Irrigation Scheduling Under a Reduced Water Allocation
A 60-hectare block of processing tomatoes is furrow-irrigated from a district canal. The grower's seasonal allocation has been cut from 6,000 to 4,200 cubic metres per hectare. Soil is a silt loam with a measured field capacity of 0.30 and permanent wilting point of 0.14 (volumetric), root zone depth 0.6 m, and the crop is at early fruit set with a crop coefficient of 1.05 and reference evapotranspiration of 7 mm/day. Furrow application efficiency has historically been 55 percent. The grower proposes to keep the same seven-day interval and simply shorten each set. Late-season heat is forecast, and the processor requires the crop to reach a soluble-solids target before a fixed harvest date.
1. Calculate the net and gross daily crop water use and the available water capacity of the root zone, then judge whether stretching the interval while shortening sets can work.
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Net crop use = ET0 x Kc = 7 x 1.05 = 7.35 mm/day. Gross requirement at 55% efficiency = 7.35 / 0.55 = 13.4 mm/day applied. Over seven days that is about 93.5 mm gross, or 935 cubic metres per hectare per week. Available water capacity = (0.30 - 0.14) x 0.6 m = 0.096 m, i.e. 96 mm. At 7.35 mm/day the crop draws down the entire root-zone reserve in roughly 13 days, and only about half of that is readily available before stress. Shortening sets while keeping the interval cuts the applied depth per event without reducing frequency, so the soil dries below the readily available threshold between irrigations. The correct move is to shorten the interval, not the set, or convert to a higher-efficiency method; spreading the same gross volume over more frequent, smaller events keeps the root zone above stress.
2. The allocation of 4,200 cubic metres per hectare equals 420 mm for the season. Using the gross daily figure, assess the season length this supports and propose two defensible responses.
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At 13.4 mm/day gross, 420 mm supports about 31 days of full irrigation. A tomato season from fruit set to harvest typically needs far more, so the allocation is binding. Two defensible responses: first, raise application efficiency, since every point of efficiency converts directly into days of supply; converting furrows to drip at 90% efficiency lowers gross use to 7.35 / 0.90 = 8.2 mm/day, stretching 420 mm to about 51 days. Second, reduce the irrigated area and concentrate the allocation on the best block, accepting a smaller but adequately watered crop rather than a larger stressed one. Deficit irrigation timed to vegetative stages, not fruit set, is a third option, but applying it at fruit set risks blossom-end rot and lost yield, so it is the weakest choice here.