Drip System Under a Six-Hour Power Window
A farmer in Solapur has 0.4 ha (4,000 m²) of rabi onion under drip. Laterals are laid 1.2 m apart on flat beds, with in-line emitters of 4 litres/hour spaced 0.4 m along each lateral. The borewell yields 25 m³/hour and the pump sets are energised only between 10 pm and 4 am. Peak crop water requirement in January is 6 mm/day. The irrigation water has an EC of 0.9 dS/m and onion is moderately sensitive, so a leaching fraction of about 15% is needed. The farmer presently runs the whole system for 90 minutes every night in a single shift. He complains that the rows farthest from the head are dry, although the water meter shows about 55 m³ used per day. He plans to run the system for 2 hours to cure the dry rows. Emitters near the head show a weak, uneven jet.
1. Calculate the system discharge, the net and gross daily water requirement and the run time needed. Is the present 90-minute shift over- or under-irrigating?
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System discharge
- Total lateral length = 4,000 m² ÷ 1.2 m = 3,333 m
- Number of emitters = 3,333 ÷ 0.4 = 8,333
- Discharge = 8,333 × 4 L/h = 33,333 L/h ≈ 33.3 m³/h
Requirement
- Net demand = 6 mm × 4,000 m² = 0.006 m × 4,000 = 24 m³/day
- With a 15% leaching fraction = 24 × 1.15 = 27.6 m³/day
- Run time = 27.6 ÷ 33.3 = 0.83 h ≈ 50 minutes
Verdict: the 90-minute shift applies about 50 m³, roughly double the requirement, and the meter reading of 55 m³ is consistent once end-flushing is added. So at field scale he is over-irrigating, not under-irrigating. The dry far rows therefore reflect non-uniform distribution, not shortage of water. Running 2 hours would double the excess at the head, leach nitrogen, waterlog the onion bed and invite bulb rot, while the far rows would still be dry.
2. Given the 25 m³/h borewell and the 10 pm-4 am power window, restate the schedule in blocks and show that it fits. Explain why longer running will not cure the dry rows.
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Splitting the block
- The system needs 33.3 m³/h, but the borewell gives only 25 m³/h, so running all laterals at once starves the far end - this alone explains part of the unevenness.
- Divide the 0.4 ha into two 0.2 ha blocks: each needs 16.7 m³/h, comfortably within the pump.
- Water per block = 27.6 ÷ 2 = 13.8 m³ → 13.8 ÷ 16.7 = 0.83 h ≈ 50 minutes per block.
- Total 1 h 40 min, plus about 10 minutes of end-flushing per block = about 2 hours, well inside the 6-hour window. He can therefore irrigate daily and still have time in hand.
Why longer running fails: the dry rows are a distribution problem, so extra minutes only add water where it already arrives. Check, in order: pressure at the first and last lateral (a drop of more than 20% means the laterals are too long or the submain too small - a 16 mm lateral at 4 L/h and 0.4 m spacing is normally limited to about 60-70 m, so a 100 m run must be split); emitter clogging, indicated by the weak uneven jet at the head - flush the lateral ends and treat for scale if present; and bed level, since even 5 cm of fall concentrates water at one end.
3. Suppose the borewell drops to 15 m³/h in April during bulb development. What would you change, and what would you refuse to change?
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Recalculate first: the crop still needs about 27.6 m³/day gross. At 15 m³/h he needs about 1.85 hours of pumping, or roughly 55 minutes per 0.2 ha block.
- Because 16.7 m³/h per block exceeds the new yield of 15 m³/h, split into three blocks of about 45 minutes each, or run two blocks with a small top-up the next night.
- Keep the 15% leaching fraction - at EC 0.9 dS/m, cutting leaching to save water simply lets salt accumulate in the bulb zone, and onion is only moderately tolerant.
- Apply a straw or crop-residue mulch to cut evaporation, irrigate only at night, and keep the beds level so no water is lost at the tail.
What not to change: do not stretch the interval to 3-4 days to compensate. Onion is shallow-rooted, and moisture stress during bulb development causes split and thick-necked bulbs, which store badly and fetch a lower price. If water truly runs short, accept a modest deficit - about 80% of ETc - spread evenly, rather than a heavy deficit at the critical stage.