Which condition will decrease water pressure in a long run of irrigation pipe?

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

Answer: (D)

In a long irrigation pipe run, the pressure you end up with is mainly determined by friction losses from the flowing water. Those losses grow when the water’s velocity is higher. When the pipe narrows along the run, the water must speed up to keep the same flow rate, which increases friction and causes a larger pressure drop as the water travels downstream. That extra drop means lower pressure at the end of the run.

If the pipe instead gets larger, the water slows down, friction losses per unit length decrease, and the downstream pressure stays higher (though there can be small loss at the expansion, the overall effect is not to reduce end pressure as much). Other scenarios like going downhill add gravitational head and tend to increase pressure, while having fewer sprinklers reduces flow and friction, also increasing pressure rather than reducing it.

So, the condition that decreases water pressure in a long run is when the pipe narrows from start to end.