When it rains here on the central coast of California (not nearly enough of late) the water that splashes down on our rooftops runs through the gutters, then the downspouts and (if you live in the city) usually down a storm drain system and out to the ocean.
Ever wonder how much water that is? I did when after just one downpour I collected enough water from an eight-foot length of gutter on my shop roof to overflow a fifty-gallon barrel in less than 1 hour!
With a little research, I found that as a rule of thumb, for every 1,000 sq ft of roof area, 1” inch of rain will produce about 625 gallons of water runoff. A slightly more precise formula to calculate runoff for a year is: A x R /12 = W.
- A = Area of ground covered by a roof in square feet (not the total area of the sloped roof)
- R = rainfall in inches per year
- W= cubic feet per year to convert to gallons multiply W by 7.5 (approximately 7.5 gallons per cubic foot of water)
My small home is about 1,000 sq ft. using the above calculation with R = 20 (the average rainfall amount in inches for where I live in Monterey County) AxR = 20,000, divided by 12 = 1,666.66 (cubic feet) x 7.5 = 12,500 gallons of water a year! That’s from just one roof on our property; we also have a shop and a barn!!
The average home in our area is about 2,000 sq ft which would add up to about 25,000 gallons a year. Think of a city block with 10 homes down a street, which would equal about 250,000 gallons a year!!!
Where’s all that water going now? Well, it doesn’t all end up in the ocean, a fair bit will get absorbed until the ground reaches “field saturation” after which no more can soak in and the water flows away.
To keep a 1,000 sq ft garden thriving requires about 100 gallons of water (a generous amount) a day. With 25,000 gallons of water that would get you through a drought period of about 250 days.
A storage tank that big is not practical for most locations. However, a tank that is only 5 ft high and 6ft on each side (or a pond 2 ft deep and 10 ft in diameter) will hold roughly 1,400 gallons of water or enough to last about two weeks.
A word of caution. Rainwater collection is for irrigation purposes only and not suitable for household use. And some states actually have laws on the books which make it illegal to collect rainwater from your own property!
With the rains coming here on the west coast ever less frequently. if we all start doing even a little rainwater collection the impact in lessening water use from groundwater and municipal sources would be significant.