Last winter was my first with a 530-gallon rain tank. I did not buy it because I thought it would save me a bunch of money on my water bill. I bought it because rain coming off my roof and running down the driveway irritated me. It seemed wasteful. Water is scarce and expensive in Southern California. But now I find myself wondering whether the rain tank will eventually pay for itself.
The tank is a Bushman Slimline, and it cost $615. The accessories needed to connect it to the gutter and direct overflow (pipes, joints, straps), plus a ball valve to distribute the collected rain into drip lines that irrigate my vegetable garden and some fruit trees, as well as the gravel for the tank’s foundation all cost about $30. So, excluding my time and excluding sales tax, the tank system cost $660.
The tank collects from a 363 square-foot section of roof covered in concrete tiles, which have an efficiency estimated at 65%, meaning 35% of the rain that falls on the roof is not shed into the tank. In other words, though 226 gallons fall on this area of roof per inch of rain, only 147 gallons are shed into the tank.
In an average year of rainfall we get 16.43 inches. So in an average year we’ll collect 2,415 gallons in the tank. If I bought 2,415 gallons of water through our municipal water district it would cost $17.81. At that rate, it will take 37 years for the tank to pay for itself. That’s depressing.
However, I’ve started to think of comparing rainwater to municipal district water as comparing apples to oranges. They’re different products. The quality of the municipal district water is far lower than the quality of the collected rain in terms of the characteristics that plants care about: chloride levels, total dissolved solids, pH, electrical conductivity, etc. And so, the value of a gallon of rainwater is higher than a gallon of municipal district water — to plants, at least (and that’s who’s getting my rain tank water).
To this point, a study of banana plants in Israel found that the highest yield that could be obtained when using “freshwater commonly used for irrigation” (similar to our municipal district water) could be obtained with about half the amount of desalinated water. In other words, if 5 units of desalinated water are needed to grow a banana, then 10 units of irrigation water would be needed to grow that same banana. The desalinated water is twice as valuable to the banana plant.
Similar to desalinated water, rain has very low levels of salts, and rain is more valuable to plants than our municipal district water. I think we already know this from experience. We give the plants in our yard a couple inches of water from a sprinkler and the plants look good, but after a real storm rolls through and drops a couple inches of steady rain our plants look amazing — they’re standing so tall and firm and shiny — and they don’t seem to need more water for weeks. There’s something magic about rain.
So, what if I value the rain that I collect in my tank at twice the price of district water? At that rate we’re looking at just under 19 years until it pays for itself. That’s still a very long time.
The price of the water from the district keeps going up though. I just got a letter informing me that the rate will increase again on September 1. This rate that becomes effective on September 1 will be a 20% increase from the rate we paid when we moved in only two years ago. That’s a 10% rise per year, so far.
If I use the rate that will be effective in September (and continue to value rain at twice as much as district water), the tank and its system will pay off in about 15 years.
Furthermore, as far as I know, the price of water in my district has never gone down. I’m guessing that it will only continue to rise. If it continues on its trajectory of 10% per year, then we’re looking at a pay-off length of ten years.
Ten years is still a long way off, but it does sound reachable. And even though I didn’t buy my rain tank as an investment (thinking it would save me a bunch of money on my water bill), I do now feel less indulgent about my purchase.
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Love reading your yard posts, Greg! I can see the making of another book for So. Californians about what works and doesn’t work in the realm of recycling water and the economics of capturing water from our limited rainfall. Hope we can get together this summer. I like the idea of the bean plants shading your windows.
I found out that you can get rebates on rain tanks. At my water district website I was redirected to this page (http://www.socalwatersmart.com/?page_id=2973). The subsidy should help out a bit with the cost. I would imagine most water districts in California should offer some sort of $ incentive the way things are politically motivated with the drought.
Thanks for the post Greg
Thanks for the link, Eric. Offsetting part of the cost might encourage more people to try out a barrel or cistern. And you never know how long this rebate offer will last.
Hi Greg, I use a rain barrel to feed my avocado tree for as long as I can, but it runs out early summer. My house has one spigot that has San Diego City water and one spigot that comes off a pipe in the house. The house has a water softener system that uses only potassium chloride pellets (no sodium chloride), so the house pipe is potassium softened water. I read that it’s fine to use this to water garden plants and potassium chloride is actually a (albeit cheap) potassium fertilizer for plants. However, I also read that chloride (in general) is not good for avocados. Do you have any thoughts about whether it would be better to water my avocado tree with the city water or the potassium softened water? I seem to get tip burn either way. I do try to leech the soil by flooding it with rain water right before the rain water runs out. Thanks!
Hi Keith,
If the water softener system adds chloride, then it’s probably worse for your avocado tree than simple city water.
Most likely, you’ll have to accept some amount of tip burn during fall and winter each year, even with good watering practices and your careful use of extra rain water. I know that this is the case for me too, unfortunately, although I’ve also noticed that certain varieties and rootstocks clearly perform better in this area.
I’m putting tanks in my place now at perth west Australia.my name is Greg alder also.unbelievable
That is funny! Putting in Bushman tanks too?
Hi Greg! My husband and I have tossed around the idea of collecting water from our roof, but we have an asphalt shingle roof (fairly new– 2 years old.) We’ve heard some people say that water collected off asphalt shingle isn’t the greatest, and may contain harmful chemicals? Have you heard this, and do you have any insights for us? I hate to see the water just run off of our property when we could be collecting some of it! Thanks.
I would not be much concerned about asphalt shingles especially if you use dirty water diverter. after a few minutes of rain the water will wash off most of the accumulated dust and degraded shingles residuals. The first few gallons of dirty water will get collected and then progressively cleaner water will be going into a tank. During a rainy seasons the roof stays fairly clean. I have been collecting rain water of my asphalt roof for many years and the water stays good even after a year in a tank especially after dirt particles sink to the bottom. I have not tested the collected rain water but it looks clean and odorless.
Hi Paul. Thanks so much for your input. That’s very helpful info!
Greg,
I have non commercial tanks to collect rain water with a total capacity of 5,500 gallons. 2,300 gallons in fiberglass pipes sank into a ground about 3 feet deep with poured concrete bottoms. all of the pipe tanks are totally leak free. 2 years ago i have constructed tank out of 6″ concrete blocks with a rebars running through every layer and every cavity. That 3200 gallon tank is in the ground. I have kept log of the building costs and i have spend $2,000 on building it. Over the projected life of 100+ years it will pay for itself many times over even if I won’t see fully realized return on investment during my lifetime. Just like buildings, bridges and other infrastructure that were build before out time tanks will keep serving future generations with quality salt free rain water and the garden and trees will reward you with extra bounty of fruits and vegetables.
All of California seems to be in a state of permanent drought (other than the most northern end of the state). Besides the monetary calculation on ROI, I think there is also the civic value of not draining hetch hetchy (or wherever your water is sourced). Wise water management benefits everyone.
I am hoping to build a house in Sonoma at some point in the future. It will have both avocado trees and water tanks.
Another reason for rain water collection is emergency. City water has been very reliable, but if there is a major disaster and city water stops flowing saved rain water can be filtered and sanitized for drinking as well as a source to keep your most important trees/plants alive during the long summer months here in San Diego.
Agreed, Rick. It is very comforting to know that you’ve got water to drink if the water stops flowing from the faucets for some reason. One time in my life I experienced a lack of drinking water and it was something I never want to go through again.
Hi Greg. I’m a rank beginner to all of this, but am about to begin a large remodel project on our 1955 home in the Modesto area that will include rain tanks. I lived in Australia for many years and friends had rain tanks on top of or beside their homes. SO GLAD that Californians are beginning to see the merits of harvesting rainwater! I am just beginning my research into rain tanks for landscape irrigation but am NOT technologically savvy. Can any contractor install and pipe rain tanks for irrigation or do I need an irrigation specialist? How do I know how many rain tanks I’ll need? The level part of the property is about 1/3 acre. Do I have to get permission from my water board (the neighborhood gets its water from two “private” wells)? All I want to do is use what water we have responsibly and keep what little rain that falls in our area for later use in my yard. It’s hard for me to know where to begin.