Planning a Water Tank for the School Garden
A school wants to install a rainwater tank for its vegetable garden. The garden needs about 120 litres per week in summer. The roof area that feeds the tank is 30 square metres. Local rainfall in summer averages 80 mm per month. The tank comes in two sizes: 1000 litres for $850 and 2000 litres for $1400. The school has a budget of $1000. A parent warns that summer rain is unreliable and the tank may run dry. The garden coordinator wants to know whether the 1000-litre tank is a sensible choice.
1. Calculate how much rainwater the roof could collect in an average summer month. Use the fact that 1 mm of rain on 1 square metre gives 1 litre.
Read the worked answer
Rainfall = 80 mm per month.
Collection = 80 mm × 30 m² = 2400 litres per month (in theory, before losses).
Real tanks lose water to splashing, first-flush diversion and evaporation, often 10–20%. At 80% efficiency: 2400 × 0.8 = 1920 litres per month.
Garden need = 120 L per week × about 4.3 weeks = about 516 litres per month.
So in an average month, the roof collects far more than the garden needs (1920 L vs 516 L). The 1000-litre tank could fill up quickly. A strong answer notes that this is an average - actual months vary a lot.
2. The school can only afford the 1000-litre tank. Is this a sensible choice? Consider both the average rainfall and the parent's warning.
Read the worked answer
On average, yes. Monthly need is about 516 L, and even at 80% efficiency the roof collects about 1920 L per month. A 1000 L tank can hold roughly two months of garden use, which gives a useful buffer.
But the parent's warning matters. Summer rain is unreliable. If a month has only 10 mm of rain, collection = 10 × 30 × 0.8 = 240 L, well below the 516 L needed. The tank would drain down. Several dry weeks in a row could empty it.
So the 1000 L tank is sensible only with a backup - for example, mains water top-up, or mulching and hand-watering to cut demand in dry spells. The 2000 L tank would give more buffer but is over budget ($1400 > $1000).
A strong answer weighs average vs worst case and recommends a risk-management step rather than a simple yes/no.
3. The coordinator suggests saving for the 2000-litre tank instead. What would the extra 1000 litres buy, and is it worth the extra $550?
Read the worked answer
The extra 1000 L is a buffer against dry spells. At 120 L per week, 1000 L lasts about 1000 ÷ 120 ≈ 8.3 weeks of garden watering with no rain.
The 1000 L tank would last about 4.2 weeks with no rain.
So the extra $550 roughly doubles the dry-spell buffer from about 4 weeks to about 8 weeks. Whether it is worth it depends on:
- How often summer dry spells exceed 4 weeks in this area.
- Whether a backup water source exists (if mains top-up is available, the buffer matters less).
- The cost per litre of buffer: $550 ÷ 1000 L = $0.55 per litre of extra storage.
A strong answer recognises that the decision depends on local risk, not just the price. If dry spells longer than 4 weeks are common, the 2000 L tank is better value despite the higher cost; if rain is frequent, the 1000 L tank plus careful watering is enough.