Monday, April 20, 2015

Update on water systems (part II)

I wrote progress report on our water systems at the beginning of January. This weekend, we had the first solid rain since then. That is over 70 days without significant rain. This weekend, I estimate we caught about 8 kL in our tanks, so the summer dry spell is officially over.

From reading the water meter, we used 8.5 kL from town water over summer to make up the shortfall in stored rainwater. I estimate that
  1. I accidentally released 3 - 4 kL from the tanks into the garden in late January
  2. The tanks at the front of the house were only connected in December, which meant we only collected 1/2 of the spring rains
Were the summer to be repeated, I think our water would last. But, can we expect future summers to be similar to the 2014/2015 summer?

This summer, versus historical summers

This summer was not unusually dry -- if anything, the rain that we had at the beginning of January was unusual and is unlikely to be repeated every year. Here's what the data say for Adelaide:

All numbers are mm of rainfall for that month. the statistics [1] were calculated over all the historical rain data I have for Adelaide (since 1884). For the purpose of this analysis I've assumed the data are stationary (ie. there's been no change in Adelaide's rainfall patterns in that time).

Assuming the rainfall distribution is normal (a wrong assumption, but probably doesn't matter for this), 35% of years will be within 1 standard deviation of the mean. Note that for the months November to March this means that 1/3 of those months can be expected to have basically no rain at all! (Note that I haven't looked at inter-dependencies in the data (ie. Does a dry February tend to follow a dry January (which it probably does)?)

For the summer just gone,  here are the numbers

Every month except January was drier than average, but none by much more than one standard deviation. In other words, it was a dry summer but it was not unusually dry.

Rainwater Model

Based on this summer's data, I've revised my rainwater model a bit. Mainly I've realised that although I have 37 kL storage, I probably only have 30 kL of usable storage. I've also increased the washing machine frequency to daily, from 0.8 times per day. Looking at the historical data, we would now have run out five years since 1884.


I set up a dripper system on the garden, and believe I wasted a lot of water there. Next summer, I want to move to wicking beds for the annual crops.
I also want to have a functioning composting toilet, which will save more water.

[1] Standard deviation is a measure of how variable the data are. In this case, it is a measure of how likely it is that a given year is a long way from the mean (average) rainfall. For normally distributed data, about 1/3 of years will be within one standard deviation of the mean, and about 7% being more than 3 standard deviations away from it. More info on wikipedia.

This article was written by Angus Wallace, and first appeared at


  1. Hi Angus. It is amazing what the long term averages tell us about climate and rainfall. This year has been a more or less average rainfall year for here. Your February sounds quite scary. However, the climate is so variable that from year to year the mean rainfall number cannot be relied upon as it at best only a guide. Glad to hear that you are thinking about your systems based on hard won experience. People don't quite seem to understand that infrastructure has to be able to survive the hardest tests and not the best conditions (or even the average) if they are to be of value in the long term.

    PS: I'm about to add another 4.4kL water tank here.

    Cheers. Chris

  2. Hi Angus, you have done all the maths here. Numbers aren't my strong suit, but I do know that I want to get water usage down, and water storage up (from zero - we can only go up from here!).

    I found some really clear plans for wicking beds the other day - thought you might be interested:

    It may take me a couple of years to get on top of all of these projects, but I will get there eventually!

  3. Hi Chris,

    Yes, I think you've hit the nail on the head there. Systems need to be designed to withstand worst case scenarios. That's one reason why it's so hard to be off-grid (that's not my goal, per se, since I live in an urban environment, but it's interesting to think about). The variability of rainfall in Adelaide is quite remarkable. I don't know if this is a general truth globally, but Adelaide's historical rainfall data are very telling...
    Cheers, Angus

  4. Hi Jo,

    Thanks for the link. To be honest, I'm not planing to worry about wicking beds much until about September -- got to get through winter first ;-) Will check that out in detail though, so thanks ;-)

    Cheers, Angus


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