because its a liquid, the rate of flow anywhere in the pipe is exactly the same. pressures and velocities may change, but flow rate is constant (unless there is a leak in the system).
That is true only when the pipe is completely filled with water or if a partially-filled pipe is able to reach a state of equilibrium.
Imagine a 100 foot long 4" pipe filled with air. For illustrative purposes, we'll make this pipe stand vertically. Ten feet down from the top of the pipe a restriction where the pipe is necked down to 1" in diameter. At the bottom of the pipe is another restriction where the pipe is necked down to 1/2" in diameter.
Now we dump as much water as we can into the top of the pipe. Because of the 1" restriction, the water will back-up until it fills the top 10' of pipe and overflows. At the same time, water is flowing through that 1" restriction. Some of it will exit the pipe at the bottom via the 1/2" restriction. But the rate-of-flow through the 1" restriction at the top is greater than the rate-of-flow through the 1/2" restriction at the bottom. The rate-of-flow into the pipe is not the same as the rate-of-flow out of the pipe. The system is not in a steady-state.
So water will begin to accumulate in the 90' section between the restrictions. The water level in that 90' center section will start to rise. As it does so, the head pressure at the bottom of the pipe will increase, and the flow through the 1/2" restriction will likewise increase. Eventually the rate-of-flow through the 1/2" restriction will match the rate-of-flow through the 1" restriction at the top and the system will reach a state of equilibrium. I pulled these numbers out of the air. Depending on the actual relative size of the restrictions and the amount of head pressure, there may be a small air column or there may be a large air column inside that pipe.*
Now lay this pipe down on a grade. Add some hills and valleys along its 100 foot length. You should be able to see that there will be air pockets within this pipe. These air pockets are what pose a challenge for wet-testing the insulation resistance with a megger.
*If the opening at the bottom of the pipe is small (big restriction) and the opening at the top is large (small restriction), then there will be no air column.
