Unit Hydrograph


  • The basis of unit hydrograph is that if two identical rainfalls occur on a drainage basin having identical conditions prior to the rains, the hydrograph of run-off from the two storms would be expect to be same.
  • In actual conditions two identical storms are rare to occur.
  • Most of the practical storms vary in duration, amount, and distribution of rainfall.
  • A unit hydrograph is a hydrograph of a rainfall of a specified duration and areal pattern, resulting in a run-off of 1 cm.
  • In other words, the total volume of water contained in this run-off hydrograph will be equal to 1 cm depth of water on the catchment.
  • Various storms of varying intensities and having different volumes of run-off, but a particular unit duration, will have the same shape of their run-off hydrographs.
  • Different unit hydrographs for different unit duration can be prepare.
  • A particular unit hydrograph when once prepare for a particular unit duration can be utilised for evaluating the run-off hydrographs of other storms of the like durations
  • The number of unit hydrographs for a given basin is theoretically infinite, since there may be one for every possible rainfall duration, and for every possible distribution pattern in the basin.
  • Practically only a certain limited number of hydrographs can be used for a given basin.

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