Māra Kai Water: Sizing a Rain Tank for a 62-Day Dry Spell
Te Māra Kai o te Awa grows vegetables for kaumātua in a town where the council restricts summer water use. The group has one cylindrical tank collecting rain from the roof of a shed.
The tank's inside diameter is 2.2 m and its inside height is 1.8 m. The shed roof that feeds it has a horizontal collecting area of 60 m².
Over a typical January the town receives 85 mm of rain, but about 20% of what lands on the roof is lost to the first flush, gutter splash and overflow before it reaches the tank.
There are 12 garden beds. In summer each bed needs 18 L of water every day. The dry spell usually runs for 62 days, and the tank is empty at the start of it. The tank has never been measured in litres, the group is unsure whether one tank is worth keeping, and the committee is meeting before the next growing season to decide whether to buy a second tank.
1. Calculate the capacity of the tank in litres, to the nearest 10 L.
Read the worked answer
Radius = 2.2 ÷ 2 = 1.1 m
Volume of a cylinder = π r² h
- V = 3.1416 × 1.1² × 1.8
- V = 3.1416 × 1.21 × 1.8
- V = 6.842 m³
Convert to litres (1 m³ = 1000 L)
- 6.842 m³ = 6,842 L ≈ 6,840 L
Use the inside diameter and height, not the outside - the wall thickness would add roughly 5% to the volume and overstate the supply. Rounding to the nearest 10 L reflects how precisely a tank this size can really be described.
2. How many full days of watering can this tank supply if it starts full?
Read the worked answer
Daily demand
- 12 beds × 18 L = 216 L per day
Days supplied
- 6,842 ÷ 216 = 31.68 days
So the tank supplies 31 full days of normal watering. The 32nd day would be partly watered and would run dry partway through.
Keep both numbers in view: 31.68 is the honest figure, but the committee can only plan on 31 guaranteed days. Rounding up to 32 would commit water the tank does not hold.
3. The dry spell is 62 days. Decide whether one tank is enough, and support your decision with figures, including a check on how much rain the roof actually delivers in January. Give a workable alternative.
Read the worked answer
Demand over the dry spell
- 62 × 216 L = 13,392 L
Supply from a full tank = 6,842 L
Shortfall = 13,392 − 6,842 = 6,550 L
One tank covers about 51% of the dry spell, so on its own it is not enough.
Rain check for January
- Gross rain on the roof = 0.085 m × 60 m² = 5.1 m³ = 5,100 L
- Captured at 80% = 4,080 L
- January use = 30 × 216 = 6,480 L
- January deficit = 2,400 L
Even in a month with rain, the roof delivers less than the beds use - so the problem is not just tank size but total supply.
Options
- Second tank: the shortfall is 6,550 L, so a second 7,000 L tank would cover the 62 days if both tanks start full. Note that refilling them depends on rain that January does not reliably provide.
- Reduce demand: 6,842 ÷ 62 = 110 L per day, which is 110 ÷ 12 ≈ 9 L per bed per day - half the current rate. Mulch and drip lines would make that realistic.
- Both: a second tank plus mulching gives a buffer for a dry January.
Recommendation: buy the second tank only if the group also cuts daily use to about 110 L, because a bigger tank does not fix a catchment that delivers 4,080 L in a month when 6,480 L is needed.