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This question is asked in the spirit of "I don't know what I don't know".

I have three raised-bed gardens, and I'd like to stop watering them with municipal water and start using collected rain.

Can a [ahem] garden-variety rain barrel supply an adequate amount of water for 12' x 8' of raised garden beds, or would we need to supplement with municipal water anyway? National Geographic estimates that a rain barrel‡ saves 1,300 gallons of water. But I can't find an estimate for how much water is consumed by plants in raised beds.

All of that builds to the real question: what are large-scale options for storing rainwater? Are doubled-up rain barrels effective? Cisterns? Some other concept I've never heard of?

‡-We'll assume 65 gal, though they don't specify

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    That 1300 gallon number sounds a little optimistic and would rely heavily on where you live. In my area, we get practically no rain in the summer when I'd need water, and we get lots of rain in the winter when I don't really need the water, so a 55 gallon rain barrel would save me roughly 55 gallons of water. – Johnny May 25 '13 at 0:47
  • You may find the Virginia Rainwater Harvesting Manual useful. Much of it deals with household use of collected water rather than for gardening, but it should give you some insight into the regulations and options. – Compro01 May 28 '13 at 20:04
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    How much rain do you get, and when? There's a big difference between "it rains about twice as much every month in the winter" and "it only rains for a month in the autumn, the rest of the year is dry" in terms of water storage requirements. – Móż Jul 9 '13 at 1:46
  • Answers the secondary question "Can a garden-variety rain barrel supply an adequate amount of water for 12' x 8' of raised garden beds?": sustainability.stackexchange.com/q/20/570 – Jeromy French Jul 17 '13 at 15:25
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From small to large (roughly speaking):

  • Barrels (usually I see 30‒65 U.S. gal (110‒250 L))
  • Plastic Intermediate bulk containers (IBC; usually I see 275 to 330 gal (1000‒1250 L))
  • Rain barrels or IBCs in series — this allows you to scale your capacity over time
  • Large above- or underground tanks / cisterns (e.g. plastic tanks are widely commercially available from 500‒2000 gallons, possibly more; ferrocement is supposed to be relatively inexpensive and you can DIY fairly large containers; really, you could build a cistern as large as you wanted — millions of gallons if you happen to be an emperor with vast resources and an endless supply of laborers at your disposal)
  • Small ponds (a ~20' (6 m) diameter pond at 6' (2 m) deep could hold a few thousand gallons)
  • Increasing water-holding capacity of the soil of a large area (“Soil scientists report that for every 1 percent of organic matter content, the soil can hold 16,500 gallons of plant-available water per acre of soil down to one foot deep.” — M. Charles Gould, Michigan State University Extension)
  • Large ponds (a 100+' (30 m) diameter pond could hold hundreds of thousands of gallons)

I suppose that any of these could really be used in series. If you had a lot of space, you could have a series of ponds, and if you needed to add capacity in the future you could just dig another pond. Substitute tank or cistern for pond if that’s what you’re using.

Or mix and match: an IBC at the house to catch the first water, let the overflow drain into an underground tank, and let the overflow from that drain into a pond. Any overflow from the pond could flow through a series of swales into soils with high organic matter content that are ready to accept tens of thousands of gallons of water per acre. Use the water from the IBC on your kitchen garden. Use the tank water for domestic use. Use the pond water for irrigating market crops. Use gravity to your advantage as much as possible.

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I'd go with a water well - let the earth do the rain water collecting.

If well is not a viable option, commercial rain water gathering systems with e.g. underground tanks do a good job.

http://en.wikipedia.org/wiki/Rainwater_harvesting

http://www.rainharvest.com/rain-harvesting.html

  • While the well is an okay option, you still tapping into the community resource. If you live on the hill or a mountain side, you’d have to dig really deep well. – theUg Jul 7 '13 at 13:32
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    @theUg I don't see it as a community resource. In rural Finland practically every household used to have a well around a hundred years ago. People didn't live close enough to each other for anything else to be practical. – jkj Jul 7 '13 at 19:03
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    In the wet boreal forestland — maybe. Closer to the equator (even in temperate climates) water is increasingly becoming a scarce resource. And I have to reiterate, what if you live on the mountain slope? – theUg Jul 7 '13 at 23:11
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I grew up on a rural property using only rainwater. I'll walk through the calculations as an example to give a general idea of what you'll need to do.

The rainfall in the region averaged about 750mm pa (temperate rainforest area).

The water tank was (using imperial measurements) 20' diameter and 6' deep, for a total volume (in roundish numbers) of 1885 cubic feet, and a capacity of 11740 gals; ~53370 liters. The construction was concrete.

With an initial roof area of around 100 m^2, the average annual water catchment was 75000 liters.

So, a lower than average rain year was likely to maintain the tank water level. More importantly, we could, with frugal water use, go an entire year without rain, using 130 l/day.

Your rainfall figures can be obtained from your local weather service.

"Plastic" water tanks up to 10000 l are easy purchased and transported where I live (Australia), YMMV. You can also consider rubber bladder tanks under the house.

You will need to measure your garden water use to know what you'll need. This will vary according to the crop plants, mulch, area, and climate. If your house has a water meter then learn to read it, and measure how much you use to water the garden. Alternatively, water by bucket and count the buckets.

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To elaborate a bit more on underground rain collection options. With plastic tanks topping out in the 2500 gallon range for underground storage, you are able to link tanks together to increase capacity. In the states a lot of lower cost options fall into where you're located. 1700 gallon tanks are spread nicely throughout the United States, which makes will call or shipping feasible. Larger tanks 2000 - 2500 gallon tanks tend to be limited to Iowa/Minnesota & Central California. Freight on these can be expensive. If you find the 1700 gallon tank nearby, linking a few of these may be a nice option. There's a nice image at the bottom of this page showing a linking possibility.

This 2650 is the biggest plastic underground tank i've come across at the moment. If anyone has anything bigger they've found let me know. Still searching.

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