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CfE H Geography: Hydrosphere Revision
Transcript of CfE H Geography: Hydrosphere Revision
THE HYDROLOGICAL CYCLE
You need to have a sound understanding of the global hydrological cycle. Recap processes including: evaporation, precipitation, transpiration, evapotranspiration, infiltration, through flow, percolation and what is meant by groundwater and surface runoff.
When looking at rivers, we look at their discharge (this is measured in cumecs per sec) and is calculated by multiplying the speed of the river by it's cross section area.
What does a STORM HYDROGRAPH tell you?
Storm hydrographs have different shapes depending on the influence of both natural factors and human activity.
As with the other Prezis, this is a tool to use alongside your revision. Use it to remind yourself of a topic before tackling revision or past/model paper questions or to summarise your learning after revising the topic.
You will need to use your class notes to effectively revise each topic.
Differing discharge rates can be taken at different points of the river and then drawn onto a storm (or flood) hydrograph. These show:
- Precipitation rates
- Baseflow (normal level of the river)
- Discharge over time
- Peak flow rate during a storm
- Basin lag time (time between peak rainfall and peak flow)
- Rising limb, falling limb and storm flow
Size and Shape
Large drainage basins tend to have
greater runoff as they get more
precipitation. Lag times tend to be longer
as it takes more time for the water to reach
the main channel. They also tend to have a higher drainage density e.g. there are more streams and more tributaries leading to the main channel.
Surface runoff in circular basins reaches the
main channel more quickly than that of
elongated drainage basins.
Soil Type and
Thick soils mean greater infiltration
rates, increasing lag time. Impermeable
soils e.g. Clay, will increase surface runoff,
meaning the water will reach the main
channel in a shorter period of time, meaning a much shorter lag time.
Steeper gradients mean water will enter the main channel more quickly than gentle gradients, increasing peak flow.
Underlying rock type
Rock type does affect soil formation,
but also has a major influence on surface
Impermeable rocks will increase soil throughflow and surface runoff as they will prevent infiltration. Permeable rocks will let water through, but it may take some time to reach the channel.
Surface drainage is low on permeable
rocks, meaning less precipitation
and surface runoff to directly
enter the river system.
Urban areas mean much of the land
is covered in tarmac or concrete, which
act in a similar way to impermeable rock -
increasing surface runoff. Sewers and drains also quicken the rate at which the water enters the main channel, especially during times of flood.
When arable land is built on and crops are removed which were important in intercepting rainfall, this can lead to higher peak flows.
Forested areas intercept a lot of rainfall,
transpire and take water up in their roots.
Afforestation delays runoff, deforested
basins are more prone to
REVISION/REVISITING: River Processes
Along a rivers profile the river erodes, transports and deposits material. You must know these processes in detail.
ABRASION / CORRASION
This is where the load of the river wears way the bedrock and banks, especially during times of flood.
Erosion of the bed and banks of the river by chemical action, such as carbon dioxide, dissolved in the water.
The sheer pressure of the water hitting off the bed and river banks.
Small particles on the river bed which bounce short distances.
Material dissolved in the water as a result of corrosion.
Fine particles, e.g. clay and silt, carried along 'suspended' in the water.
Large stones and boulders dragged along the river bed by the flowing water.
When the river slows down, it no longer has so much energy and so starts to deposit material. The largest, heaviest material is deposited first, especially when a river enters a loch or the sea and forms a delta.
Unlike the global hydrological cycle, a drainage basin is an open system with inputs and outputs such as precipitation (an input) and evaporation (an output). Water can also be stored within the drainage basin, temporarily or for longer periods of time, and also transferred by a variety of processes.
There are 4 main areas you want to focus on:
Precipitation is the only input into a drainage basin. This is normally in the form of rain but could be snow, sleet or hail. Most precipitation will take place in the upper course where the mountainous land causes relief rainfall.
Water can be stored in vegetation, soil moisture or in the ground as part of the water table. How much is stored will depend on the permeability of the ground.
This could be overland flow when it is no longer able to infiltrate the ground, so flows to the nearest river or stream.
These include evaporation from rivers/lakes and (evapo)transpiration from vegetation.
EXAM STYLE QUESTION
FACTORS AFFECTING THE HYDROLOGICAL CYCLE
Drainage basins can be affected by a number of different PHYSICAL FACTORS:
- snow will take longer to reach the steams/rivers meaning more water is held in storage
of precipitation - long or frequent spells of heavy rain will lead to the ground being saturated, meaning greater surface run-off
- drainage basins with more rivers/streams will have greater surface run-off leading to shorter lag times
– sandy soils are more porous and allow water to pass through more easily
– trees slow down interception rates more than grass and transpiration rates also vary
– chalks have a higher infiltration rate than clays
– evaporation rates are higher in warmer areas
Human factors that affect the hydrological cycle include:
– cutting down trees increases run-off, decreases
evapotranspiration and leads to more extreme river flows as water is no longer intercepted and stored by trees.
– this increases the interception rates in the drainage basin.
– taking water from the cycle can reduce river flow, lower water tables and increase evaporation/evapotranspiration by placing water in surface stores (ditches/canals), or by crops removing water from the cycle as they grow.
– removal of natural vegetation and replacement with impermeable surfaces e.g. roads and drains, speeds up overland flow and evaporation and can lead to higher river levels; it also decreases the amount of water returning to groundwater storage, possibly reducing the water table.
– silting-up of lakes, rivers and reservoirs leads to reduced storage capacity in these areas; mining may also lead to reduced vegetation cover and increased run-off, higher evapotranspiration, altering the rainfall patterns.
Dam building and reservoirs
– these increase the amount of water available for evaporation and affect the local climate.
– interception and infiltration rates are affected if there are crops in a field rather than bare soil after harvesting.
Type of precipitation
In heavy storms, rainfall is often far
greater than the
of the soil,
leading to overland flow and rapid rises in river levels.
This would produce a very
steep rising limb
short lag time
as the rainfall will move to the river quickly.
ong periods of rainfall
, the ground becomes
and overland flow increases. This again results in a very
steep rising limb
short lag time.
If there is snowfall, the potential discharge of a river is
held in storage
until the snow melts – making the
lag time longer - once temperatures rise, rapid
melting can lead to flooding.