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.
How this is tested: Paper 1 Option A opens with a data-response on a storm hydrograph or discharge graph. You Estimate or State a value (peak discharge, peak rainfall, a frequency) or work out a lag time / time gap by reading the time axis carefully. Always quote the units.
[Diagram: geo-storm-hydrograph] - Available in full study mode
[Diagram: geo-storm-hydrograph] - Available in full study mode
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.
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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.
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.
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.