Are rainwater tanks useful in Townsville?

Townsville’s ongoing drought has encouraged many of us, especially the keen gardeners, to think seriously about bores, grey water systems and rainwater tanks. This post attempts to arrive at a credible answer to the first question we must ask about tanks: are they even useful?

We have been hearing from two schools of thought on the question for as long as we have been in Townsville, more than 25 years: “Yes, of course!” and “No! The dry season is so long and so dry that no tank will last through it.” One group must be wrong, and the only way to find out is to crunch a few numbers.

Working through the facts and figures

Note that all these figures are only approximate and the tilde “~” should be read as “roughly equal to” or “round about.” My figures will probably be correct within 20% for nearly everyone, but any closer approximation would need to take into account individual circumstances.

Any bulleted line is a conclusion which follows from what’s above it.

The basics

1 mm of rain on 1 sq. m. of roof gives us 1 litre of rainwater.
Typical house roof is ~ 120 sq. m.
Annual rainfall in Townsville is ~ 1000 mm

  • If we harvested all that fell on a typical roof, we would have 120 000 litres aka 120 Kl.

Average annual household consumption (as per TCC site) ~ 600 Kl.

  • Rainwater could theoretically provide about 20% of annual consumption, which is enough to be useful.

Our own household consumption (3-person household) is ~ 130 Kl p.a. (as per our rates notice) and almost none of that is used on the garden because we have a bore. Average household usage on gardens (as per TCC site) is ~ 400 Kl; if we added that to our own actual consumption, we would be roughly in line with the TCC’s figure for average consumption, as we should be.

Rainwater tanks are not permitted to be plumbed into household systems but may be used on gardens.

  • Harvested rainwater could theoretically supply ~30% of what is normally used on gardens (which may be nearly as much as can realistically be used on gardens under Level 3 restrictions).
  • If we could harvest and use all the water from our house roof we could keep our gardens going even through Level 4 restrictions.

Rainfall patterns make a difference

However, harvesting and storing all our rainwater is only possible if we have a big enough tank so that it does not overflow in the Wet.

In a worst-case scenario (all the rain arriving in one week), that would mean we needed a 120 Kl tank. That’s two drums 4.5 m diameter and 3.5 m high, or the equivalent, since the largest standard rainwater tanks are ~ 50 Kl; not impossible but nearly.

In a best-case scenario, the tank would fill every Monday and be used every Wednesday – Sunday; our 1000 mm would arrive in 50 lots of 20 mm, each of which would deliver 2400 litres into the tank, which could be one of the smaller tanks on the market.

The reality is between these two extremes but more like the first: on average, there is less rain (in total) in the six months May-Oct than in the single months of January, February or March; and it is not unusual for the total rainfall across a four-month span to be less than 20 mm, as the BOM data below shows:

Townsville's median monthly rainfall totals, 1961 – 1990, from BOM
Townsville’s median monthly rainfall totals, 1961 – 1990, from BOM

(I have quoted median, rather than mean, rainfall totals because they give a fairer idea of what we can expect. Both ‘mean’ and ‘median’ are averages but the median is less affected by extreme values of the data – in this case, unseasonal downpours.)

Townsville rainfall 2013-17
Monthly rainfall 2013 – 2017 from BOM, highlighting each monthly total under 10 mm (bold), the extraordinary eight-month span in 2015 which brought us only 40 mm (italic), and each four or five month span totalling less than 20mm (dust-coloured background).

We would want our hypothetical tank to be large – at least 60 000 litres – and full at the end of April, so that we could use it at 10 000 litres per month for the six months to the end of October … and still have to hope for good rain in November and December.

But our big tank won’t fill in a weak Wet any more than the Ross Dam will. In fact, our tank and its rooftop catchment are faced with the same problems as Ross Dam and its catchment: (1) making the storage bigger won’t help if there isn’t enough rain to fill it in the Wet season, and (2) there is so little rain in the Dry that usage consistently exceeds replenishment, as the dam levels show.

Grey Water

Since rainwater tanks don’t look like a good solution after all, we had better look at grey water instead. It will be more consistently available and there may be more of it. I wrote about grey water at some length a year ago but at that time I didn’t attempt to estimate just how much of it might be available. I won’t repeat myself but encourage you to visit that post for context for what follows here.

According to the water calculator here, our bathroom and laundry consumption is about 100 Kl per year (about 75 and 25 Kl respectively). That happens to be almost exactly what we could hope to collect from our roof, and it can legally be recycled as grey water.

The bottom line

Two big rainwater tanks would cost about $13,000 delivered but not installed, according a quick internet search. They would also take up quite a bit of garden space, and (to add indignity to overkill) would not even fill in a weak Wet and would therefore not keep the garden going through the Dry in the years they were most needed.

Grey water, which wouldn’t encroach on the garden and wouldn’t run dry before the next Wet arrived, should also be far cheaper even if the plumbing is complicated, so it looks like a far better option.

Not even a little tank? Ever?

If you needed to replace your gutters anyway, directing them into a smallish rainwater tank (e.g. 2000 – 4000 litres) wouldn’t be a huge additional expense (maybe under $1000, especially if you picked up a secondhand tank) and would be moderately useful in helping the garden survive until the next real Wet, supplementing your grey water and the meagre allowance from Ross Dam.

Further reading

There have been several discussions about the pros and cons of rainwater tanks on the Water for Townsville Action Group facebook page – just type “tank” into the search box on that page to find them. Three posts which caught my eye:

One member said, “We use tank water in our washing machine and to flush our toilet. Imagine everyone had tank water for this alone.” This is safe enough but I’m not sure whether TCC would approve.

Another noted that, “running a sprinkler system from a rainwater tank is perfectly okay under our restrictions.” I believe (but haven’t checked) that running a sprinkler system from grey water is not okay for health reasons; if so, rainwater does have one advantage.

Another said, “grey water diversion systems range from $600-$1,100,” which gives us a ballpark figure for complete installations. The cheapest possible installation, a length of poly pipe on the washing machine outlet, might only cost $10 and might still recycle 20 Kl p.a., which must be the best value of any solution.

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