1.2 Discharge relationships with drainage basins

Question 1

What is a hydrograph

Answer 1

• A hydrograph is a graph that shows how river discharge changes over time

Question 2

What is river discharge

Answer 2

• Discharge is the volume of water passing through a cross-sectional point of the river at a given point in time, measured in cubic metres per second
• This combines the speed of the river in a given direction (velocity) with the area of a cross sectional point (area) to show how much water is travelling through an area, and how fast it’s doing so.

Question 3

What is the difference between a storm hydrograph and an annual hydrograph

Answer 3

• A hydrograph may show how a river channel changes in response to a storm event (storm hydrograph) or it may show the changes of a river channel over a year (annual hydrograph).

Question 4

Explain what a storm hydrograph is:

Answer 4

• Storm hydrographs show how a storm event (with consequent higher precipitation levels in the drainage basin) alters the discharge of a river. The hydrograph can be used to identify several components of a flood.

Question 5

In relation to a storm hydrograph: what is discharge

Answer 5

• 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). Made up of baseflow and stormflow.

Question 6

In relation to a storm hydrograph: what is the rising limb

Answer 6

• The line on the graph that represents the discharge increasing

Question 7

In relation to a storm hydrograph: what is the falling limb:

Answer 7

The line on the graph that represents the discharge decreasing

Question 8

In relation to a storm hydrograph: what is the lag time

Answer 8

• The time between peak rainfall and peak discharge

Question 9

In relation to a storm hydrograph: what is the baseflow

Answer 9

• The level of water that the river holds without contributions by overland flow. This is the ‘normal’ discharge line (without precipitation)

Question 10

In relation to a storm hydrograph: stormflow

Answer 10

• This is the additional water in the river bank during a storm, comprised of overland flow and throughflow

Question 11

In relation to a storm hydrograph: what is bankfull discharge

Answer 11

• The maximum capacity of the river
• If discharge exceeds this then the river will burst its banks and be in flood. When the discharge goes above the line labelled above, it is in flood

Question 12

What is a flashy hydrograph

Answer 12

• Short lag time and high peak discharge, most likely to occur during a storm event, with favourable drainage basin characteristics

Question 13

What is a subdued hydrograph

Answer 13

• Long lag time and low peak discharge

Question 14

What are the features of a flashy hydrograph

Answer 14

• Short lag time
• Steep rising and falling limb
• Higher flood risk
• High peak discharge

Question 15

What are the features of a subdued hydrograph

Answer 15

• Long lag time
• Gradually rising and falling limb
• Lower flood risk
• Low peak discharge

Question 16

Explain what an annual hydrograph is and what does it show

Answer 16

• Annual hydrographs show how discharge changes over a year
• The discharge is usually an average, such as a daily or weekly average
• Annual hydrographs allow different trends of a river to be seen, such as seasonal variations.

Question 17

Two main influences on hydrographs:

Answer 17

• The climate the river is in
• The drainage basin the river is part of

Question 18

State how precipitation can effect the type or shape of hydrograph:

Answer 18

• Precipitation varies with the climate, and these variations affect hydrographs
• Overall, more precipitation will lead to a flashier storm hydrograph, and will also create higher levels of discharge on an annual hydrograph

Question 19

Explain how precipitation can effect the hydrograph

Answer 19

• Storm events influence hydrographs due to the fast and heavy rainfall falling in short periods of time
• When above average amounts of precipitation fall in short periods of time, there is not enough time for the water to infiltrate, and instead it flows into the channel
• Bankfull discharge is reached quickly and any discharge above that line on the hydrograph flows outside the river channel

Question 20

Explain how the type of precipitation can effect the storm hydrograph

Answer 20

• The type of precipitation also affects discharge in a river
• If water falls as snow, for example, it can be stored as surface water for longer
• Therefore, it would take longer to reach the channel, affecting the lag time

Question 21

Explain how temperature may effect hydrographs

Answer 21

• Temperature has a direct relationship with evaporation. When temperatures are higher, water particles have more energy, meaning they are more likely to evaporate
• Therefore, in warmer climates or in warmer seasons, water stored in soil, bodies of water, on trees etc. evaporates, meaning more water can be stored in them before they reach their capacity
• When precipitation does fall (dependent on the type of precipitation), more of it can infiltrate the ground or be stored, meaning less water overall reaches the river. Therefore, the overall discharge will be lower

Question 22

Explain what antecedent moisture is:

Answer 22

• Antecedent moisture is the pre-existing level of moisture within soil before precipitation
• The level of moisture in the soil affects the amount of infiltration, which therefore affects hydrographs (as infiltration has a direct affect on river discharge)
  *

Question 23

Explain how antecedent moisture has an effect on hydrographs: highly saturated soils

Answer 23

• Highly saturated soils have a lot of water already stored, meaning the soils will reach their field capacity sooner, and water will flow as overland flow instead
• Thus, there will be more water flowing as channel flow rather than through flow or infiltration
• Higher levels of overland flow leads to higher levels of channel flow, therefore increasing the discharge in a river. This leads to a flashier hydrograph - one with higher discharge levels as well as a quicker lag time

Question 24

Explain how antecedent moisture has an effect on hydrographs: very low antecedent moisture levels

Answer 24

• If antecedent moisture is very low in soils, the soils may become too dry for water to infiltrate as soils can structurally deteriorate.
• This means more water flows as overland flow and into channels, increasing discharge
• Higher levels of overland flow leads to higher levels of channel flow, therefore increasing the discharge in a river. This leads to a flashier hydrograph - one with higher discharge levels as well as a quicker lag time

Question 25

Explain how seasonal varations can influence hydrographs: spring and summer

Answer 25

• More vegetation growth, meaning more interception by vegetation and more transpiration. Therefore, soil saturation content is lower and potentially more water can infiltrate into below ground stores
• Precipitation is usually lower in many regions, therefore leading to less saturated soil as well as less water flowing as channel flow
• If precipitation becomes very low, the ground may become hard and less permeable. This leads to more overland flow as the water cannot infiltrate, potentially leading to high discharge, especially during storm events

Question 26

Explain how seasonal varations can influence hydrographs: autumn and winter

Answer 26

• Less vegetation growth and cooler temperatures. This leads to less interception as well as less transpiration, leading to more overland flow and therefore higher discharge
• Soil may be more saturated, leading to more overland flow and higher river discharge
• Frozen ground may be impermeable and lead to overland flow. Snow takes time to melt, therefore leading to a longer lag time and more opportunity for water to infiltrate the
  ground. slowing down the processes that occur within the water cycle.

Question 27

Explain how the size of the drainage basin affects the discharge of the river/hydrograph

Answer 27

• The size of the drainage basin affects discharge in a river
• In larger drainage basins, overland flow has more distance to cover before it reaches a river, potentially leading to a longer lag time
• Furthermore, large basins cover more area, therefore there is potentially more water in this area to flow into the river, leading to a higher peak discharge

Question 28

Explain how the shape of the drainage basin affects the discharge of the river/hydrograph

Answer 28

• In circular drainage basins, different starting points of overland flow are likely to be similar distances apart, meaning they are more likely to join the river at a similar time
• This leads to a large quantity of water concentrating into an area in a short space of time, therefore increasing peak discharge as well as shortening the lag time
• In contrast, in an oval drainage basin of a similar area, the starting points of overland flow will be different distances apart, meaning they will arrive at different times. Due to this, the lag time will be longer

Question 29

Explain what the drainage density is:

Answer 29

• The drainage density is the total length of all rivers and streams in a drainage basin divided by the total area of the drainage basin
• The drainage density is an indicator of how well the drainage basin is drained

Question 30

Explain the shape of a hydrograph for a drainage basin with a high drainage density

Answer 30

• In a drainage basin with a high drainage density, there are more streams and rivers carrying water, meaning the peak discharge is high and it is reached quickly, meaning there is a short lag time
• As well as this, the amount of rivers mean the water is drained quickly, leading to a steep
  falling limb

Question 31

Explain the shape of a hydrograph for a drainage basin with a low drainage density

Answer 31

• In areas of low stream density, overland flow is more likely to infiltrate rather than make its way into channels, therefore lengthening the lag time and lowering the peak discharge, as there is less water draining into the river

Question 32

Explain how porosity and permeability of soils affects hydrographs

Answer 32

• Particles with larger pores will allow water to infiltrate faster, as there is more space available for water to enter
• Therefore, in periods of intense, heavy rainfall, sandy soils - for example - will infiltrate more water than clay soils
• Drainage basins with sandy soils would have a more subdued storm hydrograph as there is less overland flow
• The porosity of clay soils is higher than that of sandy soils as overall there is more space in between each soil particle, therefore clay soils can hold more water overall
• In periods of lighter rain (when there is time for water to infiltrate the clay soil), overland flow may be higher in areas with sandy soils, leading to higher river discharge.

Question 33

Explain how rock type affects hydrographs

Answer 33

• Similarly to soil type, rock type also affects drainage, therefore affecting river discharge within the drainage basin
• Permeable rocks such as sandstone infiltrate more water than impermeable rocks as there are spaces for water to seep into. Therefore, drainage basins with permeable rock will have less overland flow, leading to less discharge
• Furthermore, water stored in soil on top of the rocks can percolate into the rocks below, making the soil less saturated and therefore more capable of holding water.

Question 34

Explain how bumpiness or smoothness can affect hydrographs

Answer 34

• A rough, jagged topography reduces the velocity of water, as the flow of water is interrupted with bumps, rocks, debris etc
• In contrast, a smoother topography allows water to move quickly. Therefore, in areas with jagged topography, peak discharge is usually lower due to slower overland flow.

Question 35

Explain how slope can affect hydrographs

Answer 35

• A steeper slope allows water to travel faster as the force of gravity is stronger
• As well as this, water therefore has less time to infiltrate, and more of it flows as overland flow. This makes the lag time quicker and the peak discharge higher (a flashier hydrograph).

Question 36

Explain how a well vegetated drainage basin affects hydrographs

Answer 36

• A well vegetated drainage basin encourages surface water to drain into the ground rather than flowing as overland flow
• Tree and plant roots create passages for water to infiltrate into the lower layers of soil, meaning less water flows as overland flow and therefore less water goes into the river channel
• Furthermore, vegetation decreases soil moisture content as vegetation takes up water and releases it into the atmosphere through transpiration. This process is more significant in forested areas, rather than grassy fields, for example.

Question 37

Explain how land use can affect hydrographs: deforestation

Answer 37

• There is less interception by trees so surface runoff increases.
• The soil is no longer held together by roots, so soil water storage decreases
• There are fewer plants so transpiration decreases
• Overall, this leads to a flashier hydrograph, with a shorter lag time and higher peak discharge.

Question 38

Explain how land use can affect hydrographs: agriculture

Answer 38

• Livestock can trample the ground reducing infiltration as the pore spaces are
  lost
• Ploughing increases infiltration by creating a looser soil, which decreases overland flow. However, digging drainage ditches (often seen around field edges) increases surface runoff and streamflow
• Hillside terracing (for rice padi fields) increases surface water storage and therefore decreases runoff
• Irrigation (the movement of water by human intervention through tunnels and other conduits) can lead to groundwater depletion, therefore affecting the water table and therefore allowing more water to enter channels before the river is in flood.

Question 39

Explain how land use can affect hydrographs: urbanisation

Answer 39

• Roads and buildings have impermeable surfaces, reducing infiltration and increasing overland flow. This reduced lag time and increases the peak discharge
• Urban drainage reroutes large amounts of water to different areas, meaning water outside the natural drainage basin can flow into a river, increasing peak discharge.