Soccer Field Irrigation Audit with Low Pressure and Slope Runoff
A municipal soccer field is 100 m by 64 m and divided into four irrigation zones with 24 pop-up rotors. Static pressure is 4.2 bar, but dynamic pressure at the last head is 2.4 bar. Nozzle precipitation rate is 22 mm/h; triangular spacing is 18 m. The root zone is 150 mm deep with a management allowed depletion of 50%. Peak evapotranspiration is 6.5 mm/day, application efficiency is 70%, and the water window is 6 hours. The field has a 3% slope and is exposed to 15 km/h wind. After renovation, far zones have dry spots, the slope runs off, and the pump cycles rapidly. The client asks for a one-page schedule and a low-cost fix.
1. Calculate the net and gross daily irrigation requirement and the runtime per zone. Is the 6-hour window feasible?
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Net daily requirement equals ET, or 6.5 mm/day. Gross requirement = 6.5 / 0.70 = 9.3 mm/day. At a 22 mm/h precipitation rate, runtime per zone = 9.3 / 22 = 0.42 h, or about 25 minutes per zone per day. Four zones require about 101 minutes, or 1.7 hours, so a daily schedule fits the 6-hour window. If scheduling every third day, net = 19.5 mm, gross = 27.9 mm, runtime = 76 minutes per zone, and four zones total 304 minutes, or 5.1 hours. That still fits but leaves little margin for cycle-soak and wind. Maximum allowable depletion = 150 mm x 0.50 = 75 mm, which suggests a long theoretical interval, but turf quality and the 3% slope require shorter, more frequent cycles. Use daily or every-two-day scheduling, not maximum depletion.
2. Diagnose the dry spots, runoff, and pump cycling. Which problem is most urgent?
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Dry spots on far zones fit low dynamic pressure at the last head and wind. If nozzles need about 3.0 bar, 2.4 bar breaks up the pattern, shortens throw, and causes poor far coverage. An 18 m spacing in 15 km/h wind is too wide; drift causes uneven catch. Runoff on a 3% slope means application rate exceeds infiltration, so cycle-soak is needed. Pump cycling suggests zone flow below the pump minimum, a pressure-switch problem, or air in the system; rapid cycling can damage the pump and cause pressure surges. The pump cycling is most urgent because it can fail the pump and destabilize pressure. Check the pump curve, zone flows, air release, and pressure switch before changing nozzles.
3. Propose a revised schedule and low-cost hardware changes.
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Schedule: run each zone about 25 minutes per day before dawn. On the slope, split each zone into two 12-13 minute cycles separated by 45-60 minutes to allow infiltration. During 15 km/h wind, expect lower efficiency and monitor catch cans rather than simply adding water. On a three-day schedule, run about 76 minutes per zone in three cycles, but keep the total within the six-hour window. Hardware: first install pressure gauges and a pressure regulator at the valve, replace nozzles with a matched set rated for the actual dynamic pressure, and check the pump impeller and air release. Adjust head spacing or add part-circle heads at edges if coverage remains poor. Low-cost sequence: measure pressure and flow, correct pump cycling, then change nozzles; do not increase run times to compensate for a pressure problem.