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The following Warning message appears in the User Notifications list when computing:
The captured surface flow at this node does not connect a valid subsurface network. The flow is lost from the system.
This message indicates that there is no sub-surface conduit downsteam of the catchbasin (inlet) in question. So, the flow captured by the inlet does not have a place to go and thus is lost from the system.
This can happen for example if the last catchbasin in a network is at a dead end without an outfall. Meaning, the network may simply end at a catchbasin. To resolve the issue, either accept that the flow is lost, or if there is actually a pipe under the ground to carry the captured flow away which you need to account for in the model, lay out a new conduit from the catchbasin in question, to a downstream element (or outfall, to end the network).
The model is still able to compute in this situation (it does not result in a failed run) because surface-only networks are now possible. Meaning, you can simply analyze the surface gutter network without necessarily having to lay out the subsurface network. See: Analyzing surface gutter and inlet capture without subsurface conduits