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The Drainage Basin System

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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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CatchmentThe area from which water drains into a particular drainage basin.

TributaryA river which joins a larger river.

ConfluenceThe point at which two rivers join.

WatershedThe boundary dividing one drainage basin from another- a ridge of high land.

What is a drainage basin?

SourceThe upland area where the river begins.

MouthWhere the river flows into the sea, or sometimes a lake.

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Do you know your drainage basin terminology?

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The drainage basin of the Afon Caerfanell, a tributary of the River Usk, South Wales.

the watershed can be clearly seen on this photograph

Watershed

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The confluence of two rivers

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Watershed

Tributary

Match the label to the correct letter

Confluence

a

b

c

Sourced

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Sketch the river showing its catchment

Add the following labels – Watershed, Confluence, Tributary

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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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river

evapotranspiration

precipitation

water table

interception

percolation

infiltration

Drainage basin system

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Drainage basin system

inputs

precipitation

outputs

river runoff

evaporation

transpiration

stores and flows

interception

surface runoff

surface water storage

soil water storage

groundwater storage

infiltration

groundwater flow

throughflow

percolation

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Drainage basin terms

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Drainage basin system

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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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Rising limb

Falling limb

Peak discharge

Peak rainfall

Write a definition of lagtime.Calculate the lagtime shown on this hydrograph.

storm flow

normal (base) flow

Hydrographs

I = Lagtime

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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

What affects the shape of a hydrograph?

Land use

Type and amount of precipitation

Geology and soil Gradient of the valley sides

Drainage density

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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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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

Hydrographs

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What could cause 2 peaks in a hydrograph?

Hydrographs

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Compare these two hydrographs.

Which one has the largest lag time? Explain your answer.

Hydrographs

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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 dischargechanges in response to a rainfall event.The shape of a flood hydrograph is influenced by a range of physical and human factors.