Modified rational unit hydrograph method and applications

Nirajan Dhakal, Xing Fang, David B. Thompson, Theodore G. Cleveland

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8 Scopus citations


The modified rational method (MRM) is an extension of the rational method to develop triangular and trapezoidal runoff hydrographs. A trapezoidal unit hydrograph (UH) was developed from the MRM for the case when the duration of rainfall is less than the time of concentration of the watershed and is called the modified rational unit hydrograph (MRUH). The MRUH method was applied to 1400 rainfall-runoff events at 80 watersheds in Texas. Application of the MRUH method involved three steps: (a) determination of rainfall excess using the runoff coefficient; (b) determination of the MRUH using drainage area and time of concentration; and (c) simulating event runoff hydrographs. The MRUH performed as well as the Gamma function UH, Clark-HEC-1 UH and NRCS curvilinear UH methods when the same rainfall loss model was used. The MRUH method can be applied to time-variable rainfall distributions and at watersheds with drainage areas greater than typically used for the rational method (a few hundred acres).

Original languageEnglish
Pages (from-to)381-393
Number of pages13
JournalProceedings of the Institution of Civil Engineers: Water Management
Issue number7
StatePublished - Jan 1 2014


  • Aids/hydrology
  • Design methods
  • water resource/Models (physical)


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