Dry-Season Water Plan at a Butaritari Junior Secondary School
St. Michael's Junior Secondary School on Butaritari has 180 boarding students and 12 staff. Its only dry-season source is a 10,000-litre ferrocement tank fed by a 150 m² iron roof. The school expects 40 teaching days before the next reliable rain. The head teacher estimates consumption at 4 litres per person per day for drinking, cooking and handwashing. The Island Council has approved a delivery truck carrying 3,000 litres per trip, but the school's budget can pay for only six trips at $35 each. Last week a teacher measured 100 mm of rain over two days and watched water pour out of the tank overflow. The board must decide whether to ration water, buy a second tank, or ask the Council for more trips. A parent has offered a used 4,000-litre tank for $250, but it needs a $120 repair.
1. Calculate the dry-season demand and the usable supply if no extra storage is added. State whether the school faces a deficit and by how much. Show each step.
Read the worked answer
Demand: 180 students + 12 staff = 192 people. At 4 L per person per day: 192 × 4 = 768 L/day. Over 40 days: 768 × 40 = 30,720 L.
Rainfall harvest: roof area 150 m² × rainfall 0.100 m = 15 m³ = 15,000 L gross. With 85% collection efficiency: 15,000 × 0.85 = 12,750 L harvestable.
Storage limit: the tank holds only 10,000 L. If the 100 mm falls in a short period, at least 12,750 − 10,000 = 2,750 L overflows and is lost. Usable rain is therefore 10,000 L under a conservative assumption.
Truck deliveries: 6 trips × 3,000 L = 18,000 L.
Usable supply: 10,000 + 18,000 = 28,000 L.
Balance: 28,000 − 30,720 = −2,720 L. The school faces a deficit of about 2,720 L, or roughly 9% of demand. The conclusion holds unless the rain is spread out enough that the tank is drawn down during the storm; in that case more of the 12,750 L could be captured.
2. Explain why the 12,750 litres of rainfall does not all become usable supply, and recommend whether the school should accept the used tank. Use numbers.
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Rainfall is not the same as stored water. The tank can hold only 10,000 L, so the remaining 2,750 L of the 12,750 L harvest goes out the overflow. The used tank adds 4,000 L of storage for $250 + $120 = $370. If it is repaired, cleaned, covered and plumbed in before the rains, it can capture most of that overflow. Usable supply then becomes about 10,000 + 4,000 + 18,000 = 32,000 L if the new tank fills, or about 30,750 L if only the 2,750 L overflow is captured. Either figure is close to or above the 30,720 L demand, but the margin is small. Recommend accepting only if the repair is verified and the tank is installed before the first rain; otherwise it adds nothing this season. The board should also check gutters and pipe capacity, because a second tank is useless if water cannot reach it.
3. If the board cannot install extra storage before the dry season, set a daily per-person ration that leaves at least 1,000 litres in reserve over the 40 days. Justify your figure.
Read the worked answer
Without extra storage, usable supply is 28,000 L. Reserve 1,000 L, leaving 27,000 L for consumption. Divide by the number of person-days: 192 people × 40 days = 7,680 person-days. 27,000 ÷ 7,680 = 3.52 L per person per day. A ration of 3.5 L/person/day gives total use of 3.5 × 7,680 = 26,880 L, leaving 28,000 − 26,880 = 1,120 L in reserve. That meets the 1,000 L minimum. A workable allocation is 2.0 L drinking, 1.0 L cooking and 0.5 L handwashing, but 0.5 L is low for hygiene, so the school should prioritise handwashing before meals and use stored chlorinated water. If the used tank is installed, recalculate: supply rises, so the ration can move closer to 4 L. The board should publish the ration and check the tank gauge daily rather than assume the plan is working.