The big idea: River discharge is the volume of water passing a point each second, measured in cumecs (cubic metres per second, m³/s).
The storm hydrograph
- 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.
A core Option A skill: 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 core Option A content.
More often in recent papers: a [10] essay
- what controls the curve's shape
- how human activity changes it
Also: a data-response figure
- read a peak discharge, peak rainfall or lag time off it
Either way: Know the shape and always quote the units (cumecs, mm, hours).
One storm, two basins. River X (urban / impermeable / steep) peaks fast and high (point A); River Y (rural / permeable / vegetated) peaks slowly and low (point B).
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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 (flashy)
- raises the peak
- shortens the lag time
Anything that slows water down: storage, infiltration, interception (subdued)
- lowers the peak
- lengthens the lag
Factors that make a basin flashy or 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.
Seasonality: why the same river behaves differently through the year
- Wet season — soils stay saturated, so storms produce high peaks and short lag times; in monsoon regions such as South Asia, discharge rises many times over between the dry and wet seasons.
- Snowmelt — in spring, melting snow adds water to rain, giving the largest floods of the year on rivers that drain mountains.
- Frozen ground — in cold winters, frozen soil stops infiltration, so rain or melt runs straight off.
- Summer — high evaporation and plant growth take up water, so peaks are lower and base flow falls.
Discharge and hydraulic radius downstream: Hydraulic radius measures how efficient a channel is — the cross-section area divided by the wetted perimeter.
Downstream change (the Bradshaw model)
- Both discharge and hydraulic radius increase downstream.
- Tributaries add water.
- The channel gets larger and smoother, so 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. There are two recurring versions.
Physical factors shape a hydrograph
- relief, geology, soil, vegetation, rainfall, basin shape
Human activities alter it
- urbanisation, land-use change, channel work, dams
Top band needs
- Accurate terms.
- Two or more developed factors with examples.
- A weighing of their relative importance / interactions.
- A clear conclusion.
Examine how physical factors influence the shape of a river's storm hydrograph.
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