Rain catcher design proves too costly

I'm looking at arbor / pergola and trellis designs that I like and collecting them over on Pinterest.

Follow Brent's board Arbors and Trellises on Pinterest.

This one from Lowes caught my eye because it is simple and has a number of design elements that I might like to consider.  It also has a bill of materials and costs for each part of the construction.  Overall, this is just the sort of useful garden construction advice that I like to see online.  Bravo to Lowes.

However, one of the design features of the arbor is a "rain catcher" which is a section of 6" PVC pipe stood on end and fed by a downspout.  Go to the article linked above to see the construction details or just take my word for it. The BOM is reproduced below showing a total price of $125!!

Rain Catcher: $125
    1 PVC pipe (6-inch x 10-foot, #86806)
    1 PVC clean-out plugs (6-inch, #53292)
    1 PVC female adapter (6-inch, #44863)
    1 PVC sanitary tee (6-inch, #53339)
    1 PVC bushing (6- x 4-inch, #23411)
    1 PVC bushing (4- x 2-inch, #23314)
    1 PVC bushing (2- x 1/2-inch, #51013)
    1 hose bibb (1/2-inch, #21515)
    Primer (#23778)
    PVC cement (#23464)

Special Spigot Hidden behind the lattice in the center of the pergola, a rain-catcher system harvests rain from the home’s gutters. Sarah uses the rain for watering nearby plants. Rainwater Harvester: Build a water-catching system to recycle rainwater and save on utility bills. 

Above photo and caption give the idea that the ~6' tall by 6" diameter PVC pipe is adapted down to a hose spigot through a PVC tee and some reducing adaptors and hidden behind wood lattice that is integrated into the arbor support.

All fine and dandy, you say.  But wait!  How much water can that PVC pipe hold? (Does quick math: 6" diam, ~6' long * pi*r^2, then convert to gallons with the help of Google).  A whopping 8.8 gallons!  (Note: Juxtaposition of whopping and and 8.8 gallons is supposed to have a sarcastic ring to it.)

This is roof drainage, collecting over probably tens of square feet, so it wouldn't take much to fill that pipe up and overflow it.  A 100 square foot roof (very small) would require about 0.15" of rainfall to fill the pipe.

Aside: Those 8.8 gallons of water weigh in excess of 70 pounds.  Where's the support for the pipe in the construction plans?

Suppose I fill the pipe 10 times during a southern California rainy season (more times than I would expect given my gut feel about storm frequency and size) and I get 10 years of use out of it (about as long as I would estimate, making this a bounding estimate in engineering terms).  That's 8.8*10*10 = 880 gallons collected.  But it cost me $125 according to the BOM to build the darn thing, so the cost per gallon is $0.14.  That's about 50 to 100 times the cost per gallon of tap water!

And this is why small rainwater collection devices don't work well in southern California. At least it compares favorably to Dasani bottled water, which costs about $0.50 per gallon.  This isn't me making shit up - do the engineering, folks.