The Modified Curve Number method was first proposed by Paul Wisner & Associates in 1982, and was based on their research and monitoring of rural and urban catchments in Canada. This method has been used successfully in Canada for the past 35 years and has correlated well with measured flows.

Rather than having a varying IA parameter, as in the SCS method, the IA is fixed, as described above, and the CN is altered. The modified CN, called is a function of the IA, and total rainfall. is calculated as follows:

1. Select an appropriate IA (see above) for catchments being modelled.
2. Determine the SCS CN value from soils maps and/or calculations. Convert the CN (AMC II conditions) to a CN (AMC III conditions).
3. Determine the largest precipitation volume, P, for a rainfall event that would just represent AMC III soil moisture conditions. In most cases this is the 100-year storm event. For ex-ample, in Markham Ontario the 100-year storm volume for the 3-hour storm is 80 mm.
4. Calculate the soil storage S, based on the SCS Method using CN (AMC III conditions). The metric equation is S = (25400 / CN) –254 and the imperial equation is S = (1000 / CN) – 10. This will give you the soil storage during your large storm event.
5. Calculate the IA based on the SCS Method, where IA = 0.2S. Note that this relationship is also valid for the Modified CN Method because it is assumed that the runoff volume, Q, for large events is the same using both methods.
6. Determine the runoff volume, Q, based on the familiar:
Q = (P – IA)2 / (P – IA + S)
7. Next calculate S* using the above equation again but this time setting IA to the value calculated for the Modified CN method (i.e. 1.0mm to 5.0mm). This IA will be the value used in the model simulations.
8. Once you have calculated S*, calculate from the equation:
S* = (25400 / CN*) –254 metric
S* = (1000 / CN*) – 10 imperial
9. The above calculation will give you the for AMC III soil conditions. You now finally de-termine the CN* for AMC II soil conditions by using published tables relating CN for AMC II and AMC III conditions.

The above method is easily adaptable to a spreadsheet so that for future uses, you can easily and quickly calculate the once you know the IA, P, and CN.
This process has been incorporated in the Convert to tool in Visual OTTHYMO. For more information, see Appendix A.1 in the User’s Manual.