Frost protection planning in a Central Otago Pinot Noir vineyard
A 12 ha Pinot Noir vineyard in Central Otago is at budburst (E-L stage 4–5) on 12 October. A clear, still night forecast is -2.5°C at 2 m, with an inversion layer at 15 m and wind machines available on 8 ha. The vineyard has 2 frost fans (each protects 4 ha), overhead sprinklers on 4 ha (application rate 3 mm/h), and 6,000 frost cloth bags. The crop is expected to yield 8 t/ha at $2,800/t. A full frost event could destroy 60% of buds on unprotected vines. Fans cost $45/h each to run; sprinklers cost $120/h for the block; labour for cloth $1,200. Water supply is limited to 6 mm/ha/night. The bank wants a costed plan.
1. Calculate the value at risk and compare the cost of each protection method for one night.
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Total crop value = 12 ha × 8 t/ha × $2,800/t = $268,800. A full frost event could destroy 60% of buds, so the potential loss is 0.60 × $268,800 = $161,280 if no protection is used.
Running two fans for 10 hours costs 2 × $45/h × 10 = $900. Running sprinklers on 4 ha for 10 hours costs $120/h × 10 = $1,200, but the water limit is 6 mm/ha/night; at 3 mm/h the block can run only 2 hours, so the realistic sprinkler cost is $240 for the critical period. Frost cloth labour is $1,200 plus the cost of bags. The fans cover 8 ha, sprinklers 4 ha, and cloth can protect part of the remaining area. Total one-night protection cost is therefore roughly $2,340 if fans, limited sprinklers and cloth are all used-far below the $161,280 value at risk.
2. Recommend a protection strategy and justify it using inversion strength, method effectiveness and water supply.
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Use the fans as the primary method because the forecast inversion at 15 m means warmer air can be mixed down to the canopy. Run both fans from about 1 hour before the temperature reaches 0°C until after sunrise. Use sprinklers only for the 2 hours around the coldest period, because the water limit of 6 mm/ha/night prevents all-night running at 3 mm/h; starting sprinklers too early would exhaust the supply before the critical minimum. Place frost cloth on the youngest, most exposed vines and block edges where fan coverage is weakest. Do not rely on cloth alone for 12 ha; it is labour-intensive. The strategy protects the majority of buds at a cost of about $2,300, which is justified against a potential $161,000 loss.
3. Explain the limitations of sprinklers and fans in this situation and how you would monitor the event.
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Sprinklers work by releasing latent heat as water freezes, but they must keep running until ice melts; stopping early can cause more damage than no protection. The 6 mm/ha/night water limit means the block cannot be protected for a long, severe frost, and poor water distribution can leave dry patches. Fans depend on a temperature inversion; if the inversion is weak or the wind is too strong, mixing may not raise canopy temperature enough. Fans also have a limited protection radius and can be noisy. Monitoring should use temperature sensors at bud height and at 2 m, plus a check that the inversion is present. Start fans when bud temperature approaches 0°C, and record run times and water use for the bank's cost report.