Rainfall/cistern calculation - results
First, thanks to the various kind souls who sent me responses on this. I
hope over time I'll be able to "give back" with contributions of my own.
I was asked by one list member to post any response I got. I've received
two versions of this, one in English and one in metric terms, and I think
I'll quote both here. (I haven't tested the metric one.)
From Jeff Jacobsen in Washington state (US)(English measure):
>Here is what I do for my calculations:
> Roof size has to be the calculated equal to the ground area the roof
>covers. Hence, do your measurements parallel to the ground; if you
>measure at an angle your result will be too high. Determine area, i.e.
>length x width. Add all roofs or roof segments together to get your
>catchment area. If you work in inches you won't have to worry about
>conversion. Include your gutters too, since they catch water too.
>Divide by 231, the number of cubic inches in a gallon.
>This gives you the number of gallons for one inch of rainfall.
>Mulitply by annual rainfall to get annual catchment.
And from Murray Hopkins in Australia (Metric):
> If you want to do the conversions, the metric formula is:
> 1 litre of water per 1 mm of rainfall per square metre of roof.
>(4.5 litres per gallon, 305 mm per foot, approx 39 inches per metre.)
>You then need to subtract a percentage for losses. This depends upon how
>hot the roof is; how much water ends up in the gutters etc etc.
>We work on about 10% here but thats very variable.
I also got some suggestions as to storage media, from stock tanks to 40 gal.
plastic barrels to ferrocement to pre-formed large plastic tanks (we've got
one already -- they're not cheap!). We're leaning toward ferrocement at
this point, but with at least a year before we have the resources (time and
labor) to essay this, anything is still possible.
Again, thanks to those who wrote!
The First Amendment went too far. It should have said, "Congress shall
make no law".