Rainwater harvesting.

TheOracle

TheOracle

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Harvesting Rainwater can save you water, but you must first do your homework. I have build a calculator to help.

Most rainwater calculators multiply your roof area by one annual rainfall figure. That hides the thing that actually decides whether a tank works: when the rain falls relative to when you use it. This one runs a day by day tank simulation against ten years of real rainfall for your town, then tells you the smallest standard tank size that covers the share of your watering you want.

Anyone interested can have a here:
southafricafacts.co.za

Rainwater Tank Size Calculator South Africa

Same roof, same tank, same garden: 84% covered in Johannesburg, 62% in Cape Town. When the rain falls decides it. Size yours properly.
southafricafacts.co.za southafricafacts.co.za
 
Make the towns alphabetical please.
 
Seems my 2 x 3000L tanks is good enough for my 135m² roof based on this calculator. Nice job.

Though I don't think it accounts for our current weird weather though.
 
Toilets and laundry? That would be a bitch to re-plumb.
 
Quite an interesting concept, with a lot of useful details added! However it confirms my opinion that rainwater harvesting in Cape Town is a waste of time and money, due to the seasonal mismatch between rainfall and demand. Our Body Corporate bought tanks during the Day Zero crisis, but never actually used them...

P.S. Rainfall amounts vary tremendously around the Cape Metropole, but I can't expect you to model that. Not even meteorologists agree on the numbers...
 
Gordon_R said:
Quite an interesting concept, with a lot of useful details added! However it confirms my opinion that rainwater harvesting in Cape Town is a waste of time and money, due to the seasonal mismatch between rainfall and demand. Our Body Corporate bought tanks during the Day Zero crisis, but never actually used them...

P.S. Rainfall amounts vary tremendously around the Cape Metropole, but I can't expect you to model that. Not even meteorologists agree on the numbers...
Click to expand...

I think that largely applies across the board, Rainwater harvesting for Garden watering MIGHT make sense, but for normal household usage of things like toilets, etc it gets very cost prohibitive very quickly to try keep sufficient water stock for the required period.
 
ToxicBunny said:
I think that largely applies across the board, Rainwater harvesting for Garden watering MIGHT make sense, but for normal household usage of things like toilets, etc it gets very cost prohibitive very quickly to try keep sufficient water stock for the required period.
Click to expand...
Not sure about prohibitive. Storage tanks are running at less than R1 a litre and it's easy to scale provided you have space available.

We have fairly consistent rainfall throughout the year here in PE so we over anticipated our needs.
 
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ToxicBunny said:
I think that largely applies across the board, Rainwater harvesting for Garden watering MIGHT make sense, but for normal household usage of things like toilets, etc it gets very cost prohibitive very quickly to try keep sufficient water stock for the required period.
Click to expand...

The main point is that the SW-Cape has massive storage dams which harvest rainwater naturally (and store more than 18 months demand). The costs of treating and piping the water are high, but averaged over a large scale. Trying to reproduce that on a small scale only works in really isolated cases. Even Gibraltar has moved to desalination now: https://en.wikipedia.org/wiki/Water_supply_and_sanitation_in_Gibraltar
 
bwana said:
Not sure about prohibitive. Storage tanks are running at less than R1 a litre and it's easy to scale provided you have space available.

We have fairly consistent rainfall throughout the year here in PE so we over anticipated our needs.
Click to expand...

PE might be an exception then.

in eSchitweni we have 6 months (give or take) where rain is rare at best so I'd need a fairly significant level of water storage on site... somewhere in the region of 20000 - 30000l. And that ain't cheap.
 
ToxicBunny said:
PE might be an exception then.

in eSchitweni we have 6 months (give or take) where rain is rare at best so I'd need a fairly significant level of water storage on site... somewhere in the region of 20000 - 30000l. And that ain't cheap.
Click to expand...

If you read the small print in the OP report, it makes the pros and cons clear. For most cases the first 5kl is cost positive, the next 5kl is marginal, and the last 10-20kl is a waste of money. That 'wisdom' is worth more than all the complex calculations...
 
bwana said:
Not sure about prohibitive. Storage tanks are running at less than R1 a litre and it's easy to scale provided you have space available.

We have fairly consistent rainfall throughout the year here in PE so we over anticipated our needs.
Click to expand...

I think that is the whole point of the OP's calculations! Cost per liter stored is only the starting point. Even if capacity is unlimited, roof space then becomes the limiting factor.

The most important parameter is how many times per year the tank can be filled and drained, in order to make cost/benefit estimates. In CT (outside of winter) the number of good rain events (20mm+) is vanishingly small, compared to other parts of SA.
 
Gordon_R said:
I think that is the whole point of the OP's calculations! Cost per liter stored is only the starting point. Even if capacity is unlimited, roof space then becomes the limiting factor.

The most important parameter is how many times per year the tank can be filled and drained, in order to make cost/benefit estimates. In CT (outside of winter) the number of good rain events (20mm+) is vanishingly small, compared to other parts of SA.
Click to expand...
Our problem is that even though we have plenty of roof area (280m2) the roof is segmented and the down pipes distributed all around the house so having one tank farm isn't going to happen. I've got about 110m2 of my roof servicing 12000l of tanks and it's plenty for us.

EDIT: that 12000l only services our washing machine at the moment but can provide water to the entire house at the flip of a valve. Unless I can find a meter for the tanks I can only guess how much rainwater we actually use but at 50l a load and 15+ loads a week it adds up.
 
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