Sprinkler sets on a sealed sandy loam where water runs off before the profile is wetted
A 0.6 ha groundnut field (90 m × 67 m) on sandy loam in Khurda district has a basic infiltration rate of 8 mm/h measured with a double-ring infiltrometer. The farmer uses hand-move sprinklers: 3 mm single nozzle, 2.5 kg/cm² at the nozzle, 1.1 L/s per sprinkler, spacing 12 m × 12 m, 8 sprinklers per shift, and a 3 hp monoblock pump with 5 m suction lift from an open well. In the afternoon winds of 12–18 km/h the outer sprinklers throw short and the farmer compensates by running each set for 2 hours, once a week. After about 40 minutes of running, water begins to flow off the lower end of the field, the soil surface seals and small rills appear. The farmer reports poor pod filling and says the crop "dries even after irrigation". Pan evaporation at this stage is 5.5 mm/day and there is no rain.
1. Calculate the application rate and compare it with the soil's intake rate. Explain what is actually happening during the 2-hour set.
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Application rate
- Sprinkler discharge = 1.1 L/s × 3,600 = 3,960 L/h.
- Area wetted per sprinkler = 12 m × 12 m = 144 m².
- Application rate = 3,960 ÷ 144 = 27.5 mm/h.
Comparison with the soil 27.5 ÷ 8 ≈ 3.4 times the basic infiltration rate. The soil cannot accept water at the rate it arrives.
What the 2-hour set really does
- Gross applied = 27.5 mm/h × 2 h = 55 mm.
- Runoff begins as soon as surface detention is filled - typically 15–25 minutes, which matches the 40 minutes the farmer observes once the surface has sealed.
- Of the 55 mm applied, only a fraction infiltrates; the rest leaves the lower end, carrying topsoil and nutrients and causing the rills. Effective storage may be only 25–35 mm, and it is unevenly distributed, so the lower end gets more water and the upper end less.
- The surface seal is caused by the impact energy of the drops on bare sandy loam: the fine particles disperse and block the pores, so the real intake rate falls below the measured 8 mm/h as the set proceeds.
Diagnosis: the problem is the delivery rate and the set pattern, not a shortage of water. The farmer is applying nearly a week's water requirement in a single event, then losing much of it as runoff.
2. What daily crop water requirement must be met at pod development, and what irrigation interval and application depth would you prescribe? Show that pump capacity is not the limiting factor.
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Requirement
- ETc = pan evaporation × Kc = 5.5 × 1.05 ≈ 5.8 mm/day.
- With about 70% application efficiency, gross requirement ≈ 5.8 ÷ 0.70 = 8.3 mm/day.
Constraint check The system can apply 27.5 mm/h, so 8.3 mm needs only about 18 minutes of running. Capacity is therefore not the problem; the intake rate is. Applying 8.3 mm at 27.5 mm/h is still more than three times the soil's 8 mm/h, so even a short set will pond and run off unless it is pulsed.
Prescription
- Net depth per event: 15–20 mm, every 2–3 days (20 mm ÷ 5.8 mm/day ≈ 3.4 days; 15 mm ≈ 2.6 days). Use 15 mm every 2 days in hot, windy weather, 20 mm every 3 days if it is calmer.
- Because the application rate is fixed by the nozzle and spacing, deliver that depth in pulses: 10–12 minutes on, 30–40 minutes off, repeated until the depth is reached. The off period lets the sealed surface layer re-wet and the profile drain.
- After the first pulse, check with a spade: if water is still standing, lengthen the off period rather than shortening the on period.
- Weekly 55 mm, even split into two 27 mm events, is wrong: much of it leaves the field and the crop goes into stress between events because the root zone never holds enough.
Note on efficiency: using 70% is reasonable for a hand-move system in wind. If wind drift and evaporation are worse, use 60% and increase the depth.
3. The farmer blames low pressure at the far sprinklers. Check whether the 3 hp pump can run 8 sprinklers, and recommend changes to the hardware and to the operating schedule.
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Pump check
- 3 hp ≈ 2.24 kW shaft power. At about 60% overall efficiency and 25 m head (2.5 kg/cm²), Q ≈ (0.6 × 2,200) ÷ (9,810 × 25) ≈ 0.0054 m³/s ≈ 5.4 L/s.
- At 1.1 L/s per sprinkler that is roughly 4–5 sprinklers at once, not 8. Running 8 sprinklers divides the flow, so each gets far less than 1.1 L/s, pressure at the nozzle collapses well below 2.5 kg/cm², droplets become small and wind drift worsens. The farmer's observation is correct; the cause is overloading the pump, not a defective sprinkler.
Hardware and schedule changes
- Run 4 sprinklers per shift and increase the number of shifts. Check pressure with a gauge at the last sprinkler; if it is below about 2.0 kg/cm², reduce to 3 sprinklers.
- Irrigate early morning or at night, when wind is under 5 km/h. This single change usually improves uniformity more than any hardware purchase.
- Reduce the spacing in the direction of the prevailing wind (for example 12 m × 10 m) if night irrigation is impossible; do not widen spacing to cure runoff, because it damages uniformity.
- Keep the 3 mm nozzle but consider 2.5 mm if the pump cannot hold pressure - flow falls by about 30%, giving roughly 19 mm/h, still above the intake rate, so pulsing remains essential.
- Check suction lift and the intake strainer; 5 m lift with a partially blocked strainer is a common hidden cause of low pressure.
- Consider a small drop in operating pressure only after the flow is matched to the pump - pressure is needed at the nozzle for droplet size.
Summary: the fix is four-part - match sprinkler number to the pump, pulse the sets to beat the 8 mm/h intake limit, irrigate at night, and keep total depth at about 8–9 mm/day gross rather than 55 mm per week.