Irrigation Systems Design and Installation
Pressure, flow, and hardware
Design to dynamic pressure at the point of use, not the static reading at the hose bib - subtract meter, valve, backflow, and friction losses. Keep velocity under about 5 ft/s. Select backflow protection by hazard: a reduced-pressure zone (RPZ) for high hazard, a pressure vacuum breaker (PVB) where there is no backpressure risk, a double check where permitted.
Zoning and spacing
Zone by hydrozone: turf, beds, and drip belong on separate valves. Set heads head-to-head (spacing about 50% of throw diameter) and use matched precipitation nozzles - mixing sprays and rotors on one zone causes over- and under-watered areas. Drip emitters are rated in gallons per hour and suit narrow beds and containers.
Formula and worked example
Precipitation rate for a group of heads: PR (in/hr) = 96.25 × total GPM ÷ area (SF). One rotor delivering 3 GPM on a 30 ft × 30 ft spacing (900 SF): 96.25 × 3 ÷ 900 = 0.32 in/hr. Runtime to apply 1 inch = 1 ÷ 0.32 ≈ 3.1 hours per week, split across cycles to avoid runoff on slopes.
Traps
Assuming static pressure equals working pressure; ignoring the pressure cost of the meter and backflow; leaving one zone with mixed head types; omitting check valves on slopes; sizing pipe by nominal diameter rather than flow. Also expect questions on controller scheduling, rain sensors, and winterization.