A storm hydrograph shows how a river’s discharge changes after rainfall. Rainfall is usually drawn as bars, and discharge as a line, so you must read the right feature for each question.
This lesson follows tracing a landform sequence and uses the discharge ideas from channel cross-sections. It is part of rivers and drainage.
What are the parts of a hydrograph?
- Baseflow: the steady flow before the storm.
- Rising limb: the part of the line where discharge increases.
- Peak discharge: the highest point of the line.
- Falling limb: the part where discharge decreases.
- Lag time: the time from the peak rainfall to the peak discharge.
How do you find lag time?
- Find the tallest rainfall bar and read its time on the horizontal axis.
- Find the highest point of the discharge line and read its time.
- Subtract: later time minus earlier time.
- Write the units, usually hours.
Worked example
All figures are invented for practice. The table gives discharge every two hours after a storm. The tallest rainfall bar was at hour 6.
| Hour | 0 | 2 | 4 | 6 | 8 | 10 | 12 | 14 | 16 | 18 |
|---|---|---|---|---|---|---|---|---|---|---|
| Discharge (m³/s) | 2 | 2 | 3 | 6 | 14 | 28 | 21 | 12 | 7 | 4 |
Question 1, baseflow: before the rain, discharge is 2 m³/s.
Question 2, peak discharge and its time: the highest value is 28 m³/s at hour 10.
Question 3, lag time: peak rainfall at hour 6, peak discharge at hour 10. Lag = 10 − 6 = 4 hours.
Question 4, how much did discharge rise? The rise above baseflow is 28 − 2 = 26 m³/s.
Question 5, rate of rise on the rising limb: between hour 6 and hour 10, discharge went from 6 to 28. That is an increase of 22 m³/s in 4 hours, so 22 ÷ 4 = 5.5 m³/s per hour.
Check: 6 + 22 = 28, and 10 − 6 = 4. The values agree with the table.
Question 6, describe the falling limb: after the peak, discharge falls from 28 to 4 m³/s over 8 hours. It falls more slowly than it rose, as water continues to drain from the slopes and soil.
How do you use lag time to compare catchments?
A short lag time and a high, steep peak suggest water reaches the river quickly. This fits impermeable surfaces such as tarmac, steep slopes or saturated soil. A long lag time and a lower peak fit permeable soils, gentler slopes or vegetation that intercepts rain.
Be careful with the wording. The hydrograph is consistent with such causes, but it does not prove one on its own. Use phrases such as “suggests” and “is consistent with”, and name the information you would need to be sure.
The mistake to watch for
Mistaken answer: “The lag time is 10 hours because the peak discharge occurs at hour 10.”
The student read the time of the peak discharge and stopped. Lag time is a difference between two times.
Hour 10 is where the peak happens, not how long it took to get there from the rain peak. The correct answer is 10 − 6 = 4 hours.
A second common slip is to measure from the start of the rain instead of from the peak rain. Check which rainfall point your question uses and write it beside your subtraction.
Check yourself
All data are invented.
1. Peak rainfall is at 09:00 and peak discharge at 10:30. What is the lag time?
Show answer
10:30 − 09:00 = 1 hour 30 minutes (1.5 hours).
2. Catchment A has a lag time of 1 hour and a peak of 40 m³/s. Catchment B has a lag of 4 hours and a peak of 15 m³/s. Which is more likely to have more impermeable surface, and how sure can you be?
Show answer
Catchment A is more consistent with impermeable surfaces, because water reaches the river sooner and the peak is higher. You cannot be certain from the graph alone, since slope, soil moisture and rainfall amount could also differ.
3. Baseflow is 3 m³/s and peak discharge is 24 m³/s. How much higher is the peak than baseflow?
Show answer
24 − 3 = 21 m³/s.
Where this leads next
Next, evaluate a flood-related claim from supplied evidence to practise judging what data can and cannot show. Use the map scale, contour and gradient practice to strengthen slope reading, and the statistics and distribution explorer for averages. Then try the mixed practice set.
If graph reading is where marks slip, a teacher in online one-to-one Geography tuition can work through unfamiliar graphs with you.