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:

  • Geologydischarge - if the rocks under the ground are impermeable and water cannot drain through the rock layer resulting in rapid overland flow and a shorter lag time. Permeable rocks encourage a slow transfer by groundwater flow, hence a longer lag time.
  • Soil type – clay soils do not drain easily and become saturated very quickly. This results in rapid overland flow and shorter lag times. Dry soils slow down water transfer leading to longer lag times.
  • Slope - steep slopes lead to rapid water transfer and shorter lag times. Gentle slopes slow down water transfer making the lag time longer.
  • Drainage basin shape - a high density basin has more streams and rivers which speed up water transfer and shorten lag time. A low density basin has less streams and rivers leading to a slower transfer and longer lag time.
  • Antecedent conditions - wet conditions before a storm cause the ground to become saturated. This speeds up overland flow and shortens lag time. Drier conditions mean the rainfall can infiltrate into the soil slowing down lag time.
  • Vegetation - 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.
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.