The first step of any successful rainwater harvesting and collection system begins with planning.
The roof size or foot print needs to be determined first and the lay of the land including slop and size must be evaluated.
Specific city or county restrictions must be taken into consideration. We recently heard of a tragic story of an Oregon farmer being sentenced to 30 days in jail for collecting rainwater on his own property! – read the whole story here.
Unfortunately rainwater collection and usage falls in between different regulating agencies, all of which have their own different standards. For example water that is left over from residential usage is subject to the regulations by the Oregon Plumbing Board while outdoor water brought back into the home is regulated by the Oregon Department of Health and water used for lawns is regulated for the Oregon Construction Contractors Board. The red tape can be tedious to navigate but is definitely well worth it in the end.
The next step is to determine exactly how much rainwater you plan to collect subject to the total area of your catchment surface / roof size / foot print. Then it is time to consult historical rainfall records for the area of the country that you live in.
Rainfall amounts are measured in both monthly and annual amounts. Example; April – 3″ Rainfall, 2007 – 48″ Rainfall.
In order to estimate your total possible supply of stored rainwater you need the measurements of your catchment area, the local amount of rainfall on average and the rain collection system’s efficiency. Different surfaces have different runoff coefficients.
Metal, gravel and asphalt shingles have the highest runoff coefficients with a value of 0.95 on the high end and a value of 0.75 on the low end.
Lawns have the lowest runoff coefficients with the high being 0.25 and the low 0.05.
We are almost there, the last variable to consider the Safety Design Factory, Fudge Factor aka (SWAG). 0.5 as an average per sq ft. is an accepted average but the majority of professionals use a value between 0.65 and 0.95 depending on their location.
Calculating Harvested Water
Gallons of Harvested Water = Catchment Area (ft2) x rainfall depth (in.) x 0.623 x runoff coefficient x safety factor.
*It should be noted that 0.623 is a constant conversion factor that is always used in the equation.