Would a rainwater cistern save you money?

by - March 29, 2012

Once you start down the green road, it's pretty easy to get hyped about conservation - soil, water, building supplies, energy, savings. You would assume a rainwater cistern for watering your garden would save you money. This could be true, but depending on the size of your garden, the frequency of use and the cost of your system, it might actually take a very long time to see those savings.

The Biological and Agricultural Engineering Department at North Carolina State University has produced a free computer model that simulates a cistern for your garden based on historical rainfall data and and calculates how long it would take to recoup your initial cost in water savings based on the amount of city water usage replaced by your cistern. I capture water from 500 square feet of roof, and my cistern can store up to 500 gallons of water. I ran the model so that I watered my 600 sq ft garden 1" a week or 53.4 gallons a day, with waterings at 8am and 6pm during the months March through September.

Because my cistern was absolutely free, I am able to garner savings right away. However, if I had paid the $500 that systems of its size typically cost, it would take me 15 years to pay it off in water savings. The cistern replaced 57% of my city water usage. This calculation is based on whether there was water in the cistern when I was scheduled to do the twice daily waterings.

Out of curiosity, I switched over my design to a 100 gallon storage system (typical of a rain barrel or two small linked barrels) and assumed a cost of $100, I was able to start saving money on water in 6 years. The cistern replaced 31% of my city water usage.

So, before you jump the gun and ride the newest green fad, do the math!

This nifty model has rainfall data files and municipal water costs for most areas of North Carolina. I ran the model with "custom usage" rather than "irrigation" which schedules irrigation by demand based on historical PET estimates. I figured this was more representative of my watering habits.

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  1. Of course, going green is NOT just about saving money, but about saving resources. The fact is there is more and more competition for water, between communities and between states. It is a limited resource, and in times of drought -- the issue becomes huge, like in the west (Texas, California, etc) and even here east of the Mississippi -- when Atlanta had dire water issues the past several years due to the depletion of their reservoir. Tennessee and Mississippi have been fighting over rights to the underground water resources that under both states (is one state taking more than their "fair" share). Recapturing water to use for other than drinking or necessary uses, is the "right" and "green" thing to do, even if there is no monetary savings (kind of like buying "local" or organic -- not the cheapest way to go, but only if you look at the short term and just in terms of the cost of the meal.

  2. Great point, Diana. There are some areas that don't allow people to harvest their rainwater because it prevents the runoff from reaching the public supply. This is true in Colorado unless people have special permission to do so. Check out this article: http://green.blogs.nytimes.com/2009/06/29/the-legalities-of-rainwater-harvesting/.

  3. Very interesting. I would not have thought that it would take that long to recoup the cost, though thankfully rain barrels can usually be found for quite a bit cheaper with a little looking. I just got a rain barrel and am eager to put it to work!



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