I'll try to answer this question and others as I dig into rain barrels in an effort to convince myself that they are a good idea.
There's good instructions on the web for sizing and building rainwater catchment systems. The more comprehensive ones have disclaimers such as, "However, rainwater harvesting for landscape irrigation may only be practical in locations where rainwater can be collected in sufficient quantities during the time that it is needed," which I pulled from the linked web page (from Texas).
Susan Carpenter, the Realist Idealist of the LA Times ranks her three rain barrels third on a list of good eco-innovations - right behind gray water and solar photovoltaic panels and ahead of earthworks (passive garden design to capture rainwater), the Australian waterwall (a narrow rectangular rain barrel that looks like a wall), edible landscapes, and composting toilets. See Composting toilets, backyard chickens and waterwalls: Susan Carpenter's eco-living experiment
She spent $500 ($300 for two rain barrels (a third was free from the city) and $200 for installation and parts) and writes of the rain barrel experiment,
I was a rain barrel skeptic before I joined L.A.'s rainwater harvesting pilot program last fall.... Though rainwater holds such enormous potential for supplementing Southern California's dwindling reserves of imported water, rain barrels seem like such thimbles. During a normal L.A. winter, my 1,500-square-foot roof generates 13,500 gallons of water — a tidal wave compared to what a little barrel can handle.
Which is my concern exactly, but I don't think that under most circumstances that it's best mitigated by her next observations:
Having lived with rain barrels for a year, I've learned that their small size makes them manageable and affordable. The water they catch isn't stored only for summer use. It can be drained in between rains to water nearby plants. An added perk: reducing storm-water runoff to the ocean.
...The 175 gallons they hold were a lot more useful than I'd expected for feeding my exceptionally thirsty fruit plants. The water they held lasted about a month into the summer.
Under certain circumstances (for instance, your roof drains to unavoidable hardscape and then directly off property) I can see a benefit of rain barrels, but for the vast majority of suburban homes I can't see the ecological benefit over garden infiltration. There will almost NEVER be a cost savings if Ms. Carpenter's costs are typical: $500 for three unsightly barrels?
Let's try to put some numbers to my misgivings.
Let's assume that you, the rainwater stud, have 600 square feet of roof (half of a modest sized suburban home's roof area) and a 55 gallon rain barrel. Then it would take about 0.15" of rainfall to fill the barrel (using a conversion of 231 cu. in. per gallon). That means that after 0.15" of rainfall any additional rain is not captured!
Looking over the last six years of rainfall information in my own LA-area backyard (One of my not so private obsessions is rainfall statistics. I'm using my daily records of rainfall to make the tables below. You can do the same thing with a modicum of effort, an Excel spreadsheet, and the daily rainfall tallies for your area. Note to Steve Libby: Naturally I predicted the statistical utility of this data years ago when I started recording rainfall :-)
I'm also assuming I made no mistakes with the analysis, something that has not always proven to be the case, but the numbers seem to be what I expected and time is growing short so with an arrogant tip of my nose I'll take the "meets my expectation" observation as confirmation that they are indeed correct. Most of LA and a wide area of coastal southern California should be similar to what I present below. In fact, the inclusion of our driest year (06-07) and our wettest year (04-05) probably bounds the calculation for most of southern California: you should have results no worse and no better than predicted in those years.
Finally, with caveats and rambling prose out of the way, I can make the assessment of the impact that a rain barrel would have had on the rainwater stud's runoff, had he or she sited it in my backyard in any of the preceding six years:
There are two cases below. They use identical rainfall patterns (that of my back yard for the given years) and identical roof area (600 square feet). They differ in the size of the rain barrel. [Note that the number of down spouts doesn't matter. I assume ALL the rain from a 600 sq ft roof goes into the barrel or barrels. I'm writing this parenthetical note in response to a comment I received elsewhere suggesting that I add downspouts.] The upper table gives values for a 55 gallon rain barrel and the lower gives values for a 110 gallon rain barrel. In our recent wettest year, 2004-05, a 55 gallon rain barrel would only have been 11% efficient at capturing rainfall but would have been 65% efficient in our driest year (2006-07). For the case of the 110 gallon rain barrel the numbers are 19% and 99%, respectively. Average efficiencies are 0.47 for 110 gallon rain barrels and 0.29 for 55 gallon rain barrels.
|55 gallon rain barrel, 600 sq ft of roof|
|110 gallon rain barrel, 600 sq. ft of roof|
I have to admit that the efficiencies are surprisingly high, particularly with larger capacity barrels. However, our assumptions tell us that our rainwater stud isn't always using the water when it's most needed in the garden, which is between storms. Instead, over consecutive storm days that exceed capacity the barrel has to be drained to get the kind of efficiencies that you see here: you actually would have to water with your barrel while it was raining in many cases. Consider also, that 600 sq ft of roof is only half of a small 50's era SFR roof. For larger homes the runoff will increase in proportion to the roof area driving efficiencies down for the two cases set forth above. I'll leave this post as it is now, with a high likelihood of a return visit to this topic later when I can stand to think about this again.
More locally-relevant information about rain barrels is contained in a fairly thorough document that does not address rain barrel efficiency at www.larainwaterharvesting.org.