1.1 The drainage basin system

Question 1

Explain what a drainage basin is:

Answer 1

• It is the area that is drained by a river and any tributaries
• It can be classified as a natural system

Question 2

Explain what the outputs of a drainage basin system are:

Answer 2

• Outputs are where matter or energy leaves the system
• Examples include: evaporation, evapotranspiration and river discharge

Question 3

Explain what the stores of a drainage basin system are

Answer 3

• Stores are where matter or energy builds up and is stored in the system
• Examples include: interception, soil water, surface water, ground water and channel storage

Question 4

Explain what the flows above ground of a drainage basin system are

Answer 4

• Flows are where matter or energy moves in the system
• Examples include: throughfall, stemflow, overland flow, and channel flow

Question 5

Explain what the flows below ground of a drainage basin system are

Answer 5

• Flows are where matter or energy moves in the system
• Examples include: infiltration, percolation, throughflow, groundwater and baseflow

Question 6

Explain the output: evaporation

Answer 6

• The change in state of water from liquid to gas
• Evaporation occurs in the drainage basin system when water is heated by solar energy, causing it to evaporate into a gas and rise into the atmosphere
• Water is leaving the system here, which is why evaporation is an output.

Question 7

Explain the output: evapotranspiration

Answer 7

• Compromised of evaporation and transpiration
• Evaporation occurs when water is heated by the sun, causing it to become a gas and rise into the atmosphere
• Transpiration occurs in plants when they respire through their leaves, releasing water they absorb through their roots, which then evaporates due to heating by the sun.

Question 8

Explain the output: river discharge

Answer 8

• The volume of water passing through a cross-sectional point of the river at any one point in time, measured in Cubic Metres Per Second (Cumecs)
• Water leaves the basin through streams which drain the basin. These may flow as tributaries into other rivers or directly into lakes and oceans.

Question 9

Explain the stores: interception

Answer 9

• Water (e.g. precipitation) that is intercepted by plants’ branches and leaves before reaching the ground
• This water is stored short-term.

Question 10

Explain the stores: soil water

Answer 10

• Water that is stored in the upper levels of the soil, which is utilised by plants.

Question 11

Explain the stores: surface water

Answer 11

• Water stored in puddles, ponds, lakes etc.
• Dependent on the size of these stores, they can last from hours to millions of years.

Question 12

Explain the stores: groundwater

Answer 12

• Water that is stored in the pore spaces of rock or lower soil.

Question 13

Explain the stores: channel storage

Answer 13

• Water that is stored in a river’s channel.

Question 14

Explain the above ground flows: throughfall

Answer 14

• Water flows from leaves and foliage onto the ground
• This is especially prominent in areas with a large canopy that receives a lot of rainfall, like a rainforest, but will occur whenever precipitation falls onto plants and trees.

Question 15

Explain the above ground flows: stemflow

Answer 15

• Intercepted water stored on plants and trees flows down a stem onto the ground.

Question 15

Explain the above ground flows: overland flow

Answer 15

Water flows above the ground as:
* Sheetflow (lots of water flowing over a large area)
* Rills (small channels similar to streams, that are unlikely to carry water during periods where there is no rainfall)
* Overland flow will occur when water cannot infiltrate the soil.

Question 16

Explain the above ground flows: channel flow

Answer 16

• Water that moves through established channels, like streams or rivers
• Naturally, channel flow is fed by two sources: overland flow (i.e. precipitation that flows over the ground, rather than infiltrates the soil), and groundwater flow (groundwater flow contributes to channel flow where a channel intersects the water table).

Question 17

Explain the below ground flows: infiltration

Answer 17

The movement of water from above ground into the soil. Infiltration is affected by multiple factors, including:
* Permeability of soil
* Relief
* Rate of precipitation
* Saturation levels in soil

Question 17

Explain the below ground flows: percolation

Answer 17

• Water flows from the ground or soil into porous rock or rock fractures
• The percolation rate is dependent on the fractures that may be present in the rock and the permeability of the rock.

Question 18

Explain the below ground flows: throughflow

Answer 18

• Water flows through the soil and into streams or rivers
• Speed of flow is dependent on the type of soil, e.g. clay soils will have a slower flow rate than sandy soils.

Question 19

Explain the below ground flows: groundwater flow

Answer 19

• Water flows through permeable rocks, below the water table, and may also flow as springs
• Groundwater maintains water levels elsewhere

Question 20

Explain the below ground flows: baseflow

Answer 20

• The baseflow is the level of channel flow when there is no overland flow, (i.e. when no precipitation has fallen)
• This means baseflow is made up of any groundwater that contributes to channel flow (refer back to the two sources of channel flow: overland flow and groundwater flow - baseflow is the level where there is no precipitation contribution, hence only groundwater flow is contributing to the channel flow).
  In short, baseflow is the ‘normal’ or base level of water in a river, when there is no contribution from precipitation.

Question 21

Explain what groundwater is and what percentage of earths water it takes up:

Answer 21

• Groundwater is classified as any water underground, meaning it can be held in soils or in between rock spaces
• Underground water makes up a large amount of the Earth’s water stores, in fact it accounts for around 30% of all the Earth’s freshwater.

Question 22

Explain what a water table is:

Answer 22

• The water table is the level at which the pore spaces and fractures in the ground become saturated, meaning above the water table is unsaturated soil, and below the water table is saturated soil.

Question 23

Explain what will happen if the surface dips below the water table:

Answer 23

• Groundwater will fill the surface space to become surface water.

Question 24

Explain what recharge is:

Answer 24

• When groundwater levels deplete (by human extraction, or by groundwater replenishing surface water levels) groundwater can be recharged.

Question 25

Explain why groundwater levels fall:

Answer 25

• Groundwater levels fall when groundwater is being used more
• For example, in hotter temperatures, there is less rainfall (summer), so more surface water bodies are drying up and therefore groundwater will somewhat replenish these stores
• When groundwater is utilised, this can cause the water table to fall, as there is no water replenishing the used groundwater.

Question 26

What is ground water recharged by:

Answer 26

• Precipitation infiltrating the ground until it reaches groundwater. When precipitation is
  higher, and evaporation is lower, such as in winter, groundwater levels will be recharged
• Surface water bodies (such as lakes and rivers) seeping into groundwater stores.

Question 27

Explain how the type of precipitation effects groundwater recharge

Answer 27

In storm events, large amounts of rainfall quickly saturates the ground to its field capacity (no more water can infiltrate the soil) increasing overland flow. Storm events are therefore less effective at recharging water stores than prolonged rainfall.
* If 20mm of rain fell evenly over the course of 24 hours, this would infiltrate the soil and percolate into the groundwater stores as well, with low overland flow
* In 1 hour, if 20mm of rain fell, there would be less water infiltrating the soil and percolating into the rocks, reducing the replenishment of groundwater stores, but increasing overland flow.

Question 28

What is a spring

Answer 28

• A natural outlet of groundwater, i.e. it is any place where groundwater flows out onto the surface.

Question 29

Explain why springs form

Answer 29

1. Permeable rock meeting impermeable rock, causing the infiltrated water to build up and eventually escape as a spring
2. Where the water table meets the surface (especially on a hill), causing the discharge of groundwater.