GoldSim is often used to simulate the volume in a reservoir in a model employing relatively long duration time steps (e.g. 12-hr or daily). In models like these, you need to be careful how you represent physical processes that are sensitive to time, like orifice and weir discharge hydraulics to calculate withdrawals from a reservoir.
This webinar demonstrates examples that handle the physical relationships between flow and water level even for large time step models. This method is useful in preventing over estimation of spillway outflow from a reservoir and the resulting oscillations that might otherwise occur in a typical water balance model involving reservoirs or ponds.
The video recording of the presentation (from the November 2016 webinar) can be downloaded from here.