1. The Drainage Basin System

2. 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?
Learning objectives

3. What is a drainage basin?
Tributary
A river which joins a larger river.
Mouth
Where the river flows into the sea, or sometimes a lake.
Catchment
The area from which water drains into a particular drainage basin.
Watershed
The boundary dividing one drainage basin from another- a ridge of high land.
Source
The upland area where the river begins.
Confluence
The point at which two rivers join.

4. Do you know your drainage basin terminology?

5. 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

6. The confluence of two rivers

7. Match the label to the correct letter
Watershed c
Confluence
Tributary b
Source d a

8. Sketch the river showing its catchment
Add the following labels – Watershed, Confluence, Tributary

9. 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?
Learning objectives

10. 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!

11. The water cycle – more detailed

12. The water cycle – definitions!

13. The drainage basin

14. Drainage basin system precipitation evapotranspiration interception
infiltration
surface runoff
throughflow
percolation
river
water table
groundwater flow

15. Drainage basin system inputs precipitation stores and flows
interception
surface runoff
surface water storage
soil water storage
groundwater storage
infiltration
groundwater flow
throughflow
percolation
outputs
river runoff
evaporation
transpiration

16. Drainage basin terms

17. Drainage basin system

18. 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?
Learning objectives

19. What is a flood hydrograph?

20. Hydrographs I = Lagtime Write a definition of lagtime.
Peak discharge
Falling limb
Rising limb
storm flow
normal (base) flow
I = Lagtime
Peak rainfall
Write a definition of lagtime.
Calculate the lagtime shown on this hydrograph.

21. 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?
Learning objectives

22. What affects the shape of a hydrograph?
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

23. 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

24. 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.

25. 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

26. Hydrographs
What could cause 2 peaks in a hydrograph?

27. Hydrographs Compare these two hydrographs.
Which one has the largest lag time?
Why might this be the case.

28. What do you know about drainage basins?

29. 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.