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Spec mapping (AQA 7037): Paper 1, §3.1.1 — Runoff variation and the flood hydrograph; the physical and human factors that affect the storm hydrograph. Synoptic links: directly underpins river-flood hazard assessment and management (§3.1.5 Hazards) and connects to the drainage-basin water balance (Lesson 3) and to flood-management strategy (Lesson 10). AOs: AO1 (hydrograph anatomy and controls), AO2 (explaining a catchment's response from its characteristics), AO3 (measuring lag time, peak discharge, and runoff from a graph; flood-frequency calculation).
Spec note on skills: the hydrograph is the topic's premier AO3 resource, so this lesson deliberately emphasises quantitative manipulation — extracting lag time, peak discharge, stormflow, runoff volume, and return periods — alongside the AO1 explanation of controls. Practising the calculations until they are automatic frees up thinking time in the exam for the harder explanatory and evaluative demands.
The storm hydrograph is the single most important analytical tool in catchment hydrology. It records how a river responds to an individual rainfall event, distilling the entire cascading system of Lesson 3 into one curve that reveals both the speed and the magnitude of the basin's response. In effect, the hydrograph is the output signal of the drainage-basin system: everything covered in Lesson 3 — the partitioning of rainfall between fast overland flow and slow subsurface routes — is encoded in its shape, so learning to read a hydrograph is learning to read the behaviour of the whole basin at a glance. Reading hydrographs, extracting quantities from them, and explaining the factors that govern their shape are core AO3/AO1 skills for AQA Paper 1, and the hydrograph recurs throughout the flood-hazard and management material.
Key Definition: A storm hydrograph (flood hydrograph) plots river discharge (m³ s⁻¹, cumecs) on the y-axis against time on the x-axis for a single precipitation event, usually with the causative rainfall shown as a bar chart (hyetograph) along the top. It shows how a river's flow rises above and then returns to its normal baseflow in response to that storm.
| Component | Definition |
|---|---|
| Peak discharge (Q_p) | The maximum flow reached during the event (cumecs). The flood's severity scales with it |
| Lag time | The interval between peak rainfall and peak discharge — the headline measure of response speed |
| Rising limb | The ascending part of the curve as discharge increases after rainfall begins to arrive |
| Falling (recession) limb | The descending part as discharge declines back towards baseflow |
| Baseflow | The background discharge sustained by slow groundwater seepage between storms |
| Stormflow (quickflow) | The discharge above baseflow attributable directly to the storm (overland flow + rapid throughflow) |
| Bankfull discharge | The flow that exactly fills the channel; any excess spills onto the floodplain (overbank flooding) |
| Approach segment | The pre-storm baseflow level before the rising limb begins |
| Storm runoff | Total volume produced by the storm = the area under the curve and above baseflow |
graph TD
subgraph hg["Storm Hydrograph Sequence"]
R["Rainfall (hyetograph bars)"] --> RL["Rising limb: discharge increases"]
RL --> PD["Peak discharge Qp"]
PD --> FL["Recession (falling) limb"]
FL --> BF["Return to baseflow"]
end
subgraph meas["Key Measurements"]
LT["Lag time = peak rainfall to peak discharge"]
SF["Stormflow = discharge above baseflow"]
end
The steepness of the rising limb records how fast water arrives; the height of the peak records how much arrives at once; the gentleness of the recession limb records how long stored water continues to drain (a long, gentle recession indicates a large slow-release groundwater store sustaining the river).
Lag time arises because water does not reach the channel instantaneously — it must travel through the cascade of stores described in Lesson 3. The fastest water (overland flow near the channel) arrives within minutes; water that infiltrates, moves as throughflow, and emerges as baseflow may take days. The hydrograph peak occurs when the combined arrival of water from all pathways is greatest, which is necessarily after peak rainfall because the slower pathways are still delivering water from earlier in the storm. A basin dominated by fast overland flow has a short lag (the contributions bunch together near the start); a basin dominated by slow subsurface flow has a long lag (the contributions spread out over days). Lag time is thus a direct readout of the balance of pathways — which is why it is the headline diagnostic of catchment behaviour.
The shape of a hydrograph is a fingerprint of the catchment and of the dominant pathways by which water reaches the channel (Lesson 3).
A flashy hydrograph has a short lag time (often < 6 hours), a steep rising limb, a high peak discharge relative to baseflow, a steep recession limb, and high flood risk. It signals that water is reaching the channel rapidly, predominantly via overland flow and (in towns) storm drains, rather than slow throughflow and baseflow. Flashy responses characterise small, steep, circular, impermeable, sparsely-vegetated or heavily urbanised catchments — Boscastle being the textbook example.
A subdued hydrograph has a long lag time (often > 24 hours), a gentle rising limb, a low peak relative to a high baseflow, an extended gentle recession, and low flood risk. It signals that water takes slow subsurface routes — infiltration → throughflow → percolation → baseflow — through permeable rocks and soils. Subdued responses characterise large, gently-sloping, elongated, permeable (chalk/limestone), well-vegetated catchments.
It is worth stressing that "flashy" and "subdued" are ends of a continuum, not two boxes — most real hydrographs sit somewhere between, and the same basin can produce a flashier response in winter (saturated ground) than in summer (dry ground able to absorb the first rain). The shape also depends on the scale of the storm: a small storm onto a permeable basin may be entirely absorbed (an almost flat hydrograph), while an extreme storm onto the same basin can overwhelm its infiltration and storage to produce a surprisingly peaky response. This is why hydrograph interpretation is never a matter of simply labelling a basin "flashy" or "subdued"; it requires reasoning about how this basin will partition this storm under these antecedent conditions — exactly the integrated thinking the higher mark bands reward.
| Factor | Effect | Explanation |
|---|---|---|
| Basin area | Small → flashy | Shorter travel distances; water concentrates at the gauge quickly |
| Basin shape | Circular → flashy; elongated → subdued | Circular: all tributaries deliver to the outlet simultaneously. Elongated: staggered arrivals spread the peak |
| Drainage density | High → flashy | More channel length per km² collects water faster |
| Slope/relief | Steep → flashy | Gravity accelerates overland flow; less time to infiltrate |
| Rock type | Impermeable (granite, clay, slate) → flashy; permeable (chalk, limestone) → subdued | Impermeable rock forces water over the surface; permeable rock stores and slowly releases it as baseflow |
| Soil type | Thin clay → flashy; deep sand → subdued | Clay has low infiltration capacity (~5 mm hr⁻¹); sand admits rapid infiltration (~25 mm hr⁻¹) |
| Channel efficiency | Smooth, straight, steep channels → flashy | Water is conveyed to the gauge more efficiently |
| Factor | Pushes towards flashy | Pushes towards subdued |
|---|---|---|
| Vegetation cover | Sparse/absent — minimal interception | Dense forest canopy — high interception (25–45% of rainfall; Calder, 1990) |
| Urban development | Impermeable surfaces (40–80% of city area) + storm drains | Rural land with high infiltration capacity |
| Agriculture | Compacted plough pans; field underdrains accelerate flow | Permanent grassland/woodland with good soil structure |
| Deforestation vs afforestation | Removes interception, transpiration, root macropores | Afforestation restores all three, lengthening lag time |
| Factor | Effect |
|---|---|
| Rainfall intensity | High intensity exceeds infiltration capacity → Hortonian overland flow → flashy |
| Rainfall duration | Prolonged rain saturates soil → saturation-excess overland flow → rising flashiness through the event |
| Rainfall type | Convective (intense, localised) → flashy; frontal (steady, widespread) → more subdued |
| Antecedent conditions | Saturated ground → flashy; dry ground → subdued (soil absorbs the first rain) |
| Snowmelt | Rapid spring thaw, especially rain-on-snow over frozen ground, produces very high discharges |
| Temperature | Frozen (impermeable) ground promotes overland flow regardless of soil type |
G.E. Hollis (1975) produced a classic quantification of urbanisation's effect on flood peaks, with a counter-intuitive and exam-friendly finding:
The management implication is subtle: urbanisation disproportionately increases the frequency of nuisance flooding, not merely the severity of catastrophic events. It is the everyday 1-to-5-year flood that urban development most inflates.
Urban development alters almost every term in the drainage-basin system at once, and every change pushes the hydrograph towards flashiness:
The combined effect is a shorter lag time, a steeper rising limb, a higher peak, and a faster recession than the same rainfall would produce over rural land. This is why retrofitting Sustainable Drainage Systems (SuDS) — permeable paving, green roofs, swales, rain gardens, and detention basins — is now central to urban flood management: each device deliberately re-introduces a store or a slow pathway that conventional development removed, flattening the urban hydrograph back towards its rural form. The same logic, applied at catchment scale, underlies Natural Flood Management (Lesson 10).
On 16 August 2004 a flash flood devastated the village of Boscastle, Cornwall — the definitive UK example of an extreme flashy hydrograph.
Exam Tip: Boscastle is a high-yield, versatile case study — usable for hydrograph shape (extremely flashy), drainage-basin characteristics, flood-risk factors, and management. Always deploy the precise figures: ~185 mm in 5 hours, ~140 m³ s⁻¹ peak, < 2 hr lag. Contrast it with the more subdued response a permeable, vegetated, low-relief basin would have shown under the same rainfall.
It is important not to confuse the storm hydrograph (the response to a single event, over hours to days) with the river regime (the annual pattern of discharge through the year, reflecting climate and seasonal water balance). They answer different questions at different timescales.
| Storm hydrograph | River regime (annual hydrograph) | |
|---|---|---|
| Timescale | Hours to days | A whole year |
| Driven by | A single rainfall/snowmelt event | Seasonal climate: precipitation, temperature, evapotranspiration, snowmelt |
| Shows | Lag time, peak, limbs of one flood | Months of high and low flow |
| Example pattern | The flashy Boscastle response | A UK river: winter high flow (surplus), summer low flow (deficit) |
A simple regime has one peak per year — for instance, a UK lowland river peaking in winter when precipitation is high and evapotranspiration low, and falling to a summer minimum sustained by baseflow. A complex regime has several peaks reflecting different inputs — for example, a snowmelt peak in late spring superimposed on a rainfall-driven winter peak in an alpine or upland catchment. The regime is essentially the seasonal expression of the water balance from Lesson 3: the surplus season produces high flow, the deficit season low flow. Both the storm hydrograph and the regime matter for management — the regime tells you when reliable supply and seasonal flooding occur, while the storm hydrograph tells you how the river will respond to the next individual storm.
To quantify how much of the flood was direct stormflow, hydrologists separate it from baseflow:
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