• shows a hydrograph for any isolated duration of rainfall.
  • A is the point from which hydrograph starts rising.
  • The hydrograph continues to rise at a very steep rate till peak point B is reached.
  • After this flood discharge starts receding.
  • AB limb of the hydrograph is called rising limb and BD limbs as the receding limb.
  • On limb BD, there is a point C known as point of inflection.
  • It has already been stated in this article, that the hydrographs have three types of flows, over land flow (surface runoff), Interflow (influent streams or subsurface flow) and ground water flow.
  • Overland flow and interflow are generally grouped together and this combined flow is known as Direct run-off. During floods the streams contribute ground water to the soil but during low water flows, streams derive most of its water from ground water.
  • See Figs 6.14, 6.15 and 6.16. Direct run-off and ground water run-off in a particular hydrograph can be separated by the following methods:




Rise in water level and process of entrance and exit of ground water.

  • Extend the recession line of previous flow hydrograph to a point lying exactly on a vertical line drawn through
  • The peak of the hydrograph i.e. point B of Fig. 6.17. Let this point be E.
  • Select a point D on the recession limb of the present hydrograph N days after the peak. Join point E with D.
  • The value of N in days can be found out as follows:

N = 0.826 A1/5

A = catchment area in sq. km.

  • Thus AED (Fig. 6.17) is the dividing line between ground water and Direct runoff.
  • The area ABCDEA represents the volume of direct run-off and area below line AED represents ground water.
  • 2. Refer Fig. 6.17. The base flow is obtain by simply drawing a line.