All of the following will not decrease the water pressure in a long run of irrigation pipe except?

• A. Going from a smaller pipe to a larger pipe
• B. Going from a larger pipe to a smaller size pipe
• C. Fewer sprinklers on a run
• D. A pipe going down hill

Answer: (A)

Pressure in a long irrigation pipe mainly drops due to friction losses along the line, and those losses depend on how fast the water is moving and the pipe size. When you go from a smaller pipe to a larger pipe, the velocity drops because the same flow spreads over a bigger area, so friction losses go down and pressure is preserved or even increased downstream. Moving from a larger pipe to a smaller one makes the water speed up, which ramps up friction losses and lowers the pressure downstream.

Having fewer sprinklers on a run reduces the overall flow, so there’s less friction loss and the pressure drop is smaller, which helps maintain pressure. A pipe that runs downhill gains hydrostatic head, boosting pressure at the downstream end rather than reducing it.

So the change that actually decreases water pressure in the long run is reducing the pipe size along the way—going from a larger pipe to a smaller size.