Choosing a planting window after a false start in Salima
A smallholder in Salima District farms 0.8 ha of hybrid maize. She planted on 20 November, after 92 mm of rain in November, close to the long-term mean of 135 mm. From 1 to 18 December only 18 mm fell, then 21 days passed with no rain. Maximum temperatures reached 33°C and estimated evaporation was 6.5 mm per day. Her maize is at the six-leaf stage; lower leaves are wilting, and the soil is moist only in the top 8 cm. The 125-day hybrid needs about 500 mm of seasonal water. She has 25 kg of seed for a 90-day short-season variety, enough for the same area, but replanting now would cost about MWK 95,000 for seed and labour. Long-term records show December mean rainfall of 250 mm and January of 280 mm; the rains usually continue into March. She must decide whether to replant, wait, or split the plot.
1. Compare the November–December rainfall with the long-term pattern and assess the water balance for the crop.
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November rainfall was 92 mm against a mean of 135 mm, about 68% of normal, so the start was already slightly dry. December is far more serious: 18 mm to 18 December against a monthly mean of 250 mm. A 21-day dry spell at 33°C and 6.5 mm/day evaporation removes about 137 mm of water from an exposed soil surface. The soil is moist only to 8 cm, while maize at six leaves has roots exploring deeper. The crop has received little water since planting and is wilting. A 125-day hybrid needs about 500 mm; the remaining season would need well-timed January–March rain. January mean of 280 mm and rains into March show the season is not over, but the crop's yield potential is already reduced. The water balance is negative: rainfall since planting is below evaporation plus crop demand. Waiting might save cost but risks losing the remaining potential; replanting now uses residual moisture but depends on the next rain.
2. Explain the dry spell using the position of the ITCZ and the influence of the subtropical high.
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The dry spell is consistent with the ITCZ moving south and then weakening or shifting. In November the ITCZ lies near Malawi, giving convection and the 92 mm. If the ITCZ is displaced north or the subtropical high strengthens south of the country, subsidence and stable air suppress convection. The result is clear skies, high maximum temperatures and strong evaporation. A 21-day break is not unusual within a rainy season, but its timing at the crop's establishment stage makes it damaging. The high evaporation rate is both a cause and an amplifier: it dries the topsoil quickly, so even a short break becomes an agricultural drought. The long-term December mean shows that December should normally supply a large share of the seasonal total, so 18 mm is a strong negative anomaly. This is a rainfall-variability problem, not necessarily a permanent climate shift, but it shows why planting dates and variety choice matter.
3. Recommend and justify a decision: replant the whole plot, wait, or split. Include risk management.
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Split the plot. Replanting all 0.8 ha now is risky because the next rain is not guaranteed and the 90-day variety yields less. Waiting on the whole plot risks losing everything if the dry spell continues. A split spreads risk: keep the healthier part of the hybrid if it can recover, and replant perhaps 0.3–0.4 ha with the short-season variety when 20–25 mm of rain has soaked the soil. Use the remaining seed efficiently and plant on the contour with basins or tied ridges to capture rain. Do not plant into dry soil merely because the calendar says so. Set a decision rule: if no effective rain by a stated date, replant more; if rain comes, top-dress and weed. Record costs: MWK 95,000 for the whole area is a real burden, so partial replanting limits loss. Seek advice on a drought-tolerant variety and consider mulching. The aim is not to predict the season perfectly but to avoid a total loss.