The Use Case "Impose Flow Regime" is expressed as an action by the End-User.

I think that the end-user will configure the use of the HPM by the UO through the parameterization/edition of the UO. This is an action (Use-Case) which is common to any Unit Operation. After that the End-user disappears. This decision to impose the flow regime could be a decision that takes place at configuration time (specific Hydro Use case) of the HPM. So indeed this "configuration" is a pre-condition that is in fact the general Use Case of any UO. But it could be made in different ways. Why so? In the configuration Use Case for an HPM, one can impose a specific constant flow regime or mention that the flow regime, whatever it is, will be imposed by the Unit Operation to the HPM. In the second case, the flow regime may be switched from one to another by the Unit Operation acting with its own rules.

Anyway the Unit Operation is then the only Actor acting upon the HPM. Whatever happens in the events before the Use Case, the End-User participates to other Use Cases not to this Use Case.

As a consequence of this Use Case: while it has not been said explicitely in the HPM definition (glossary) that the HPM delivers the flow regime, we know it is often the case. So if there is a specific Use Case that is here to modify this behaviour, it should be clearly mentioned that the expected behaviour of an HPM is to deliver a flow regime.

Exact, I made a mistake with this use case and what was made in previous one.
In this use case, the only actor is the <Unit>, <End> acting in a previous one i.e. the HPM configuration stage.