A hydrograph shows discharge through time. A short lag and high peak suggest rapid delivery, but several basin and storm conditions can cause them.
A storm hydrograph plots discharge against time. The rising limb records increasing flow as storm water reaches the channel. Peak discharge is the highest flow rate. The recession limb records the decline as rapid inputs end and stored water drains more slowly.
Compare it with rainfall measured over the same event. Peak rainfall is the highest rainfall rate or interval total shown. Lag time is the time between peak rainfall and peak discharge. State the convention and interval: an hourly rain total is not an instantaneous measurement.
Intense or prolonged rain, saturated soil, low permeability and efficient drainage networks can produce a steep rising limb and a high, early peak. Urban drains often shorten travel time. Wetlands, permeable ground and available reservoir storage may delay delivery and reduce the peak.
Basin size and shape also matter. Water from a compact basin may arrive more synchronously than from an elongated one, other factors being equal. However, storm location, channel routing and previous wetness can reverse a simple expectation. Compare similar events before attributing a difference solely to land use.
Step by step
Locate both peaks
Read peak rainfall and peak discharge from their own series and units.
Measure the lag
Subtract rainfall-peak time from discharge-peak time.
Explain the shape
Link the timing to pathways, storage and the storm rather than just naming urbanisation.
Worked example: A four-hour lag
In the illustrative event below, peak hourly rainfall occurs at hour 2 and peak discharge at hour 6. Lag time is 4 hours. The discharge line alone cannot tell us whether paving, saturated soil or another factor caused that response.
Watch out for this
The highest rainfall value and highest discharge value use the same units.
Rainfall is commonly mm per interval; discharge is cubic metres per second. Compare their timing, not their numerical magnitudes.
Check your understanding
Rainfall peaks at 14:00 and discharge at 19:00. What is the lag time?
- 19 hours.
- 33 hours.
- 5 hours.