HOA Irrigation Audit: Mixed Heads, Runoff, and a Defensible Schedule
A 1.6 ha homeowners association common area has 0.9 ha of tall fescue lawn and 0.7 ha of shrub beds. One 12-zone controller serves the site. Static pressure at the meter is 520 kPa and dynamic pressure is 380 kPa at 34 m3/h. Symptoms include dry spots on lawn zone 3 near a 12% slope, runoff from zone 5 within four minutes, overspray onto walks from bed zones, a water bill 38% above last year, and pump short-cycling. The lawn zones mix 15 m throw rotors spaced 12 m apart with pop-up sprays on the same valve. Bed zones use the same rotors as turf. No check valves or pressure regulation are installed. The controller has only one start time and a 30% seasonal adjust. July reference evapotranspiration is 6 mm/day, and the fescue crop coefficient is 0.8. The association wants a repair list and a defensible watering schedule before the next board meeting.
1. Prioritize the irrigation audit findings and propose a retrofit scope that addresses the symptoms.
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Prioritize safety and water waste first, then scheduling. The mixed 15 m rotors and sprays on one valve create different precipitation rates, so part of the zone is overwatered and part is dry; that explains the dry spots and runoff. Dynamic pressure of 380 kPa is too high for pop-up sprays, causing misting and overspray. No check valves allow low-head drainage and short-cycling, and using turf rotors in beds oversprays walks. The 12% slope needs separate control and cycle-and-soak. Retrofit scope: install a pressure-regulating master valve or zone regulators at 300 kPa, replace sprays with matched nozzles, and set rotor and spray zones separately. Add check valves to all heads on slopes. Convert bed zones to pressure-compensating drip with a filter and flush valves. Split the slope into its own zone and add a smart controller with flow monitoring and a rain sensor. Repair the pump cycling by matching zone flow to the pump curve. Then commission with a catch-can audit and a distribution uniformity test before setting the schedule.
2. Using a measured application rate of 12 mm/h and a distribution uniformity of 0.70, calculate a July schedule for the lawn, and explain how you would adjust it for the slope and the association's three-day watering window.
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July ETc = ETo x Kc = 6 mm/day x 0.8 = 4.8 mm/day. With DU 0.70 and application rate 12 mm/h, the runtime to replace daily ETc is 4.8 / (0.70 x 12) = 0.57 h, or 34 minutes per day. For a three-day window, weekly crop water use is 4.8 x 7 = 33.6 mm; per event that is 33.6 / 3 = 11.2 mm. Runtime per event = 11.2 / (0.70 x 12) = 1.33 h, or 80 minutes. Do not apply 80 minutes continuously on the 12% slope. Use cycle-and-soak: three cycles of about 27 minutes with 60 minutes between cycles, or four cycles of 20 minutes, and stop if runoff begins. Set the controller to the three allowed days and use a 100% seasonal adjust in July, not 30%. Check catch-can depths after the first week; if the lowest cans are below 70% of the average, increase runtime by 10% and re-audit. For beds, use a separate drip runtime based on emitter output, not the lawn schedule.