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The Drainage Basin System
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Learning objectives
What is a drainage basin?
How does the drainage basin system operate?
What is a flood hydrograph?
What factors influence the shape of a flood hydrograph?
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What is a drainage basin?
Mouth
Where the river
flows into the sea,
or sometimes a
lake.
Tributary
A river which joins a larger river.
Watershed
The boundary
dividing one
drainage basin
from another- a
ridge of high
land.
Confluence
The point at which two rivers join.
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Catchment
The area from
which water drains
into a particular
drainage basin.
Source
The upland area
where the river
begins.
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Do you know your drainage basin terminology?
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Watershed
The drainage basin of the Afon Caerfanell, a tributary of
the River Usk, South Wales.
the watershed can
be clearly seen on
this photograph
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The confluence of two rivers
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Match the label to the correct letter
Watershed
c
Confluence
Tributary
b
Source
d
a
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Sketch the river showing its catchment
Add the following labels – Watershed, Confluence, Tributary
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Learning objectives
What is a drainage basin?
How does the drainage basin system operate?
What is a flood hydrograph?
What factors influence the shape of a flood hydrograph?
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How does the drainage basin system operate?
A drainage basin system is a part of the world’s
hydrological (water) cycle:
The hydrological cycle is a closed system.
The drainage basin is an open system.
You should be able to explain why once you have seen
the following slides!
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The water cycle – more detailed
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The water cycle – definitions!
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The drainage basin
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Drainage basin system
precipitation
interception
evapotranspiration
infiltration
percolation
water table
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river
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Drainage basin system
inputs
precipitation
stores and flows
interception
outputs
river runoff
surface water storage
evaporation
surface runoff
transpiration
infiltration
soil water storage
throughflow
percolation
groundwater storage
groundwater flow
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Drainage basin terms
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Drainage basin system
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Learning objectives
What is a drainage basin?
How does the drainage basin system operate?
What is a flood hydrograph?
What factors influence the shape of a flood hydrograph?
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What is a flood hydrograph?
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Hydrographs
Peak discharge
Rising
limb
Falling
limb
storm flow
normal (base) flow
I = Lagtime
Peak rainfall
Write a definition of lagtime.
Calculate the lagtime shown on this hydrograph.
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Learning objectives
What is a drainage basin?
How does the drainage basin system operate?
What is a flood hydrograph?
What factors influence the shape of a flood
hydrograph?
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Hydrographs
Geology and soil
Gradient of the valley sides
What affects the shape
of a hydrograph?
Land use
Drainage density
Type and amount of precipitation
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How do trees reduce flooding?
trees intercept the rain
evapotranspiration
reduces the amount
of water that
reaches the river
water is taken through the roots
and so less enters the river
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What factors influence the shape of a hydrograph?
The hydrographs ‘a’ and ‘b’ have been produced from the
same storm event but from different drainage basins.
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Hydrographs
Imagine that these two areas have had the same
amount of rain over a 24-hour period. Draw a possible
hydrograph for each of these areas.
A
B
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Hydrographs
What could cause 2 peaks in a hydrograph?
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Hydrographs
Compare these two hydrographs.
Which one has the largest lag time?
Why might this be the case.
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What do you know about drainage basins?
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Key ideas
A drainage basin is an area of land drained by a main
river and its tributaries.
The hydrological (water) cycle is the global circulation of
water. It is a closed system.
A drainage basin system is one part of the hydrological
cycle. It is an open system as it has inputs and
outputs.
A flood hydrograph shows how a river’s discharge
changes in response to a rainfall event.
The shape of a flood hydrograph is influenced by a
range of physical and human factors.
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