Thursday, November 14, 2013

Permaculture Design: Passive Rainwater Harvesting

About a year ago, I first heard about and then met Josh Robinson, founder of the San Diego Sustainable Living Institute. Actually, I read about him even before that, because his work was featured in one of my favorite books, Gaia's Garden. Josh is an expert in permaculture, a form of organic agriculture that was new to me at the time.

I knew this man had coaxed lush green plants and lots of food out of the desert in Tucson, AZ, before he moved to San Diego - without irrigation! - but I had never really seen him in action until this weekend, when I attended his workshops on passive rainwater harvesting and pond construction in Escondido, CA. All I have to say is: WOW!

This is a good read no matter where you live. Permaculture is applicable to all climates, not just those of us who live in bone dry areas like San Diego.

Josh began the workshop with some math. To find out how much rain falls on your roof (or on any space), multiply the square footage of your roof (or that space) by the number of feet of rain you get. Then, to translate that into gallons, multiply the number you've got by 7.48.

We get about 14" of rain per year, Josh said, so let's round it to 12" and call it a foot. And let's say we're looking at a roof that is 1000 square feet.

1000 sq ft * 1 ft rain * 7.48 gallons/cu ft = 7480 gallons of water

To put that another way, for every inch of rain that falls, that means 623.33 gallons of water fall on each 1000 square feet of space.

It's pretty common around here to get these little 55 gallon rain barrels. You put them near the downspout of your gutter and they gather all of the rain that falls on your house. You do the math. 55 gallon rain barrel... 623.33 gallons of water per inch of rain for a 1000 square foot roof... the rain barrel is going to fill up in the first few minutes of a rainstorm and the rest of the water is lost.

Of course, you can get a larger rain barrel. Thousands of gallons, even. And they are really, really expensive.

The point of the workshop is to see how to capture all of that water in addition to gathering it in rain barrels.

Josh also reminded us that in addition to the water from rainfall, a family can obtain more water by routing greywater into their yard. Just by doing laundry-to-landscape (i.e. routing the water from your washing machine out into the yard), an average family of four can obtain an extra 16,400 gallons of water per year. That figure is obtained by the average of 400 loads of laundry times 41 gallons of water used per load. If you do this, you want to use laundry detergent that is low in salts and then put that water into a spot somewhere where the laundry water can be watered down and diluted by salt-free rainwater.

Next, Josh did two demos. Most cities are designed like the upside down muffin tin he held up. That is, the houses are on higher ground and yards, driveways, and roads all channel water into lower ground, where it goes into the gutter. So what happens when it rains? Josh poured water over the muffin tin to simulate this. Of course, all the water ran off. In a typical year in dry San Diego, that's thousands of gallons of water lost for each property.

Josh Robinson
When it rains, landscapes shaped like this upside down muffin tin capture no water

What if we re-shaped our landscape so it looked like a right-side-up muffin tin? Josh flipped over the muffin tin and poured water on that.

Josh Robinson

Some "rain" still ran off, but some of it was captured by the muffin tin. That's the basic idea of rainwater harvesting.

Now he gave us an even more realistic demo. Here's a yard and a house, with two rain barrels. But what if the soil and the yard could also hold water? To simulate soil that soaks up water like a sponge, he used... a sponge. Or four of them actually. Then it rained.

Josh Robinson
Josh shows his scale model of a rain barrel, which he's putting next to the house.

Josh Robinson
It's raining!

After the rainstorm caused by the watering can, Josh poured the water from the rain barrel and squeezed the water from the sponges into a beaker to show how much water was caught. Answer: A lot of water.

Josh Robinson

OK, so aside from the rain barrels, how do we passively capture rainwater?

Scenario 1: Flat Ground

On flat ground, to gather water, dig a basin. The water that falls into the basin seeps into the ground instead of running off.

Josh Robinson
Basin filling up with water

To keep the water from evaporating, fill the basin with mulch. The mulch harbors beneficial organisms, especially fungi, which trees love. It slowly breaks down, adding nutrients to the soil. And it keeps the soil nice and moist.

Josh Robinson
Diagram of basin with mulch in it.

Then - and this is key - plant trees. You can plant trees like willows that don't mind having their feet wet directly in the basin, or you can plant trees that don't tolerate waterlogging next to the basin. Their roots will find that basin and suck up and use all of that water.

Josh Robinson
Plant trees, says Josh

Scenario 2: Sloped Ground

What if the ground is sloping down a hill? Obviously, if you do nothing, the water is going to run off, carrying some of your soil with it. In this case, you dig a swale. A swale is a ditch dug along the contour of the hill. That means that the bottom of the swale is completely flat, so that the water will spread out along its entirety instead of running to the lowest point.

When you dig your swale, obviously you'll be removing a bunch of soil and it has to go somewhere. You place the soil on the downhill side of your swale, creating a berm. It's a nice little barrier to give you extra insurance that the water you catch in your swale won't keep flowing down the hill.

Josh Robinson
Josh points to a swale and berm he drew

Diagram of a Swale and Berm
Close up of swale and berm

The swale fills up with water, which seeps into the ground. You plant on the downhill side of the swale. Again, plant trees. The tree roots will find the water reservoir in the soil.

Josh Robinson

Scenario 3: Really Steep Slope

If the slope is really, really steep, a swale won't work. Josh said that you know it's too steep for a swale when you toss the soil you dig on the ground below the swale to make a berm, and the soil just falls down the hill. Then it's time for terraces.

Diagram of Terraces
Terraces on steep ground

Hands On
We ended the workshop with a hands-on lesson. The workshop after ours would be a hands on laundry-to-landscape workshop. The place we were landscaping would actually begin receiving greywater from a washing machine very soon. And, as you can see, we were working on a slope:

Digging Swales
Sloped land

In this area, there would ultimately be a path with metal arches over it, and the arches would support vines. Five arches, ten vines. On either side of the path, we would make five small swales and berms. And they would capture the laundry water and rainwater, providing water to the vines. (I don't know what kind of vines were planned, but I could imagine using passionfruit, grape, jasmine, or clematis. The former are edible, the latter smell nice.)

So, we started digging. Then, to stabilize the lower side of each swale, we lined it with rocks.

Digging Swales
Lining a swale with rocks

Digging Swales
Another view of the swale. The rocks are on the downhill side. The bottom of the swale should be level ground so that the water spreads out along it evenly.

Digging Swales
A row of swales on the uphill side of the path. The downhill side of each swale is lined with rocks. The vines will be planted in between the swales.

So that's the basic version of passive rainwater harvesting. In a second post, I will detail the workshop Josh did on how to build a pond and a swale. It will show how to use some tools to make sure your swales are built on level ground.

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