The Storm Hydrograph

The storm hydrograph is one of the most important analytical tools in hydrology. It shows how a river responds to a single rainfall event, revealing the speed and magnitude of the catchment's response. Interpreting hydrographs and explaining the factors that control their shape is a core skill for AQA A-Level Geography Paper 1.

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What Is a Storm Hydrograph?

Key Definition: A storm hydrograph (flood hydrograph) is a graph showing river discharge (m³/s) on the y-axis against time on the x-axis, illustrating the change in a river's flow in response to a precipitation event.

Key Components

Component	Definition
Peak discharge (Qp)	The maximum flow reached during the storm event, measured in cumecs (m³/s)
Lag time	The delay between peak rainfall intensity and peak discharge — a critical indicator of catchment response speed
Rising limb	The ascending portion of the hydrograph as discharge increases following rainfall
Falling limb (recession limb)	The descending portion as discharge decreases back towards baseflow
Baseflow	The normal background discharge sustained by slow groundwater seepage
Stormflow (quickflow)	The discharge above baseflow attributable directly to the storm event
Bankfull discharge	The flow at which the channel is completely full — any increase causes overbank flooding
Time to peak	The duration from the start of rainfall to peak discharge
Storm run-off	The total volume of water produced by the storm, represented by the area under the hydrograph curve above baseflow

graph TD
    subgraph "Storm Hydrograph Components"
        R["Rainfall bar chart
(plotted above x-axis)"] --> RL["Rising Limb
(discharge increases)"]
        RL --> PD["Peak Discharge
(maximum flow)"]
        PD --> FL["Falling Limb / Recession
(discharge decreases)"]
        FL --> BF["Return to Baseflow"]
    end
    subgraph "Key Measurements"
        LT["Lag Time = time between
peak rainfall and peak discharge"]
        SF["Stormflow = discharge
above baseflow"]
    end

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Flashy vs Subdued Hydrographs

The shape of a storm hydrograph tells us a great deal about the catchment characteristics and the dominant pathways by which water reaches the river channel.

Flashy (Peaky) Hydrograph

A flashy hydrograph has:

• Short lag time (often < 6 hours)
• Steep rising limb
• High peak discharge relative to baseflow
• Steep falling limb
• High flood risk

This shape indicates that water reaches the channel rapidly, predominantly via surface runoff and storm drains rather than throughflow or baseflow.

Subdued (Flat) Hydrograph

A subdued hydrograph has:

• Long lag time (often > 24 hours)
• Gentle rising limb
• Low peak discharge relative to baseflow
• Gentle, extended falling limb
• Low flood risk

This shape indicates that water takes longer to reach the channel, moving predominantly via throughflow, percolation, and baseflow through permeable soils and rocks.

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Factors Affecting Hydrograph Shape

Physical Factors

Factor	Effect on Hydrograph	Explanation
Basin area	Small basin → flashy	Water travels shorter distances to the channel
Basin shape	Circular → flashy; Elongated → subdued	Circular basins: tributaries converge simultaneously. Elongated: staggered arrival of tributary flows
Drainage density	High → flashy	More channel length per km² means water enters channels more quickly
Slope angle	Steep → flashy	Gravity accelerates surface runoff; less time for infiltration
Rock type	Impermeable (granite, clay) → flashy; Permeable (chalk, limestone) → subdued	Impermeable rocks prevent infiltration, forcing water over the surface. Permeable rocks absorb and slowly release water
Soil type	Thin, clay-rich → flashy; Deep, sandy → subdued	Clay has low infiltration capacity; sand allows rapid percolation
Channel characteristics	Smooth, straight, steep-gradient channels → flashy	Water moves more efficiently through the channel network

Vegetation and Land Use

Factor	Flashy Hydrograph	Subdued Hydrograph
Vegetation cover	Sparse or absent — minimal interception	Dense forest canopy — high interception (25–45% of rainfall, Calder, 1990)
Urban development	Impermeable surfaces (40–80% coverage in cities)	Rural land with high infiltration capacity
Agricultural land	Compacted plough pans; field drains accelerate flow	Permanent grassland or woodland with good soil structure
Deforestation	Removes interception, transpiration, root uptake	Afforestation restores these processes

Meteorological Factors