Drip section design on a 0.6 ha banana block with an 8-hour power supply
Vikas Kulkarni has 0.6 ha of Grand Naine banana at Barshi, Solapur, planted at 1.5 m x 1.5 m (4,444 plants/ha) and irrigated from a 12 m open well. In October the well sustains about 20,000 L/h and the pump runs only during the 8 hours of agricultural power supply. The drip system has 4 pressure-compensating drippers of 4 L/h per plant, a sand filter rated at 10 m3/h and a 130-micron screen filter. At present the entire 0.6 ha is operated as a single section. October pan evaporation is 5 mm/day, the crop coefficient is 1.0, and he wants to apply the full daily crop water requirement with 90% application efficiency. The neighbouring furrow-irrigated banana of the same age receives 45 mm every 6 days, and the tail of that field stays waterlogged for a day after each irrigation.
1. Calculate the system discharge for a single section and decide whether the well and power supply can support it. Then design the number of sections and the daily running time.
Read the worked answer
Dripper discharge. 0.6 ha carries 0.6 x 4,444 = 2,666 plants. At 16 L/h per plant (4 drippers x 4 L/h): 2,666 x 16 = 42,700 L/h, i.e. about 42.7 m3/h for one section.
Can the well support it? No. The well gives about 20 m3/h, so a single-section run would draw the well down faster than it recovers, the pump would suck air and the filters would never backwash properly.
Section design. Each section must stay comfortably below 20 m3/h. Three sections of 0.2 ha give 0.2 x 4,444 x 16 = 14.2 m3/h per section, which leaves a safe margin. Two sections would need 21.3 m3/h each, slightly above the well yield - too close to the limit. Use three sections.
Daily requirement. 5 mm over 0.6 ha = 5 L/m2 x 6,000 m2 = 30 m3; at 90% efficiency, 30/0.9 = 33.3 m3/day. Running sections one at a time at 14.2 m3/h, the total pumping time is 33.3/14.2 = about 2.4 hours, well inside the 8-hour window - roughly 45-50 minutes per section.
Caveat: the sand filter rated at 10 m3/h is undersized even for a 14.2 m3/h section. It must be upgraded to at least 20 m3/h (or two filters in parallel), otherwise the pressure drop will starve the drippers.
2. What would go wrong if he continued to run the whole block as one section? Mention the filter, the pump and the drippers specifically.
Read the worked answer
Running 42.7 m3/h against a 20 m3/h well means the demand exceeds recharge for the whole run.
- Well and pump: the water level falls below the suction, the pump begins to cavitate, deliver less, and may run dry and burn out its winding. Frequent start-stop on a falling water table is the usual failure mode.
- Filters: a sand filter rated 10 m3/h forced to pass 42.7 m3/h churns its media, loses the graded bed, and passes dirt downstream; the backwash cycle also needs a flow and pressure it will not get. Screen filters then block within a few days.
- Drippers: pressure-compensating drippers work within a stated pressure range, typically around 0.5-3.5 bar. Starved of pressure they drop below that range, stop compensating, and the tail-end plants receive far less than the head-end ones. The block then looks uneven and the farmer typically over-runs the system, making the drawdown worse.
- Water and salt distribution: non-uniform application creates wet and dry zones, uneven bunch weights, and salt build-up at the drier margins.
The correct response is not a longer run but a section plan plus a properly sized filter.
3. Compare his daily water use with the neighbour's furrow practice and comment on the waterlogged tail of the furrow field.
Read the worked answer
Neighbour's furrow: 45 mm every 6 days = 45/6 = 7.5 mm/day. Vikas's drip target is 5 mm/day at the crop. So the furrow field is applying about 50% more water than the crop can use - and that is before accounting for the fact that furrow application efficiency is commonly 50-60% against the 90% assumed for drip, which makes the gap in useful water even wider.
The waterlogged tail has three costs:
- Aeration: banana roots need oxygen; a day of standing water in the root zone suppresses root activity and invites Erwinia soft rot and Pectobacterium-type problems.
- Leaching and nutrient loss: the excess water carries nitrate and potassium below the root zone, so the neighbour pays for fertilizer he never uses.
- Salt and disease pattern: waterlogging at the tail is nearly always a levelling and stream-size problem, not a water-quantity problem. Shortening the run, reducing the stream and re-levelling the tail is cheaper than drilling another well.
Vikas should not copy the 45 mm dose. His correct comparison is the 5 mm/day crop demand, and he should schedule by pan evaporation and check the dripper output at the far end with a measuring can once a fortnight.