Hydrographs & factors affecting their shape | AQA GCSE 9-1 Geography

What are hydrographs and what are the factors affecting their shape?
This is the forty-eighth video for the AQA GCSE 9-1 Geography course, and the fifteenth video of the Physical Landscapes topic.
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Video Script:
Hydrographs show the relationship between rainfall and river discharge over time.
Peak discharge is the maximum amount of water in the river, measured in cumecs.
Peak rainfall is the maximum amount of rainfall, measured in millimetres.
The lag time is the time between the peak rainfall and peak discharge. A shorter lag time means a greater risk of flooding.
The rising limb is where the discharge is increasing and the falling limb is where it is decreasing.
The base flow is the normal discharge of the river.
The shape of hydrographs are affected by various factors:
The characteristics of the drainage basin.
Smaller drainage basins have shorter lag times as the water has less distance to travel to the river, and circular drainage basins have the same effect as all of the water will reach the river channel around the same time.
A high drainage density also results in a shorter lag time, as water can reach the river channel more efficiently.
Steep relief reduces infiltration, increasing runoff. The steep gradients also mean runoff will travel faster and this can be exacerbated by agriculture if plough lines are vertical.
Impermeable rock types reduce the lag time as they don’t allow infiltration, increasing surface runoff. The same thing happens when prolonged rainfall causes soil to become saturated.
Heavy rainfall can cause a very short lag time and very high peak discharge.
Snowy areas have a long lag time as snow takes a long time to melt.
Urban areas have higher rates of surface runoff due to the abundance of impermeable surface. Their drainage systems also take water directly to the river channel. This results in a shorter lag time and higher peak discharge.
Finally, forested areas have longer lag time as interception is higher and roots increase infiltration, reducing surface runoff.
A hydrograph with a steep rising limb, high peak discharge and short lag time is a flashy hydrograph.
A gentle hydrograph instead has a low rising limb, low peak discharge and long lag time.
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GCSE Geography
17/05/2023
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