IGCSE Geography / River environments

Storm hydrographs revision notes

Edexcel International GCSE Geography (4GE1), chapter 1: River environments, specification point 1.1c, 1.S1. Free lesson.

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A storm hydrograph shows rainfall during a storm and how river discharge responds afterwards. Reading its parts and lag time helps explain why some rivers rise quickly and are more likely to flood.

How to read a storm hydrograph

A storm hydrograph shows one storm and the river’s response. Time runs along the bottom. Rainfall is shown as bars, in millimetres, and discharge as points joined into a line. Discharge is the volume of water passing one point each second, measured in cumecs (cubic metres per second). The tallest rainfall bar is the peak rainfall. The highest point on the discharge line is the peak discharge. The rising limb is where discharge climbs towards its peak. The falling limb, also called the recession limb, is where discharge drops after the peak. Baseflow is water that…

How to find lag time

Lag time is the time between peak rainfall and peak discharge. It shows how long the river takes to reach its highest discharge after the heaviest rain. Both ends of the interval are peaks, not the start of the rain. Read the time of peak rainfall and the time of peak discharge from the graph or table. Subtract the time of peak rainfall from the time of peak discharge. Use the times, not the rainfall or discharge values.

What makes a hydrograph flashy or subdued

Two rivers can receive the same storm but respond differently. A flashy hydrograph has a short lag time, a high peak and steep limbs. A subdued hydrograph has a long lag time, a low peak and gentle limbs. Flashy rivers are more likely to flood. The key is how quickly rain reaches the river. Steep slopes encourage fast surface runoff. Gentle slopes give rain more time to soak in. Impermeable rock, such as clay or granite, does not let water pass through easily. Permeable rock, such as chalk, lets water sink in. Saturated or frozen soil cannot take in much more water, so rain runs off.…

How a town can change a storm hydrograph

Building a town adds impermeable surfaces and drains. Rain cannot infiltrate through tarmac or concrete, so more runs over the surface and drains carry it quickly to the river. The river responds faster, producing a shorter lag time and a higher peak discharge. For a three-mark explanation, link the surface to the movement of water, then to the hydrograph: impermeable surface, faster runoff to the river, shorter lag time and higher peak discharge.

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