Rain Barrels Part 2: Amount

Water, water, everywhere...

You will be surprised how much a half of an inch of rain on your roof equals in gallons. Your house square footage is a good rough estimate for you roof area. If you have a 2,000-square-foot house (and thus the same roof area) a 1/2 inch of rain will yield 620 gallons of water. In Wisconsin we get about 23 inches of rain (35in if you include snow) meaning about 28,400 gallons each year. Good news is that this over 7 months. Remember that most houses have 4 downspouts so 1/2 inch of rain = 155 gallons of water per downspout.

Estimating:

The website save-the-rain.com is a great way to start estimating how much water your roof gets. They use the  metric system on that site, but you can always use onlineconversion.com to convert it to English Units.

You might also take a look at the rssweather.com to know what your monthly weather is like. Remember if you you get snow where you live, your you probably won't be using your rain barrels. Frozen water expands and water that freezes in your rain barrels is not good.

Needs:

What are you going to do with all that water? I'll leave the details of that answer to you. Don't get too over whelmed with this large amount of water. Remember most occurrences of rain are 1/2in - 4in at a time. After it rains then there are little periods of drought (good time to use that captured water).

I mainly use it to water plants. Last year had about 100 sq ft of garden space and about 180 of rain barrel storage. That is 1.8 gallons of rain storage per square foot of garden space. I didn't use any city water and I was over watering my plants. I think 1.5gal/sq would better estimate of your garden water needs (assuming you live in USDA Zone 5a area). This could even be reduced, if garden water retention techniques were used.

Remember that normally, with out any rain barrels, the water is sent to into the yard. I think the 28,400 gallons each year explains why my sump pump is always running... When I made my first rain barrel, I forgot to add an overflow hose to let extra water out of my rain barrel. After a 2 inch rain storm ( 420gal of water per downspout) filled a 35 gallon trash can rain barrel and I found the lid to my rain barrel had popped off. I would recommend that there is always a for excess water to move out of the rain barrels and away from the house.

Resources:
Roof Rain Water Calculator: http://www.save-the-rain.com/world-bank/

Online Conversion: http://www.onlineconversion.com/

Math for gal/yr :http://www.portlandonline.com/bps/index.cfm?c=ecbbd&a=bbehfa

Collection Area (sq. ft) x Rainfall (in/yr.) / 12 (in/ft) X 7.43 (Gallons/Cubic Foot) = Gallons/Year


Weather Data: http://www.rssweather.com/climate/