When using the SCS Curve Number Method, IA should be set to 0.2S where S is the soil storage (a function of CN). Bear in mind that this method may underestimate the peak flow for small storms because the initial abstraction is higher than the total rainfall, which is not accurate. A literature review of this method has found that for lower CN values, a lower IA should be used. Suggests guidelines are as follows:
CN ≤ 70 IA = 0.075S
CN > 70 ≤ 80 IA = 0.10S
CN > 80 ≤ 90 IA = 0.15S
CN > 90 IA = 0.2S
Please note that the above guidelines are for the SCS Method only, where the SCS Curve Num-ber is used to define the soil type.