Within InfoWorks CS there can be situations whereby a small cut out of an existing network run individually may produce difference results for that area as within a larger network.
Typically the things to check when looking into differences in results between two networks would include looking at the simulation parameters that have been set, the hydraulic run schedules as well as a comparison between two networks. Usually after these steps have been taken differences may be identified that answer questions with regard to differing simulation results.
There have been some cases however where a single model has included two or more discrete network areas and when one is run in isolation the results for this differ to the same area in the larger model. After investigation it may become apparent that the simulation parameters are the same, the network objects are the same and the run schedules are the same yet results vary by more than the tolerance for volume balance value being used.
The usual reason for this is particularly difficult to identify in versions prior to 12.5, as here the functionality causing the discrepancy is enabled by default. The areal reduction factor that is used in rainfall smoothing is usually the cause and would indeed differ between the two networks in question. This is dependent on the total runoff area within the network, this means the sum of all the areas for the subcatchments. As a consequence, when using the larger network, albeit the one with two discrete networks incorporated then the total area will be greater than that for the network with only the single area.
The areal reduction factor used for rainfall smoothing will cause a different inflow profile for the subcatchments, the routing will be affected and hence different results produced (even though the volume will remain the same).
It is worth noting that the impact of this will be particularly apparent when only a small section of the total network is looked at in isolation as this is when the difference in areal reduction factor will be the greatest.