You won’t spend too much time in the company of permaculturists before you hear the word ‘swale’. It is a permaculture mainstay for some, and a point of contention for others.
For me, a swale is another useful tool in the permaculture tool box. So what is a swale? Read on, and I’ll give you a little introduction.
The purpose of a swale
If you live on an urban property with a slight slope to it, you might notice that when it rains, the water just runs right off down the incline, and flows into storm drains. What a waste!
Swales are used to capture that rainwater, and soak it into the soil. The soil is the best rainwater catchment system there is. But the key is to create lots of organic matter in your soil, so it can hold lots of water. That’s why compost and mulch are so important.
Anyway, back to swales. A swale is a sort of ditch, which is dug ‘on contour’, to catch the rainwater before it runs away, and soak it into the soil. The more water that is soaked into the soil, the less supplemental water is needed to water what is growing in the soil. Makes perfect sense, right?
Swales not only hydrate the soil, but they can contribute to refreshing the underground aquifer, springs, and creeks on the property.
Storing rainwater in the soil
There are a lot of different designs for rainwater storage tanks; and many configurations and set-ups. But if you want the best rainwater catchment system, dig a swale, and a pond, if you can.
A swale will soak rain right into the soil where it belongs. It will stop water from running down the slope and off your property; and will spread it out evenly across the landscape, so you don’t have any soggy spots.
Rain barrels need to be cleaned often, as they grow all kinds of slimy stuff. They can also get clogged up, and that’s a real pain in the butt to deal with. And unless they are very large containers, the potential for gravity fed watering is pretty pathetic. There just isn’t enough pressure from a small barrel.
Besides, in order to have enough water stored to make a difference, you’d need quite a lot of barrels. And those barrels would be taking up good growing ground! Better to just store the water in the ground using swales.
Anatomy of a swale
A swale can be many different sizes, but is generally from 2 to 6 feet wide, depending on where it is being use. For an urban lot, a small swale is probably plenty. But if you have a large property, go for those big swales.
A swale is dug on contour. This will ensure that it is level all along it’s length. And that is very important, as it ensures that the water soaks in evenly. If the swale is lower at one end, or lower in the middle somewhere, all the water will flow there. This totally defeats the purpose of spreading water over the landscape. So make sure that the bottom of your swale is level all the way along.
The soil that you dig out of your swale goes on the downhill side. This is called a berm. You should plant your berm right away, with plants that will hold the soil together and add fertility. Legumes are often used.
Generally speaking, a swale is twice as wide as it is deep. So if you want to dig a 2-foot wide swale for your yard, dig it 1 foot deep, on contour, with the berm on the downhill side. If you dig a swale that is 6 feet wide, make it 3 feet deep. (See illustration below)
How many swales, and how big?
The size and number of swales to put on a property depends on a few things:
- How big is the property?
- How big is the watershed?
- How much rainfall do you get?
- What is the slope?
The higher the number of the first three answers, the bigger and more numerous the swales can be. Obviously, if you’re on a small suburban lot, one or two small swales will suffice.
If you have a large acreage or farm to hydrate, you can add more swales, at much larger sizes. You can utilize these swales to implement a food forest, hydrate the soil for gardens, and mitigate flood damage.
Now let’s take a look at how a swale does it’s magic. As you can see from the illustration below, the water hits the swale, either from direct rainfall or from the watershed above it, and spreads out evenly along the swale because the bottom is level all the way along.
As the water soaks into the ground, it creates what is called a ‘plume’ of water. This water is now available to the plants growing near the swale, rather than just running off down the property. Each time there is rain, more water is added to this plume and it gets bigger. It starts to go further and further down the slope, as well as down into the aquifer.
As the plume grows, this water is available to more and more plants. It can water the trees along the swale, as well as annual gardens planted between the swales. Or you can gradually fill this all in with food forest plants. Swales create lots of options.
Whatever you decide to do, you can see how beneficial a swale can be to a property. If you have an urban lot, your lawn is probably inhibiting rainwater from penetrating to the soil; it just runs right off into the gutters, and is washed away before it can be used by the land.
But if you put in a swale part way down your yard, this will catch and soak the rainwater into the soil, which will benefit your whole yard. Now plant a few fruit bushes or trees along that swale, and you’ll have the beginnings of an amazing edible landscape.
Swales and trees
According to Geoff Lawton, permaculture teacher extraordinaire, swales are tree-growing systems. They are especially useful in arid conditions where it is difficult to get things growing.
Swales will enable you to get water right to where it’s needed, and soak it into the landscape. The trees planted along the berms will have better access to water, and will grow faster.
As the trees grow, they will create shade and mulch, helping to stop evaporation and hold more moisture in the soil. This is the great benefit of tree-growing swale systems in arid lands.
As the swale soaks more and more water into the soil, you can start to plant other things, such as annual and perennial gardens, downhill from the swale berm. And it goes on and on from there.
When NOT to use a swale
As cool as swales are, there are times when they aren’t a great idea. For instance, swales should not be built on a slope greater than about 15%. They can cause the soil to shift and slide down the hill.
A landslide would put a real kink in your gardening plans. If you need to plant on a steep slope, you’re best to implement terraces.
Because a swale is designed to hydrate soils, if the soil is already saturated, there really is no need for a swale; and it could be more detrimental than helpful. A landscape can be saturated because of a very high water table; so it’s best to check that out beforehand.
If you have very soggy and saturated land, you can drain it using swale diversion ditches, which are dug so that the bottoms are not level, but have about a 1-2% slope. This will passively remove the water to a more desirable place, like a pond or rain garden, without causing erosion.
Geoff Lawton is a big proponent of swales, and has used them brilliantly on his own property, as well as in places such as Jordan. If you haven’t heard about the Greening the Desert projects, you need to check that out.
Swales can be the answer to a gardener or farmer’s prayers; or they can be complete disasters. It all depends on where you live, what your soil and water table is like, how steep the property is, and many other circumstances. Using the guidelines I’ve laid out, you will have a good idea if swales are right for you.
I hope this article gave you a bit of an understanding of how swales are built and how they work. If you have any questions or comments, please leave them in the comments section below. I’m always happy to hear from you, and help where I can!
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