I actually love rain harvesting. I think my rainwater barrel is super cool and a good metaphor for water conservation.
But I find it interesting to actually run the math and see how little water they hold, and I wonder if they are taking the limelight away from more important organic gardening practices…
Rain Harvesting Mathematics 101
If your roof is 1000 square feet (100 square metres) and you get 1 inch of rain on a spring Friday afternoon (don’t worry, it had to get it out of its system before the sunny, warm weekend ahead), you will have 625 gallons of water coming off that roof.
I don’t know about where you live, but where I live, 1 inch of rain is common.
Let’s say you are rain harvesting into a 60 gallon rainwater barrel that will get nice and full and you can use it when organic gardening to water your tomatoes another day. But 90% of that water is going elsewhere, maybe into your city’s sewer system. Your barrel can only take 1/10 inch of rain from that 1000 square foot roof.
So while I like the 60 gallon rainwater barrel, I think there are better solutions.
Rain Harvesting Into Cisterns
A 600 gallon rainwater cistern could handle almost a full inch and a 1500 gallon cistern could easily take 2 inches. Think of it as a giant rainwater barrel. I think every house should have one of these, the size dependant on the amount of rainfall in your area.
They can be a bit tricky to fit into the landscape and some people think they are an eyesore, but they can be cleverly hidden or even buried.
Rain Harvesting Into Ponds
A more attractive organic gardening solution for rain harvesting would be to build a small pond to catch the water. A pond the size of a king size bed and 2 feet deep would hold your inch of water and if constructed thoughtfully, would provide drinking water for insects and animals.
Every organic garden should have some open water in it for this purpose. Mosquitoes aren’t a problem if you keep the water moving and put some effective microorganisms in it, but that’s for another article.
While your rain harvesting into a pond, you might as well bring your greywater into it to be cleaned and used as well, rather than piping it to the sewer or septic system. Your sink, shower and laundry water can all be put to use in the garden. Make sure to use non-toxic soaps, shampoos and detergents.