The choice of a transform method should be coordinated with the purposes of the study and desired level of accuracy.  When hydrologic data are carefully selected and used, the results obtained through the use of unit hydrograph transform methods are generally acceptable for practical purposes.  In fact, throughout the world, hydrologic modeling guidelines specify or recommend the use of unit hydrograph theory (Institute of Hydrology, 1999) & (The Institution of Engineers, Australia, 2001). 

Initial estimates of transform parameters should be subjected to a model calibration process where computed outputs are compared against observed data and model parameters are modified in order to achieve an adequate fit.  Also, best estimate parameters derived through the aforementioned model calibration process should be tested through a model validation process where computed results, without any further parameter modifications, are used to compute outputs which are compared against observed data for independent events that were not considered during model calibration.  Storm events used during both model calibration and validation should be approximately equal to the magnitude of events that are being considered within the particular application.

The following table contains a list of various advantages and disadvantages regarding the aforementioned transform methods available for use within HEC-HMS.  However, these are only guidelines and should be supplemented by knowledge of, and experience with, the methods and the watershed in question.

Method

Advantages

Disadvantages

User-Specified Unit Hydrograph

  • Well established and documented method.
  • Requires the use of observed data to derive an empirical unit hydrograph.
  • Difficult to calibrate derived unit hydrographs.
  • Difficult to apply to ungaged areas due to lack of direct physical relationship of parameters and watershed properties.
  • Shortening the duration of excess precipitation is difficult and can lead to numerical oscillations.

User-Specified S-Graph

  • Parameters can be regionalized allowing for estimation within ungaged watersheds.
  • Similar to disadvantages of the user-specified unit hydrograph method.
  • Not well documented nor widely used.

Clark

  • "Mature" method that has been used successfully in thousands of studies throughout the U.S.
  • Well established, widely accepted for use, easy to set up and use.
  • Parameters can be regionalized and related to measurable basin characteristics.
  • Parameters can be varied with excess-precipitation rate for use within extreme event simulation when using the Variable Clark option.
  • Default time-area histogram may be inappropriate for use within some watersheds (though, a user-specified time-area histogram can be used).
  • Cannot be used with gridded snowmelt processes.
  • When using the Variable Clark option, requires development of Variable Tc and R curves.

Modified Clark

  • Similar to advantages of the Clark method.
  • Uses a site-specific time-area histogram.
  • Can be used with gridded snowmelt processes.
  • Similar to disadvantages of the Clark method.
  • Requires a gridded definition of travel time.
  • Parameters cannot be varied with excess-precipitation rate for use within extreme event simulation.

Snyder

  • Similar to advantages of the Clark method.
  • Similar to disadvantages of the Clark method.
  • Only key unit hydrograph characteristics are defined; HEC-HMS implementation determines equivalent Clark unit hydrograph parameters in order to define a “complete” unit hydrograph.
  • Parameters cannot be varied with excess-precipitation rate for use within extreme event simulation.

SCS

  • Similar to advantages of the Clark method.
  • Similar to disadvantages of the Clark method.
  • Parameters cannot be varied with excess-precipitation rate for use within extreme event simulation.
  • Only a limited number of unit hydrograph shapes can be derived using this method.

Kinematic Wave

  • Predicted values are in accordance with open channel flow theory.
  • Parameters can be related to measurable basin parameters.
  • Method is less parsimonious than unit hydrograph methods; it requires many more parameters.
  • Not as widely used as unit hydrograph methods.
  • Only appropriate for use in steep watersheds (slopes > 10 ft/mi)
  • Relatively small modeling elements are required to adequately determine representative parameters.

2D Diffusion Wave

  • Predicted values are in accordance with open channel flow theory.
  • Can be much more accurate than unit hydrograph theory, especially when extrapolating.
  • Allows for:
    • Unstructured and structured meshes.
    • Large cell sizes while still retaining detailed terrain information.
  • Much more computationally intense when compared to other transform unit hydrograph theory