The Drainage Basin System

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

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.

Do you know your drainage basin terminology?

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

The confluence of two rivers

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

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

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

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!

The water cycle – more detailed

The water cycle – definitions!

The drainage basin

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

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

Drainage basin terms

Drainage basin system

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

What is a flood hydrograph?

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.

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

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

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

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.

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

Hydrographs What could cause 2 peaks in a hydrograph?

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

What do you know about drainage basins?

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.