As much as I want to believe that my exhaustive arborist knowledge and skills are the saving grace for all trees I encounter, the reality is rain does more good than anything I can dream up. Unless… if I could provide trees with rainwater on demand! Hello rainwater harvesting. Did you know that irrigation water from a well or the city’s supply is vastly different than rainwater? The chemistry is WAY different. Our ground water and city water is very hard; pH is usually around 8 or higher. Rainwater generally has a neutral pH from dissolving nitrogen gases an carbon dioxide from the atmosphere. I like this analogy, “City water irrigation is like life support as rainwater is like the fountain of youth.”
Irrigation is not a replacement for rain. It is only a supplement during drought. Rainwater is MUCH better for trees and all plants.
The city of Austin has a pretty great incentive for installing rainwater collection systems. In a nut-shell, they’ll pay half the cost or $0.50 per gallon of storage tank size, whichever is samller. Go here to check out all the details for yourself. They have programs that range from single barrels to large and complex harvesting systems.
Technology behind large systems is ancient (over 4,000 years old) and very complex. In fact, too complicated to write about in a quick blog post. The Texas Water Development Board wrote an 88 page manual about all the factors one should consider when designing and water harvesting system. You can read the document here. I highly recommend reading this document in its entirety if you are seriously considering a water collection system. Ultimately, there are four main components: catchment surface, plumbing, treatment and storage. Depending on your needs, other components that may be added. Storage is the most expensive part and requires the most thought. I’m going to put a system in my new house; here’s the math I came up with to help decide what storage to do.
- My roof is about 2,000 sq ft of catchment area. A catchment surface will harvest about .50 gallon of water per sq ft on catchment area per inch of rain in an ideal world = I can catch 1,000 gallons rainwater per inch of rain.
- The Austin area gets about 34 inches of rain per year on average. 34 inches of rain X 1,000 gallons per inch = 34,000 gallons harvestable rainwater per year.
- Composite shingles reduce efficiency and some water will be lost to splash and other factors. I’ll probably get about 75% efficiency.
- 34,000 gallons X 75% = 25,500 gallons of rainwater harvested per year.
- I want to account for quarterly recharge cycles. 25,500 divided by 4 = 6,375 ideal storage capacity. Lets go with 6,000 for a round number.
Your imagination is the extent of options available for storage tanks. I like the solution offered by http://www.RainXchange.com. They have a really neat modular product that installs underground and is scalable to the size of storage you want. These guys also have a great calculator on their site to help you determine how much supplemental water you’ll need to augment your landscape irrigation needs.
I found this cool youtube video of a 30,000 gallon underground rainwater storage tank installation
httpv://www.youtube.com/watch?v=YdwKDVzh1cs