Water and Carbon Cycles
Runoff Variation and the Flood Hydrograph
Introduction
Runoff is the flow of water over or through the ground towards rivers and streams. Its rate and timing vary between drainage basins and individual storm events, depending on physical and human factors.
A flood hydrograph (also called a storm hydrograph) shows how a river’s discharge changes over time following a rainfall event.
Hydrographs help geographers understand how quickly precipitation is transferred to the river channel, which is vital for flood forecasting and management.
1. River Regimes vs Storm Hydrographs
River regimes show seasonal discharge changes, while storm hydrographs illustrate the immediate response of a drainage basin to rainfall.
| Term | Definition | Timescale |
|---|---|---|
| River Regime | The annual pattern of discharge in a river, showing seasonal variations over a year (e.g. high flow in winter, low in summer). | Long-term (months to years) |
| Storm or Flood Hydrograph | A short-term graph showing how river discharge responds to a single rainfall event or storm. | Short-term (hours to days) |
2. Components of a Flood Hydrograph
A flood hydrograph plots river discharge (m³/s) against time (hours or days) after a rainfall event. It usually includes two graphs: rainfall (bar chart) and discharge (line graph).
Flood hydrograph
| Component | Definition |
|---|---|
| Peak Rainfall | The time of highest rainfall intensity. |
| Peak Discharge (Flood Peak) | The maximum flow in the river after the rainfall event. |
| Lag Time | The delay between peak rainfall and peak discharge — indicates how quickly water reaches the channel. |
| Rising Limb | The upward slope showing increasing discharge as rainfall enters the river. |
| Falling Limb (Recession Limb) | The downward slope as surface runoff and throughflow decrease. |
| Baseflow | The normal river flow supplied by groundwater. |
| Stormflow | The additional flow from precipitation reaching the channel as surface and subsurface flow. |
3. Factors Influencing Runoff and Hydrograph Shape
Runoff response differs between drainage basins because of a combination of physical and human factors.
| Factor | Influence on Runoff | Effect on Hydrograph |
|---|---|---|
| Basin Size | Small basins transfer water quickly; large basins respond more slowly. | Small → short lag time; large → longer lag time. |
| Basin Shape | Circular basins concentrate flow; elongated basins spread runoff over time. | Circular → flashy; elongated → subdued. |
| Relief (Gradient) | Steep slopes increase surface runoff; gentle slopes encourage infiltration. | Steep → flashy; gentle → subdued. |
| Soil Type | Impermeable soils (clay) reduce infiltration; permeable soils (sandy) absorb water. | Clay → flashy; chalk → subdued. |
| Rock Type (Geology) | Impermeable rocks promote surface flow; permeable rocks encourage groundwater flow. | Impermeable → flashy; permeable → subdued. |
| Vegetation | Dense vegetation intercepts and delays runoff. | Forested → longer lag time. |
| Land Use | Urbanisation creates impermeable surfaces and drains. | Urban → short lag time, high peak. |
| Antecedent Moisture | Wet or frozen soils prevent infiltration. | Saturated → flashy response. |
| Precipitation Intensity & Duration | Heavy, prolonged rain increases surface runoff. | Intense → steep rising limb. |
| Temperature | Snowmelt or frozen ground can increase runoff rates. | Rapid melt → flashy. |
| Drainage Density | High density = fast runoff; low density = slower flow. | High → short lag time. |
4. Flashy vs Subdued Hydrographs
The shape of a river’s flood hydrograph reflects how quickly precipitation is transferred to the river channel.
A flashy hydrograph shows a rapid response to rainfall. Discharge rises steeply to a high peak before falling quickly.
A flashy hydrograph
A subdued hydrograph, by contrast, indicates a slow response. Discharge increases and declines gradually, with a lower peak and longer lag time. These differences are controlled by the physical characteristics of the drainage basin, weather conditions, and human influences such as land use.
A subdued hydrograph
| Factor | Flashy Hydrograph | Subdued Hydrograph |
|---|---|---|
| Lag Time | Short | Long |
| Rising Limb | Steep | Gentle |
| Peak Discharge | High | Low |
| Falling Limb | Steep | Gradual |
| Basin Shape | Circular | Elongated |
| Relief | Steep | Gentle |
| Soil/Geology | Impermeable | Permeable |
| Urbanisation | High | Low |
| Antecedent Moisture | Saturated/frozen | Dry |
| Drainage Density | High | Low |
| Precipitation type | Intense rainfall | Light, prolonged rainfall |
| Vegetation | Sparse or cleared | Dense forest cover |
5. Human Influences on Runoff and Hydrographs
Human activities modify natural processes and alter the shape of flood hydrographs.
| Human Activity | Hydrological Impact | Resulting Hydrograph Effect |
|---|---|---|
| Urbanisation | Increases impermeable surfaces; storm drains speed runoff. | Steeper rising limb; shorter lag time. |
| Deforestation | Reduces interception and evapotranspiration. | Increases surface runoff; flashier response. |
| Agriculture | Soil compaction and drainage systems modify infiltration. | Higher runoff rates, especially after rainfall. |
| River Management | Dams delay peak discharge; channelisation speeds flow downstream. | Can reduce local flood peaks but transfer risk downstream. |
| Climate Change | Alters rainfall intensity and snowmelt timing. | Greater runoff variability; more frequent flash floods. |
6. Case Study Examples
Boscastle, Cornwall (2004) – Flashy Hydrograph
- Basin size: Very small catchment (~20 km²) on the River Valency.
- Rainfall: Over 60 mm in two hours on 16 August 2004.
- Physical factors: Steep slopes, impermeable slate geology, a narrow valley, and saturated ground from previous rainfall.
- Human factors: Urbanised valley floor and limited infiltration due to roads and buildings.
- Outcome: Discharge on the River Valency rose from 5 m³/s to 140 m³/s within 60 minutes, producing a lag time of less than one hour.
- Hydrograph characteristics: Extremely flashy — very steep rising limb, short lag time, high peak discharge, and rapid fall in flow after the storm.
River Eden, Cumbria (Storm Desmond, 2015) – Subdued Hydrograph
- Basin size: Large drainage basin (~2,400 km²).
- Rainfall: Prolonged rainfall associated with Storm Desmond in December 2015 (over 300 mm in 48 hours in upland areas).
- Physical factors: Mixed geology — permeable sandstone and limestone in lower catchment, impermeable volcanic rock in upper reaches.
- Human factors: Urbanisation in Carlisle increased surface runoff; deforestation in the uplands reduced interception.
- Outcome: Soils became saturated after weeks of heavy rainfall, producing very high river discharge.
- Hydrograph characteristics: Subdued — long lag time, gradual rising limb, moderate peak discharge, and an extended falling limb as groundwater and throughflow sustained flow for several days.
7. Hydrographs and Water Balance
Flood hydrographs link directly to the drainage basin water balance:
- During water surplus, saturated soils produce more surface runoff → flashy response.
- During water deficit, soils absorb rainfall → subdued hydrograph.
- Seasonal water balance variation helps explain differences in hydrograph behaviour across the year.
8. Interpreting Hydrographs in the Exam
Key Skills
- Identify and label lag time, rising limb, peak discharge, and baseflow.
- Explain why hydrograph shape varies using physical and human factors.
- Use specific examples (e.g. Boscastle 2004, River Eden 2015).
- Apply systems language: link inputs (precipitation) → flows (runoff) → outputs (discharge).
Common Command Words
- Describe: Identify key hydrograph features.
- Explain: Link features to causal factors (e.g. geology, land use).
- Compare: Contrast two hydrographs, using terminology such as flashy and subdued.
Summary
- Runoff variation depends on basin characteristics, precipitation type, soil moisture, and human activity.
- Flood hydrographs show the response of river discharge to rainfall.
- Flashy hydrographs have short lag times and high peaks; subdued hydrographs have gentle slopes and longer delays.
- Human modification (urbanisation, deforestation) can make hydrographs flashier and increase flood risk.
- Hydrograph analysis supports flood prediction, management, and risk reduction.
Exam Tip
When answering hydrograph questions:
- Label key features clearly (lag time, peak discharge, baseflow).
- Use case study data (e.g. “River Valency discharge rose from 5 to 140 m³/s”).
- Refer to both physical and human factors.
- For 6–9 mark questions, compare two contrasting hydrographs and explain the reasons for differences.