A tomato drip block where the top rows wilt and the bottom rows drown
A 0.8 ha tomato block (80 m × 100 m) in Ganjam district is fed by a 5 hp submersible in a borewell at the lowest corner of the field. The farmer laid 16 mm inline laterals with 2 L/h drippers at 40 cm, laterals 1.5 m apart and 80 m long, running upslope. The block rises about 6 m from the sub-main to the far end. A pressure gauge just downstream of the screen filter reads 1.5 bar when the block valve is fully open, and a temporary gauge teed into the far end of a top-row lateral reads 0.3 bar. Dripper discharge collected near the sub-main averages 2.3 L/h; at the far end of the same lateral it averages about 1.0 L/h. Tomato plants in the bottom rows show waterlogging and standing water between beds, while plants in the top 10 rows wilt by afternoon although the pump runs 90 minutes daily. The whole block is irrigated in one shift. The farmer has replaced three top-row laterals with new ones of the same specification and the wilting returned within a week. The dealer says the pump is the problem and has quoted a 7.5 hp replacement.
1. Using the two gauge readings, separate the pressure loss into its elevation and friction parts, and test the emission figures against them. Is the dealer's diagnosis correct?
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Head budget along the lateral
- Gauge difference = 1.5 − 0.3 = 1.2 bar ≈ 12.2 m of water.
- Static elevation from sub-main to far end = 6 m = 0.59 bar.
- Therefore friction over 80 m ≈ 0.61 bar (6.2 m), i.e. about 0.078 m per metre.
Is that friction believable? Each lateral carries 80 m ÷ 0.4 m = 200 emitters × 2 L/h = 400 L/h. In a 16 mm lateral (ID about 13.8 mm) that is roughly 0.75 m/s, and 6 m of loss in 80 m at that flow is on the high side but within the range shown on manufacturer charts for a long lateral at full flow. So the number is consistent with the layout, not with a fault.
Check the drippers against the pressures Discharge varies as the square root of pressure: (1.0 ÷ 2.3)² = 0.19. Pressure ratio = 0.3 ÷ 1.5 = 0.20. The two match almost exactly. The drippers are behaving normally; they are simply being fed at one-fifth of the intended pressure at the far end. That also explains why replacing laterals changed nothing.
Conclusion: the dealer is wrong. The pump is delivering 1.5 bar at the filter; the loss is created inside the block by elevation plus lateral friction, made worse because all 53 laterals run together. A 7.5 hp pump would raise inlet pressure but would increase waterlogging in the bottom rows, stress fittings and still leave poor uniformity, because there is no pressure compensation.
2. Propose two hydraulic corrections, with the pressure each one must achieve, and state which you would recommend first.
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Option A - halve the lateral run with a mid sub-main (recommended first) Run a second sub-main along the middle of the block so laterals are 40 m, not 80 m.
- Elevation component falls from 6 m to 3 m (0.3 bar).
- Friction on half the length at half the flow falls to roughly one-quarter to one-third, about 1.5–2 m (0.15–0.20 bar).
- New far-end pressure ≈ 1.5 − 0.3 − 0.2 = about 1.0 bar, which is normal working pressure, giving an emission uniformity in an acceptable range. Cost is roughly 100 m of extra sub-main plus tees and a valve - a fraction of a new pump, and it also makes zonal operation easier.
Option B - pressure-compensating (PC) drippers PC drippers hold a fixed flow down to about 1.0 bar. Required sub-main pressure = 1.0 (far end minimum) + 0.6 (elevation) + 0.2–0.3 (friction) = 1.8–1.9 bar. The existing pump may reach this if the block is split into two zones, because halving the flow moves the pump up its curve; this must be checked against the pump curve and filter losses. Penalties: PC drippers need finer filtration (120–150 mesh) and regular flushing, and cost more per emitter.
Option C - shorter or larger laterals: 50 m laterals, or 20 mm laterals at the same emitter spacing, reduce friction but do nothing for the 6 m elevation.
Recommendation: Option A first - it fixes the hydraulics at low cost and keeps the existing drippers. Add a permanent pressure gauge at the far end of the worst lateral and a valve or bypass to regulate inlet pressure. If the farmer later wants PC drippers, the same mid sub-main still helps.
3. What operating and crop advice would you give, and how would you answer the farmer's plan to buy the 7.5 hp pump?
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Operating advice
- Split the block into two shifts (about 26–27 laterals each). Inlet pressure will rise when flow halves, so regulate it with the bypass or a valve and re-check the far-end gauge; do not simply open everything.
- Flush each lateral at the end, one at a time, at about 0.5 bar above operating pressure - monthly, after fertigation, and whenever the screen clogs.
- Keep the screen at 120 mesh or finer and clean it on a pressure differential of about 0.3–0.5 bar.
Crop advice
- The two symptoms have opposite causes: bottom rows are over-irrigated (standing water, risk of Phytophthora and Fusarium root rots), top rows are under-irrigated. Correcting the hydraulics fixes both.
- Sanity-check the 90-minute run. With tomato Kc around 1.15 and pan evaporation near 5 mm/day, ETc ≈ 5.8 mm/day = 58 m³/ha/day, about 46 m³/day for 0.8 ha. Split this into two or three shorter pulses rather than one long run, and confirm with a soil-moisture check at 15 cm.
- Inspect the bottom rows for root disease before the next irrigation and improve drainage between beds.
On the 7.5 hp quote Tell the farmer plainly that the pump is not the cause and that a bigger pump will make the bottom-row waterlogging worse, raise power bills, and may exceed the pressure rating of the 16 mm laterals and fittings. Ask the dealer for the pump curve at the design head. Spend the money on the mid sub-main, gauges and zoning first; consider a VFD only after the block hydraulics are corrected. Get the corrected design in writing, with the predicted far-end pressure.