Selecting a loss method and estimating the required parameters are critical steps in developing program input. Not all loss methods can be used with all transforms. For instance, the gridded loss methods can only be used with the ModClark transform. Table 16 lists some positive and negative aspects of the alternatives. However, these are only guidelines and should be supplemented by knowledge of, and experience with, the methods and the watershed. League and Freeze (1985) point out that "In many ways, hydrologic modeling is more an art than a science, and it is likely to remain so. Predictive hydrologic modeling is normally carried out on a given catchment using a specific method under the supervision of an individual hydrologist. The usefulness of the results depends in large measure on the talents and experience of the hydrologist and …understanding of the mathematical nuances of the particular method and the hydrologic nuances of the particular catchment. It is unlikely that the results of an objective analysis of modeling methods…can ever be substituted for the subjective talents of an experienced modeler."

The following table contains a list of various advantages and disadvantages regarding the aforementioned loss 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

Initial and Constant

  • "Mature" method that has been used successfully in thousands of studies throughout the U.S.
  • Easy to set up and use.
  • Parameters can be related to predominant soil textures and estimated using multiple literature sources.
  • Method is parsimonious; it includes only a few parameters necessary to explain the variation of runoff volume.
  • Difficult to apply to ungaged areas due to lack of direct physical relationship of parameters and watershed properties.
  • Method may be too simple to predict losses within event, even if it does predict total losses well.
  • Does not allow for continuous simulation.
  • Does not allow for surface storage to occur prior to soil saturation.

Deficit and Constant

  • Similar to advantages of the Initial and Constant method.
  • Method is scalable in that it allows for continuous simulation (but is not required for use).
  • Similar to disadvantages of the Initial and Constant method.

Green and Ampt

  • Parameters can be related to predominant soil textures and estimated using multiple literature sources.
  • Predicted values are in accordance with classical unsaturated flow theory (good for ungaged watersheds).
  • Allows for surface storage to occur prior to soil saturation.
  • Not widely used, so less mature.
  • Not as much experience in professional community as simpler methods.
  • Less parsimonious than simpler methods.
  • Does not allow for continuous simulation.

Layered Green and Ampt

  • Similar to advantages of the Green and Ampt method.
  • Allows for continuous simulation.
  • Similar to disadvantages of the Green and Ampt method.
  • Requires more parameters than the Green and Ampt method.

SCS Curve Number

  • Simple, predictable, and stable method.
  • Relies on only one parameter, which varies as a function of soil group, land use and treatment, surface condition, and antecedent moisture condition.
  • Features readily understood and well-documented.
  • Well established method widely accepted for use in U.S. and abroad.
  • Parameters can be related to predominant soil group/land use and estimated using multiple literature sources.
  • Predicted values not in accordance with classical unsaturated flow theory (infiltration rate will approach zero during a storm of long duration rather than a constant rate).
  • Developed with data from small agricultural watersheds in midwestern U.S., so applicability elsewhere is uncertain.
  • Default initial abstraction (0.2*S) does not depend upon storm characteristics or timing.
  • Rainfall intensity is not considered when computing losses (i.e., the same loss volume will be calculated for 1 in rainfall distribution over 1 hour or 1 day).
  • Does not allow for continuous simulation.
  • Does not allow for surface storage to occur prior to soil saturation.

Exponential

  • Predicted values are in accordance with classical unsaturated flow theory.
  • Similar to disadvantages of the Green and Ampt method.
  • Requires more parameters than Green and Ampt.
  • Parameters cannot be related to predominant soil textures and estimated using (not good for ungaged watersheds).
  • Does not allow for continuous simulation.
  • Does not allow for surface storage to occur prior to soil saturation.

Smith Parlange

  • Similar to advantages of the Green and Ampt method.
  • Similar to disadvantages of the Green and Ampt method.
  • Requires more parameters than the Green and Ampt method.
  • Does not allow for continuous simulation.

Soil Moisture Accounting

  • Parameters can be estimated for ungaged watersheds from information about soils.
  • Predicted values are in accordance with classical unsaturated flow theory (good for ungaged watersheds).
  • Allows for continuous simulation.
  • Allows for surface storage to occur prior to soil saturation.
  • Not widely used, so less mature, not as much experience in professional community.
  • Features not widely understood.
  • Less parsimonious than simple empirical methods.