Irrigation Zone and Pressure Problem
A contractor is troubleshooting a newly installed drip and spray system on a 1-acre site. The 3/4-inch water meter provides 62 psi static pressure and a safe available flow of 12 gpm. Zone 3 contains 18 pop-up spray heads, each with a 1.5 gpm nozzle, on 1/2-inch poly laterals totaling 150 feet. Zone 4 contains 10 rotor heads at 3 gpm each. The controller runs both zones simultaneously at 6:00 a.m. The client reports misting from Zone 3 heads, dry corners in Zone 4, and water running off the slope in Zone 4. The contractor proposes increasing the controller run time.
1. Calculate whether the zones can run together and diagnose the symptoms.
Read the worked answer
Run the numbers: Zone 3 = 18 × 1.5 = 27 gpm; Zone 4 = 10 × 3 = 30 gpm. Together they demand 57 gpm, but the meter's safe available flow is 12 gpm. Either zone alone exceeds 12 gpm, so the problem is not run time; it is hydraulic design. In 1/2-inch poly at 27 gpm, velocity is very high-roughly 15–20 ft/s-causing large friction loss and misting at the sprays. Zone 4's dry corners are from low pressure and flow, and runoff on the slope is from application rate exceeding infiltration.
2. Propose a hydraulic redesign.
Read the worked answer
Split Zone 3 into at least three valves of about 9 gpm each, or replace nozzles with lower-flow, matched-precipitation heads. Split Zone 4 into three valves of about 10 gpm each. Do not mix sprays and rotors on one valve; their precipitation rates differ. Use pressure-regulating heads or a pressure regulator at the valve, and set sprays near 30 psi and rotors per the manufacturer. Add cycle-and-soak for the slope. Keep each valve below 12 gpm with a 10–20% safety margin.
3. How should you respond to the contractor's proposal to increase run time?
Read the worked answer
Increasing run time would make it worse: higher runtime does not create more available flow, and it increases runoff on the slope and misting at high pressure. It can also operate nozzles outside their designed pressure range and waste water. The correct fix is hydraulic redesign: separate valves, lower-flow nozzles, pressure regulation, and a cycle-soak schedule. Then adjust run times based on soil moisture or evapotranspiration, not on the symptom.