Permaculture Blog

How much rainwater could you collect?

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If you haven’t yet worked out how much rain you can collect from your roof, the answer might be surprise you.

Our roofs are an under-utilised resource, how many people do you know with waterbutts connecting up to their downpipe? Well, maybe a few, you are on a permaculture blog after all!



Here in the UK, it’s not an uncommon sight to see small rivers of clear water rushing down our roads and pavements when it’s raining. This water runs off our pavements, roads and driveways and rushes straight into the network of drains running underneath our towns and cities, to be processed by the water companies. Some water falls directly onto permeable surfaces and soaks directly into the ground. A lot of water we never even see, as it runs straight off our rooftops, down the pipe and into the public sewer.

Then, when the sun comes out we fill up our watering cans from taps and plug our hoses in to give our thirsty gardens a much needed drink. Silly, isn’t it? Even a small waterbutt can hold 100l of water. That’s a tiny amount, but it’s enough to keep pots and seedlings watered while you await the next rainfall. Imagine if you collected all of the rainfall off your roof!

To calculate how much water you could collect off your roof at home, you need a few numbers. You need to determine the total roof area, the amount of local rainfall, and the roof material run-off coefficient (this is the proportion of the rainwater that runs off as opposed to soaking in and re-evaporating later). Outbuildings can add a significant amount of additional collecting surface.

The image below shows a water map of Trelowia, you can see how I calculated the total number of litres under the photo, the total amount might surprise you!


Trelowia Water Calcuation Map

Here are the sums for Trelowia:

Total roof surface = 343m"
Annual rainfall = 1.25m/yr
Slate tiles run-off coefficient (efficiency) = 0.9
Total collectable rainwater = 343m" x 1.25m/yr x 0.9
= 386m3/yr = 386,000 litres/year

Now that is a lot! Most of us would be hard pressed to make good use of all that - even if flushing large-cistern toilets. The previous stewards here installed just 100 litres. Enough for a small backyard maybe but certainly not enough to take advantage of this abundant and life giving resource.

Next time you take a walk round your neighbourhood have a look at the available surface area on the top of the buildings. Imagine how much water and energy we could save if everyone collected enough for use in their homes. It would significantly reduce the stress on our water supply.


How much rainwater could you collect?


If you're interested in learning more about how to apply permaculture design skills to your own site, check out our new online course, Design Your Site with Permaculture.


DESIGN YOUR SITE WITH PERMACULTURE
If you're interested in learning more about how to apply permaculture design skills to your own site, check out our new online course, Design Your Site with Permaculture.

Tags: permaculture water water saving water harvesting site design analysis

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