1.5 ha of Onion, 48,000 Litres a Day: Allocating a Deficit

Peak demand

• 4.5 mm/day = 4.5 litres per m². Over 15,000 m² (1.5 ha) that is 67,500 L/day of net crop use.
• At 90% drip efficiency the gross requirement is 67,500 / 0.90 = 75,000 L/day.
• Supply is 8,000 × 6 = 48,000 L/day. Deficit = 27,000 L/day, about 36% of gross demand (29% of net).
• At 90% efficiency the well can meet full demand on only 48,000 × 0.90 / 45,000 ≈ 0.96 ha.

Spreading 48,000 L over 1.5 ha gives about 3.2 mm/day gross, roughly 2.9 mm net - a 35% deficit. Onion can survive that at bulbing only with mulch and careful management, and not for a whole month.

Where the water goes first

1. Fix the system before allocating anything. Clean the filters, acid-flush the laterals and check emitter discharge in the uneven block. Lifting that block from about 60% to 90% efficiency frees water for the whole field at no extra cost.
2. Then give full water to the 1.0 ha that reached bulb formation earliest and has the better soil and the more uniform drip block.
3. Run the remaining 0.5 ha on a planned deficit (70–80% of ET) with mulch.
4. Never spread the same small depth over all 1.5 ha. That gives every plant a deficit and secures no full yield anywhere, whereas concentrating water secures a marketable crop on part of the area.

Using the 5 cm canal turn

50 mm = 50 litres/m². At 4.5 mm/day that is about 11 days of full ET for 1 ha, or 22 days at half duty. It must be timed to coincide with the start of bulb formation, the most sensitive stage, not spread thin across the whole area.

15-day plan

• Days 1–2: clean filters, flush laterals, repair the uneven block, and run 10 mm through it to check uniformity.
• Mulch the deficit 0.5 ha with about 5 t/ha of crop residue. This alone cuts evaporation by roughly 20–30%, worth about 1 mm/day - the cheapest water on the farm.
• Irrigate the 1.0 ha at full ET (4–5 mm/day net) in two or three pulses; run the 0.5 ha at no more than 70–80% of ET on alternate days.
• Canal water: apply the 50 mm to the 1.0 ha in two splits of 25 mm at the start of bulb formation, so it infiltrates rather than running off or percolating below the root zone.
• Stop irrigation 10–12 days before harvest. Last year's splitting and rot came from heavy late water after the unseasonal shower; mature bulbs need a dry finish.
• Check moisture at 15 cm before every extra turn: if a soil ball holds together when squeezed, skip the turn. Irrigating by surface appearance, as last year, is what wasted the well.

Watershed package, in this order

1. Treat the catchment first: contour trenches, continuous contour bunds and recharge pits on the upper fields, plus gully plugs. A pond built before the catchment is treated silts up within 3–4 years.
2. Check dam or cement nala bund on the second-order stream. This is what actually slows the flow and recharges the aquifer that feeds the borewells.
3. Farm pond of about 1,000 m³ for protective irrigation. At 45 m³ per ha per day at peak ET, 1,000 m³ is roughly 22 days of full water for 1 ha, or a 5 cm supplemental dose over 2 ha. It is a life-saver at bulb formation, not a substitute for the well.
4. Recharge shaft near the borewell, plus a simple measuring device (marked stick or V-notch) so the group knows what it actually has.

Does joining pay?

• A 1,000 m³ pond with earthwork and lining costs roughly ₹8–12 lakh before subsidy. An onion crop on 1 ha grosses ₹2.5–3.5 lakh; protecting a quarter of the yield is worth ₹60,000–90,000 a year. On one farm the payback is 5–10 years; shared by 4–5 farmers it is much shorter.
• The non-monetised benefits - recharge of drinking-water wells, reduced soil loss, one protective irrigation at the critical stage - are what justify a group investment.

Conditions before joining: a written water-sharing and maintenance rule, a silt-trap cleaning schedule, and an agreement not to expand the irrigated area beyond what the watershed can recharge. Without those, the pond becomes a silt trap and the borewells keep falling.