Frost risk and site choice in an upper Assam tea estate
Three sections of a tea estate in upper Assam are being considered for 40 ha of frost-sensitive clonal bushes. The riverside flat lies at 78 m, the mid-slope at 95 m and the ridge at 125 m. On clear, calm January nights after rain, temperatures at 05:30 average 3.2°C on the flat, 5.1°C on the mid-slope and 6.0°C on the ridge; the dew point is 2.8°C. Frost damage to young buds occurs when canopy temperature falls to 0°C or below. The estate records four to six frost nights per season. It owns sprinklers that can protect only 12 ha and is considering a wind machine for the flat. The flat also has the deepest soil and the best road access; the ridge soil is shallow and stony.
1. Explain the temperature pattern on the three sections and identify the process responsible.
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The pattern is a radiation inversion produced by nocturnal cooling under clear, calm skies. The ground loses heat rapidly, and the air in contact with it cools. Cold, dense air then drains downslope and collects on the flat, while the ridge remains in better-mixed, relatively warmer air. The figures fit this: the flat is coldest at 3.2°C, the mid-slope is 5.1°C and the ridge is 6.0°C. The flat is only 0.4°C above the dew point, so a slight further fall can bring the canopy to 0°C or below and deposit frost. The flat is therefore the frost pocket, not the ridge. The mid-slope is a transition zone, and the ridge is warmest but has the poorest soil.
2. Recommend where to plant the 40 ha of frost-sensitive clonal bushes and how to deploy the 12 ha of sprinklers. Justify using both frost and site data.
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Put the most frost-sensitive clonal blocks on the mid-slope, where the average minimum is 5.1°C and cold air drains away, and keep the shallow, stony ridge for hardier or drought-tolerant bushes. Use the flat only for protected blocks or frost-hardy material. Deploy the 12 ha of sprinklers on the flat, because that is where frost frequency and severity are greatest; sprinklers protect by releasing latent heat as water freezes and by keeping canopy temperature near 0°C. The flat also has the best soil and road access, so protecting it gives the highest return. Do not spread the 12 ha thinly over all 40 ha: concentrate them on the youngest, most valuable clonal bushes on the flat, and use the mid-slope as the main expansion area.
3. A manager argues that one wind machine on the flat will protect the whole 40 ha. Assess this claim.
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The claim is not credible. A wind machine mixes warmer air from aloft with the cold surface layer, but its effective radius is only a few hectares, not 40 ha, and its performance depends on the strength and depth of the inversion. Under a strong, shallow inversion it may simply stir the cold layer without raising canopy temperature enough. It also creates noise and needs reliable power. It could be tested on the flat alongside temperature loggers, but it should not be counted as protection for the mid-slope or ridge. The main protection should be site selection plus the 12 ha of sprinklers; a wind machine is at most a supplement for a small, high-value block.