Why did the river rise?
Two nearby catchments received the same rain. Their rivers responded differently. Find out why.
Read the resource
| Catchment | Rain in 6 hours (mm) | Built surface (%) | Lag time (hours) | Peak flow (m³/s) |
|---|---|---|---|---|
| Alder | 42 | 12 | 5 | 38 |
| Brook | 42 | 61 | 2 | 74 |
The catchments have the same area, slope and rock type in this simplified comparison. Built surfaces are mostly impermeable. Lag time runs from peak rainfall to peak river flow.
Before you answer
Find the command, the subject and any limits: place, time, one or two reasons, and which resource to use.
For an explanation, connect the cause to the outcome. For a judgement, use evidence to explain how effective, important or convincing something is.
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A fresh resource
| Catchment | Rain (mm) | Built surface (%) | Lag time (hours) |
|---|---|---|---|
| Cedar | 30 | 18 | 4 |
| Dale | 55 | 62 | 2 |
Try it yourself
Explain why Dale might have a shorter lag time. Then explain why these data cannot isolate the effect of built surfaces.
One possible answer
Dale has more built surfaces, which can reduce infiltration and speed up runoff. However, it also received more rain. The comparison changes two factors, so their separate effects cannot be established from these data alone.
Check your answer: did you address the question, use accurate evidence and explain the link?