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

The bar chart shows rainfall. The line graph shows river discharge. The time between peak rainfall and peak discharge is the lag time. The rising limb and falling limb are on either side.
  • Peakdischarge - maximum amount of water held in the channel.
  • Peak rainfall – maximum amount of rainfall (millimetres).
  • 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 short 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 drainage basin
  • Vegetation
  • Valley side steepness
  • Soil type
Small drainage basin: short lag times, large basin: long lag times. Bare vegetation: short, forests: long. Steep valleys: average, gentle valleys: long. Impermeable soil: short, permeable: long.