A hydrograph shows how river discharge changes over time after rain. To interpret it, read the key points, compare them, then give a reason for the difference.
This lesson is part of river processes and sustainable management. The comparison method also works on graph evidence from other chapters.
What are the parts of a hydrograph?
A storm hydrograph has a few features, each with a simple meaning.
- Peak discharge: the highest flow reached.
- Lag time: the delay between peak rainfall and peak discharge.
- Rising limb: the climb in discharge after rain starts.
- Falling limb: the drop back toward normal flow.
- Base flow: the steady flow from groundwater before and after the storm.
Worked example: two invented catchments
Both catchments get the same storm. The figures are original and invented, not from a real river.
| Feature | Catchment A (mostly forest) | Catchment B (mostly paved town) |
|---|---|---|
| Time of peak rainfall | 0600 | 0600 |
| Time of peak discharge | 1200 | 0800 |
| Lag time | 6 hours | 2 hours |
| Peak discharge | 40 m³/s | 95 m³/s |
| Base flow | 10 m³/s | 10 m³/s |
Lag time for A is 1200 − 0600 = 6 hours. For B it is 0800 − 0600 = 2 hours. The rise above base flow is 40 − 10 = 30 m³/s in A and 95 − 10 = 85 m³/s in B.
Turn the table into a three-step answer
Describe. Catchment B has a peak discharge of 95 m³/s, more than double the 40 m³/s in A, and its peak arrives 4 hours sooner.
Compare. B has a much steeper rising limb and a shorter lag time than A, although the rainfall peak is the same time.
Explain. In B, paved surfaces and drains send rainwater quickly to the channel as surface runoff, so the river responds fast. In A, leaves catch rain and roots help water soak into the soil, so water takes longer to reach the river.
The mistake that costs marks
A common slip is to write “the river will flood next week” from a hydrograph. The graph shows one supplied storm, so that conclusion goes beyond the evidence.
A safer closing sentence reads: “The data show that catchment B responds faster and reaches a higher peak, so it has a higher chance of the river rising above its banks in a storm like this one.” The wording stays tied to the graph and avoids a dated forecast.
Check yourself
In an invented catchment, rainfall peaks at 1000 and discharge peaks at 1300. Base flow is 15 m³/s and peak discharge is 75 m³/s. State the lag time and the rise above base flow, then name one factor that could shorten the lag time.
Answer
Lag time: 1300 − 1000 = 3 hours.
Rise above base flow: 75 − 15 = 60 m³/s.
Lag time shortens if the catchment has more paved surfaces, steeper slopes or already saturated soil. Each factor makes surface runoff reach the channel faster.
What to study next
Next, use the same describe, compare, explain steps on evaluating a river-management trade-off from a balanced classroom case, then try the river processes practice set.
If you want a teacher to check your graph answers line by line, see online one-to-one Geography tuition.