The system is shown on the figure, where scheme of longitudinal profile is given. There is a spring sourse (the reservoir) with variable capacity, e.g. very small during the summer and significantly bigger during the another seasons. The pipeline consists by two pipes - the first one is with very big diameter. When the spring flow rate is enough small (Q3 and Q4) there is some length in the first pipe with free surface flow. At the end of second pipe there is a valve, which is partially open with one steady position during the whole year (it could be modeled with general purpose valve with some minor loss coefficient). The second pipe fills the tank from top.
Is there possibility to model correctly the system with Watergems (or Sewergems or something else), i.e. to see where the flow became pressurized - point 1 or point 2 depending of flow rate of the spring. And to model correctly the connection point of reservoir with first pipe – especially when water levels in spring reservoir is lower than top of the pipe (level 3 and level 4). May be for such a spring and pipe after it, the reservoir is not appropriate element. And/or the first pipe has to be divided further to small sections to see where is the transition between free surface and pressurized flow?