Monday, 3 May 2021

Water catchment calculations

Its been a dry spring here in the Gulf Islands of British Columbia this year which we're told is unusual, however it has taught me a few things about our water catchment system.

We have 3 roofs catching water and diverting it to our approximately 10ft x 30ft x 5ft cistern. 

*All numbers are approximate. Note: ' ~' means 'about'. 

The main house roof square footprint = 750 sq ft.

The cottage house roof square footprint = 300 sq ft. (has its own well and does not use cistern)

The wood shed roof square footprint = 100 sq ft.

Total = 1150 sq ft

When you check the weather (in Canada) they say rainfall in mm. It will rain ~1mm, 1-3mm, 5-10mm, etc.. but how is that calculated? Turns out this is how much rain will fall per square yard. Note: These are both in different measurement types.... Metric and Imperial, of course.

Because of my age I 'think' most things in Imperial (inches) but some things in Metric (kilometers) Canada switched to metric in 1975 according to Google but I remember it happening around 1983... probably because we moved back and forth to England so often when I was a kid. But I digress...

So it turns out 1 inch of rain = 1.56 Gallons of water per square foot.   

This means that our 1,150 sq ft of water collecting roof gives us 1,794 Gallons per inch of rain or 72 Gallons per mm. For perspective, an oil drum holds 55 gallons. We get our rainfall in mm, there are 25mm per inch and as a general rule when it rains it is usually ~1mm or 1-3mm and far less often 5-10mm. In our neck of the woods our average rainfall is around 37 inches per year mostly spring, fall and winter with generally zero rain in July.

Our cistern holds roughly 12,000 U.S. Gallons which requires 6.67 inches of rainfall from our roof.

Now I have always been known for my baths, I shower every morning for washing and bath every night for relaxing -- or at least I did before we left to go #RVLife and now in BC with water shortages - in summer I limit myself to 2 baths per week.

The clawfoot tub holds around 32 Gallons without a person in it and in the 'off season' ie summer we have shallower baths so I guesstimate about 10 gallons per bath, or 20 gallons per week. 

The rainfall requirements for my baths therefore is 1mm per month.

Our cistern is lower at the moment than it was this time last year. I monitor the water level each week on a spreadsheet so we know what to expect. When we moved in May 16th it was 43 inches. Last Monday it was 34.5 inches, we did have a fair bit of rain this week and I checked it around Friday and it was at 36 inches, we've even had a bit of rain since then. I just checked the weather history and we had 4.9mm last week. so about 1/5 inch. Today before we leave for Nanaimo I will check it again and record it. 

I'm tracking this info mostly so that if we build a house again in future we will know:

a) how much water we use     b) how big a cistern we require to be comfortable

External cisterns

I note that lots of houses around this area have external cisterns, usually 2 or 3 that hold 1,500-2,500 Gallons... we'd need at least 15,000 stored Gallons in order not to have to worry about water,  so I don't know how people manage with that - often with just a small house roof footprint too. It seems its not so much a water shortage situation out here as it is a catchment holding shortage.

According to Google the typical family of 4 uses 12,000 Gallons per month, so 6,000 Gallons for a couple. The trick is to collect enough in the wet months to hold you over during the dry months.


  1. In Bermuda all Homes are built atop a large Cistern that collects the Rain from the White Painted Roofs. The trick is to cut back on your Usage to make it last. Living in an RV is a great way to learn that.
    Be Safe and Enjoy!

    It's about time.

    1. Yes that 2.5 years was a very handy lesson in water conservation. We have become very adept at it. We save bath and shower water for our flowers/plants.

  2. Well done Katie, that's a lot of computing. :) More storage = more baths. :)


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