As we continue on our path of becoming a non-toxic household, it is also important to try to reduce our carbon footprint while doing so. Taking care of our environment and taking care of our health and the health of our children are not always part and parcel, although there is often an overlap. With the population growth rate getting exponentially higher, we need to consider small, daily things we can do at home to limit our consumption and nurture our planet. Our household has not yet embarked upon the journey of harvesting rainwater for our irrigation, but perhaps this will be a 2013 goal as we are just getting our garden started. I’m inspired. This is a great project for the entire family and an incredible teaching moment for our children. We love outside projects! Water is one of our most precious resources and to teach the importance of it at such a young age is invaluable. Hayley Diamond, our guest author, will walk us through how to harvest rainwater to use for irrigation and other daily uses.

Rainwater Harvesting For Your Home 

By Guest Author (and dear friend), Hayley Diamond, Clean Water Act Program Specialist, Hawaii

Rainwater harvesting is the practice of collecting rainwater from hard surfaces, such as roofs, in order to use the water for gardening, washing and toilet flushing. This sustainable practice conserves potable (drinking) water supplies and the energy involved in treating and transporting potable water and prevents water quality impairments that can be associated with stormwater runoff. Throughout history and across cultures rainwater has been valued as a resource:

• Evidence of rainwater harvesting dates back to 1500 BC (if they could do it, why can’t we?)

• Ancient Roman dwellings had cisterns for capturing stormwater runoff from paved courtyards to augment water supplied by the city’s aqueducts

• Kalahari Bushmen of South Africa collected, stored and buried rainwater in ostrich eggs

Sizing a rainwater harvesting system for your home depends on how much water you need and how much space is available for storage. On average, lawn requires 1 inch of water per week. To calculate the water demand you would multiply the square footage of your lawn by 1/12 (1 inch converted to feet) by a conversion factor of 7.48 (gallons per cubic feet).

You typically will not need water during the rainier months, especially if the end use is to irrigate your landscape. It is best to size the system’s storage capacity based on your non-potable water needs and the average annual rainfall of the drier months and to ensure that there is an overflow hose from the system directed away from the foundation of your home to a vegetated area. Rainwater can be stored in 55 gallon barrels or in larger cisterns (typically 500 gallons and greater). Multiple barrels can be connected to increase your storage capacity.

Locate your rainwater harvesting system near to where you will be using the water and your roof downspouts. Place barrels on sturdy, level surfaces. If you do not plan to have a submersible pump, then you may try elevating the barrel(s) to increase water pressure. Typically, a 4-inch pipe delivers water from the roof downspout to the storage barrel(s). Please note, typically a 4-inch PVC pipe would be used for this, but since we know that PVC is toxic, carcinogenic, and not a “green” option, try to opt for polyethylene and metal pipes, or even black metal drain pipes.

To prevent mosquito breeding place mosquito screen over open areas like the overflow hose. To maintain your system, it is advised to empty and rinse the inside of the barrel(s) with water annually and to allow a few days for drying out. Also, periodically remove debris from roof gutters.

To calculate how much rainwater your rooftop will produce in a month, multiply the square footage of your roof by the average monthly rainfall (in feet) by a conversion factor of 7.48 (gallons per cubic feet) by a safety factor of 0.95. For example, in January it rains an average of 4.7 inches in San Francisco, and a home with a 1,000 square foot roof, can yield 2,783 gallons of rainwater for storage! 

More resources and diagrams on how to build your own rainwater harvesting system are available online on the San Francisco Public Utilities Commission’s website.

Rainwater Harvesting Quick Facts 

  • 600 gallons of rainwater can be harvested from a 1,000 square foot roof after a 1 inch rain
  • 600 gallons of rainwater can be used to flush a toilet 375 times, wash a car five times or water a 960 square foot lawn for one week

Click here to see how to install your own barrel.

Please let us know if you have any questions in the comments.

Good luck and happy harvesting!

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