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撑到几月 Will the tank last?

Not "is the tank big enough" — in how many of the last thirty summers was it not.

A roof, a tank and a household are three numbers. Whether they are enough is not a fourth number, it is a rate of failure over the rainfall record — and the useful form of the answer is how many loads of water you would have bought, in which years, and what that costs against a bigger tank.

Nothing is uploaded. The arithmetic runs on the server for one request and nothing is written down.

The house

m² L L × $

450 L a day against 541 L a day of average supply. A full tank is 67 days of use with no rain at all — and that number, not the annual total, is what decides most of what follows.

The rain

These four records are simulated, not observed. They are built to have the right number of wet days, the right depth on a wet day and — the part that decides everything here — dry spells of the right length. They are not Kaitaia's actual rain. For a real answer, paste a real record: NIWA's CliFlo gives daily rainfall for any station, free, back decades.

Reading 11,323 days, 1990-07-01 to 2021-06-30: 1,445 mm a year over 131 wet days a year. Simulated.

Short in 12 of 30 years

12 of 30 is 40% — about 4 years in 10. On 30 years of record the honest range is 23% to 59%: wide enough that 6 years in 10 and 1 year in 4 are the same evidence.

0.7loads a year, on average $161a year, on average $6603 load(s), the worst year on record

Nobody pays the average. In 18 of the 30 years the bill was nothing and in the worst it was $660 — the mean is a planning number, not a year anyone lived through. Water is carted by the load, so any shortfall at all costs a whole delivery.

Every year in the record

Each bar is one water year, July to June. Full height is a tank that never went below full; the floor is a tank that emptied. A red bar is a year that ran short, and its depth below the line is how many loads it took.

full empty1991/92: lowest 14,081 L1992/93: lowest 17,665 L1993/94: lowest 0 L — first short 2 April, 2 load(s), $4401994/95: lowest 0 L — first short 26 January, 2 load(s), $4401995/96: lowest 18,587 L1996/97: lowest 0 L — first short 17 September, 1 load(s), $2201997/98: lowest 0 L — first short 29 March, 1 load(s), $2201998/99: lowest 0 L — first short 3 March, 3 load(s), $6601999/00: lowest 0 L — first short 3 July, 2 load(s), $4402000/01: lowest 11,820 L2001/02: lowest 20,860 L2002/03: lowest 3,466 L2003/04: lowest 0 L — first short 24 March, 1 load(s), $2202004/05: lowest 1,426 L2005/06: lowest 15,235 L2006/07: lowest 0 L — first short 8 April, 2 load(s), $4402007/08: lowest 0 L — first short 12 September, 2 load(s), $4402008/09: lowest 3,965 L2009/10: lowest 19,096 L2010/11: lowest 10,106 L2011/12: lowest 4,495 L2012/13: lowest 0 L — first short 27 April, 1 load(s), $2202013/14: lowest 2,284 L2014/15: lowest 0 L — first short 3 March, 2 load(s), $4402015/16: lowest 0 L — first short 16 January, 3 load(s), $6602016/17: lowest 5,075 L2017/18: lowest 9,916 L2018/19: lowest 5,708 L2019/20: lowest 7,914 L2020/21: lowest 2,645 L1991/922006/072020/21

The first water year of the record is not shown and not counted: the tank started at whatever the form said, so for those months the answer is about that guess rather than about the climate. The longest run with no rain at all in this record is 49 days, and a full tank of 30,000 L at 450 L a day lasts 67 days.

Which thing is too small

The tank, not the roof

38,952 L a year — 20% of everything the roof caught — went out the overflow because the tank was already full. That water arrived and there was nowhere to keep it. Storage is the lever here.

Each of these is the smallest change that gets through every year in this record on its own, with the other two left alone.

A bigger tank75,000 L up from 30,000 L — 2.5× the storage, and 167 days of use instead of 67
More roof270 m² up from 170 m², so another 100 m² — a shed or a carport plumbed in counts, if it actually drains to the tank
Less water115 L each a day down from 150 L — 23% less, which is a shower head and a washing machine, not a lifestyle

Carting, or a bigger tank? Another 45,000 L is roughly $8,400 — a poly tank at about $0.16 a litre supplied, plus around $1,200 for the base, the delivery and the plumbing. Carting has been costing $161 a year on average, so on money alone the tank takes about 52 years to pay for itself.

Which means carting is the cheaper answer here, and that is worth saying plainly because it is not what anyone selling tanks will tell you. The reasons to buy the tank anyway are not money: a tanker that is booked out for a fortnight in a drought year is booked out for everyone at once, some councils restrict carting when the town supply is under pressure, and a delivery needs a truck that can reach the tank.

The years it ran short

Water yearFirst shortDaysLitres short LoadsCostLongest dry spell
1993/942 April2910,638 2$44024 days
1994/9526 January4618,890 2$44016 days
1996/9717 September208,139 1$22015 days
1997/9829 March62,491 1$22021 days
1998/993 March5320,454 3$66018 days
1999/003 July3913,465 2$44021 days
2003/0424 March145,202 1$22020 days
2006/078 April2711,143 2$44020 days
2007/0812 September4718,234 2$44016 days
2012/1327 April249,556 1$22049 days
2014/153 March4216,774 2$44016 days
2015/1616 January5922,802 3$66015 days

"Litres short" is what the household wanted and the tank did not have. It becomes whole loads of 10,000 L because that is how water is sold, which is why a year that was 400 L short and a year that was 9,000 L short cost exactly the same.

What it does not know

The record is the answer's ceiling. Thirty years of rain is thirty observations of a once-a-decade drought — that is three. Every interval on this page is wide for that reason, and no amount of arithmetic narrows it.

The past is not the forecast. These are the summers that happened, run against your roof. They are the best guide available and they are not a promise, and a changing climate is precisely a reason the older years may be the wrong ones.

It assumes you keep using the same amount. Real households cut back when the gauge drops, which is why a house that this page says runs out every year often does not quite. Cutting back is the shortage, though — it has just been paid for in showers rather than in dollars.

Roof area is the catchment, not the footprint. Use the plan area under the roof that actually drains to the tank: a pitched roof catches the rain that falls on its shadow, not on its surface, so the pitch does not add anything.