1.2.2 Drainage basins

Question 1

What is a drainage basin?

Answer 1

The area of land drained by a river and its tributaries.

This is also called local hydrological cycles.

Question 2

What is the boundary of a drainage basin called?

Answer 2

• The watershed.

Any precipitation falling beyond the watershed enters a different drainage basin.

Question 3

Are drainage basins open or closed systems?

Answer 3

Open - they have inputs and outputs.

Water comes into the system as precipitation and leaves via evaporation, transpiration and river discharge.

Question 4

Typical drainage basin system diagram

See reverse

Answer 4

Question 5

Inputs in drainage basins

Answer 5

• Precipitation

Question 6

Outputs in drainage basins

Answer 6

• Evaporation
• Transpiration
• Evapotranspiration
• Channel flow (discharge)

Question 7

Stores in drainage basins

Answer 7

• Interception
• Vegetation storage
• Surface storage
• Soil storage
• Groundwater storage
• Channel storage

Question 8

Flows in drainage basins

Answer 8

• Infiltration
• Overland flow (surface runoff)
• Throughfall
• Stemflow
• Throughflow
• Percolation
• Groundwater flow
• Baseflow
• Interflow
• Channel flow (discharge)

Question 9

[Inputs in drainage basins]

Precipitation

Answer 9

Precipitation includes all the ways moisture comes out of the atmosphere. It is mainly rain, but it also includes other types, like snow, hail, dew and frost.

Question 10

[Stores in drainage basins]

Interception storage

Answer 10

When precipitation lands on vegetation or other structures (e.g. buildings and concrete/tarmac surfaces) before it reaches the soil.

Interception store is only temporary because the collected water may evaporate quickly or fall from leaves are throughfall.

Question 11

[Stores in drainage basins]

Vegetation storage

Answer 11

Water that’s been taken up by plants. It is all the water contained in plants at any one time.

Question 12

[Stores in drainage basins]

Surface storage

Answer 12

Surface storage includes water in puddles, ponds and lakes.

Question 13

[Stores in drainage basins]

Soil storage

Answer 13

Moisture in the soil.

Question 14

[Stores in drainage basins]

Groundwater storage

Answer 14

Water stored in the ground in rocks.

The water table is the top surface of the zone of saturation – the zone of soil/rock where all the pores in the soil or rock are full of water. Porous rocks that can hold water are called aquifers.

Question 15

[Stores in drainage basins]

Channel storage

Answer 15

The water held in a river or stream channel.

Question 16

[Outputs in drainage basins]

Evaporation

Answer 16

Water turning into water vapour.

Question 17

[Outputs in drainage basins]

Transpiration

Answer 17

Transpiration is evaporation from within leaves – plants and trees take up water through their roots and transport it to their leaves where it evaporates into the atmosphere.

Question 18

[Outputs in drainage basins]

Evapotranspiration

Answer 18

The process of evaporation and transpiration together.

Question 19

[Outputs in drainage basins]

Channel flow (discharge)

Answer 19

The water flowing in the river or stream itself.

Question 20

[Flows in drainage basins]

Infiltration

Answer 20

Water soaking into the soil.

Infiltration rates are influenced by soil type, soil structure and how much water’s already in the soil.

Question 21

[Flows in drainage basins]

Overland flow (surface runoff)

Answer 21

Water flowing over the land.

It can flow over the whole surface or in channels. It happens because rain is falling on the ground faster than infiltration can occur.

Question 22

[Flows in drainage basins]

Throughfall

Answer 22

Water dropping from one leaf (or part of a plant) to another.

Question 23

[Flows in drainage basins]

Stemflow

Answer 23

Water running down a plant stem or a tree trunk.

Question 24

[Flows in drainage basins]

Throughflow

Answer 24

Water moving slowly downhill through the soil.

It can happen through ‘pipes’ (either man-made or cracks in the soil/animal burrows).

Question 25

[Flows in drainage basins]

Percolation

Answer 25

Water seeping down through soil into the water table.

Question 26

[Flows in drainage basins]

Groundwater flow

Answer 26

Water flowing (slowly) below the water table through permeable rock. Water flows slowly through most rocks, but flows quickly through rocks that are highly permeable.

Question 27

[Flows in drainage basins]

Baseflow

Answer 27

Groundwater flow that feeds into rivers through river banks and river beds.

Question 28

[Flows in drainage basins]

Interflow

Answer 28

Water flowing downhill through permeable rock above the water table.

Question 29

[Flows in drainage basins]

Water balance

Answer 29

The balance between inputs (precipitation) and outputs (channel flow and evaporation).

The water balance affects how much water is stored in the basin.

Question 30

How do wet seasons affect the water balance?

Answer 30

In wet seasons, precipitation exceeds evapotranspiration. This creates a water surplus. The ground stores fill with water so there’s more surface runoff and higher discharge, so river levels rise.

Question 31

How do dry seasons affect the water balance?

Answer 31

In drier seasons, precipitation is lower than evapotranspiration. Ground stores are depleted as some water is used (e.g. by plants and humans) and some flows into the river channel, but isn’t replaced by precipitation.

So, at the end of the dry season, there is a deficit (shortage) of water in the ground. The ground stores are recharged in the next wet season (i.e. autumn).

Question 32

What do we call it when there is enough water?

Answer 32

A positive water balance (surplus).

Question 33

What do we call it when there is not enough water?

Answer 33

A negative water balance (deficit).

Question 34

Discharge

Answer 34

The ‘volume of water passing a measuring point in a given time’.

Question 35

What do river regimes show?

Answer 35

The variation in a river’s discharge over a year. They allow for comparison between different rivers.

Question 36

Water balance diagram

Answer 36