Can a 5 cm Shallow Tube Well Irrigate 1.5 ha of Boro Rice?
A farmer in Kamrup owns a 5 cm diameter shallow tube well and 0.8 ha of boro rice. A field test with a bucket and stopwatch shows the well delivers 6 litres per second at the outlet. The pump is run for 8 hours a day on diesel. The field has a percolation rate of 4 mm/day and peak evapotranspiration in April is 6 mm/day; field application efficiency is estimated at 60%. The farmer irrigates by flooding basins and usually keeps 5 cm of standing water. He is offered an additional 0.7 ha of adjacent land on lease for the same boro season and asks whether his existing pump can irrigate 1.5 ha in total, and how many hours he would have to run it during the peak period. Diesel costs Rs 92 per litre and the pump consumes 1.2 litres per hour.
1. Calculate the gross daily water requirement for 1.5 ha at peak and the hours the pump must run. State whether the existing 8-hour schedule is adequate.
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Peak daily net water use is evapotranspiration plus percolation: 6 + 4 = 10 mm/day. Over 1.5 ha (15,000 m2), net requirement = 0.010 m × 15,000 m2 = 150 m3/day.
With 60% field application efficiency, gross requirement = 150/0.60 = 250 m3/day. Pump discharge is 6 L/s = 21.6 m3/hour. Required running time = 250/21.6 = 11.6 hours/day.
An 8-hour schedule delivers 8 × 21.6 = 172.8 m3/day gross, which is about 77 m3 short of the peak requirement. The same calculation for the original 0.8 ha gives 80 m3 net, 133 m3 gross and about 6.2 hours/day, so the present schedule works only for 0.8 ha. The 5 cm standing water is a buffer, not a substitute for the daily deficit; if the shortfall continues, the water level will fall and stress will appear. The existing schedule is not adequate for 1.5 ha.
2. Evaluate the options: extend pumping hours, improve application efficiency, reduce the area, or develop another source. Support the comparison with diesel cost and agronomic risk.
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Extending to 11.6 hours/day requires 13.9 litres of diesel per day (11.6 × 1.2), costing about Rs 1,281/day; over a 90-day peak season this is roughly Rs 115,000. At 8 hours the cost is about Rs 883/day, so the extra 3.6 hours costs about Rs 398/day, or Rs 35,800 over the season. That is likely cheaper than developing a new tube well, but the well must actually sustain 11.6 hours without excessive drawdown.
Improving application efficiency from 60% to 75% cuts gross demand to 200 m3/day and running time to 9.3 hours, saving about 2.3 hours/day. Land levelling, smaller basins, well-sealed bunds and alternate wetting and drying are practical. Reducing the leased area to about 1.0 ha would bring demand within about 7.7 hours/day. A second source or a larger pump is justified only if the lease adds enough net income to cover the extra diesel and capital. The agronomic risk is that a marginal shortfall at panicle initiation causes proportionally larger yield loss than the same water deficit at tillering.
3. What practical measurements would you make before advising on the expansion, and what water-management practices would you prioritise?
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Measure actual discharge at the field outlet, not at the pump, using a calibrated container and stopwatch; repeat at least three times. Record the static and pumping water levels, hours actually run, area commanded, depth applied and time per basin. Use a drum or ring infiltrometer to check percolation, and check distribution uniformity by measuring water depth at several points after irrigation. Confirm the pump's fuel consumption and the reliability of the diesel supply.
Prioritise laser or precision land levelling, repairing bunds and inlet channels, reducing plot size, and adopting alternate wetting and drying with a field water tube rather than continuous deep flooding. Irrigate at critical stages and avoid keeping unnecessary depth during tillering. Keep a simple daily log of hours, fuel and water depth so the farmer can see whether the expanded area is being served before the crop reaches the reproductive stage.