Wen water flows out of a tank through a horizontal pipe, the pressure of the water decreases linearly in the direction of flow. This is seen in the levels of water in the equidistant vertical glass pipes.

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.