Watering Off a Newly Planted Windbreak Before a Heatwave
You are on a 240-hectare mixed farm. On Tuesday a contractor planted 600 native tube-stock seedlings along a 400 m fence line as a windbreak. The soil is a sandy loam and the paddock has had 18 mm of rain in the past three weeks. The forecast for the next five days is 36-39 degrees Celsius with hot northerly winds and no meaningful rain. The farm has one 5,000 L water cart and a pump that delivers about 200 L per minute. The owner asks you to 'get the trees through the week' but also needs the cart on Wednesday morning to water the weaner cattle in a distant paddock for two hours. You have one offsider available on Wednesday only.
1. Prioritise the week's watering. Explain how you would allocate the cart, the offsider and the available water, and justify the trade-offs.
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First, work out the constraint. At 200 L/min the cart fills in 25 minutes. If each seedling needs about 5 L to wet the root zone in sandy loam (higher than clay because water drains through), 600 seedlings need roughly 3,000 L per full watering - under one cart load, but only if delivery is efficient.
Prioritise by risk, not by convenience. The seedlings are the most vulnerable asset: newly planted tube stock has a small root ball and no established access to subsoil moisture, while the weaners have access to a paddock and can be watered with a trough top-up rather than a full cart run. So:
- Tuesday afternoon: fill the cart and water the most exposed section first - the northern end of the windbreak, which takes the hot northerly wind and dries fastest. Water half the row (300 seedlings) at about 5 L each.
- Wednesday morning: the owner's cattle run takes two hours, so the offsider does that while you hand-water the remaining 300 seedlings from a knapsack or a second fill. Do not send the cart to the cattle and the trees at the same time - one cart, one job.
- Wednesday afternoon: second fill, re-water the northern section if the soil is already dry 50 mm down (check by hand), otherwise move to the southern section.
- Thursday-Friday: one deep watering per day on the worst section, early morning or late afternoon to cut evaporation.
Justify the trade-off: the weaners can be managed with a trough and shade, but a seedling that dries out in a 38-degree wind is dead and cannot be replaced cheaply. The cart and offsider are the bottleneck, so the plan protects the highest-loss asset first and uses the coolest hours to make each litre go further.
2. The offsider suggests watering at midday because 'that's when it's hottest and they need it most'. How would you respond, and what would you check before the next watering?
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Respond with the reason, not just the rule. Midday watering loses a large share of the water to evaporation before it reaches the root zone, and the hot wind can dry the surface within hours. Early morning is best because the plant takes up water before the heat peaks and the soil surface stays moist longer; late afternoon is second best. Watering at midday also risks scorching leaves if water sits on them in full sun.
Then check before the next run:
- Soil moisture: push a finger or a probe 50-100 mm down near several seedlings. If it is moist, skip that section and move the water to a drier one.
- Plant condition: wilting, dull or curled leaves, and browned tips tell you which rows are failing. Do not assume the whole row is the same.
- Wind and forecast: if the northerly is strong, water earlier and consider a light mulch or shade cloth on the worst section.
- Water quality and volume: confirm the cart is clean and the pump is delivering at the expected rate; a slow fill eats the schedule.
- Records: note date, section, litres applied and plant condition so the next person can see what has already been done.
The judgement point: the offsider's instinct is about care, but the exam wants you to weigh water efficiency, plant physiology and the limited cart against the emotional pull of 'doing something when it's hottest'.