in the long plots we ask for two capacities, we get pfc vs r.pfc for pipes they are the same , but for open channels such a trapezoidal they are different. there seems to be no literature on this.
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PFC is just full pipe capacity, and it is not part of the ICM InfoWorks solution. It is a reference value that users can easily understand as it is based on Manning's equation. Regarding your question, it is probably a case where the PFC calcuated on the link grid uses a different cross-sectional area than the cross-sectional area during the simulation.