This model fills a bathtub, while outflow occurs at a flow determined by the volume of water (or pressure at outlet) of the bathtub. It could just as well be a dam or lake that is modeled.
Exercise 1. 'Water
in bathtub' is the stock, measured as a volume of water, in L. The bath is initially empty. The bath receives water from the taps ('Variable inflow from tap', in
L/min) and loses water to the drain ('Constant outflow to drain', in
L/min). 'Twiddling the taps' automatically switches the inflow from 40
L/min to 20 L/min at 5 minute (300 second) intervals. The outflow
remains constant at 30 L/min.
