What is meant by excess runoff?

Excess runoff is water reaching the surface faster than it can infiltrate or be stored, so it flows overland and raises river discharge sharply.

Describe the monsoon and its effect on runoff.

In South Asia the winds reverse in summer to bring moist air off the Indian Ocean, delivering most of the year's rainfall in about four months, so rivers rise enormously and floodplains are inundated.

How does snowmelt generate excess runoff?

A rapid thaw releases a whole winter of precipitation within days, often on to ground that is still frozen and cannot absorb it.

WJEC A-Level Geography 601-8455-3

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Water and Carbon Cycles

Excess runoff and flooding

3 things to know
A2 Unit 3

Overview

Excess runoff, how prolonged precipitation causes flooding and monsoon and its effect on runoff.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Explain how prolonged precipitation causes flooding.

Key term

Why can one intense storm flood a catchment when days of drizzle would not?

Key term

Why does baked, dry ground shed water during a summer storm?

Questions from this subtopic

Each one with the model answer underneath.

1
What is meant by excess runoff?
Model answer
Excess runoff is water reaching the surface faster than it can infiltrate or be stored, so it flows overland and raises river discharge sharply.
2
Explain how prolonged precipitation causes flooding.
Model answer
Days of rain fill the soil moisture store to capacity, and once the soil is saturated any further rain runs straight off into the channel.
3
Why can one intense storm flood a catchment when days of drizzle would not?
Model answer
The rainfall rate exceeds the infiltration capacity of the surface, so overland flow begins within minutes even though the total that falls may be small.
4
Why does baked, dry ground shed water during a summer storm?
Model answer
A hard, cracked and crusted surface has a very low infiltration rate, so heavy rain runs off it instead of soaking in.
5
Describe the monsoon and its effect on runoff.
Model answer
In South Asia the winds reverse in summer to bring moist air off the Indian Ocean, delivering most of the year's rainfall in about four months, so rivers rise enormously and floodplains are inundated.
6
How does snowmelt generate excess runoff?
Model answer
A rapid thaw releases a whole winter of precipitation within days, often on to ground that is still frozen and cannot absorb it.

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