When computing or validating a model, the following user notification is generated: "Link diameter (or rise) is smaller than the diameter (or rise) of the upstream."
This is a warning-level validation message indicating that there are conduits in the model where the diameter or rise of the conduit decreases as you move downstream. Since loading typically increases as flow travels downstream, it is typical that diameter or rise of conduits in the system will increase as you move downstream. This message is generated so that you can review the input to make sure that it is accurate.
If the conduit diameter does decrease as you move downstream, the model will still compute with accurate results based on the input into the system.