In both the United States and Canada, either the Horton’s Method (LOSS = 1) or the CN Method (LOSS = 2) are commonly used for urban catchments. The Proportional Loss Method (LOSS = 3) has been successfully used in France for urban catchments. While the selection of Loss Routine can be somewhat arbitrary and at the discretion of the user, there are a few things to keep in mind when choosing a loss routine.
Horton’s Method is what is used in the SWMM model, therefore if the user is comparing results with a SWMM based model, or working in a watershed where the overall model used was SWMM, then this method may be the most appropriate. However, the user should bear in mind that for longer duration storms (greater than or equal to 12 hours) the Horton’s Method may not accurately predict the runoff from pervious areas. We have seen cases where the model simulates no runoff from a previous area during a 12-hour 100-year storm. This is clearly erroneous. The CN Method does not have any limitations with respect to storm length and often yields more conservative results as compared to Horton’s Method.
If the user selects the CN Method, then the IA parameter should be set somewhere between 1.5 mm and 5 mm. Note that this is a different value than what would be used for a rural catchment with the same CN value. An urban catchment generally has less pervious depression storage than the same catchment in its rural state.