1.2 Discharge relationships within drainage basins

Question 1

What is a hydrograph?

Answer 1

graph that shows how river discharge changes over time

Question 2

What is discharge?

Answer 2

• 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 3

What is the rising limb?

Answer 3

line on the graph that represents the discharge increasing

Question 4

What is the lag time?

Answer 4

time between peak rainfall and peak discharge

Question 5

What is the falling limb?

Answer 5

line on the graph that represents discharge decreasing

Question 6

What is baseflow?

Answer 6

normal day-to-day level of water in a stream that comes from groundwater seeping into the river bed

Question 7

What is stormflow?

Answer 7

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

Question 8

What is bankfull discharge?

Answer 8

maximum capacity of the river
- if discharge exceeds this then river will burst its banks and be in flood

Question 9

How do you work out discharge?

Answer 9

areaxvelocity

Question 10

What is a flashy hydrograph?

Answer 10

short lag time
high peak discharge
steep rising limb and falling limb
higher flood risk

Question 11

What is a subdued hydrograph?

Answer 11

long lag time
low peak discharge
lower flood risk
gradually rising and falling limb

Question 12

What does a storm hydrograph show?

Answer 12

• how a storm event alters the discharge of a river

Question 13

What does an annual hydrography show?

Answer 13

shows how discharge changes over a year

Question 14

How can Precipitation influence a hydrograph?

Answer 14

Precipitation type/intensity:
- more precipitation=flashier storm hydrography
- create higher levels of discharge on annual hydrography
- during storm events there is fast and heavy rainfall in short periods of time
- not enough time for the water to infiltrate and instead it flows into the channel
- bankfull discharge reached quickly and any discharge above that line on hydrograph flows outside river channel
- type of precipitation: if water falls as snow can be stored as surface water for longer
- so increased lag time

Question 15

What climate factors affect a hydrograph?

Answer 15

Precipitation
Temperature
Wind and humidity
Antecedent moisture

Question 16

How does temperature affect hydrographs?

Answer 16

• higher temps = more evaporation
• in warmer climates, water stores in soil, on trees etc, evaporates
• means more water can be stored in them before they reach their capacity
• when precipitation does fall more of it can infiltrate the ground or be stored
• means less water overall reaches river
• overall discharge lower

Question 17

How does size affect a drainage basin?

Answer 17

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

Question 18

How does shape affect a drainage basin?

Answer 18

• in circular drainage basins, different starting points of overland flow likely to be similar distances apart, likely to join river at similar time
• leads to large quantity of water concentrating into an area in a short space of time, increasing peak discharge and shortening lag time
• in oval drainage basin of similar area, starting points of overland flow will be different distances apart, meaning will arrive at different times
• longer lag time

Question 19

How does drainage density affect the hydrograph?

Answer 19

• high drainage densities mean that water reaches river channels quickly
• water will have a relatively short distance to flow to a river channel, which will produce a rapid response
• a flashy hydrograph - giving high, sharp flood peaks, with short lag times
• low drainage densities water will reach river channels more slowly
• long distance to flow to river channel, will produce much slower response - subdued hydrograph - giving longer lag times and lower flood peaks

Question 20

How do you find drainage denisty?

Answer 20

total length of all rivers and streams in a drainage basin divided by the total area of the drainage basin

Question 21

How does porosity and permeability of soils affect the hydrograph?

Answer 21

• particles with larger pores will allow water to infiltrate faster, as more space for water to enter
• porosity of clay soils is higher than that of sandy soils
• in periods of intense, heavy rainfall, sandy soils infiltrate more water than clay soils, so would have a more subdues storm hydrograph as there is less overland flow

Question 22

What is the difference between infiltration and percolation?

Answer 22

infiltration - actual entry of water into the surface of the soil

percolation - downward movement of infiltrated water through the pores and spaces of soil once the water has actually entered the soil

Question 23

How does rock type affect the shape of a hydrograph?

Answer 23

• permeable rocks, e.g sandstone infiltrate more water than impermeable rocks as there are spaces for water to seep into
• so 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 24

How do slopes affect a hydrograph?

Answer 24

• steeper slope allows water to travel faster as the force of gravity is stronger
• water has less time to infiltrate, more flows as overland flow
• makes lag time quicker and peak discharge higher

Question 25

How does vegetation affect a hydrograph?

Answer 25

• dense vegetation encourages both greater interception and infiltration
• slow down overland flow producing subdued hydrograph
• sparse vegetation leads to less interception and infiltration which will speed up the arrival of water into river channel
• leads to flashy hydrograph

Question 26

How does land use affect shape of a hydrograph?

Answer 26

Deforestation:
- less interception by trees so surface runoff increases
- soil no longer held together by roots, soil water storage decreases
- fewer plants so transpiration decreases
- leads to flashier hydrograph

Agriculture:
- livestock trample the ground reducing infiltration as pore spaces are lost
- ploughing increases infiltration by creating looser soil, decreases overland flow
- irrigation can lead to groundwater depletion, affecting the water table and allowing more water to enter channels before river is in flood

Urbanisation:
- roads/buildings have impermeable surfaces, reducing infiltration and increasing overland flow
- reduced lag time and increases peak discharge
- urban drainage reroutes large amounts of water to different areas, meaning water outside natural drainage basin can flow into a river, increasing peak discharge

Question 27

Explain how recurrence intervals can be used in flood prediction an prevention (6)

Answer 27

Recurrence levels are statistical measures used to estimate the frequency at which a certain magnitude of flood is likely to occur

Can be used in flood prediction:

1. Risk assessment:

Can quantify risk of flooding, e.g 100-year flood has a 1% chane of occurring in any given year

**Mapping flood zones **

• Trigger points: if river levels reach a threshold that corresponds to a 10-year flood, alerts can be issued to warn communities

Improving forecast models

Question 28

Is climate the most significant factor which influences discharge within a drainage basin

Answer 28

Climate:

Precipitation type:
- Rain will increased discharge quickly, whereas snow will delay the impact

• Precipitation intensity:
  When intensity exceeds infiltration capacity = overland flow and so rapid rises in discharge

Temperature:
- in equatorial climate may have constant annual pattern and so constant discharge levels

Tropical climate such as the amazon basin, high levels of precipitation contribute to high discharge rates

Antecedent moisture/climate:
Affect soils infiltration capacity

Other factors:

Topography:

• mountainous regions like the Himalayas, steep slopes and high elevations influence discharge patterns. Precipitation falling on these slopes can quickly generate runoff, leading to flashy hydrographs with rapid rises and falls in river discharge

Land use:
- urbanized drainage basins, impervious surfaces such as roads, buildings, and parking lots can increase surface runoff and alter discharge patterns

e.g Bangladesh