Wednesday, January 06, 2010

Ark Water Tank

These days I consider a rainwater catchment tank an integral part of any structure.  Last winter I built the Ark; this winter I'm installing the tank.  This one is 1100 gallons, which I bought from Western Tank conveniently located just a few miles down the road.  I chose the low-profile model, since I didn't want to block the view from the window.  Since I wanted a bit of water pressure, I put the tank on a platform, which as Laura demonstrates, is about 5 1/2 feet tall.  Since 1100 gallons of water weighs 9185 pounds, I built a stout platform using treated 6x6 timbers and 2x6 diagonal bracing.  No more railroad ties for me!

Now I have to install the guttering and downspout, and figure out a way to handle the overflow.  Then I'll lay an underground water line to the Ark coldframe, and to a little garden area barely visible in the lower-right corner of the picture.

The Ark roof is 16x24 feet, which works out to 384 square feet.  Assuming 8" of rain a year, the Ark collects 256.1 cubic feet of rain a year.  One cubic foot = 7.481 gallons.  So in an average year, the Ark roof will collect a little over 1900 gallons of water, or nearly double the tank's capacity.  The conclusion I draw from this is that surely I can fill the tank every year. 

An interesting piece of data is to calculate the amount of water from 1" of rain.  From this, you can easily figure out how much water has been added to your tank after any rain.  One inch is 1/12 of a foot, so in this case we multiply 384 square feet by 1/12 and get 32 cubic feet.  Multiply 32 by 7.481 and get 239.4 gallons from 1" of rain.  If, for example, we have 0.2" of rain, multiply 239.4 x 0.2 and get 47.9 gallons.  How about a 2 1/2" gullywasher?  239.4 x 2.5 = 598.5 gallons, over half a tank.  Aren't we glad we went to grade school?