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A hydrograph shows how a river responds to a period of rainfall.

• Peak discharge - maximum amount of water held in the channel (measured in cumecs).
• Peak rainfall - maximum amount of rainfall (mm).
• Lag time - the time taken between peak rainfall and peak discharge.
• Rising limb - shows the increase in discharge on a hydrograph.
• Falling limb - shows the return of discharge to normal/base flow on a hydrograph.
• Base flow - the normal discharge of the river.

The lag time can be short or long depending on different factors. For example, if there is no vegetation in an area, the water runs off into the river quicker, therefore it would have a short lag time. Alternatively, if there is plenty of vegetation in the area, the lag time would be longer as the plants would intercept the rainfall. A shorrt lag time means water is reaching the river quickly, so there is a greater chance of a flood.

Factors influencing lag time include:

• size of the drainage basin
• vegetation
• valley side steepness
• soil type