Wednesday, 20 August 2014

Rain water harvesting


Our new allotment is completely off-grid, there are no amenities provided on site. For people who dream of being self-sufficient in the future, and took on the allotment to hone our self-sufficiency skills, this is really great news.

One of the key issues we will face is ensuring that we have water. It's difficult to know in advance how much water we will need since the amount will vary depending on the number of raised beds, type of crops and the conditions each summer.


Exeter receives about 1,000 mm of rainfall each year, anywhere between 30 and 130 mm per month.

Annual Rainfall / Precipication Exeter at Airport (EXT): 972 mm

If we collect 1,000 mm (1m) of rain per year from a area that is 1metre by 1 metre then we will have collected one cubic metre of water (1,000 litres)

So, initially that is exactly what we intend to do.

Intermediate bulk containers

Conveniently, Intermediate bulk container (IBC) are designed to hold 1,000 litres.  An IBC is a "reusable industrial containers designed for the transport and storage of bulk liquid". A suitable food-grade IBC may have been used to transport fruit juice for packing and sale in the destination country.


Even though we're only intending to store water on the allotment for garden use (and not for a home grey water system), we'll still be following BS8515:2009.  In practice this will mean collecting water from a greater area to ensure that we regularly flush out the tank and ensuring that the tank is insulated so that the temperature does not rise too high since this would increase the risk of bacteria growth.

  • It may be worth reading the FAO's Introduction to irrigation
  • More to understand on Annual Rainwater Yield (ARY), Annual Rainwater Demand (ARD) and risks like Legionnaires Disease


  1. Oooh, but they are heavy!!!

  2. But not so bad when you realise that you can roll them on soft ground

  3. Just catching up - I missed this post for some reason. I certainly looks like a good idea and presumably it's well under way now. Question: would bacterial growth actually provide nutrients for the soil?

  4. Hi GB, it sounds plausible - but I'd be too concerned by something like legionnaires to ever risk bacteria and stagnant water. One day I'd like to process captured rain water through something like a slow sand filter or a distiller so that it's drinkable.