WJEC (B) GCSE Geography
Theme 2 Topic 2
Understanding
hydrographs
Hodder Education Revision Lessons
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WJEC (B) GCSE Geography
Theme 2 Topic 2
Understanding hydrographs
In an exam question you
will need to show you can
interpret a hydrograph.
In other words, what
does the hydrograph tell
us?
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WJEC (B) GCSE Geography
Theme 2 Topic 2
Lag time
Lag time is the time
between peak rainfall and
peak discharge. The longer
the lag time, the longer it
has taken for the rainfall to
reach the river.
What could make the lag
time longer (i.e. what would
slow down the journey of
rainfall to the river)?
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WJEC (B) GCSE Geography
Theme 2 Topic 2
Longer lag time
• Trees could intercept the water by holding it on their leaves.
• Trees could soak up the water through their roots.
• The water could soak into the soil and travel to the river as
throughflow or groundwater flow.
• The water could be held in a reservoir and slowly released by
a dam.
• The water could be used by farmers or industry and then put
back into the river later.
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WJEC (B) GCSE Geography
Theme 2 Topic 2
Lag time
What could make the lag
time shorter (i.e. what
could speed up the
journey of rainfall to the
river)?
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WJEC (B) GCSE Geography
Theme 2 Topic 2
Shorter lag time
What could make the lag time shorter (i.e. what could speed up the
journey of rainfall to the river)?
• There might not be any plants to intercept the rainfall due to
deforestation or after crops have been harvested.
• The soil could be frozen so the water cannot infiltrate.
• The ground could have been tarmacked so that water cannot
infiltrate.
• The soil could have been compacted, e.g. by cattle, so rain cannot
infiltrate.
• The ground could be hard after a dry spell, so it is hard for the rain
to infiltrate.
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WJEC (B) GCSE Geography
Theme 2 Topic 2
Lag time
A short lag time can
lead to flooding.
Copy the diagram
and complete it to
show which factors
might increase the
risk of flooding.
What might
increase
flood risk?
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WJEC (B) GCSE Geography
Theme 2 Topic 2
Factors that might increase the risk of flooding
Lots of tarmac
Deforestation
Compacted soil
What might
increase
flood risk?
Heavy rain
Dry ground
Frozen ground
Impermeable
rock
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WJEC (B) GCSE Geography
Theme 2 Topic 2
Key terms
Match the key terms and definitions. Just write the letters and
numbers.
1 Lag time
A Water being held or kept
2 Peak discharge
B Time between peak rainfall and peak discharge
3 Discharge
C Rock that allows water to flow through
4 Base flow
D The transfer of water
5 Rising limb
E Normal discharge of a river
6 Falling limb
F Increase in discharge shown on a hydrograph
7 Impermeable
G Decrease in discharge shown on a hydrograph
8 Permeable
H Rock that doesn’t allow water to flow through
9 Store
I Amount of water at a point on a river at a given time,
measured in cumecs
10 Flow
J The maximum amount of water held by a river after a
storm
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WJEC (B) GCSE Geography
Theme 2 Topic 2
Key terms
1 Lag time = B Time between peak rainfall and peak discharge
2 Peak discharge = J The maximum amount of water held by a river after a
storm
3 Discharge = I Amount of water at a point on a river at a given time,
measured in cumecs
4 Base flow = E Normal discharge of a river
5 Rising limb = F Increase in discharge shown on a hydrograph
6 Falling limb = G Decrease in discharge shown on a hydrograph
7 Impermeable = H Rock that doesn’t allow water to flow through
8 Permeable = C Rock that allows water to flow through
9 Store = A Water being held or kept
10 Flow = D The transfer of water
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