My region gets an average of 12 inches of rain per year, and 24 inches of snow per year. Nearly all of the rain falls in huge monsoons at the end of the summer, so I have to store everything that falls, and can't expect to collect some and use some, then collect more and use it later, I'll be collecting it all at once.

  • My local laws do permit collection.
  • I would like to collect as much possible water in a giant basin, but need to calculate the size needed.

Assuming my house is 1000 square feet, and I try to collect from all angles of the roof, how can I estimate how large a collection basin I'll need? Can I expect to also collect from the snow?

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    If you were just counting the rain, 1000 square feet x 1 foot = 1000 cubic feet. Snow converts to water at between 5 and 15 inches of snow per inch of water depending on how billowy vs packed it is. 8 gallons to a cubic foot. Keep in mind a cubic foot weighs 60 lbs. – Harper - Reinstate Monica Nov 19 '20 at 20:32
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    What are you going to do about losses due to evaporation? – jwh20 Nov 19 '20 at 21:00
  • I thought you already had a pond to collect it. Guess I was wrong. Just curious what part of the world gets monsoon rain and 24" of snow?!?! – FreeMan Nov 19 '20 at 21:40
  • First estimate how much water you need ; I think you will find it impractical = expensive to collect anything close to what you want . 1 cubic ft = 7.5 gallons. – blacksmith37 Nov 19 '20 at 22:13
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    Your looking at close to 8000 gal storage capacity... at about 80% collection efficiency. An 8000 gal tank is a cylinder 6 feet deep and 15 feet across (think a decent sized swimming pool)! This will give you about 22 gal a day of water, which is very thrifty... Which is why most rainwater collection schemes rely on recharge and use not collecting in one month for the entire years use. – mark f Dec 16 '20 at 21:40

Measurements and observations

Precipitation is measured with units of length rather than volume. At first blush that seems odd: why would we measure a quantity of water in inches or millimeters rather than cups, gallons, or liters? The answer is that volume depends on surface area.

When weather observers in your region say that the average rainfall is "12 inches per year" what that means is that if you set out an open-top container of arbitrary size with walls perpendicular to the earth, and if we assume no wind during the rainfall events, then water will accumulate 12 inches deep in that container. In theory it doesn't matter whether the container is an inch or a mile across. Square or round. Man-made container or natural pond or ephemeral pool in the sandstone.

Weather observation standards call for a rain gauge to be placed out in the open, spaced away from any obstruction by at least two times the height of the obstruction.

Wind puts a bit of a wrench in the works, as do obstructions like buildings or trees. If the collection surface isn't horizontal that changes things too. Consider that when rain falls while wind is blowing, one face of a building will get fully wet even though there's a roof overhang (the windward side) while the opposite face of the building will remain entirely dry, as will some amount of ground beyond the building (the leeward side). A rain collection made on the leeward side of an obstruction will take in less than a collection on the windward side. A surface that is tilted to be more normal (perpendicular) to the rainfall vector will collect more rain, while a surface that is tilted to be more parallel to the rainfall vector may collect much less.

Applied to rooftop collection

The simplest way to estimate the storage volume needed is to find the projected surface area of the roof and multiply by the average annual precipitation. By "projected surface area" I mean the amount of ground covered by the roof -- the amount of floor coverings you'd need under the roof, rather than the amount of shingles you'd need to cover the roof.

This simple method overlooks some obvious sources of error. If there is a tree or other building nearby it'll block some rain from reaching the roof surface. If there are winds present while the rain falls, a stronger wind may result in less rain collection. If the roof is pitched rather than flat then losses to wind will increase with roof steepness. Orientation of the building's larger or smaller roof faces with reference to prevailing wind direction will also play a part.

Bottom line

Water not collected because the container is already full is a missed opportunity. Storage containers have cost, as does the time to do careful engineering accounting for wind, roof slopes, and so on. Only you can determine how much cost can be supported today by the possibility of catching or missing some water in the future.

Start with the simple calculation: build a container with cubic foot volume equal to 1000 sq ft * (1 ft of rain + X ft of snow water content).

If containers are costly then start a little smaller than this and add when experience shows that your roof requires it. On the other hand, if excess capacity now is cheap and later expansion of the storage is costly, then oversize it a bit.

  • 98% of "it's more complicated" averages right out again in most locations, so 1000 cubic feet / 7500 gallons is a fairly solid estimate barring some oddly consistent weather pattern and a wildly asymmetric roof being affected by it. On the third hand to the extent that the annual average may involve some significant variation year to year averaging to 12", more capacity is indeed good, if affordable. – Ecnerwal Dec 16 '20 at 23:35

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