12. Landscape Sustainability: Rainwater Harvesting
Sustainability in the landscape may seem easy – green is green, right? – but there is a lot more to it than that. I’ve already talked about what sustainability means to me and there are many specific areas where a landscape can be more sustainable; this time let’s talk about rainwater harvesting.
The sustainable attitude towards the rain falling on your property is to take care of it on site rather than piping it away to cause problems elsewhere. I’ve already gotten into the different things you can do to manage your stormwater on the ground. When it comes to rain falling on your roof or other raised paved areas, the best thing you can do is make use of this valuable resource by collecting it and storing for use later.
The focus here is rainwater harvesting for garden use, also referred to as non-potable uses (not for human consumption). These systems do not typically require complicated engineering, and we are lucky to now have great resources available for basic residential systems. In California, the challenge is our extremely long dry season, which means that it takes quite a bit of storage capacity to last all summer and fall, and large tanks can get expensive. But that doesn’t mean its not worth it to have a system that provides you with sustainable and free water during dry spells and into some of the summer months.
Typical System Components: 1. Collection Surface
Your system starts with the collection surface, the roof or other paved area and the point that the water flows to, usually a gutter. The material of the roof surface and the debris that collect on it are important to keep in mind. Most homes have roofs with asphalt shingles which add small particles and pollutants to the rain water. A metal roof is preferable, but any surface can be dealt with, at least for garden use. If you have trees overhanging your roof, you will end up with fallen leaves, twigs, and other debris that needs to be kept out of your piping. This means screens on your gutters that are securely attached and well maintained. You can also install a debris separator just below a cut-off downspout to create a single point for screening.
2. First Flush Device
From the gutters, the rainwater flows to some type of first-flush device. This diverts a certain amount of the initial rainfall away from your storage tank to reduce pollutants. Especially after a long period of no rain, the first bit of water that flows across your roof will pick up quite a lot of pollutants and debris, so the idea is to let that water flow to the storm drain or landscape as it would have before you had a collection system, keeping pollutants and pipe-clogging particles out of your storage tank. The typical residential system uses a pretty ingenious mechanical device that has a floating ball in a vertical pipe that seals the connection once the pipe is full. The exact volume of the first flush should be calculated by an experienced professional specifically for your project, but generally ranges from 1 to 10 gallons per 1,000 s.f. of collection area.
3. Storage Tank
Ultimately, the rainwater reaches a storage tank. I won’t get into the calculations for sizing a tank, but your basic yield is about 600 gallons per 1” of rain falling on 1000 square feet. One important thing to keep in mind that water is heavy, about 8.35 pounds per gallon. Try to locate your tank so that if a disaster was to occur and it burst or fell over, it wouldn't be catastrophic to the occupants of the building or other valuable equipment. If there is any doubt, you should consult with an architect or engineer. Another potential hazard is growth of bacteria or insects in your tank. Mosquitoes make quick use of still water to breed, so the tank needs to be sealed against this. There also needs to be an overflow outlet at the top so that when the tank fills up, the water has somewhere to go, ideally into a bioswale or rain garden.
4. Distribution System
Downstream of the tank you have your distribution system, which can be like typical irrigation system. Ideally you are able to collect and store the rainwater uphill of your garden so that you can gravity feed irrigation lines. If that’s not possible, then you will need to have a powered pump that either pressurizes your lines, or takes the collected water to a storage tank near the high point of your yard for gravity feeding. Of course a pump will add cost and maintenance time, but there are some DC solar pumps made for this type of use that are relatively simple. There are some complications with connecting to a standard drip irrigation system, especially in California's summer-dry climate where you will most likely need to be able to switch over to another water source once your tank runs dry. I am still learning about all the technical details for these systems. Certainly the most low-tech and cost effective distribution is resorting to good old manual labor and have a faucet at the bottom of the tank that can be used to fill watering cans.
Rainwater Harvesting for Indoor Use
The simplest rainwater harvesting systems are used to irrigate the garden, but don’t forget the possibility of bringing it back into your house for flushing toilets, or if it is clean enough, dish washing and laundry. A properly designed system can even produce drinkable water, but this gets more technical because of the obvious hazards for human health. You would definitely need to look into local government regulations and work with an experienced professional for these types of systems.
In many parts of the world people have used low-tech rainwater collection systems like these successfully for generations. There is no doubt that clean water is globally becoming scarcer every year, and so becomes more valuable and politically charged. Especially in the case of a brand new home and garden, a rainwater harvesting system really should be part of your design, even if you’re just allowing for a future installation.
References: (some of these references include their own longer lists that I also drew upon)
Water Management, Occidental Arts and Ecology Center workshop led by Brock Dolman, fall 2012
Water Harvesting in California: Obstacles and Opportunities, published by The Water Institute at OAEC, https://oaec.org/our-work/projects-and-partnerships/water-institute/publications/
Slow it. Spread it. Sink it., published by the Southern Sonoma County Resource Conservation District, https://sonomarcd.org/resources/
Urban Farmer Store literature and website www.urbanfarmerstore.com
An rainwater tank supplier with a particularly informative web site is www.supatank.com.au