A continuous flow simulation model has been developed to relate causative rainfall to sewer inflow and infiltration. It addresses many of the shortcomings of traditional analytical methods. In the model, I/I is composed of several different flow components, each of which is a function of antecedent precipitation. These I/I flow components can be associated with individual physical elements of the sewer system, and thus, aid decision-makers in prioritizing rehabilitation efforts. The model can be used to synthesize a flow database beyond the scope of available for measurements. This perments the comparison of different sewer systems or sewer systems before and after rehabilitation. Because the model can synthesize a long flow record, a probability of occurrence can be associated with a particular storm flow event. Utilizing a probability approach allows decision makers the chance to make decisions while being informed of the potential risk associated with their decision.