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# Rain & Gutter Output (Math?)

## Rain & Gutter Output (Math?)

#1
03-02-03, 02:09 AM
magister
Visiting Guest
Posts: n/a
Rain & Gutter Output (Math?)

I'm currently trying to size a rain and gutter fed cistern for use in irrigation; It should also be noted that I am in a desert environment; My instincts tell me that a simple equation of rainfall totals multiplied by the square footage of the roof might be oversized due to evaporative loss and any absorption experienced by the standard asphalt shingles; Has anyone come up with or is aware of any formula more exact for calculating gutter output than the one I currently possess?

R

#2
03-02-03, 06:21 AM
Member
Join Date: Dec 1999
Location: South Dakota
Posts: 2,999
I don't know what formula your using, but in rainfall you always begin with and acre inch of water. An acre inch is the amount of water to raise 43560 sq ft one inch. Now you take your roof square footage and divide to see what percentage you have. If you have a 4/12 pitch or greater, forget evaporation or absorbtion. Now just figure the runoff, which will be 99.999%. I think your figureing too much. If you want the maximum runoff to your cistern, just use the big gutters instead of the normal house size gutters.

#3
03-02-03, 08:34 AM
magister
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Posts: n/a
Thanks Jack; As usual, you come through;
I should experience little or no loss;
And, I should calculate using a percentage of acre inches;

According to the Cooperative Extension Service of University of Missouri, an acre-inch of water is 27, 154 gallons and now I should be armed with enough info to calculate how large a cistern I should build to retain all of the run-off and better predict how many gallons of irrigation, I should be able to get from rainwater, alone. Of course it helps that I also have access to a ten-year record of daily precipitation, though it isn't specific to my house, it is from close enough down the road that I am using their records in a variety of calculations.

And again; Thanks Jack;
R

#4
03-03-03, 07:03 PM
Member
Join Date: Dec 1999
Location: South Dakota
Posts: 2,999
Yes, they gave you the right amount of gallons. I guess I should have done that also. Have a good day.