Storm Hydrographs

Showing a river’s response to rain

Rain falling across a drainage basin does not all reach its river immediately. Some travels quickly over the ground as surface run-off. Some moves more slowly through soil as throughflow, or through rocks as groundwater flow.

A storm hydrograph shows how river discharge changes during and after rainfall. Discharge means the volume of water passing a point in the river each second, measured in cubic metres per second ($\text{m}^3/\text{s}$). It is not simply the river’s depth.

The graph combines two different measurements: rainfall bars, measured in millimetres, and a discharge line, measured in cubic metres per second. Both use the same horizontal time axis, allowing you to compare when rain falls with when the river responds.

Turning a table into a hydrograph

Rainfall is the total for the one-hour period ending at the stated time; discharge is a reading taken at that time. The zero-hour row records conditions before the storm.

Start with a horizontal axis labelled Time since start (hours), running from 0 to 8 with evenly spaced hourly marks. Add a left-hand vertical axis for Discharge (m³/s) from 0 to 20, and a right-hand vertical axis for Rainfall (mm) from 0 to 10. The two scales are independent: the same height on the page does not mean the two quantities are equal.

Draw the rainfall bars first, using the rainfall scale. The first bar covers hours 0–1 and reaches 10 mm; the next covers hours 1–2 and reaches 6 mm; the third covers hours 2–3 and reaches 2 mm. Later periods have no rainfall.

Next, plot every discharge reading using the discharge scale. For example, the point at hour 2 is 9 m³/s, and the point at hour 4 is 20 m³/s. Join successive points with a line in time order. Give the graph a title and make the bars and line clearly distinguishable.

Reading and annotating the finished graph

The rising limb is the section where discharge increases towards its maximum. In this example, discharge rises from 4 m³/s at hour 0 to the peak discharge of 20 m³/s at hour 4. The , also called the recession limb, follows as discharge decreases.

The largest rainfall total is 10 mm in the hour ending at hour 1. Using the recording times in this table, the lag time is:

$$\text{lag time} = 4 - 1 = 3\text{ hours}$$

Annotate both peaks with their values and times. Show lag time with a horizontal double-headed arrow between their time positions: it measures a time interval, not the vertical distance between the bars and line. With real data, check whether rainfall times label the beginning, middle or end of each recording interval.

Explaining the shape

Discharge can continue rising after rainfall stops because water is still travelling through the drainage basin. Faster surface run-off helps produce the rising limb. Slower throughflow and groundwater flow continue supplying the river as discharge falls back towards its background base flow.

A steep rising limb and short lag time indicate a rapid response. Impermeable surfaces such as roads can encourage this by increasing surface run-off. A gentler rising limb and longer lag time indicate a slower response. Use the plotted data to describe the actual shape rather than assuming every storm produces an identical curve.

The same drawing method transfers to measured rainfall and discharge data: retain their units, recording intervals and time labels.