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.

River discharge

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

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:

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.

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.

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:

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.

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.

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.