a liquid
- Happens at the dew point and leads to cloud formation
Evaporation
- Liquid water to water vapour
- Heat energy hits the water and is absorbed as latent heat causing evaporation
(rather than heating up the water)
Ablation= the loss of ice from ice sheets, glaciers etc due to a combination of melting,
evaporation and sublimation (it is a cyrospheric process)
Interception
- Vegetation intercepts precipitation and stores it temporarily on leaves, branches
stems etc
- Interception rates decrease as the duration and intensity of rainfall increase (as
vegetation becomes more saturated)
Interception loss
- Eventually the moisture evaporates, falls to the ground known as throughfall (water
that is briefly intercepted before reaching the ground) or flows down the trunk or
stem (known as stemflow)
Factors that affect the rate of interception loss:
- Wind speed
- Vegetation type (eg; greater from grasses have a lower interception loss rate than
trees that have a larger surface area)
- Tree species (eg; evergreens have higher interception loss rates than deciduous
plants as they have leaves all year round)
, Rain fall that does not enter storage follows one of two flowpaths to streams or rivers;
- Infiltration (through the soil, known as throughflow)
- Overland flow (across the ground surface)
Infiltration capacity= the maximum rate at which soil can absorb rain
There are two conflicting ideas to explain flowpaths
- One argues overland flow takes place when rainfall exceeds infiltration capacity and
soils become saturated (Saturated overland flow)
- The other argues that rainfall regardless of its intensity always infiltrates the soil
When permeable rocks lie below soils, water can percolate to them and then migrate slowly
through the rock pores and joints as groundwater flow, eventually emerging at the surface
as springs or seepages
Cloud formation
- Form when water vapour reaches its dew point (the atmospheric temperature
where water droplets condense), the dew point is related to humidity (the higher
the humidity the higher the dew point)
Cooling occurs when air rises which happens due to;
- Warm air rises as it is lighter (higher energy meaning particles spread out and are
more buoyant than the air around them, this is known as atmospheric instability and
results in air rising freely as a convection current), water vapour continues to rise as
long as its internal temperature is higher than the surrounding atmosphere
- Air masses are forced to rise as they cross a mountain barrier or turbulence forces
their ascent ascent
As air particles get higher up there are less above them so pressure falls and the air cools by
adiabatic expansion
The lapse rate= the rate at which an atmospheric variable (normally temperature) in the
Earth’s atmosphere changes with altitude
Environmental lapse rate (ELR)= the vertical temperature profile of the lower atmosphere at
any given time, on average the temperature falls by 6.5 degrees for every km of height
gained
Dry adiabatic lapse rate (DALR)= the rate at which a parcel of dry air cools due to adiabatic
expansion which takes place due to a fall in pressure as air particles have risen to their dew
point. Cooling caused by adiabatic expansion is roughly 10 degrees per kilometre (has to be
less than 100% humidity so that condensation is not taking place).
Saturated adiabatic lapse rate (SALR)= the rate at which a saturated parcel of air cools (eg;
one in which condensation is occurring) cools as it rises through the atmosphere. Because
condensation releases latent heat, the SALR is lower than the DALR at around 7 degrees per
km.
Latent heat= the heat given out to the atmosphere when condensation occur
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