Irrigation Hydraulics and System Design Fundamentals
Pressure, flow, and elevation
Every design decision starts with the water supply. Static pressure is measured with no water moving; dynamic (working) pressure is what remains while a zone runs. The difference - lost to friction, elevation, the meter, and the backflow assembly - is what actually reaches the heads.
Convert elevation with 1 psi ≈ 2.31 ft of water, or 1 ft ≈ 0.433 psi. A head 20 ft above the source loses about 8.7 psi; the same head below the source gains it.
Friction loss climbs roughly with the square of flow, so oversizing a zone is safer than undersizing. Keep velocity at or below about 5 ft/s in laterals (7 ft/s in mains) to limit water hammer and pipe wear. A common design target is under 10% pressure variation between the first and last head on a zone; beyond that, far heads throw short and coverage becomes uneven.
Precipitation rate ties flow to area: PR (in/hr) = 96.3 × GPM ÷ area (ft²). Matching precipitation rate across heads on a zone is what prevents wet and dry stripes. Avoid mixing rotors and sprays on one zone unless run times are managed separately.
Worked example: 12 GPM over a 1,000 ft² lawn gives PR = 96.3 × 12 ÷ 1,000 ≈ 1.16 in/hr. To apply 1 inch, run time = 60 ÷ 1.16 ≈ 52 minutes - split into two cycles on clay soil to avoid runoff.