A reservoir is similar to an assigned pressure. With the known reservoir height, the pressure can be obtained at the junction and the flow can be solved directly (Wylie, et al, 1993Wylie, E.B., V.L. Streeter & L. Suo, Fluid Transients in Systems, Prentice Hall, Englewood Hills, New Jersey, 1993., pp. 43):

Then the flow rate is obtained by employing the compatibility equations.

If the pressure is known at the upstream end of the pipe, the negative compatibility equation is used (Equation 2).

Conversely, if the pressure is known at the downstream end, the positive compatibility equation is used (Equation 1).

If the reservoir height changes with time, then a current height is obtained for the current time step and used in the equations.

Reservoir Vapor Cavitation Theory

Infinite Reservoir junctions cannot reach vapor pressure because the pressure is specified and is therefore always above vapor pressure.

Any short-lived vapor cavities that develop at a Finite Open Tank junction are assumed to have a negligible effect compared to the tank itself. Conceptually, vapor bubbles would rise by buoyancy to the open tank surface and evaporate at the surface.

Therefore, vapor cavitation is ignored at finite open tanks.