The Drainage Basin System These icons indicate that

The Drainage Basin System These icons indicate that teacher’s notes or useful web addresses are available in the Notes Page. This icon indicates that the slide contains activities created in Flash. These activities are not editable. For more detailed instructions, see the Getting Started presentation. 1 of 29 © Boardworks Ltd 2005

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? 2 of 29 © Boardworks Ltd 2005

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. 3 of 29 Catchment The area from which water drains into a particular drainage basin. Source The upland area where the river begins. © Boardworks Ltd 2005

Do you know your drainage basin terminology? 4 of 29 © Boardworks Ltd 2005

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 5 of 29 © Boardworks Ltd 2005

The confluence of two rivers 6 of 29 © Boardworks Ltd 2005

Match the label to the correct letter Watershed c Confluence Tributary b Source d a 7 of 29 © Boardworks Ltd 2005

Sketch the river showing its catchment Add the following labels – Watershed, Confluence, Tributary 8 of 29 © Boardworks Ltd 2005

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? 9 of 29 © Boardworks Ltd 2005

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! 10 of 29 © Boardworks Ltd 2005

The water cycle – more detailed 11 of 29 © Boardworks Ltd 2005

The water cycle – definitions! 12 of 29 © Boardworks Ltd 2005

The drainage basin 13 of 29 © Boardworks Ltd 2005

Drainage basin system precipitation interception evapotranspiration infiltration surfa ce ru throug hflow percolation water table 14 of 29 noff river groundwater flow © Boardworks Ltd 2005

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 15 of 29 © Boardworks Ltd 2005

Drainage basin terms 16 of 29 © Boardworks Ltd 2005

Drainage basin system 17 of 29 © Boardworks Ltd 2005

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? 18 of 29 © Boardworks Ltd 2005

What is a flood hydrograph? 19 of 29 © Boardworks Ltd 2005

Hydrographs Peak discharge Rising limb I = Lagtime Falling limb storm flow normal (base) flow Peak rainfall Write a definition of lagtime. Calculate the lagtime shown on this hydrograph. 20 of 29 © Boardworks Ltd 2005

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? 21 of 29 © Boardworks Ltd 2005

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 22 of 29 © Boardworks Ltd 2005

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 23 of 29 © Boardworks Ltd 2005

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. 24 of 29 © Boardworks Ltd 2005

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 25 of 29 © Boardworks Ltd 2005

Hydrographs What could cause 2 peaks in a hydrograph? 26 of 29 © Boardworks Ltd 2005

Hydrographs Compare these two hydrographs. Which one has the largest lag time? Why might this be the case. 27 of 29 © Boardworks Ltd 2005

What do you know about drainage basins? 28 of 29 © Boardworks Ltd 2005

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. 29 of 29 © Boardworks Ltd 2005