grafcarat6500lAs promised in my previous post about drinking water, here are my thoughts on our rainwater storage.  In our current home town of Brisbane, Australia, the dams that supply our drinking water recently bottomed out at around 14% capacity. The severe water restrictions that we’ve had for years have meant that even in our inner-city suburb about half the houses have rainwater tanks, as without them you are effectively not allowed to water your garden. These aren’t your British-style 200 litre water-butt either. Hardly any would be less than 3000 litres, most would be 5000 litres, and quite a few have 10,000 litres. We’ve currently got two 3000 litre tanks, plumbed into to an underground weeper hose that runs through all the garden beds, controlled by a programmable timer that runs the two watering circuits on alternate days. This has ensured that we have kept our lush tropical garden alive through a seven year drought.

What it doesn’t do is supply any water to the inside of the house. For a while we did use it for our washing machine (a separate tap that we connected the cold water hose to) but as this required a manual changeover it only happened rarely.

To peak-oil-proof our water supply I want our UK system to be fully plumbed in to the house and able to supply our basic, emergency  requirements, and ideally all our requirements. I’m going to try to do this relatively scientifically – based on the Tank Size Calculator from RainWater Harvesting. Manchester provides a very different scenario to our Brisbane rainfall – there’s 400mm less a year for a start, and it is more evenly spread thoughout the year, minimising the need for a huge tank to cope with peaks and troughs.

With our roof area (about 100m2) and Manchester’s annual rainfall (806mm) we can expect to collect around 64,500 litres/year (at 80% efficiency). That sounds like a huge amount, but when you consider that our current water consumption is around 250 litres a day, we currently use 91,250 litres/year! So to be self sufficient in water with our current roof size we’ll need to reduce our in-home water consumption by around 30% – that looks like the subject of another post!

So what size tank do we need to make sure that we collect as much water as we can, without wasting any? I concocted my own monthly water use chart, based on meeting a target of 170 litres/day (then found out that there’s a very good one in the RanWater Harvesting spreadsheet!).  From looking at this (below) I’ll have a shortfall of 3000 litres in the first year – so in this table I’ve pre-filled the tanks with 3000 litres to ensure that they don’t drop below zero. In reality this “top up” would happen in stages during the year depending on our actual daily consumption to give us the chance to be extra-economical as the levels drop. 

  Rainfall /mm Water Collected /litres Available Litres
/day
Water Used
(170 l/day)
Net
Water
Storage at Start   & End of      Month
Jan 71.5    5,720 184.5     5,270 450  3,000     3,450
Feb 51.8    4,144 148.0     4,760 -616  3,450     2,834
Mar 64    5,120 165.2     5,270 -150  2,834     2,684
Apr 49.1    3,928 130.9     5,100 -1172  2,684     1,512
May 53.8    4,304 138.8     5,270 -966  1,512       546
Jun 66.8    5,344 178.1     5,100 244     546       790
Jul 59.5    4,760 153.5     5,270 -510     790       280
Aug 70.9    5,672 183.0     5,270 402     280       682
Sep 69.9    5,592 147.2     5,100 492     682     1,174
Oct 86    6,880 221.9     5,270    1,610  1,174     2,784
Nov 81.9    6,552 211.4     5,100    1,452  2,784     4,236
Dec 81.4    6,512 210.1     5,270    1,242  4,236     5,478
Year 806.6  64,528 176.8   62,050   2,478    

In this our water storage peaks at around 5,500 litres, so unless we manage to keep at or below our tight 170l/day target for several years there is limited benefit to installing a tank any larger than 6,000 litres

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