Sunday, April 20, 2014

Rainwater modeling

I have spent a fair amount of time thinking about water. My family and I live in Adelaide, where water is hard to come by. The energy intensity of Adelaide's water supply is about 0.8 - 1.8 kWh/kL [1], while Adelaide's new desalination plant is expected to use 3.5 - 5.5 kWh/kL [2]. Since the average household uses about 191 kL per year, the energy associated with this is between 152 - 1000 kWh per year -- a significant amount of energy.
Thus, we want to use less of this expensive resource, and more of the free water that falls on our roof and currently goes to waste. For this reason, we have installed 35 kL of rainwater tanks, and they are currently collecting from the back part of the house, the pergola and the garage. As part of my plans for developing this system, I wanted to understand what sort of performance we can expect from this system when it is completed. In other words, do we have enough storage so that we will not run out of water?

How much storage is required?

I went to the Australian Bureau of Meteorology and downloaded the all the monthly rainfall data for Adelaide [3]. Then I built a simple model that estimates our monthly average consumption. The model pretends that we installed our tanks when rainfall records began (in 1884). I then ran the model through all the data to see how our system would have performed historically. Thus we have 129 years' real data to test things.
  1. We collect water from every roof on our land (not currently the case, but a near- to medium-term goal)
  2. We collect 85% of the water that falls on these surfaces (the rest splashes, evaporates, etc)
  3. There are many assumptions about how much we use, but it seems to be approximately similar to the bills we get from our utility (although, our usage in summer may be higher than this model suggests).


With our system as is (when the above assumptions are met), we would have run out of water in 100 out of 129 years.
If we capture grey water from the kitchen sink and washing machine, and use that instead of potable water for watering the garden, then we would have run out of water in 41 our of 129 years.
If in addition to the grey water, we move to a dry toilet (eg. composting) then we would have run out of water in three years.
If we increase our water storage to 45 kL, but retain the flushing toilet, we would have run out of water in 17 out of 129 years. All my modelling was performed in a simple spreadsheet that can be downloaded here [4] (the model was created in a spreadsheet in LibreOffice [5] -- a free office program that you may install for no cost if you want) -- you can play with the numbers yourself. Please let me know in comments or by email if you find any problems.

Modeled rainwater captured, in storage and consumption on a month-by-month  basis for an average year (rainfall averaged monthly since records began in 1884)


If the goal is not to run out of water, it is much more effective to reduce consumption than to increase storage. Also, capturing water from a larger roof area is also much more effective than increasing storage. Grey water is very powerful here, as we will be able to use water twice: once in the home (washing, etc) and then again in the garden. This is useful provided the use of grey water in the garden offsets water we would otherwise use in the garden.

Other considerations

I have also been reading about the energy cost of pumping domestic rainwater. CSIRO have published a paper on this [6], which says that having a pump that directly pressurises the pipes is a very energy expensive way to run things and results in energy consumption that is close to desalination.
I am hoping, as much as possible, to run the garden on gravity-fed water
wherever possible to mitigate this. Also, instead of having a system where a pump directly pressurises our existing water pipes in the house, I'm hoping to create a header tank up high, and then gravity feed the house [EDIT: I have decided not to do this, and have instead installed a variable speed pump. I will discuss this in a future post]. The problem is that, because we live in an urban setting, I'm limited by Council regulations how elevated such a tank can be. I will most more on this in another document.


[1] Climate Change and Water: International Perspectives on Mitigation...  edited by Carol Howe, Joel B. Smith, MS. Jim Henderson

This post was written by Angus Wallace and first appeared at


  1. Hi Angus. Very sound conclusions and astute observations. It may be worthwhile thinking about small 12V DC pumps too as they operate using 1/2inch black and green strip ag pipe? Dunno. I have two of them operating at a flow rate of 17L/min + 60PSI and they have some serious pressure. Interestingly, those pumps use about 8A at 12V which is equal to about 100W (I rounded up as A x V = W). That is stuff all power. The grey water reuse is a good idea too. Here there is a worm farm which treats the black water too and the wildlife loves the herbage that grows there during the hottest part of summer.

    I've read that elevated tanks need at least 70m fall in order to replicate normal household pressure using gravity? Dunno.



  2. Hi Chris,
    Thanks. In the end, I have gone for a variable-speed pump that only works as hard as the water demand requires. I'm also running the house at a lower pressure than before, but not at a gravity fed pressure. It's at about 25 PSI, compared with about 55 PSI on mains water, and about 5 PSI on gravity fed (at this lower use and pressure, it's using about 60 W when working, which isn't much). I think you're right about the header tank height -- we've got about 50 m head of pressure on mains here, which would need a header tank way up high! I was toying with the idea of replumbing with wide guage copper (on a low pressure system), but I decided that it wasn't worth it -- the expense of doing it, and the embodied energy in the copper, would far outweigh any energy efficiency gains I'd make. If I was building a new house or replumbing anyway I would probably go for gravity fed because, as you observe, it would allow direct solar powered DC pumps to be used to fill the tank when the sun is shining, and thus be completely decoupled from the house's power supply. I like independent, simple and redundant systems! ;-)

    I'd be interested to hear more about your black-water treatment. I'm pretty sure I've read your blog post in the past, but can't remember the detail. will have to re-read, but I'm not sure it's something I can get away with in suburbia! ;-)

    Cheers, Angus


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