Swales are tree growing systems that are built on contour. What exactly does that definition mean? Let’s break it down.
First, a definition of contour. Contour maps show the topography of a given area. They tell you, visually, how steep the slope is. By looking at a contour map you can identify where hills, valleys, and cliffs are.
Understanding contour maps can be a bit difficult at first. The following video explains it really well.
So, saying that swales are built on contour means that they follow an individual contour line drawn on a map. If you walk along a swale, it should be perfectly flat, never going up or down hill.
Next, let’s talk about a swale’s #1 purpose: to grow trees. Once you’ve outlined your contour, you create a swale by scooping up a mound of dirt along the contour line. (Also, a side note: Whenever you build any sort of earthworks, seed it with a cover crop as soon as possible to prevent erosion and to crowd out any unwanted volunteers/weeds.) Make sure that the mound stays loose enough that a tree can grow on it. You always, always, always plant swales with trees. In certain arid conditions, you may plant the tree inside the swale instead of on the mound, but there are always trees.
Since visual aids are so useful, here’s a video of Geoff Lawton explaining swales with a miniature “sandbox” version.
Swales help trees grow by catching water. Thanks to gravity, water always runs down hill. In other words, water flows perpendicular to the contour lines. A swale, which is built along a contour line, blocks the water’s downward path. If you’ve built your swale on contour, the water will pool along the level surface behind the swale. At this point, the water can seep into the soil to water the trees. The water also deposits any nutrients or organic matter it picked up while flowing down hill.
Like many things in permaculture, swales are a system. The swale does benefit the trees, but the trees also benefit the swale. As the trees grow, their canopy shades the swale’s catchment and reduces evaporation. The tree roots stabilize the swale mound. The trees also create micro-climates for other plants and provide habitat for animals.
Because they are systems, swales will evolve over time. In particularly wet climates, a swale may eventually evolve into a chinampa. On steeper slows, swales will fill in with organic debris and end up looking more like terraces. In more arid climates, as swales mature, their anti-evaporation effect will create an oasis for other plants. Wherever they are, swales hydrate the land, recharge springs and aquifers, and nurture an abundance of life.
Here’s one more video with Geoff, next to a full-sized swale this time.
How far apart should you build your swales?
Bill Mollison’s Permaculture Design Manual says that in the most arid climates (10 inches of rain per year or less), swales should be 60 feet apart. In the most humid (50 inches of rain per year or more) the swales should be 12 feet apart. What if the climate you are working in fall somewhere in the middle? A bit of extrapolation gives this formula:
Distance Between Swales (in feet) = 72 - 1.2*(inches of rain per year)
As an example, if you are designing swales in northern Utah where it rains 18 inches/year, you will get
Distance Between Swales (in feet) = 72 - 1.2*(18) = 50.4 ft
So you put about 50 feet between swales. (And don’t forget that the swale itself will take up around 5 to 15 feet).
That’s a basic overview of swales. If you have any questions or noticed something important that I missed, please let me know!