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This lesson covers how river discharge responds to precipitation events (storm hydrographs) and how it varies over the course of a year (river regimes). It addresses Edexcel A-Level Geography (9GE0) Paper 1, Topic 5, Enquiry Question 1, focusing on temporal variations in the water cycle at the drainage basin scale.
A storm hydrograph (also called a flood hydrograph) is a graph showing how the discharge of a river at a particular gauging station changes over time in response to a rainfall event (storm).
| Component | Definition |
|---|---|
| Discharge (Q) | Volume of water passing the gauging station per second (m³/s, cumecs) — plotted on the y-axis |
| Rainfall | Usually shown as a bar chart at the top of the graph (mm/hr) |
| Baseflow | The normal (background) discharge of the river, sustained by groundwater seepage. Shown as a gently rising line before the storm and a separation line during the event |
| Rising limb | The section of the hydrograph where discharge is increasing — water from the storm is reaching the channel |
| Peak discharge | The maximum discharge reached during the event — the top of the hydrograph curve |
| Peak rainfall | The period of highest rainfall intensity on the bar chart |
| Lag time | The time difference between peak rainfall and peak discharge — a crucial measure of how quickly the basin responds |
| Falling limb (recession limb) | The section of the hydrograph where discharge is decreasing — the storm has passed and water is draining from the basin |
| Storm flow (quickflow) | The discharge above baseflow that is directly attributable to the storm event — includes overland flow, throughflow and direct channel precipitation |
| Approach segment | The discharge before the storm event arrives |
The shape of a hydrograph tells us a great deal about the drainage basin characteristics and the nature of the rainfall event.
| Hydrograph Shape | Characteristics | Typical Causes |
|---|---|---|
| Flashy (steep, peaked) | Short lag time, high peak discharge, steep rising and falling limbs | Impermeable surfaces, steep slopes, urbanisation, deforestation, intense rainfall, circular basin shape, high drainage density |
| Subdued (flat, broad) | Long lag time, low peak discharge, gentle rising and falling limbs | Permeable geology, deep soils, dense vegetation, gentle slopes, elongated basin shape, low drainage density, low-intensity rainfall |
Exam Tip: When describing a storm hydrograph, always use specific terminology: lag time (not "delay"), rising limb (not "going up"), peak discharge (not "highest point"), falling limb (not "going down"). Using precise geographical vocabulary gains marks.
The shape of a storm hydrograph is determined by the interaction of rainfall characteristics and basin characteristics.
| Factor | Effect on Hydrograph |
|---|---|
| Intensity | High-intensity rainfall → infiltration capacity exceeded → more overland flow → shorter lag time, higher peak |
| Duration | Prolonged rainfall → soil becomes saturated → increased overland flow → sustained high discharge |
| Type | Rain produces immediate response; snow accumulates and produces delayed response on melting |
| Antecedent conditions | Rainfall on already-wet ground → reduced infiltration → shorter lag time, higher peak |
| Spatial distribution | Rainfall concentrated in the lower basin reaches the channel faster than rainfall in the upper basin |
| Factor | Short Lag Time / High Peak | Long Lag Time / Low Peak |
|---|---|---|
| Geology | Impermeable (clay, granite) | Permeable (chalk, sandstone) |
| Soil type | Clay soils, thin soils | Sandy soils, deep soils |
| Slope | Steep gradients | Gentle gradients |
| Vegetation | Sparse or absent | Dense forest cover |
| Basin shape | Circular (concentrated) | Elongated (dispersed) |
| Basin size | Small | Large |
| Drainage density | High (many channels per km²) | Low |
| Urbanisation | High % impermeable cover | Low % impermeable cover |
| Land use | Arable farmland, deforested | Forested, wetland |
Urbanisation has a particularly dramatic effect on storm hydrographs. Studies of paired basins (one urbanised, one rural) consistently show:
Exam Tip: A common exam question asks you to compare two hydrographs. Structure your answer systematically: compare lag time, peak discharge, rising limb gradient, falling limb gradient and baseflow. Then explain the differences using basin and rainfall characteristics.
A river regime is the expected pattern of discharge through the year. It is shown as a graph of mean monthly discharge over a 12-month period (typically averaged over 20–30 years of data).
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