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3a. Human factors can disturb and enhance the natural processes and stores…
3a. Human factors can disturb and enhance the natural processes and stores in the water and carbon cycles
Dynamic equilibrium
In the short term, inputs, outputs and stores of water and carbon will fluctuate year to year. In the long term flows and stores usually maintain a balance, allowing a system to retain its stability
Land use changes
Urbanisation
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Artificial surfaces are largely impermeable so they allow little or no infiltration and provide minimal water storage capacity to buffer run-off
Urban areas have drainage systems to remove surface water rapidly. A high proportion of water from precipitation flows quickly into streams and rivers, leading to a rapid rise in water level
Encroaches on floodplains which reduces water storage capacity in drainage basins, increasing river flow and flood risks
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Development of factories and homes, along with increased vehicle use, results. in the release of CO2 into the atmosphere by burning fossil fuels
Farming
Water cycle
Crop irrigation diverts surface water from rivers and groundwater to cultivated land as its lost to evaporation and in soil drainage
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Carbon cycle
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Soil carbon storage is also reduced by ploughing and the exposure of soil organic matter to oxidation
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Changes may be less apparent on pasture land or where farming replaces natural grassland Eg. The NPP of annual crops in the Great Plains exceeds that of the original Prairie grasslands - However there is a lack of biodiversity
Forestry
Water cycle
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Streams draining plantations typically have relatively long lag times, low peak flows and low total discharge
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Water extraction
Is extracted from surface groundwater to meet public, industrial and agricultural demand
Direct human intervention in the water cycle changes the dynamics of river flow and groundwater storage
River Kennet
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Rates of groundwater extraction have exceeded rates of recharge, and the falling water table has reduced flows in the Kennet by 10-14%
During the 2003 drought, flows fell by 20%
Lower flows have reduced flooding and temporary areas of standing water and wetlands on the Kennet's floodplain
Lower groundwater levels have caused springs and seepages to dry up and reduce the incidence of saturated overland flow on the Marlborough Downs
Artesian basins
Sedimentary rocks form a syncline an aquifer which may contain groundwater is under high artesian pressure
If this groundwater is tapped by a well or borehole, water will flow to the surface under its own pressure
The level to which water will rise - The potentiometric surface - is determined by the height of the water table in areas of recharge on the edges of the basin
Over exploitation in the 19th century and start of the 20th century caused a drastic fall in the water table. In central London it fell by nearly 90m
Aquifers
The height of the water table fluctuates seasonally and is also affected by periods of rainfall, drought and abstraction
Permeable or porous water-bearing rocks in which groundwater can be extracted from for public supply
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Feedback loops
Water cycle
Rising temperatures means evaporation increases and the atmosphere holds more vapour. This means more cloud cover and more precipitation = Positive feedback
More vapour increases absorption of long-wave radiation from the Earth, causing further rises in temperature
More water vapour = increased cloud cover which reflects more solar radiation which causes temperatures to fall
Drainage basins
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System responds to above average precipitation by increasing river flow and evaporation; where excess water will recharge aquifers increasing water storage in permeable rocks
During droughts he system adjusts to lower precipitation by reducing run-off and evapotranspiration , while the water table falls helping to conserve groundwater stores
Carbon cycle
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Human activity through burning fossil fuels has increased the concentration of co2 in the atmosphere, the acidity of the oceans and the flux of carbon between major stores
Negative feedback could use carbon fertilisation where excess co2 is extracted from the atmosphere into the biosphere which would then find its way into the long-term storage in soils
Positive feedback : Global warming = increased rate of decomposition releasing more co2 to the atmosphere increasing the enhanced greenhouse effect