Cisterns:
We have a client who challenges us with her visions. We have worked with her for years, and she still comes up with new dreams for how she wants to live in her Lumpkin County home.
In 2020, we shared with you the story of replacing her lawn with perennials. https://artofstonegardening.com/the-path-from-lawn-to-wonderland. She is interested in native plants and sustainability. Transforming her space from grass to perennials was one of the first two projects of this kind for Art of Stone Gardening. In late winter 2022 we reported on the progress of that transition https://artofstonegardening.com/if-at-first-you-dont-succseed. In early winter of 2022, Art of Stone Gardening began working with her to develop a water retention system for her very special and beautiful garden.
This story is not so much about art, stonework or gardening. It is about hydraulic engineering and water systems. Hard-core garden techies will enjoy reading about this unique and challenging project.
In 2021, our client came to us, wanting to capture rainwater into a series of cisterns and use the water to irrigate her garden. Cisterns, which have been used since ancient times, are waterproof receptacles for holding water.
Questions:
Our first question was, “How do we figure out how big the cisterns need to be??”
The second question, once we had a sense of the size of the cisterns – and they are huge – was, “Where can they be “hidden” that is not too far from the house?”
The third question was, “Since what we want to do is to water plants with this rain water, how many plants or how large of a space can one cistern water?”
Calculating the Answers:
I think many people, including me, would be surprised by the amount of water cascading off a rooftop. When I first started gardening, I added a rain barrel to one of my downspouts. The biggest problem with the barrel was that it overflowed almost immediately. Most homeowner rain barrels hold 50 to 90 gallons of water.
To put this into perspective, most showers lasting 8 minutes use around 17 gallons of water. Here is a link to an interesting site developed by the City of Portland, OR with information on household water usage. (https://www.portland.gov/water/water-efficiency-programs/save-water-home#:~:text=toilet%20replacement%20rebate-,Shower%20and%20bath%20water%20use,lasts%20for%20around%20eight%20minutes.)
How much water does 1” of rain generate on the roof of a 7,000 square foot house? It is 2,182 gallons. I calculated the surface of the roof (see below) and plugged in the numbers using this calculator from the U.S. Geologic Survey. (https://water.usgs.gov/edu/activity-howmuchrain.html)
Every square foot of roof collects .6234 gallons of water from a 1” rainfall; however, the actual amount depends on whether all the downspouts are used. Some of the water may be lost on the sides of the house without downspouts, or if the velocity of the water is too fast it will over-shoot the gutters and not necessarily collect in the downspouts. So, for this 7,000 square foot house, the roof might be 3,500 square feet. We connected only the front downspouts – approximately 1,700 square foot roof space x .6234 = 1,060 gallons of water.
We purchased three 1,500-gallon cisterns. To capture the output of four of the downspouts, we installed a main tank and an overflow tank. The third tank was placed in another area to capture the remaining two downspouts.
Each week, most plants need 1” of irrigation. To calculate the amount of water required, multiply the square footage of the planting area by gallons per cubic feet. For example, one of our client’s planting area is 20’x50’, which equals 1,000 square feet or 83 cubic feet. Multiply that by gallons per cubic feet and you have: 83 cubic feet x 7.48 gallons per cubic foot = 620 gallons. 620 gallons of water are needed each week. Each 1,500-gallon cistern will water the area for around 2 weeks, more or less.
We connected the tanks to the client’s existing irrigation system and added a feature that switches to city water if the cistern is dry, which is super important when connecting a cistern to an irrigation system. Being prepared for the tanks’ overflow is equally important. Before the tank tops out there is an overflow pipe that drains into the woods. We use pop-ups on the end of the pipe that gradually disperse the water into the soil and woods.
If you’ve read this far, you need to scroll down to see how we put our calculations into action.
Thank you for your interest in this technical project. If you’ve ever wondered what kind of business does this kind of specialized work, now you know. Art of Stone Gardening is a small, locally-owned business that offers solutions to a range of projects. Give us a call if there is a project we can help you with.
