Testing a Dealer's Sprinkler Claim Against a Low-Yielding Well
Mahendra has 1.5 ha of cumin on a 150 m × 100 m field in Barmer district. His dug well yields about 6,000 L/h in the rabi season and the pump can run 8 hours a day. The soil is sandy loam with an infiltration rate of 15 mm/h. Cumin at branching uses about 5 mm of water a day at peak. A dealer offers a sprinkler system designed for 3 kg/cm² at the pump, with sprinklers spaced 12 m × 12 m, and claims it will irrigate 1 ha in 4 hours. Mahendra has enough money for either the sprinkler system or a second 5 HP pump, but not both. He currently uses border irrigation with 25 m × 10 m borders. The well has never been tested for long-duration pumping.
1. Test the dealer's claim with a calculation. Is 1 ha in 4 hours physically possible with this well?
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A 25 mm application on 1 ha is 0.025 m × 10,000 m² = 250 m³. With a well yield of 6,000 L/h, or 6 m³/h, the time needed is 250 ÷ 6 = about 41.7 hours. In 4 hours the well can supply only 24 m³, which spread over 1 ha is 2.4 mm, not 25 mm. The dealer's claim is therefore physically impossible with this well unless another water source is connected. The claim may refer to the sprinkler hardware's capacity at a higher pump discharge, not to Mahendra's well. There is also a crop-level check: 5 mm/day over 1.5 ha is 75 m³/day, while the well gives only 48 m³/day in 8 hours. Even without losses, the well cannot meet peak demand over the whole 1.5 ha. The sprinkler can improve uniformity and efficiency, but it cannot create water.
2. Design a feasible sprinkler layout and operating schedule. State the assumptions.
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Assume the well can sustain 6 m³/h for 8 hours and the pump delivers the required pressure. With 12 m × 12 m spacing, each sprinkler covers 144 m². If 10 sprinklers of 600 L/h each are operated at a time, the total is 6 m³/h and the application rate is 600 L/h ÷ 144 m² = 4.17 mm/h, which is below the 15 mm/h infiltration rate, so runoff should be limited. The set area is 10 × 144 = 1,440 m², or 0.144 ha. A 25 mm application on that set requires 0.025 × 1,440 = 36 m³, or 6 hours at 6 m³/h. Seven sets cover about 1 ha, so one set per day takes seven days to apply 25 mm, which is only about 3.6 mm/day. That is below the 5 mm/day peak demand. The practical conclusion is to install sprinklers on about 1 ha, mulch heavily, and accept mild deficit irrigation, while keeping the remaining 0.5 ha out of the peak-demand crop or under a lower-water strategy. Check the well during a continuous 8-hour test before committing.
3. Should Mahendra buy the sprinkler or the second pump? Give a reasoned recommendation, including long-duration well testing.
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A second 5 HP pump does not increase the well's yield. If the aquifer cannot sustain 6 m³/h, a larger pump may only lower the water level, increase suction lift and risk the well drying. The sprinkler is the better investment only if it is matched to the available water, because it improves application efficiency and uniformity compared with long borders. The recommendation is: first carry out an 8-10 hour pumping test and record the discharge and drawdown. If the specific capacity is stable, buy the sprinkler for about 1 ha, not 1.5 ha, and mulch the field. If the well yield falls sharply during the test, do not expand the irrigated area; improve water harvesting or reduce the cumin area instead. The border system can be retained on the remaining area. Buying the second pump without a well test would only add cost and risk, not water.