🌧 Peak rainfall
The time when the most rain falls. It is the tallest bar on the graph.
River Landscapes » Flood Hydrographs
What you'll learn this session
Study time: 30 minutes
AQA spec: 3.1.3.3
- What a flood hydrograph shows and how to read one
- The key parts: peak rainfall, peak discharge, lag time, rising limb, falling limb and base flow
- The difference between a flashy and a gentle hydrograph
- How to describe the link between precipitation and discharge
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Unlock This CourseWhat is a hydrograph?
After heavy rain the river rises to a peak, which a hydrograph shows
When it rains, a river does not rise at once. The water has to run off the land and flow into the channel first. A hydrograph is a graph that shows exactly how a river reacts to a rainstorm.
It shows two things on the same graph:
- Precipitation (rainfall) is drawn as bars. The bars show how much rain fell each hour.
- Discharge is drawn as a line. It shows how much water flows past a point in the river.
The bottom axis shows time, for example in hours or days. The left axis shows discharge. Discharge is measured in cumecs, which means cubic metres per second (m3/s). One cumec is one cubic metre of water passing a point every second.
Key terms:
- Hydrograph: a graph showing the relationship between precipitation and river discharge over time.
- Discharge: the volume of water passing a point in a river each second, measured in cumecs.
The parts of a hydrograph
A flood hydrograph has a clear pattern. Learn each part and what it means.
🌊 Peak discharge
The highest point on the discharge line. This is when the river is at its fullest and flooding is most likely.
⏱ Lag time
The time between peak rainfall and peak discharge. It is the delay while rain travels to the river.
📈 Rising limb
The part of the discharge line that goes up. The river is filling as water reaches the channel.
📉 Falling limb
The part of the discharge line that goes down. The river is draining back to normal as the storm water passes.
💧 Base flow
The normal level of the river fed by groundwater, before and after the storm. It is the line the discharge starts and ends on.
Key terms:
- Peak rainfall: the time of the heaviest rainfall.
- Peak discharge: the highest discharge reached in the storm.
- Lag time: the time between peak rainfall and peak discharge.
- Rising limb: the part of the hydrograph where discharge increases.
- Falling limb: the part of the hydrograph where discharge decreases.
- Base flow: the normal, everyday flow of a river, fed by groundwater.
Why is there a lag time?
Rain that lands far from the river takes time to get there. Some falls on leaves and is held for a while. Some soaks into the soil and moves slowly underground. Water that runs over the surface arrives first. Water that moves through the soil arrives later. This is why the peak of the river comes after the peak of the rain.
The rising limb is steep when lots of water arrives quickly. The falling limb is usually gentler, because water keeps draining in from the soil and the slopes for some time after the rain stops.
Flashy and gentle hydrographs
Steep, rocky land makes a flashy hydrograph with a quick peak
Hydrographs for different rivers, or for the same river in different conditions, can look very different. Geographers describe two main shapes.
⚡ Flashy hydrograph
A short lag time, a steep rising limb and a high peak discharge. Water reaches the river quickly and in large amounts. The risk of flooding is high.
🍃 Gentle hydrograph
A long lag time, a gentle rising limb and a low peak discharge. Water reaches the river slowly and in smaller amounts. The risk of flooding is lower.
Key terms:
- Flashy hydrograph: a hydrograph with a short lag time, steep rising limb and high peak discharge.
- Gentle hydrograph: a hydrograph with a long lag time, gentle rising limb and low peak discharge.
Remember, the flood risk factors from the last lesson decide which shape a river has. A hydrograph is only flashy or gentle when you compare it with another one.
How to read a hydrograph
Follow these steps in the exam.
- Check the axes and the units. Read the labels and the scale.
- Find the tallest rainfall bar. This is the peak rainfall. Note its time.
- Find the highest point of the discharge line. This is the peak discharge. Note its time and value.
- Work out the lag time by subtracting the time of peak rainfall from the time of peak discharge.
- Look at the steepness of the rising limb and the height of the peak. Decide if the hydrograph is flashy or gentle.
Worked example
A storm gives peak rainfall at 06:00. The river reaches its peak discharge of 80 cumecs at 09:00. Before the storm, the base flow was 10 cumecs.
Lag time = 09:00 - 06:00 = 3 hours.
The discharge rose by 80 - 10 = 70 cumecs above base flow.
Another river has a lag time of 10 hours and a peak of 20 cumecs. The river with the 3 hour lag and the 80 cumec peak is the flashier one.
Describing the link between rainfall and discharge
You may be asked to describe the relationship shown. Use the key terms and use numbers from the graph. For example: "Discharge rises sharply after the heaviest rainfall. The peak discharge is 80 cumecs, three hours after the peak rainfall. It then falls slowly back to the base flow."
Always give the idea (what happens) and the evidence (a figure from the graph).
Common mistakes
Students often say the lag time is the time between the start of the rain and the start of the rise. It is the time between peak rainfall and peak discharge. Another mistake is to mix up the limbs: the rising limb goes up, the falling limb goes down. Finally, do not forget the units. Discharge is in cumecs, not millimetres.
Exam-style question
The hydrograph shows a storm on a river. Peak rainfall was at 12:00. Peak discharge of 45 cumecs was at 14:00. Base flow was 5 cumecs.
(a) State the lag time. [1 mark]
(b) Explain why there is a lag time between peak rainfall and peak discharge. [2 marks]
(c) Describe the shape of a flashy hydrograph. [3 marks]
Model answer
(a) 2 hours (1).
(b) The rain takes time to reach the river (1). It has to flow over the land, drip off plants or move through the soil before it enters the channel (1).
(c) It has a short lag time (1), a steep rising limb (1) and a high peak discharge (1).
Exam tip
For (a), subtract the two times and write the unit (hours). A number with no unit can lose the mark.