Why worry about pipe size in a residential irrigation system?
Correct pipe sizing leads to a more efficient distribution of water throughout the irrigated area, by sizing pipe correctly you will have less runoff, and even application of water (less swampy wet spots). In addition the likelyhood of water hammer is greatly reduced when operating at 5 FPS or less IE: less broken sprinkler lines and fittings, greater life expectancy on valves & other components.
