About a month ago I did a talk at our local garden club, Jamberoo Community Growers, about using swales for water harvesting. Below are the notes from my talk – please let me know if you find them useful!
What is a swale?
- A perfectly level ditch, following the contour of the land.
(diagram from Toby Hemenway’s excellent book, Gaia’s Garden)
What does a swale do?
- Water harvesting: captures surface water flowing downhill during rain.
- Slows the movement of surface water across the landscape, putting it to use rather than just letting it flow off.
- Spreads the water evenly along the length of the swale. Water will naturally flow into gullies and away from ridges – swales redistribute it more evenly.
- Holds water for some time (usually days, up to a week) after a rain event, allowing it to slowly infiltrate into the ground.
- Hydrates the slope below the swale, storing water in the soils and sediments underground.
- You won’t need to water even the most fussy plants on a swale for a few weeks after a rain event – more hardy plants won’t ever need watering.
How do you build a swale?
- Start by marking out where it will go – dumpy level/a-frame, clear hose, builder’s level on plank, laser level. Use little flags or pegs to mark the uphill edge of the swale.
- Dig a trench along the marked line, piling the removed soil on the downhill side of the trench to make a mound.
- Think about what will happen if the swale overflows – plan a spillway that won’t erode during high flow, and ensure that the water that exits won’t cause problems downhill.
- Make the bottom of the trench level.
- Crack/chip the bottom of the swale to aid water infiltration.
- You can completely fill the swale with mulch (leaves, sticks, wood, bark, etc) (good for smaller swales) or leave a trench/gullly that will fill with water.
- Mulch the swale mound well to prevent weeds and erosion.
- Plant legumes on the mound (pigeon pea, cow pea, clover, tagasaste, wattle, etc) to build soil nitrogen and act as nursery trees for your crop species. These can be “chopped and dropped” as mulch during the life of the swale.
- Plant fruit and nut trees on the top of the mound (for plants that like drainage) and below the mound (for plants that like moisture).
- Build successive swales down a slope, giving consideration to how they overflow into one another.
- Space neighbouring swales widely on shallow slopes, closer together on steeper slopes.
- Important: Plant out and mulch the swale immediately after construction so you don’t have to fight with weeds later!
Potential problems with swales
- If you don’t get the levels right, you can get a heavy flow of water along your swale and concentrate it to the overflow point, which might cause problems downhill. Even worse, it might burst through the mound somewhere along the swale and erode it out.
- If you don’t plant trees below the swale, you’ll cause waterlogging by infiltrating more water than grasses and small plants can absorb.
- High evaporation can cause salt buildup. You need to ensure the base of the swale is not sealed like a dam, but allows slow infiltration of water. Trees help take up the water and shade the trench to reduce evaporation. Water lost to evaporation is water not used!
- Get the overflow point right! Make sure it won’t erode during high rainfall (e.g. line a sill with rocks or pavers, or use buried overflow pipe through the mound). Plan where you’re directing the overflow water, and ensure a heavy storm won’t cause downhill problems.
- Don’t allow plants to grow thickly and clog up your overflow points.
More swale ideas
- Make swales a generous width, fill them with wood chips or green waste mulch to level the surface, and use them as access paths. Water will be held in the mulch, reducing evaporation and encouraging beneficial fungi (mycorrhiza), but the top layer will be dry for walking on.
- Similar to above, dig ditches on contour in your vegetable garden beds and fill them with mulch as access paths.
- Create crescent-shaped mini-swales for individual trees. They’ll collect water (possibly from a wide area) and concentrate it for infiltration just uphill of the tree.
- Direct swale overflows into water features, dams, rain gardens, banana circles, or (if you don’t have any other options) stormwater drains.
- Direct greywater (from laundry, outdoor showers, etc) into a slotted irrigation pipe that runs along the bottom of a swale. Cover the pipe with lots of wood chips and mulch, so the greywater won’t sit at the surface.
- Road or driveway runoff and rainwater tank overflows can also be directed into swales.
- On rocky ground that can’t be dug (or if you don’t have the time and energy!), place fallen trees, branches, sticks and rocks along contours (across the slope) and fill behind them with mulch, compost, dirt, rocks, etc. When it rains, more silt will wash down the slope and build up behind the branches, creating a swale-like mound.
- Plant water-loving plants (taro, rice, kangkong, lillies) in the swales, and moisture-loving plants (asaparagus, mints, blueberries, bananas, ginger) just above the swale flood level.
- In large swale systems, small dams or pools can be created by deepening and widening the trench at intervals. This would create a string of ponds that link up during rain events, much like billabongs along a river.
- Swales are starting to be used in urban planning to reduce water flows, nurture street trees, and reduce the amount of water going into stormwater systems.
- Aim to keep all the water that falls on your land, on your land!
- Geoff Lawton, in a podcast interview with Paul Wheaton, said that it takes 7 years for a swale system to stabilise and fully rehydrate the landscape below it.