The big idea: River discharge is the volume of water passing a point each second, measured in cumecs (cubic metres per second, m³/s).
A storm hydrograph is a graph that shows how a river's discharge responds to a storm over time. Rainfall is plotted as bars at the top; the discharge curve below shows the river rising and then falling.
Reading a hydrograph is a core Option A skill — you must find the peak discharge, the lag time, and describe the limbs.
Key terms on a storm hydrograph
- Discharge — the volume of water passing a point per second (cumecs, m³/s).
- Peak discharge — the highest discharge the river reaches after the storm.
- Lag time — the gap between peak rainfall and peak discharge (how fast the basin responds).
- Rising limb — the steep climb of the curve as run-off reaches the river.
- Falling limb (recession limb) — the gentler fall as the river drains back down.
- Base flow — the steady background discharge from groundwater between storms.
Flashy vs subdued: A short lag time + high peak = a flashy basin — water reaches the river fast (urban, impermeable, steep). It is flood-prone.
A long lag time + lower peak = a subdued basin — water reaches the river slowly (rural, permeable, vegetated, flat). It floods less.
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How this is tested: A storm hydrograph (rainfall bars above, a discharge curve below) is the figure you read most in Option A data-response. Expect a short Estimate or State part — read off a peak discharge, a peak rainfall or a lag time — then a longer Outline on what controls the curve's shape. The single mark is usually won or lost on the units (cumecs, mm, hours), so always quote them.
Storm hydrograph of a flashy basin: discharge peaks soon after the rain (short lag time) and reaches a high peak.
Interactive diagram
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Storm hydrograph of a subdued basin: the same storm gives a long lag time and a much lower, gentler peak.
Interactive diagram
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Lag time = the gap between the two peaks: Find the time of peak rainfall (the tallest bar) and the time of peak discharge (the top of the curve). The lag time is the gap between them — read both off the time axis and subtract.
Using the flashy-basin hydrograph above: (a) state the peak rainfall; (b) estimate the peak discharge; (c) estimate the lag time of the basin.
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The shape of a hydrograph depends on how fast rain reaches the river. Anything that speeds run-off raises the peak and shortens the lag time (flashy); anything that slows water down (storage, infiltration, interception) lowers the peak and lengthens the lag (subdued).
| Factor | Flashy (short lag, high peak) | Subdued (long lag, lower peak) |
|---|---|---|
| Relief (slope) | Steep — fast overland flow | Gentle — water moves slowly |
| Geology | Impermeable rock — no infiltration | Permeable rock — water soaks in |
| Soil | Thin / saturated — little storage | Deep, dry soil — stores water |
| Vegetation | Sparse — little interception | Dense forest — intercepts + uses water |
| Drainage density | High — many channels feed the river fast | Low — fewer channels |
| Land use | Urban — impermeable concrete + drains | Rural / farmland — more infiltration |
Why urbanisation makes a basin flashy
- Impermeable surfaces — concrete and tarmac stop water infiltrating, so it runs off fast.
- Drains and gutters — carry run-off straight to the river, cutting the lag time.
- Less vegetation — little interception, so more rain reaches the ground and the channel.
A small drainage basin saw its discharge rise after the land use changed. Outline two land-use changes that could raise discharge, and explain how each increases run-off.
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Always give the mechanism: Don't just name a factor — explain how it changes the run-off. Urbanisation → impermeable surfaces → less infiltration → faster run-off → shorter lag, higher peak.
Channels and dams change the shape too: Human structures alter a hydrograph directly. Channel straightening on parts of the lower Mississippi speeds flow and raises the downstream peak. A dam and reservoir (e.g. the Hoover Dam on the Colorado) does the opposite — it stores flood water and releases it slowly, lowering and delaying the peak.
Discharge and hydraulic radius downstream: Hydraulic radius measures how efficient a channel is — the cross-section area divided by the wetted perimeter. Both discharge and hydraulic radius increase downstream (the Bradshaw model): tributaries add water, the channel gets larger and smoother, friction falls, so the river carries more water more efficiently.
Outline how river discharge and hydraulic radius are related as you move downstream along a river.
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How this is tested — the [10] Examine essay: Paper 1 Option A ends with a 10-mark Examine essay, marked on markbands. Two recurring versions: how physical factors (relief, geology, soil, vegetation, rainfall, basin shape) shape a hydrograph, and how human activities (urbanisation, land-use change, channel work, dams) alter it.
Top band needs: accurate terms, two or more developed factors with examples, a weighing of their relative importance / interactions, and a clear conclusion.
Examine how physical factors influence the shape of a river's storm hydrograph.
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