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Reclaiming rain water for lawn watering

Postby rtaylor6971 » Tue Jul 31, 2007 8:07 pm

I want to collect rain water from my metal roof and store it in a huge water tank. How big of a tank would I need to have enough pressure to activate a water standard water sprinker? I don't know the formula to compute this.

Thank You,
Randy Taylor
Randy Taylor

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Postby ChrisG » Tue Jul 31, 2007 10:46 pm

Hi Randy,
For gravity-driven flow, the water pressure depends on the height of the water surface in the tank above the outlet at the sprinkler. The size and shape of the tank will affect how quickly the water surface drops as the tank empties, but the volume of the tank does not directly affect the pressure. Here is a page that describes the basic equations and has a calculator to convert between pounds/ square inches and inches of water.
I don't know how much pressure your particular sprinklers require to operate, but lets assume they can 20 psi. Twenty psi is equivalent to 554 inches, or 46 feet of water.
So, unless you live in a 5 story building, you will probably need to use a booster pump, or a different type of irrigation, such as drip irrigation.

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Postby Craig_Bridge » Wed Aug 01, 2007 2:26 pm

Sizing of the tank should be based on water usage or water collection which ever is the limiting factor.

How many gallons does it take to water your lawn? How often do you need to water? How long between rains? This will give you an idea of how much water you would like to store for the usage.

The other side of it is how much water can you collect from the typical rain? You can look up average rainfall in inches or cm by month and multiply by the area of the roof to get volume estimates.

Something else you might want to consider is changing your vegetation to require less water and to provide runoff reduction so you retain the use of the rain longer. This is often a more cost effective solution than sprinkler systems and storage tanks.

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Postby zzzzdoc » Sun Aug 05, 2007 6:50 am

The entire island nation of Bermuda collects water this way, both for household use, and irrigation. Some google searching might reveal some interesting results. That being said, I'm sure they use booster pumps, as Chris has said will be necessary.
Alan Lichtenstein, MD

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