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I am building a small building on top of a hill. The roof area is 600 square feet. In my area, we get about 39 inches of rain each year, which amounts to 600*38/12 = 1950 cubic feet ~ 14500 gallons of rainwater.

If I wanted to store all of this water in a pond, should I just make a pond with 14500 gallon capacity? Should I use slightly less than this, due to groundwater infiltration/ evaporation? Should I make more than this for multiyear storage?

I am also planning on using the pond to irrigate some perennial plantings (chestnuts, fruit trees, berries, etc) during dry spells.

If anyone has personal experience, that would be great, but I am most interested in manuals or reference materials for such a project.

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  • If you want long term storage you'll probably need to line it somehow,depending on your soil type.
    – Chris H
    Commented Mar 21, 2016 at 21:46
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    It depends on the weather you expect. If you're worried about a multi-year drought then several years of storage is a good idea. Also, if you get periodic major storms you will need to think very carefully about how to handle high inflow and outflow - it might even be necessary to have a liner than extends down the outside all round for time when the whole pond is overflowing. But if you get regular showers during droughts, even a few hundred litres might be enough (in Melbourne Australia during the "millenium drought" we never needed more than about 3kl to get 100l/day because of that)
    – Móż
    Commented Mar 21, 2016 at 22:24

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Start here: Quality Farm Dugouts

When doing this for a farm, the usual calculation (Central Alberta Climate) is to plan for 2 years use, plus 2 years evaporation.

Dugouts should be as deep as practical to excavate to minimize surface area. That usually translates into 15 to 18 feet which is what most trackhoes can handle. Here a trackhoe costs you about $500 to the point where they unload. In a day's activity they can move a bucket load -- 2/3 to 2 cubic yards -- every 15 seconds.

Dugouts are almost never filled by roof runoff, but rather by land runoff. The usual figure used is that there will be at LEAST 1" of spring runoff. This is a low estimate. 2.5" is more typical. But that means that a 15 foot deep 1 acre pond needs 15 * 12 = 180 acres drainage to fill it. This ratio will fill it 95% of the time. There are charts for this ratio for various parts of the province.

Here in Alberta we get about 20 inches of precip per year, but the net level drop from an open body of water is between 36 and 48 inches per year. (This is mostly evaporation, but also from trees on the edge.)

Here, if you are going to make a dugout larger than 6000 cubic meters (200,000 cubic feet, 1.6 million gallons) or if wish to site it or fill it from an ephemeral stream you need a permit.

One thing to check is how much water you are using right now. Off hand 15,000 gallons per year sounds too small. The usual standard is something like 100 g/day/person, but that includes a lot of flushing. But at that rate one person is using 36,000 g/year and that doesn't count exterior use.

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  • Thanks Sherwood! BTW my father in law Willie Johnston once taught with you at St. John's school. I am sure he would be interested to hear what you are up to.
    – Steven Gubkin
    Commented Mar 23, 2016 at 13:54
  • Small world. My email: sgbotsford at gmail.com
    – Sherwood Botsford
    Commented Mar 24, 2016 at 2:27

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