Rainwater Cistern Update

by - January 07, 2014

With the temperature plummeting to 9 degrees Fahrenheit this morning, I was a little concerned about the submersible pump in the bottom of our cistern. Thanks to recent rains, the cistern was full, and thanks to it being black and located on the east side of the house, there was no ice buildup at all.

So to refresh, we have a 500 gallon rainwater cistern that collects runoff from about 440 square feet of roof surface and a submersible pump, installed as a demonstration for an NCSU BAE class.

A 2" rainfall would more than fill the tank.

2in x 1ft/12in 440sqft x  x 7.48gal/cuft =  572 gallons.

Over the past year, Raleigh has had 50.8 inches of rainfall. That's a total of 13,933 gallons of water that traveled through my system and was either used in the garden, stored in the tank or was diverted around to the front of the yard where the overflow outlet empties. Sadly, for most of the past year, the cistern wasn't used on the garden because the pump wasn't working. It needed a good cleaning and now is running again.

Assuming my garden needs 1 - 2 inches a week of water, and assuming it's 400 square feet, this translates to about 250 - 500 gallons of water a week, or draining down the cistern weekly. I can count on rain making up for a chunk of this need. Rainy weeks are a time of gain and storage for the cistern, since there is less demand, and dry weeks are a time of loss for the cistern. Multiple dry weeks may require additional sources like city water for irrigation.

So what is the cost of city water? One CCF (100 cubic feet) is equivalent to 748 gallons of water. Raleigh has the following tiered rates:
  • Tier 1 = 0-4 CCF billed at $2.28
  • Tier 2 = 5-10 CCF billed at $3.80
  • Tier 3 = 11+ CCF billed at $5.07
The average American family can use about 400 gallons of water per day just for indoor use, which equates to about 16 CCF per month. So, as an extreme example, if in one really hot, dry month, if I were to use 2,000 gallons of city water on my small garden (4 weeks x 500 gallons/week), that's 2.7 CCF (2,000 gallons x 1 CCF/750 gal), and I'd automatically be charged at Tier 3, $0.0065/gal, plus the associated sewage rate of $0.00373/gal , which would cost an additional $20.46 to water the garden for that month.

Here's where we get fancy and use NCSU BAE's Rainfall Harvester with historical rainfall and PET data for my location. Assuming I grow nothing but corn, irrigate from March to September, I capture 46% of runoff from the roof, use 4,786 gallons of captured rain water on the garden, save $49 a year. The cistern overflows 52% of the time it rains. This assumes I can pump it all the way dry and only run the cistern when needed. The reality is, I'm terrible at timing irrigation. I usually water right before it rains. Also, our family uses about 8CCF of water per month (150 - 200 gallons per day), so we're usually Tier 1 for the first half the month and Tier 2 for the second half. That's not a huge yearly savings, but I'll take it.

I should be getting a meter for the cistern, and I'm looking forward to the upcoming season and reporting back on my actual water usage from the cistern.

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  1. Wow, I am glad you have all the stats!!!! I could never calculate all. Amazing that the rainwater you collect is only a small portion of your roof!!!!!! Way to go on saving $$$$$.



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