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Rain Catch Calculator

Discussion in 'Preparedness & Survival' started by MrB+, Mar 15, 2011.

  1. MrB+

    MrB+ Portland area Member

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    Learned about this site the other day (on TSP,) thought it was cute enough to pass along -- http://www.save-the-rain.com.

    It reports in metric, but you can do the math pretty easily. You feed it your address and it brings up a google-maps satellite view. You zoom in and draw an outline around your roof and it takes that and rainfall for your area and tells you how much rain you could harvest in a year. Wish it reported by month, since the Dec-Jan rainfall doesn't help my garden much, but free is a good price.

  2. Decker

    Decker My house Active Member

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    Nice find.

  3. MrB+

    MrB+ Portland area Member

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    And this site -- www.worldclimate.com -- has rainfall broken down by month for many cities. You'll have to do your own legwork converting inches in a month to gallons off your roof, but at least all the data is there to estimate how much rain *you* might get off your roof, in say, July.

    The first calculator sez I might harvest 98340 liters / yr (26,000 gal) from the back half of my roof. Checking the average-by-month chart, for the rain gauge at the KGW weather station, it looks like the dry months fall to ~500 gal.

    jan 14%
    feb 12
    mar 11
    apr 8 2080 gal
    may 6 1560
    jun 4 1040
    jul 2 520 = ~ 17 gal/day
    aug 2 520
    sep 4 1040
    oct 7 1820
    nov 14
    dec 16

    So... I'd been envisioning a catch system with 3 barrels, ~ 150 gal. It would appear that if the rain in July/August is spread across the weeks, 150 gallons might not be way off the mark for reservoir size. That averages out to 17 gal/day or 8 days of storage.

    Next question: Will 17 gal/day be enough for a half-dozen raised boxes? Or do I need to capture some of the water off the front half of the roof and add more impound? Seems like not enough, but it'd sure offset a fraction of the city-water usage. More collection/impound might be in order.

  4. 19 Adam

    19 Adam rural Clackamas County, Oregon Bronze Supporter Bronze Supporter

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    Very good info!

    It's great to live in the Pacific Northwest where things are green and with very little preparation you can have plenty of drinking water.