SwalesNovember 11th, 2014. Post by Quinta do Vale
Swales – level ditches dug to follow the contours of the land – are one of the principal ingredients of permaculture earthworks which are, by and large, recipes for catching and holding rainwater runoff and encouraging it to slowly infiltrate the soil rather than being lost to the nearest river. Because they’re level, swales don’t channel the water away but hold it in situ until it soaks into the soil. They can be dug to any sort of scale and used alone or, as part of an integrated water catchment system over an entire property, in combination with other elements like ponds, infiltration basins and dams.
On narrow terraces and steep mountain slopes with thin soils – ie. here – swales are not something you can use on a large scale, but they can still be useful. When I dug the lower ponds, the effect on the ability of the surrounding soil to support abundant growth was immediate and impressive, but it didn’t extend too far along the terrace. Just 2 metres away the soil was so dry in summer it barely supported a few grasses and wildflowers and would turn to dust in your hand and blow away. So after working out the contours of the terrace, I decided to extend the area of hydration much further along by using the ponds to feed small swales.
Since the slope of the terrace dictated that the two ponds are on two different levels, I would be able to run two main feed swales out across the terrace, one from each pond. The contour lines run diagonally across the terrace, so by using constructed spillways, I could create more swales below the lower pond level, potentially taking the water right along the terrace as far as an outcrop of bedrock where the terrace turns a corner and changes orientation.
To begin with, the presence of a temporary winter bed of cabbages and onions at the back of the terrace prevented me taking the swales the full width of the terrace, but they’ve now been dug right the way across.
I started work back in January after doing some remedial work on the ponds. The unlined ponds have been stable for a good couple of years now but a persistent problem has been burrowing rodents breaching the ponds and draining them out though their runs (usually through a terrace wall, so not to be encouraged). I used some of the slightly mangled corrugated metal sheeting we’d used as formwork for the water tanks cut down into lengths of roughly 3/4 metre and sunk vertically around the most vulnerable bits of the banks to create a physical barrier before backfilling with soil. Not pretty, but pretty effective. There’s only been one breach since, and it opened into one of the swales.
Rather than landscape the terrace into the classic swale and berm – the berm being formed on the downhill side of the swale by the soil dug to make the swale – I created mini terraces, retained on the downhill side by posts and sawmill offcuts and levelled the soil out in between to create growing beds. The area had already been planted with several apple trees the preceding year, so the trees were interplanted with shrubs such as redcurrant, goji berry and Buddleia, and comfrey as mulch and soil improver.
In May I extended the system by adding a third swale downhill of the second and also fed from the bottom pond. This time I had some help. With this swale, we used the classic berm along most of its length, largely because it’s too close to the terrace edge to do otherwise.
The swales were filled with prunings and mulch to cut down evaporative losses and we constructed mini chinampa-style trellises with bamboo and chicken wire to support the squashes we then planted on the downhill banks of the swales. A handful of compost was added to each squash planting and a lot of mulch but no other work was done to the soil. I wanted to see what it was capable of producing at this stage with just the benefit of the swales. The remainder of the beds were planted with self-seeding annuals such as amaranths and red orach, Welsh bunching onions, cabbages and courgettes.
The squash harvest was reasonable for essentially unfertilised ground.
I’ve been encouraged enough by progress so far to start on extending the system further. At the end of July, I lost all the chickens to local free-ranging dogs who managed to unlatch the fastening on the gate to the compound (at shoulder height) and then break open the gate. The compound has now been dismantled and I’m enlarging it to take in a wider area with a downhill boundary which extends all the way to the previously mentioned outcrop of bedrock. The lower main feed swale will fill a small duck pond, which will then drain into a further swale running the length of the perimeter fence. Outside the fence I’ll grow many more varieties of squash and other species which thrive on the nitrogen-rich run-off from the poultry pen while also providing summer shading for the birds.
Increasing the water holding capacity of the soil in any location is about more than just getting water to it though. Improvements need to be made to the soil structure, massively increasing the organic content and the depth of the topsoil, encouraging the presence and build-up of soil organisms, and planting soil building vegetation amongst the food plants.
I don’t necessarily see the ponds feeding the swales on a permanent basis. Once the soil becomes capable of holding considerably more moisture than it can at present – to the point where it’s able to sustain a reasonable level of moisture throughout Portugal’s hot summers – then the connection to the ponds becomes superfluous, at which point the supply for the duck pond may well be piped instead. The swales themselves will be maintained to continue to collect and infiltrate surface run-off through the winters.