1.2 Discharge relationships within drainage basins Flashcards
What is a hydrograph?
graph that shows how river discharge changes over time
What is discharge?
- 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
What is the rising limb?
line on the graph that represents the discharge increasing
What is the lag time?
time between peak rainfall and peak discharge
What is the falling limb?
line on the graph that represents discharge decreasing
What is baseflow?
normal day-to-day level of water in a stream that comes from groundwater seeping into the river bed rather than surface runoff
What is stormflow?
additional water in the river bank during a storm, comprised of overland flow and throughflow
What is bankfull discharge?
maximum capacity of the river
- if discharge exceeds this then river will burst its banks and be in flood
How do you work out discharge?
areaxvelocity
What is a flashy hydrograph?
short lag time
high peak discharge
steep rising limb and falling limb
higher flood risk
What is a subdued hydrograph?
long lag time
low peak discharge
lower flood risk
gradually rising and falling limb
What does a storm hydrograph show?
- how a storm event alters the discharge of a river
What does an annual hydrography show?
shows how discharge changes over a year
How can Precipitation influence a hydrograph?
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
What climate factors affect a hydrograph?
Precipitation
Temperature
Wind and humidity
Antecedent moisture
How does temperature affect hydrographs?
- 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
How does size affect a drainage basin?
- 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
How does shape affect a drainage basin?
- 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
How does drainage density affect the hydrograph?
- 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
How do you find drainage denisty?
total length of all rivers and streams in a drainage basin divided by the total area of the drainage basin
How does porosity and permeability of soils affect the hydrograph?
- particles with larger pores will allow water to infiltrate faster, as more space for water to enter
- porosity of clay soils is low 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
What is the difference between infiltration and percolation?
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
How does rock type affect the shape of a hydrograph?
- 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
How do slopes affect a hydrograph?
- 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
How does vegetation affect a hydrograph?
- 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
How does land use affect shape of a hydrograph?
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
Explain how recurrence intervals can be used in flood prediction an prevention (6)
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:
- 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
Is climate the most significant factor which influences discharge within a drainage basin
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
