Sprinkler Layout in Wind on a Clay Loam
Priya manages a 0.9 ha wheat block, 90 m × 100 m, on clay loam that begins to pond at about 8 mm/h. Peak crop use is 6 mm/day. Afternoon wind runs 15–25 km/h. A 10 HP submersible delivers 15 m³/h at 3.5 bar at the pump head, and the field rises 2 m from the pump to the far corner. The contractor proposes 4 mm nozzles rated 1.5 m³/h at 3.0 bar with a 12 m wetted diameter, mounted every 12 m on 100 m laterals, laterals 18 m apart, one lateral running at a time. Priya wants to irrigate in the afternoon because that is when labour is available.
1. Check the proposed spacing and application rate. Does the layout work agronomically, and what would you change?
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Application rate at 12 m × 18 m: 1,500 L/h ÷ (12 × 18 = 216 m²) = 6.9 mm/h - under the 8 mm/h infiltration limit, so on paper it does not pond.
But the spacing fails the overlap test. A 12 m wetted diameter means a 6 m throw radius. Laterals 18 m apart leave strips roughly 6 m wide that receive little or no water between lines, and in a 15–25 km/h wind the pattern is pushed further downwind, so the downwind edge of each strip is starved while the upwind edge ponds. Wheat will show visible striping and yield loss long before the farmer notices the filter gauge.
Change: bring lateral spacing in to about 12 m (and, if the wind is strong at the chosen time, to 9–12 m across the wind), which raises the application rate to about 10.4 mm/h at 12 × 12. That is above the 8 mm/h limit, so pair the closer spacing with cyclic (pulse) operation - for example 1.2 h on, about 1 h off, 1.2 h on - so the effective application rate stays under infiltration while overlap is adequate. Irrigating at night or early morning is the other half of the fix.
2. Calculate set time, number of sets and daily pumping hours for the corrected layout, and check the pump capacity.
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Sprinklers per lateral: 100 m at 12 m spacing gives 9 sprinklers × 1.5 m³/h = 13.5 m³/h, inside the 15 m³/h the pump delivers at 3.5 bar. One lateral at a time is the right choice.
Laterals: field width 90 m at 12 m spacing needs 8 lateral positions.
Depth per cycle: at 6 mm/day a 4-day cycle needs 24 mm net; with a 75% application efficiency the gross depth is about 32 mm. On-time = 32 ÷ 10.4 ≈ 3.1 h, split into two pulses of about 1.5 h with a rest interval, so about 4.5 h elapsed per set.
Cycle load: 8 sets × 3.1 h = 25 h of pump run per cycle, roughly 6 h/day, but about 9–10 h/day of elapsed time because of pulsing.
Pressure check: 3.0 bar at the nozzle + ~1.4 m lateral friction (with multi-outlet factor) + 1 m riser + 2 m elevation ≈ 3.44 bar needed against 3.5 bar available - no margin. Put a gauge on the last sprinkler of the longest lateral; if it reads below about 2.8 bar, drop to 8 sprinklers per lateral.
3. Priya insists on afternoon irrigation. Explain the consequences and what you would tell her to do instead, including any scheduling change.
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In 15–25 km/h afternoon wind, a substantial share of the spray drifts off the target strip and evaporates before landing. The result is a distorted pattern (wet upwind, dry downwind), lower application efficiency, and the need for longer sets to put the same depth into the root zone. Combined with the day's highest evaporation, the afternoon option wastes both water and pump hours.
What to tell her:
- Shift the sets to night or early morning (roughly 8 pm–8 am), when wind and evaporation are at their lowest; the pump runs the same hours, so labour can be scheduled as a night shift with a single operator and a timer rather than casual labour on call all afternoon.
- If some afternoon running is unavoidable for the last set, accept only that one set and increase the depth slightly to compensate for drift loss.
- Keep the pulse pattern in the afternoon too - the infiltration limit does not change with wind, and drift makes ponding at the upwind edge more likely.
Scheduling note: with night running, apply 24–25 mm every 4 days rather than small daily doses; this matches the pump's realistic daily window and keeps the labour requirement low.