If the pressure indicated by the vertical pipes is extrapolated backwards to the point of the inlet to the hotizontal pipe, we see that the pressure there is lower than at the bottom of the tank (as indicated by the water level in the tank). Therefore, there mus be a jump of pressure from the tank to the inlet of the pipe. This jump is related to the change of flow speed of the fluid from the tank to the pipe.

In this movie, the phenomenon can be observerd in the course of time as the level if liquid in the tank decreases. Since the flow through the pipe and therefore the flow speed in the pipe decreases as well, we expect a change in the pressure jump from the tank to the pipe.

Time step = 4.0 s.