How to read a storm hydrograph
Rain falls first. The river often peaks later. Why?
A storm hydrograph shows how river discharge changes during and after rainfall. Lag time is the time between peak rainfall and peak discharge.
Check both axes
Rainfall bars and the discharge line may use different vertical axes. Read the labels and units before comparing their heights. A taller rainfall bar is not a larger discharge.
Follow the water
Water takes time to reach the channel. Some soaks into soil or is held by vegetation. Fast surface runoff can produce an earlier peak than slower routes through soil.
Worked example
| Invented storm record |
|---|
| Peak rainfall: hour 2 · Peak discharge: hour 7 · Peak discharge value: 24 m³/s |
- Find the largest rainfall bar and read its time: hour 2.
- Find the highest point on the discharge line and read its time: hour 7.
- Lag time = 7 − 2 = 5 hours. The peak discharge is 24 m³/s; it is not the lag time.
A short lag time does not prove that building roads caused flooding. Rainfall intensity, soil wetness, slope and the basin’s size can also affect the response. Compare storms carefully.
Check you’ve got it
A second storm has peak rainfall at hour 4 and peak discharge at hour 8. Which storm has the shorter lag time?
Check your answer
The second storm: 8 − 4 = 4 hours, compared with 5 hours in the worked example.
Try it yourself
Use this new resource. Check your answers, then try another set.
River hydrographs
Blue bars show rainfall in the hour ending at each labelled time. The green line shows discharge at that time. Peak lag is the difference between the hours of maximum rainfall and maximum discharge.
Scroll the diagram sideways to see the full resource.
| Hour | Rainfall in preceding hour (mm) | Discharge (m³/s) |
|---|---|---|
| 0 | 0 | 5 |
| 1 | 4 | 5 |
| 2 | 11 | 5 |
| 3 | 6 | 5 |
| 4 | 0 | 7 |
| 5 | 0 | 13 |
| 6 | 0 | 19 |
| 7 | 0 | 13 |
| 8 | 0 | 7 |
| 9 | 0 | 5 |
| 10 | 0 | 5 |
| 11 | 0 | 5 |
| 12 | 0 | 5 |
Resource description
Hour 0: 0 mm rainfall, 5 m³/s discharge; Hour 1: 4 mm rainfall, 5 m³/s discharge; Hour 2: 11 mm rainfall, 5 m³/s discharge; Hour 3: 6 mm rainfall, 5 m³/s discharge; Hour 4: 0 mm rainfall, 7 m³/s discharge; Hour 5: 0 mm rainfall, 13 m³/s discharge; Hour 6: 0 mm rainfall, 19 m³/s discharge; Hour 7: 0 mm rainfall, 13 m³/s discharge; Hour 8: 0 mm rainfall, 7 m³/s discharge; Hour 9: 0 mm rainfall, 5 m³/s discharge; Hour 10: 0 mm rainfall, 5 m³/s discharge; Hour 11: 0 mm rainfall, 5 m³/s discharge; Hour 12: 0 mm rainfall, 5 m³/s discharge
Your course and question context
Next: Grid references
Four and six figure grid references → · Choose a skills workout