Converting a 6 ha Cotton Block from Flood to Pressurised Irrigation on a Weak Borewell
A 6.0 ha block of cotton on 1.2 m rows, on a deep sandy loam (field capacity 26% v/v, permanent wilting point 12% v/v, root zone 0.9 m), is to be converted from flood to pressurised irrigation. Peak crop water use is 6.8 mm/day, application efficiency of the existing flood system is 55% and of the proposed sprinkler 75%; rainfall in the peak month is negligible. The borewell delivers a measured 22 m3/h at 18 m drawdown. The pump curve gives 22 m3/h at 32 m total head and 30 m3/h at 24 m head. The distribution main is 350 m of 90 mm HDPE with a friction loss of 6.5 m at 22 m3/h; the sprinkler laterals need 28 m at the nozzle. Power costs Rs 7.20/kWh with pump efficiency 58%. The state gives 60% capital subsidy on drip (Rs 95,000/ha) but none on sprinklers (Rs 42,000/ha). Under flood the farmer pays a flat water charge of Rs 1,800/ha/season; under pressurised irrigation only energy is charged.
1. Compute total available water, allowable depletion, peak net and gross requirement per round, and the irrigation interval. Then state whether the borewell can meet the peak demand for the whole block.
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TAW = (0.26 - 0.12) x 0.9 m = 0.126 m = 126 mm. At 50% allowable depletion, the manageable deficit is 63 mm; at 6.8 mm/day that gives an interval of 63/6.8 = 9.3 days, so about 9 days per round. Net requirement per round = 630 m3/ha x 6 ha = 3,780 m3; gross at 75% efficiency = 5,040 m3. Running hours needed = 5,040/22 = 229 h spread over 9 days, which is about 25.5 h/day of pumping. That is beyond what a single borewell can sustain with rest periods, so the system is short by roughly 10-15% at peak even in the best case. Options in order of cost: split the block into two sets of about 2.7-3.0 ha on alternate rounds, accept a small stress by stretching the interval, or add a second source. Note what the conversion buys: flood at 55% efficiency needs 630/0.55 = 1,145 m3/ha, nearly double the water for the same crop, so pressurising releases the shortfall before any new well is drilled.
2. Check whether the existing pump can operate the sprinkler system, and compute the pumping power and per-round energy cost.
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Total head = static 18 m + main friction 6.5 m + nozzle pressure 28 m + about 1-2 m of fittings = roughly 54 m. The pump curve offers 32 m at 22 m3/h - a 22 m shortfall. Running it further out along the curve (24 m head at 30 m3/h) makes the mismatch worse, so there is no operating point that satisfies both the 28 m nozzle pressure and the quoted flow: the pump is undersized for this system and will simply deliver less water at a distorted nozzle pattern. Remedies are a booster pump, low-pressure sprinklers (15-20 m), or shorter laterals to cut friction. Power = pgQH/n = 1000 x 9.81 x (22/3600) x 54 / 0.58 = about 5.6 kW. Over a 229 h round, energy is about 1,280 kWh, or roughly Rs 9,200 per round for the whole 6 ha, which is about Rs 1,540/ha/round. Over six peak rounds that is around Rs 9,200/ha/season of energy - several times the Rs 1,800/ha flat flood charge, so the water charge comparison alone understates the true operating cost.
3. Recommend between drip and sprinkler for this block, using the subsidy, the water constraint and the pump finding, and list the design conditions that must be met.
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After subsidy, drip costs Rs 95,000 minus 60% = Rs 38,000/ha against Rs 42,000/ha for unsubsidised sprinkler, so drip is also the cheaper option as well as the more water-efficient one - decisive here, because the pump is already short at peak and drip typically saves 35-45% over sprinklers on cotton. It also wets only the root zone along the 1.2 m rows, limiting boll rot and inter-row weed germination. The pump finding reverses the usual argument: the existing pump fails at the 54 m sprinkler head but works comfortably at the 10-12 m head needed by drip, so no booster is required. Design conditions: one lateral per row with 2 L/h drippers at 0.6 m spacing; sand and screen filtration because the borewell carries silt; a fertigation unit, since district groundwater nitrate is high and N can be spoon-fed; and an EC check, because drip does not leach salts laterally the way flood does. Keep a small sprinkler or hose set for germination and emergency.