Selecting a loss model and estimating the model parameters are critical steps in estimating runoff. Some pros and cons of the different alternatives are shown in Table 4-28. These are guidelines and should be used as such. The designer should be familiar with the models and the watershed where applied to determine which loss model is most appropriate.

Table 4-28: Comparison of Different Loss Models, Based on USACE 2000

Model	Pros	Cons
Initial and constant-loss rate	Has been successfully applied in many studies throughout the US. Easy to set up and use. Model only requires a few parameters to explain the variation of runoff parameters.	Difficult to apply to ungauged areas due to lack of direct physical relationship of parameters and watershed properties. Model may be too simple to predict losses within event, even if it does predict total losses well.
Texas initial and constant-loss rate	Developed specifically from Texas watershed data for application to sites in Texas. Method is product of recent and extensive research. Simple to apply.	Method is dependent on NRCS CN. Relatively new method, and not yet widely used.
NRCS CN	Simple, predictable, and stable. Relies on only one parameter, which varies as a function of soil group, land use, surface condition, and antecedent moisture condition. Widely accepted and applied throughout the U.S.	Predicted values not in accordance with classical unsaturated flow theory. Infiltration rate will approach zero during a storm of long duration, rather than constant rate as expected. Developed with data from small agricultural watersheds in midwestern US, so applicability elsewhere is uncertain. Default initial abstraction (0.2S) does not depend upon storm characteristics or timing. Thus, if used with design storm, abstraction will be same with 0.5 AEP storm and 0.01 AEP storm. Rainfall intensity not considered.
Green and Ampt	Parameters can be estimated for ungauged watersheds from information about soils.	Not widely used, less experience in professional community.