7.1.5

Storm Hydrographs

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Hydrographs

Hydrographs show how river discharges changes over time at a particular point in a river. Increased runoff leads to a reduced lag time and greater discharge, producing a steep hydrograph because more water gets into the river channel quicker.

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River discharge

  • River discharge is the volume of water flowing in a river each second and is measured in cumecs (m3/s).
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Hydrographs

  • Hydrographs show how river discharge changes over time at a particular point of a river. Hydrographs have the following features:
    • Peak discharge: maximum discharge in the period of time.
    • Peak rainfall: maximum rainfall in the period of time.
    • Lag time: the interval between peak rainfall and discharge.
    • Rising limb: when the discharge is rising.
    • Falling limb: when the discharge is falling.

Physical Factors Affecting Storm Hydrographs

The following physical factors affect the shape of a hydrograph:

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Size, shape and drainage density

  • The larger the catchment area of the river basin, the higher the peak discharge but the longer the lag time.
  • The more circular the catchment area shape, the shorter the lag time.
  • Denser drainage networks transport water more efficiently, increasing the flow and peak discharge.
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Rock type and soil

  • If the rock and soil is permeable (i.e. Limestone), precipitation will be able to infiltrate and percolate which lengthens the lag time and reduces the peak discharge.
  • Impermeable rocks and soils (i.e. clay) stop precipitation infiltrating so surface runoff increases. Lag time is reduced and peak discharge is increased in these circumstances.
  • If the soil is fully saturated or frozen by antecedent conditions, there will not be any infiltration leading to increased surface runoff, increasing the peak discharge.
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Relief and vegetation

  • The steeper the catchment of the river basin, the quicker the water reaches the river which increases peak discharge and reduces lag time.
  • River catchments with high levels of vegetation will see high interception and evapotranspiration.
  • Tree roots help to promote infiltration and results in catchments with more vegetation having a lower surface runoff, and reduced peak discharge.

Human Factors Affecting Storm Hydrographs

The following are human factors that influence the peak discharge and the lag time of storm hydrographs:

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Land use

  • Areas that remain forested or as their natural habitat will see reduced peak discharge due to an increase in infiltration and interception of precipitation by vegetation.
    • There will be less interception in areas that have been deforested. This will increase the peak discharge.
  • Ploughing of agricultural land increases surface runoff. This causes lag time to reduce and peak discharge to increase.
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Urbanisation

  • Urbanisation leads to an increase in the number of impermeable surfaces (tarmac, concrete and tiles).
  • Impermeable surfaces lead to a decrease in infiltration, an increase in surface runoff and ultimately a reduction in the lag time. A shorter lag time means a greater peak discharge.
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Water management

  • Dams and reservoirs can be used to regulate the flow of the river by storing water and choosing when to release it downstream.
  • Abstraction of aquifers lowers groundwater levels and increases percolation and infiltration when rainfall happens.

Jump to other topics

1Tectonic Processes & Hazards

2Option 2A: Glaciated Landscapes & Change

3Option 2B: Coastal Landscapes & Change

4Globalisation

5Option 4A: Regenerating Places

6Option 4B: Diverse Places

7The Water Cycle & Water Insecurity (A2 only)

8The Carbon Cycle & Energy Security (A2 only)

9Superpowers (A2 only)

10Option 8A: Health & Human Rights (A2 only)

11Option 8B: Migration & Identity (A2 only)

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