Station Five: Dry Turf and Runoff at the Same Time
A 12-station rotor system irrigates 1.4 ha of established Kentucky bluegrass at a commercial site. The property manager reports wilted, dry turf on station 5 (the front entry island), while runoff pours across the sidewalk within four minutes of that station starting. A catch-can audit on station 5 used 20 cans on a 4.5 m triangular grid over a 15-minute run: mean catch 4.2 mm, low-quarter mean 2.6 mm. Static pressure at the quick coupler is 480 kPa, but the rotor nozzles are rated at 310 kPa. At the island tip, one head is a mismatched spray head, another is tilted about 10°, and a third is buried 30 mm below grade. The controller runs station 5 for 20 minutes three times per week in July, when reference ET is about 6 mm/day. The soil is a silt loam with an infiltration rate of about 6 mm/h.
1. Calculate the distribution uniformity and application rate for station 5 and interpret both against the site's water demand.
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Uniformity. DU = 100 × (low-quarter mean ÷ overall mean) = 100 × 2.6 ÷ 4.2 = 61.9%, about 62%.
Interpretation: a DU below 70% is generally regarded as poor and below 60% as unacceptable for turf irrigation. Here the driest quarter of the zone receives only about 62% of the average applied, and the wettest areas receive well above the average. To satisfy the driest turf you must over-irrigate the rest - which is exactly what the runoff across the sidewalk shows.
Application rate. 4.2 mm in 15 minutes = 4.2 ÷ 15 × 60 = 16.8 mm/h.
That rate is about 2.8× the soil's 6 mm/h infiltration rate. Runoff in four minutes is not a surprise: 4 minutes × 16.8 mm/h ≈ 1.1 mm applied, but with a tilted, mismatched and buried head the local delivery at the island tip is much higher, and the soil surface is likely sealed. So there are two separate problems: poor uniformity and an application rate above infiltration.
Against demand. Peak ET 6 mm/day × 7 = 42 mm/week. The current schedule delivers 20 min × 3 = 60 min/week × 16.8 mm/h = 16.8 mm/week - only about 40% of ET, and only about 62% of that reaches the driest quarter, i.e. roughly 10 mm/week there. The turf is simultaneously under-watered and over-watered depending on position. The schedule, not just the hardware, is wrong.
2. Rank the corrective actions you would take before the next audit and explain what each one fixes.
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Ranked by effect per dollar and by how much they distort the audit:
- Fix the three defective heads. Replace the mismatched spray head with the correct matched-precipitation rotor nozzle for that arc and spacing; raise the buried head to grade on a swing joint; correct the 10° tilt. A mixed spray/rotor on the same zone has a completely different precipitation rate, so it will always produce a wet/dry pattern no schedule can cure.
- Regulate pressure at the zone. 480 kPa static versus a 310 kPa nozzle rating means roughly 55% over-pressure. Excess pressure shrinks droplet size and produces fogging and wind drift, throws the stream beyond the spacing so head-to-head coverage is lost, and increases flow and runoff. Fit a pressure-regulating valve or PRS stems, then verify with a gauge at a running head, not at a static coupler.
- Check head spacing and alignment. Confirm the island heads are spaced head-to-head (roughly the nozzle's throw) and square to the boundaries. On an island, corner heads should be reduced-arc nozzles with matched precipitation, not full-circle heads turned down.
- Re-audit after 1-3 are done. Any DU measured before the hardware is corrected is measuring the defects, not the design. Target DU above 70%, ideally 75%+.
- Address infiltration. On a 6 mm/h silt loam, plan cycle-and-soak, and consider core aeration and topdressing for the compacted island. Converting the island beds to drip and shrinking the turf area is worth costing if the island stays difficult.
- Then rewrite the schedule using the post-repair numbers, not the old 16.8 mm/h.
Fixing the schedule first would simply redistribute water on a system that cannot apply it evenly.
3. Write a defensible irrigation schedule for station 5 using the measured data, accounting for the low uniformity and the soil's infiltration rate.
Read the worked answer
Step 1 - weekly crop demand. Peak ET 6 mm/day × 7 days = 42 mm/week. Use a crop coefficient near 0.8 for maintained bluegrass if you want to refine it (≈34 mm), but for a first pass use the full ET and adjust from visual/soil-moisture checks.
Step 2 - gross up for uniformity. Net requirement ÷ DU: 42 ÷ 0.62 = 68 mm/week gross with the current hardware. Note this is the argument for repairing uniformity first - going from DU 62% to 78% would cut the gross figure to about 54 mm/week, a saving of roughly 20% of the water on that zone with better turf.
Step 3 - run time. At 16.8 mm/h: 68 ÷ 16.8 = 4.05 hours = about 243 minutes per week.
Step 4 - split for infiltration. Maximum application per cycle before runoff on a 6 mm/h soil is about 6 mm, i.e. 6 ÷ 16.8 × 60 ≈ 21 minutes. Be conservative on a sloping island: use 12-15 minute cycles with a soak interval of 60-90 minutes. Spread the 243 minutes over three irrigation days: about 81 minutes per day, delivered as 3 cycles of 27 minutes or, more safely, 4 cycles of 20 minutes with 60-90 minute soaks, which keeps each application near the infiltration capacity.
Step 5 - timing and control. Run between 03:00 and 07:00 so the canopy dries early. Replace the fixed July schedule with a controller adjusted weekly from ET data or a soil-moisture sensor, and cut back automatically after 10 mm+ of rain. Add a rain sensor and a monthly catch-can spot check on the worst zone.
Step 6 - verify. Probe the soil 100-150 mm deep before the next cycle; if it is still moist, skip. Log run times and any runoff observations, and re-audit DU after the hardware repairs so the schedule can be recalculated from real numbers.