Edexcel GCSE Geography A (1GA0) — The Physical Environment
Every time a heavy rainstorm hits, a river's water level doesn't rise instantly. A storm hydrograph is a composite graph showing the relationship between rainfall and river discharge over a short period, usually 24 to 48 hours following a specific storm event. It does NOT show the average flow over a full year, which is instead known as a river regime.
These graphs use two vertical y-axes to display different datasets simultaneously. The primary (left) y-axis measures discharge in cumecs (, or cubic metres per second). The secondary (right) y-axis measures rainfall (or precipitation) in millimetres (mm), while time is plotted on the x-axis, typically in hours or days.
Creating a hydrograph is like assembling two different graphs onto a single piece of paper. When asked to complete or draw a hydrograph in an exam, you must follow sequential steps to ensure clarity and accuracy.
First, set up your axes by labeling the x-axis "Time (hours)", the primary y-axis "Discharge (cumecs)", and the secondary y-axis "Rainfall (mm)". Then, use the data to plot rainfall as a bar chart aligned with the correct time intervals. Crucially, rainfall is NEVER plotted as a continuous line graph.
Next, plot the discharge data points using the discharge axis scale and connect them with a smooth, continuous line. Draw a dotted or dashed horizontal line beneath the main curve to represent the base flow, which is the normal day-to-day discharge supplied by groundwater, separate from the extra storm flow.
Finally, annotate the distinct features of the curve. Label the rising limb, which is the upward-sloping period of the line graph showing increasing discharge as surface runoff reaches the channel. Label the falling limb (sometimes called the recessional limb), which is the downward-sloping period where discharge decreases back to normal.
The most critical measurement on a hydrograph is the delay between the heaviest rain and the highest river level. To accurately read a hydrograph, you must identify its peak values. Peak rainfall is the highest intensity of rainfall recorded, visually represented by the tallest bar on the chart. Peak discharge is the maximum point of river flow, shown by the highest peak of the line graph.
The lag time is the time interval between peak rainfall and peak discharge. When annotating a graph, draw a clear horizontal, double-headed arrow exactly between the midpoint of the peak rainfall bar and the peak of the discharge curve. If a river exceeds its bankfull discharge at this peak, it will overflow its banks and cause flooding.
How exactly do geographers calculate the delay before a flood? Calculating lag time requires careful reading of the x-axis. The time for peak rainfall must always be taken from the midpoint of the highest rainfall bar, not the start or end of the rain.
Worked Example:
Calculate the lag time if the tallest rainfall bar spans from 14:00 to 15:00, and the peak of the discharge curve occurs at 19:30.
Step 1: Identify the time of peak discharge.
Step 2: Identify the midpoint of the peak rainfall bar.
Step 3: Substitute into the formula and calculate the difference.
Not all river catchments react to storms in the same way. A flashy hydrograph has a steep rising limb, high peak discharge, and a short lag time. These are typical of urban areas with impermeable artificial surfaces or steep-sided valleys where water reaches the river rapidly.
In contrast, a subdued hydrograph (or gentle hydrograph) has a gentle rising limb, lower peak discharge, and a longer lag time. These occur in forested or rural areas where vegetation increases interception, or over permeable rock, slowing down the water's journey to the river.
Students frequently lose marks by calculating lag time from the start of the rain. Examiners strictly require you to measure from the midpoint of the peak rainfall bar.
When defining lag time in an exam, always specify that it is the time between PEAK rainfall and PEAK discharge; answers that simply say 'the time between rain and discharge' will score zero.
If asked to 'complete' a hydrograph, ensure your rainfall bars and discharge line are visually distinct and accurately scaled against their specific, separate y-axes.
In calculation questions for lag time, always display your working clearly (e.g., 10 hours - 3 hours = 7 hours) to guarantee full marks even if you make a minor mental arithmetic error.
If asked to compare two hydrographs (e.g., Urban vs. Rural), use comparative connectives like 'whereas' or 'in contrast' to explicitly contrast their lag times, peak discharges, and limb steepness.
Storm hydrograph
A composite graph showing the relationship between rainfall and river discharge over a short period (usually 24–48 hours) following a specific storm event.
Discharge
The volume of water flowing through a river channel at any given point per second.
Cumecs
The unit of measurement for river discharge, standing for cubic metres per second ().
Lag time
The time interval or delay between peak rainfall and peak discharge.
Rising limb
The upward-sloping period of the line graph showing increasing river discharge as rainwater and surface runoff reach the channel.
Falling limb
The downward-sloping period of the line graph where discharge decreases and the river returns to its normal base flow.
Peak rainfall
The highest amount or intensity of rainfall recorded during a storm event, shown by the tallest bar on the chart.
Peak discharge
The point of maximum river flow during or after a storm, shown by the highest point of the line graph.
Base flow
The normal day-to-day discharge of a river, primarily supplied by groundwater seeping into the channel.
Bankfull discharge
The maximum discharge or water level a river can hold before it overflows its banks and causes flooding.
Storm flow
The additional river discharge resulting directly from a specific rainfall event, consisting of surface runoff and throughflow.
Flashy hydrograph
A storm hydrograph characterised by a steep rising limb, high peak discharge, and a short lag time.
Subdued hydrograph
A storm hydrograph characterised by a gentle rising limb, lower peak discharge, and a longer lag time.
Put your knowledge into practice — try past paper questions for Geography A
Storm hydrograph
A composite graph showing the relationship between rainfall and river discharge over a short period (usually 24–48 hours) following a specific storm event.
Discharge
The volume of water flowing through a river channel at any given point per second.
Cumecs
The unit of measurement for river discharge, standing for cubic metres per second ().
Lag time
The time interval or delay between peak rainfall and peak discharge.
Rising limb
The upward-sloping period of the line graph showing increasing river discharge as rainwater and surface runoff reach the channel.
Falling limb
The downward-sloping period of the line graph where discharge decreases and the river returns to its normal base flow.
Peak rainfall
The highest amount or intensity of rainfall recorded during a storm event, shown by the tallest bar on the chart.
Peak discharge
The point of maximum river flow during or after a storm, shown by the highest point of the line graph.
Base flow
The normal day-to-day discharge of a river, primarily supplied by groundwater seeping into the channel.
Bankfull discharge
The maximum discharge or water level a river can hold before it overflows its banks and causes flooding.
Storm flow
The additional river discharge resulting directly from a specific rainfall event, consisting of surface runoff and throughflow.
Flashy hydrograph
A storm hydrograph characterised by a steep rising limb, high peak discharge, and a short lag time.
Subdued hydrograph
A storm hydrograph characterised by a gentle rising limb, lower peak discharge, and a longer lag time.
