rainwater formula

[gailmom.livejournal.com]

Just a quick post so I'll have this here for later, since one of our goals is to harvest rainwater.

F=footprint of roof (which is equal to square feet of ground covered)

R= average rainfall per year, in inches

to convert cubic feet to gallons, multiply by 7.5

So, according to food not lawns by H.C. Flores, who apparently got this formula from Toby Hemenway who wrote Gaia's Garden (I note this because the formula doesn't actually make sense to me, so I can't confirm it's right):

7.5(FR/12)=gallons per year of water that your roof can catch.

According to this site, our area gets 40 inches of rain per year. (srsly? ok). Let's say my roof covers 1800 sq ft (which isn't actually accurate, since that is the square footage of our home's living space, and we also have guttered roof over the garage, but close enough for now)...

Therefore 7.5[ (1800X40)/12]= (where the heck is my calculator....brb)

ok, here it is.

Holy @(%^#@%(@*#&$

*ahem*

So that would be roughly 45,000 gallons of rainwater we are currently not doing anything with.....huh. Yeah, I'd say it would be worthwhile to put rainbarrels on our list of "good stuff to save for sooner rather than later". Yup. yes. absolutely. wow.

Comments

[goudananda.livejournal.com]

Cisterns and large plastic tanks seem the best. If you can get a 5 or 10k tank you can see how you'd be able to irrigate on a small scale. Regular 55gallon drums go around and folks seem to prefer the blue #2 and some hardware to make them work. I may be able to give you all the opaque white barrels you want for free, only catch is you'll need to pick them up in Austin. If you're interested let me know. They're 55gallon drums as well.

I want to catch water both at work and here but I'm pretty...busy, generally. :P

[gailmom.livejournal.com]

We are absolutely interested. We want to have, eventually, at least two 55 gallon barrels on each downspout, which adds up to 14 barrels, so any we can get for free (or still cheap after borrowing or renting a truck and grabbing the handy excuse to visit you guys) would be a good thing. :)

[goudananda.livejournal.com]

You may have to do some research, the opaque barrels can promote algae growth but...once you're around that, you're fine.

[gailmom.livejournal.com]

I was presuming I could paint them.

[feuervogel]

Wow. I wonder how much rainwater my house could be collecting! Our area won't allow cisterns, so it'll have to be rainbarrels.

[seabream.livejournal.com]

*waves*, hi gailmom, we haven't met, but I spoke a bit with gryphynshadow at Fourth Street last year. I really like this project you're doing.

Re: Formula for anyone who's curious.
Breaking it down: There are ~7.5 gallons in a cubic foot. In order to get the volume of the water you've got to convert from ft^3 to G, you take the area that the rain is going to fall in, and find out how tall that area would be if all the precipitation for the year was in that notional container at one time. For the volume to be in cubic feet, you would need to multiply the area (in ft^2, or length ft x width ft) by the height in feet. Since there amount of rain you have is given in inches, you need to divide it by 12 to convert it to feet. Since the formula is entirely multiplication and division, the brackets are actually mathematically un-necessary, and I presume are there for the purpose of conceptual separation of the conversion factor.

So the formula is 7.5 gallons per cubic foot x area in sq ft x height of rainfall in inches / the number of inches in a foot.

[gryphynshadow.livejournal.com]

*waves back* Hi!

Aren't the most interesting things still coming out of 4th street?!

:)

Thanks for that explanation; I got most of it, but I have to admit to some mild confusion. I figured it was just a case of 'take it on faith, like physics formulas that never made sense in college...' Glad to know there's actually a sound reason for those numbers in that order. :D

[seabream.livejournal.com]

And I'm looking forward to the upcoming one. Will you be there?

You're welcome. Glad to be of use.