To accelerate the water in a circuit and to overcome frictional forces while maintaining a steady flow, you use a pump. Pumps create pressure gradients. To accelerate a column of water, a pump either increases the pressure on one side of the column, or decrease the pressure on the other side of the column.
Consider a simple U-shaped pipe filled with water. The water level is the same in each leg and the pressure on each surface is the atmospheric pressure. How can you make the water level rise in the right leg and sink in the left leg?
In a vertical, water-filled pipe gravity creates a pressure gradient in the water. The water below has to support the weight of the water above. The pressure exerted by 10 m or 33 feet of water overhead is equal to the atmospheric pressure at sea level. One way of maintaining pressure in plumbing is to have tall columns of water connected to the pipes. Many municipalities use a water tower build at a relative high site within their service region to maintain pressure in the water mains.
Suppose a U-shaped piece of pipe is completely submerged in water, filled with water, and then turned upside down under water. As you slowly pull the top of U-shaped piece of pipe out of the water, the water does not run out of the pipe. Why?
Air cannot enter the pipe. As the water starts running out of the pipe, a near vacuum is created in the topmost region of the inverted U. The pressure here drops to near zero. The atmospheric pressure on the surface of the water in the bucket pushes the water into the U-shaped pipe.
If a U-shaped hose or pipe connects a liquid-filled container at a higher altitude to a container at a lower altitude over a barrier, the liquid can be siphoned into the container at lower altitude. Atmospheric pressure helps to push the liquid over the barrier. In the diagram below P1 > P2, and the fluid is siphoned from the left to the right bucket.
We have = Ptop + ρhg.
Here Pbelow = 1 atm, Ptop = P1 = 1 atm + ρh1g on the left side and Ptop = P2 = 1 atm + ρh2g on the right side.
The flush toilet (Youtube)
Investigating Pressure (Experiments you can try at home.)