ᐅ How long can rainwater be stored in a rainwater harvesting tank?

We have a plot of land in Vienna measuring 450m2 (4844 sq ft) and a roof area of about 100m2 (1076 sq ft) – our house construction will start this September.

Although everyone advises against buying a rainwater cistern (they say it’s never cost-effective), I simply want to make use of the water resource (for now only for the garden – not for the house), but maybe that will change later.

So – I have the option to buy an 8000L (2100 gallons) cistern for the same price as a 4000L (1050 gallons) one.

My idea is that with an 8000L system I could build up a water surplus over the winter and start spring with a full tank, then gradually use the buffer until summer.

In your opinion, does this make sense or is it better to just buy the 4000L?

Thank you in advance for your feedback and best regards from Vienna,
stefansteiner

I can only share some information about the durability of water in a pool: if properly maintained (regularly filtered, cleaned, and pH-balanced), it can last up to 2 years. Personally, I would change it after no more than 1-3 months, but that might be a subjective feeling :-)

I think the idea is good; however, 4,000 liters (1,057 gallons) might not be enough for the entire summer, depending on the weather.
There is no issue with storage durability. It just tends to develop a slight odor if it sits for a longer period, but this is not a problem for garden irrigation.

As far as I know, there are also 6,000-liter (1,585 gallons) cisterns available.

stefansteiner schrieb:
So – I have the option to buy an 8000-liter (2100-gallon) cistern for the same price as a 4000-liter (1050-gallon) one.

Generally, bigger is better than smaller. If the cistern is needed, there is a high chance the smaller one will run empty, especially during dry periods in summer.

However, the overall costs of the project are not only determined by the container itself but also by necessary excavation, transportation, etc. So having the same price for 4000 liters (1050 gallons) versus 8000 liters (2100 gallons) is unlikely.

I just checked the precipitation data for Vienna. Even with storage over winter, I think the smaller cistern won’t fill up with a 100 m² (1076 ft²) roof area (which is somewhat small already).

Over the entire year, you can expect roughly 6200 liters (1640 gallons) of rainwater on the roof in total. The 8000-liter (2100-gallon) container will never be completely full.

I have 6,000 liters (1,585 gallons) for a very small garden. In summer, it sometimes runs dry, which means that either the consumption is higher or the roof area doesn’t provide enough.

Hello,

Alex85 schrieb:
For a 100 sqm (1,076 sq ft) roof (which is a bit small?!), it won’t fill up completely. Over the whole year, a total (!) of about 6,200 liters of rainfall can be expected on the roof. The 8,000-liter (2,113 gallons) tank will never be full.

It’s news to me that Vienna is located in the Sahel zone... The annual rainfall is more likely around 620 liters per sqm (20 gallons per sq ft), which equals 62,000 liters (16,379 gallons) on the roof per year. So you can definitely fill the 8,000-liter (2,113 gallons) cistern.

And it will also empty quite quickly. Our irrigation system pumps about 1,200 liters (317 gallons) of water into the garden nearly every day. The garden isn’t huge either, with 260 sqm (2,799 sq ft) of lawn plus 60 sqm (646 sq ft) of planted beds.

We don’t have a cistern ourselves, as it would never pay off. In summer it would almost always be empty, and I didn’t want to deal with managing another system. Plus, with cisterns you need to install filters in the irrigation setup and so on. With clean potable water, you don’t have these issues.

And no, even if children are still taught differently in school; Central Europe is not a water-scarce region. We have more than enough drinking water here, and saving water locally won’t cause the Sahara to get more rain.

On top of that, producing about 250 kg (550 lbs) of polyethylene, which your 8,000-liter (2,113 gallons) cistern consists of, likely consumes a considerable amount of drinking water too.

So,
Advice 1: Forget the cistern nonsense; it doesn’t help the environment or your budget.
Advice 2: If you absolutely want to do it anyway, get the large 8,000-liter (2,113 gallons) cistern.

Best regards,
Andreas