Cost Effective Rain Water Storage: I Crunch The Numbers
Most everyone in Southern California who lives in a house has a driveway--a slab of concrete that functions as a way to get the car from the street to the garage (most everyone in Southern California has a car unless they like to stay at home all the time). There is a large proportion of cars that sit only on the driveway, never in the garage, because the garage is full of stuff. However, cars in the garage or on the driveway, there's unused space under nearly every one's driveway--a wonderful place for a cistern to store rain water, lots and lots of rain water.
When we bought our house in 1999, I asked the landscape architect about the viability of putting a rain water cistern under the driveway. She looked at me like I had an extra ear growing out of my forehead. Intimidated, I dropped the idea. What I should have done was gotten a different landscape architect--maybe.
Watching thousands of gallons of perfectly good rain water rush down our drainage culvert on the way back to the ocean every year pains me no end.
I save as much rain water as I can from every storm, and distribute it to the most arid parts of the garden when everything starts to dry out. In this way, I can usually avoid using the sprinkler/drip system for half of fall, all of winter, and the first part of spring. Can I do better? Yes, but...
I started to think about the cost of storage vs. the cost of water.
The problem with saving rain water is that water takes up a lot of room, and there's usually not a good place to store it for longer than a few days or weeks. Water is also very heavy.
Storage options are frustrating. The rain barrels on the market are expensive relative to the amount of rain they can hold. There is a kind that resembles a terra cotta oil-jar. They are gorgeous, but expensive when you calculate the amount of storage space vs. the cost of the water you are saving. Our for-profit water company charges us about $6 for 100 cubic feet of water (748 gallons). One of those very pretty oil-jarish rain barrels costs $150 and holds 65 gallons of water. Rough calculations determine that I would need to spend $300 on two containers to hold a little over a dollar's worth of water. There's more to the calculation, though.
Let's say the containers lasted 10 years, which would make the cost $30/year. If I filled and distributed, filled and distributed rainwater say 5 times per year (an optimistic number), that would make my cost to save rain water only about six times as expensive as buying the water from the water company. Which doesn't quite make sense. Any other options?
To go a cheaper route, there are plastic garbage barrels at about 10 dollars. They hold about 40 gallons, so I would be spending about $30 to store that dollar's worth of water. These barrels only seem to last about two years. Either a hole develops in the bottom, or one of the handles rips off, so I'm not going further with the calculations on those, because there is a sturdier, cheaper option: recycled plastic food grade 55 gallon drums.
On craigslist, the going rate is about $10/drum, or an initial cost of a little over $20 for that dollar's worth of water. If you phone around to car washes (they buy their detergent in them) you may be able to get some for free. Five fillings a year, pro-rated over 5 years of use (barrels get brittle over time), the price starts to look reasonable, at a par or slightly less than buying it from the water company. Plus you can use them for five years, and when they do get brittle, you can still recycle them, which is "green", isn't it? These drums look like the best choice when you think about the cost.
I also looked around the web at the cost of cisterns. A quality fiberglass tank, capacity 3000 gallons, that could be buried under my driveway can be had for about $7,000 to hold (by rough calculations) $24 worth of water, not including the cost of digging up the driveway, digging a hole for the cistern, disposing of the displaced soil, and replacing the driveway. Let us say (too) conservatively, $3000 for that to give us a round number of $10,000 to hold $24 worth of water.
But we'll prorate over 30 years, (though the tank can last much longer than that), costing us around $350/year. If we were able to fill/distribute 5 times, we'd be spending $350/year for water we can buy at $120. Of course, water prices will increase, inflation, drought-restrictions and all that. But the fact is that all in all, the water company can deliver water cheaper than you can store it.
So maybe the landscape architect looked at me like I was nuts because I was indeed nuts.
Or perhaps the moral of the story is to get some food-grade 55 gallon drums off craigslist if you want cost-effective rain water storage. They are pretty easy to plumb up yourself with common plumbing parts so the water can run from your downspouts into the drums and thence out a valve to your hose, and the drums don't take up an unreasonable amount of space. You can paint them up pretty, too. At this point it looks like the best option.
Three other points: first, my water is very expensive because our water company is for-profit, so if you raise an eyebrow at my prices, realize that your ROI may be even lower than mine if your water is cheaper, and it probably is.
Second, I should also mention that looking for an old but still viable water storage tank on craigslist or freecycle or is another potentially inexpensive option. There are also these square plastic tanks encased (for strength) in metal cages--you can sometimes find those as well. Food processors buy vegetable oil, syrup and suchlike in tanks and you may be able to find some. I think they hold about 250 gallons, but are a lot harder to move around than drums.
Last, I have neglected to mention that of course rainwater is magic, you can water and water with imported water and never get the plant growth that a few gallons of rain will give you. But you already knew that, right?