Instantaneous Unit Hydrograph

A number of unit hydrographs can be drawn for a catchment corresponding to different durations. The shape of unit hydrographs depends on duration of D. Figure below shows a typical variation of shape of unit hydrographs for different values of D. As the value of D will decrease, intensity of rainfall excess being equal to 1D increases and unit hydrograph can also be drawn for a duration of rainfall approximately equal to zero. Thus, limiting case of a unit hydrograph of zero duration is known as instantaneous unit hydrograph (IUH). Thus, of IUH is a fictious unit hydrograph which represents surface runoff from catchment due to an instantaneous precipitation of rainfall excess volume of 1 cm. Let, ordinate of IUH is denoted by u(t ). Then, some of the properties of IUH can be listed as follows:

NOTE:

• It occurs when D->0 is used for IUH, it is also known as Time-area histogram method.
• It is assumed that the rainfall excess first undergoes pure translation and then attenuation.

OTHER TYPES OF HYDROGRAPH

A plot of the discharge in a stream plotted against time chronologically is called a hydrograph. Depending upon the unit of time involved, we have

1. Annual Hydrograph: It shows variation of daily, weekly or mean flow of any number of successive days over a year.
2. Monthly Hydrograph: It shows the variation of daily mean flow over a month.
3. Seasonal Hydrograph: It shows the variation of the discharge in a particular season such as the monsoon season or dry season.
4. Flood hydrograph or hydrographs due to storm: It shows stream flow due to a storm over a catchment hydrograph 1, 2 and 3 as above are called long term hydrograph and used for long term studies like
   • Calculating the surface water potential of stream
   • Reservoir studies and
   • Drought studies

Flood hydrograph on the other hand is used to study the flooding characteristics of a stream. It is a short term study.

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