by Rob Avis
Michelle, Rowen and I were driving home from a vacation in the mountains when we passed by a swale on a farmer’s field in the middle of Alberta cattle country. Naturally, it piqued my curiosity and I had to stop the car to investigate. It was such a great example of how this simple technique can catch and store water on a large scale, we decided to make a short video about it….
Rob walking along a swale after a huge rain
event at the Permaculture Research Institute
Simply put, swales are water-harvesting ditches, built on the contour of a landscape. Most ditches are designed to move water away from an area, so the bottom of the ditch is built on a modest slope, usually between 200:1 to 400:1.
Swales, however, are flat on the bottom because they’re designed to do the opposite; they slow water down to a standstill, eliminate erosion, infiltrate the surrounding area with water, and recharge the groundwater table. When water moves along the flat bottom of a swale, it fills it up like a bathtub — that is, all parts of the bath tub fill at the same rate. The water in a swale is therefore passive; it doesn’t flow the way it would on a slope.
To install a swale, we have to find a contour line. A contour is a horizontal line (with a constant elevation), along a landscape.
To better understand contour, imagine walking on a hill. If you are walking up the hill you will be putting most of your weight on your toes, if you are walking down a hill you will be putting most of your weight on your heels and if you are walking along the contour of the hill, you will be placing an even amount of weight on your heels and your toes. It is this contour line that we need to find when designing and building swales. A variety of survey tools such as transits, laser levels, water levels or A-frame levels are used to find contour lines.
Dam Filling Devices
The swale you saw in the video was being used to carry water across the landscape — from the ditch by the road to the dam far in the distance. But how?
Well, we know water always goes to the lowest possible point. So the swale is built lower than the ditch. When water flows from the ditch, it moves across the swale and fills up the dam. Once the dam is full, the extra water will sit in the swale, or get pushed back into the ditch if it gets high enough. In this way, the swale system is self-regulating. Once the available water has reached an equilibrium, meaning it has filled the lowest point and has no where else to go, it just sits there, unmoving. And as it sits, it slowly seeps into the surrounding landscape, hydrating the soil and recharging the water table below.
Rob explaining a mini-swale to students — looks like he could use
some of his own swales to help revegetate his head!
In this way, the swale fulfills three important functions: it carries water from the ditch to fill the dam, it rehydrates the landscape, and it prevents the dam from overflowing by acting as a channel back to the ditch.
Swales are great for filling dams anywhere except for arid or hyper-arid environments, where they would dry up too quickly.
Establishing Trees — No Watering Required
In my video, I showed how wet the top of the swale is. This was due to capillary action pulling water from the back side of the swale into the soft mound. (Capillary action is the phenomenon where liquid flows upward through narrow spaces against gravity — you can see this phenomenon when you dip the end of a paper towel in water and watch the water climb up it).
Diagram by: Adrian Buckley of
Big Sky Permaculture – thanks, Adrian!
Because the capillary action is so effective in the soil, these mounds along the swale can provide enough water to establish a tree system with little to no additional irrigation. Melt water that would normally be lost to the ground is captured and stored. Trees established on top of a swale at the right time of year (when the swale wall is wet) have a much higher chance of success than trees in the middle of a field. Once one row of trees is established, you can start to revegetate lower parts of the field.
Conventional wisdom says that you need more than 15” or 381 mm of annual rainfall to establish trees. This is not necessary when you design a swale system to aid with water catchment, because it effectively concentrates and holds the available water in that area.
Even permaculture founder Bill Mollison demonstrates that farms can be placed under 25% forest cover without a reduction in output. This is due to the increase in yields from the farm as well as the creation of additional habitat for predatory birds, insects and mammals that keep the the “pests” in balance.
So now you can see why we had to pull over — swales, though simple, can open up the floodgates to healthier landscapes and food systems!