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?
One of the underlying assumptions for the EPANET solver that WaterGEMS and WaterCAD is based on is that the pipes are always pressurized. Because of that, you will not be able to directly model a pipe emptying or becoming partially full.
WaterGEMS assumes the pipes are always full. If you wanted to model partially full pipes, a program like SewerGEMS would be a better option. See this link for details.
In a closed system, flow will move from an elevated reservoir or tank to satisfy demands in the system. Flow, velocity, and pressure or headloss would be reported in the model elements.
As for considering reservoir element for spring, you could create separate scenarios considering different levels/elevations in reservoirs for different seasons.
Bentley Technical Suppport