Calculating rainfall catchment potential

Q. I’d like to hear more about your water catchment plans. Do you plan to use the water for all your household needs, or just gardening? What’s your catchment square footage? What’s your average annual rainfall? Do you know how much water you use currently?

A. We are still in the beginning stages of this next step in our “water & waste” journey. First, we plan on taking care of the gardening water needs.

Yes, we do have a WATER CHART (pdf) Unfortunately it hasn’t been updated since 2004, but it certainly gives you an idea. During the 2003-2004 season, there was a flurry of planting activity especially fruit trees and berries. Now that the plants are established they require less water. Even though we’ve been conscious of water and for decades have been practicing the “mellow yellow” flush rule, installed two low flush toilets, only washing full loads, wearing clothes until they are really dirty, taking weekly baths and reusing laundry water, these past two years we’ve implemented even more water saving practices (toilet lid sink, clay pot irrigation, outdoor shower, simple grey water reuse practices such as bucket water collection from bathroom and garage sinks). With the completion of the grey water system that will take care of our tub water, installation of the compost and dual flush toilets we should save even more on our water use.

We estimate that over 80% of our household water use goes into watering the garden, and that’s where we want to concentrate the reduction measures.

With basic calculations we figured that we could catch over 30,000 gallons of water from our house roof alone (not including garage — an additional 500 sq ft). Here’s how:

For every inch of rain that falls on a catchment area of 1,000 square feet, you can expect to collect approximately 600 gallons of rainwater.

To calculate the square feet of our house’s catchment area, we measured the area of the outside walls and then include the overhang of any eaves..

(L) x (W) = 3,000 sq ft

Since one inch of rainfall provides approximately 600 gallons of water for a 1,000 square foot catchment area, and our house has a 3,000 square foot catchment area, multiply 600 gallons by 3.

600 gal x 3 = 1,800 gallons

With an average rainfall of 20 inches per year (check average rainfall totals in your area), we have the potential to collect 36,000 gallons of water in one year.

1,800 gal x 20 inches of rain = 36,000 gal

That’s the easy part. But what about cost and storage space? With cisterns priced at about 75 cents to $1.00 per gallon, we are trying to figure out how many gallons (without breaking the bank) would suffice to reduce significantly our dependence on the municipal water source.

Round tanks are fine if you have the space. I wish someone in the U.S. would come up with more efficiently use of space square tank system. Some Aussie had the brains and did just that – perfect for tight urban areas! America is so far behind in such useful eco-inventions.

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