In a year of average rainfall in San Diego, an impressive 12,000 gallons of water fall on the roof of a 2,000 square foot house. But then where does it go? For many of us, it runs off our sloped yards and clay soil, down the driveway, and ultimately into the sewer. It’s wasted. And if it takes pollutants with it into our waterways, it’s worse than wasted.
Just one inch of water falling on an acre adds up to 27,000 gallons of water. “It doesn’t seem like we get a lot of rain, and then it rains and a lot of it runs off,” says Josh Robinson, director of the San Diego Sustainable Living Institute and a designer at Ecology Artisans.
“If you look at San Diego as a whole and you kind of look at the square footage of the area, we actually receive more rainfall in a year than all of the residents of San Diego consume in a year.
There are huge amounts of water falling down and often it’s misappropriated.”
How do we capture that water and put it to work for us?
That’s what Robinson aimed to teach when I first heard him speak about passive rainwater harvesting several years ago at a San Diego Permaculture Convergence.
To start, he gave a demonstration with a watering can and a muffin tin. First, he held the muffin tin upside down, explaining that this is how our neighborhoods are designed so that the water runs off. He poured water on the muffin tin from the watering can, and the water mostly ran off. He then explained that when we achieve passive rainwater harvesting, our yards can be more like the muffin tin right side up. When he poured water on it again, the depression in the tin for each muffin held water.
With a potential 12,000 gallons of water per year or more falling on a roof and more on other nonpermeable surfaces like patios and sidewalks, each home receives far more water than it can capture in a 50 gallon rain barrel. And while one can invest in larger (and more expensive) rain barrels, passive rainwater harvesting is another option to consider.
Essentially, Robinson explains, passive rainwater is “when you’re harvesting rainwater without using tanks and just using the soil itself as your tank.” When he does this on his own property or helps a client do so, he begins with a few questions.
He’ll start first by asking, what are you trying to achieve? You might not be able to get a bumper crop of tomatoes all summer with only passive methods, but passive rainwater harvesting can work very well for native plants, fruit trees, and other perennials.
Second, Robinson will look at the yard itself and work out where they need to move water from and to. What is the square footage of the nonpermeable surfaces, like roofs? How much water could it potentially receive? What about the square footage of the soil that will receive the water?
After that, he considers the type of soil. Water seeps through sandy soils quickly, but much more slowly through heavy clay soils.
There are two scenarios one must work to avoid.
First, you don’t want to end up with pools of standing water for more than three days, because you don’t want mosquitoes. And you probably don’t want standing water near your home’s foundation at all.
Second, you do not want erosion, which can occur if you direct too much water into too small an area and it runs off quickly, taking soil with it.
Passive rainwater harvesting does not necessarily require major earthworks. Often, less is more.
“What we’re really trying to do is slow water down, spread it into the areas we want it, and sink it into the ground,” says Robinson. While it can require earthworks, Robinson says he’s “coming to realize that the least amount we can do can almost be better.”
That can mean simply building up the soil, because soils rich in organic matter can absorb more water than depleted soils. “To build soil, you need food for the microorganisms,” organic matter rich in nitrogen and carbon, “and you need water,” he says. “Any soil, once you start getting compost and mulch in there will begin to act like loam, and that’s what we want.”
Robinson continues, offering other simple strategies. “Maybe mulching can prevent evaporation and runoff. Or maybe burying a few logs or some rocks and blocking that flow of water so it can’t just run down hill. I’m also moving towards vegetative strategies, like planting sedge grasses or shrubs that would also block that flow of runoff. So if those work, great.”
If not, that’s when he begins to examine the topography of the area to consider swales and berms or other options.
No matter what, when passive rainwater harvesting, it’s important to plan for overflows.
You can also integrate passive rainwater harvesting into other water-wise plans, such as using greywater, planting native plants or succulents, shading, mulching, or the more conventional type of rainwater harvesting done with a tank.
After taking Robinson’s workshop, in which he unveiled an ambitious design that included a pond, several overflow pools, and a channel that led water to ultimately spread out over a fan shaped area as it went down a slope, I put a much simpler mini strategy in place in my apartment’s tiny front yard.
I simply dug a ditch along the contour of land and filled it with straw mulch, placing the soil I removed on the downhill side of the ditch to create a berm. I planted either side of the ditch with plants like rosemary that can handle both wet and dry periods during the year. Then, over the course of two years, I tossed in my coffee grounds, and let the rainwater run down my yard and into my tiny swale.
When I moved out, I was both delighted and sad to discover I was leaving fantastic loamy soil that had built up in my ditch in just that short period of time, and during the height of the drought.
Robinson recommends that those looking for more information should check Brad Lancaster’s website (harvestingrainwater.com) or his books, two volumes called Rainwater Harvesting for Drylands and Beyond. If doing it yourself seems daunting, you can hire a contractor like Ecology Artisans that specializes in sustainable landscape design.