Scheduling Drip for Tomato When the Canal Supply Is Cut
Meera grows 0.5 ha of tomato on a sandy loam under drip. Laterals are 1.2 m apart, emitters 0.6 m apart, each delivering 4 L/h, and there is one emitter per plant. At mid-season, reference evapotranspiration is 6.5 mm/day, the crop coefficient is 1.1, and 8 mm of effective rainfall is expected during the week. The drip system's application efficiency is 90%, and a 10% leaching fraction is considered necessary because the irrigation water has moderate salinity. The canal outlet is closed for the next 10 days. A community borewell can supply only 20 m3/day to her farm. Meera wants to run the drip for 2 hours every morning, as usual, and use the borewell water only on the tomato. She asks whether that will be enough and whether she should remove some plants.
1. Calculate the approximate daily gross irrigation requirement for the 0.5 ha block and compare it with the borewell supply. Show your reasoning.
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First convert the weather data to a depth over the field. ETc = 6.5 mm/day × 1.1 = 7.15 mm/day. If the 8 mm effective rainfall is spread over the week, it contributes about 1.1 mm/day, so the net crop water need is about 6.0 mm/day; without counting it, the figure is 7.15 mm/day. The gross requirement must cover the 10% leaching fraction and the 90% application efficiency: gross ≈ 6.0/[0.90 × (1 − 0.10)] = 7.4 mm/day, or about 8.8 mm/day if rainfall is ignored. Over 0.5 ha (5,000 m2), 7.4 mm/day is 37 m3/day (44 m3/day in the no-rain case). The borewell supplies 20 m3/day, which is only about half of the gross requirement. Meera's normal 2-hour run also needs checking: one emitter per plant at 0.6 m × 1.2 m spacing gives about 6,944 plants, each at 4 L/h, so the system would deliver about 27.8 m3/h, and 2 hours would be about 55.6 m3-far more than the borewell can supply. The numbers show the usual schedule cannot be sustained.
2. Recommend a deficit-irrigation and water-management strategy for the 10 days. What should she prioritize and what should she avoid?
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With only 20 m3/day, she cannot meet full ETc. Spread the 20 m3 over the block and operate in short, frequent pulses to keep the root zone moist. If the whole block is wetted, 20 m3/day over 0.5 ha is 4 mm/day, about 55% of the gross requirement. That is a deficit strategy, not a full supply. Mulch the beds with straw or plastic to cut evaporation; check and repair leaks; lower the operating pressure if emitters are over-delivering; and use the rainfall when it comes. Prioritize the flowering and fruit-set period: do not allow severe stress then, because fruit set and fruit size are set at that stage. If the crop is in vegetative growth, a 50–60% deficit is less damaging. She should not remove plants at mid-season; that reduces yield potential and does not save much water unless the stand is unusually dense. Instead, consider alternate-lateral irrigation or partial root-zone drying, but only if she can monitor the crop. Avoid the usual 2-hour run because it will empty the borewell and may waterlog part of the field unevenly.
3. What simple field measurements would tell her whether the strategy is working?
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Simple checks: read the water meter before and after each irrigation; catch emitter output in cups at several points to find clogging or pressure differences; install tensiometers at 20 and 40 cm in representative beds; record plant wilting, leaf rolling, flower drop and fruit set; and track rainfall. A weekly check of soil moisture by hand at the root zone is enough to decide whether the next pulse should be increased or deferred. Also record the borewell yield and the hours run, because the strategy depends on a fixed daily volume. If tensiometer readings show the soil is drying beyond the safe threshold during flowering, she should shift water from the least critical block to the flowering block, even if that means some vegetative stress elsewhere.