Before we explain the method of use of unit hydrograph to estimate the run-off from a basin let us first of all learn some important termsĀ  follow:


1. Hydrograph. It is graphical relation between discharge or flow, against time at a particular point of a stream or river.

  • Hydrograph represents the time
  • distribution of total run-off at the point of measurement.
  • As volume of run-off, discharge or flow, is obtained by multiplying discharge with time, the area under the hydrographs gives the volume of flow during that period.
  • The hydrographs have three types of flows:

(i) Surface run-off or water flowing in the stream or river.

(ii) Sub-surface storm flow i.e. infiltrated water in the top layers of soil.

  • This water reaches the streams within short time.
  • It is also known as inter-flow or influent stream.

(iii) Ground water flow or water contributed as underground flow from the ground water reservoir fed by infiltration from previous rainfall and also some by the storm in question.

  • At the start of any hydrograph, there is contribution to the run-off from the ground water reservoir accumulated in the soil during previous rainfall.
  • Due to fresh rainfall when water level in the streams rises, they contribute water to the ground water.
  • The contribution of ground water to surface flow may be represented by the dotted line

Ground water inflow and outflow.


  • Because it is not possible to determine, the actual shape of dotted line curve and also since in any major flood rise
  • The ground water table contribution is a small percentage of the total flow, it is sufficient to assume this line as straight line.
  • The surface run off is above this line whereas ground water flow lies below this line.