The Great British Swale Off.

By Andy Williams.

Let’s face it, swales are exciting. It’s one of the things that really engages people when they first come across permaculture design, and with good reason. They’re easily constructed even with hand tools, don’t require inputs or maintenance if properly installed and really work. Swales have become somewhat unfashionable lately however, with their suitability for temperate climates being brought into question. As someone who has installed them on a wet Scottish field, I thought I’d add my opinion to the debate.

The fact you’re reading this suggests you know what a swale is, but in case you don’t they’re basically a ditch and berm dug along the contour of land, to catch overland flow of water and hold it until it’s absorbed into the ground. They’re used extensively in areas of low rainfall, or where rains come in infrequent but large events. Swales recharge aquifers, and can bring dried up springs back to life downslope. All good you might think, but swales are not without controversy. Some permaculture ‘celebrities’ are huge advocates for swales in wet climates, whereas other big names insist that other techniques can achieve the same result more effectively. So how is anyone supposed to know which advice is right? I’m no expert (and frankly, avoid anyone who claims they are; they tend to be keyboard warriors) but I’ll explain my reasons for installing ours.

A large part of the site design is transitioning from an open, exposed field into a silvopasture/agroforestry system of perennial productive trees and shrubs over pasture, essentially a cool temperate savanna system. Our field is a foot of compacted topsoil over solid clay. In winter, every step squelched underfoot. At first glance we’re the last place on earth you’d think would need more water in the soil, however the water holding capacity of our land isn’t great. Once that foot of topsoil is saturated, any rain becomes overland flow. And after a few weeks without rain, the water that was being held in the soil begins to run out and plant growth slows. If we put in drainage, as is common here, it would help keep the fields more usable in wet conditions but in summer we’d still have no resiliency to drought. Drainage ditches do exactly that, drain. Summer or winter, gravity still works, and any overland flow runs away. Without drainage however, the topsoil is too wet in winter for most productive tree species. Building berms or mounds to plant on is one solution for that, which also has the advantage of providing deeper topsoil for getting trees established. What we could do with then, is a drainage ditch that we can ‘switch off’ when we want, combined with a berm for planting on. That sounds an awful lot like a swale now doesn’t it? We’ll come back to the ‘switch off’ feature shortly.

We initially installed one swale with a retention pond at its end, for wildlife habitat. It was meant to be an experiment, to see how it performed over the summer. This was the summer of 2018 however, so we watched it do nothing but bake in the sunshine. For three months. Day after day, warm winds desiccated the exposed soil. The swale ditch itself was bare clay at its bottom, and soon resembled fired terracotta. In February we’d planted a row of hybrid willow cuttings down the southeastern boundary of the property. By autumn they’d reached knee high. Hardly impressive for willow under normal circumstances, but during the summer of 2018 just surviving was impressive. We’d put more of the same variety onto the swale berm in May, not really expecting much from them. They didn’t just grow, they flew up, putting on 8 feet of growth by autumn. In hard packed, poor soil.

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The only place on the croft that things were thriving was on the swale. Spare tomato plants we’d put out just to avoid throwing them in the compost outyielded the ones we kept by a factor of three. Purple sprouting broccoli grew huge and healthy. All with zero irrigation, zero fertilisers and no attention whatsoever, through 60mph winds even. I suspect that there are a few factors coming in to play here. In the case of the willow, being planted into bare soil, without grass competition, will have certainly made a huge difference. The vegetables will have done better with the protection of the berm. But no matter how beneficial the microclimate, nothing grows well without water. Without rain, we were relying entirely on dew fall. Once the sun was up, about 1.30am in midsummer, that dew was hit by intense sun. But on the south side of the berm, that moist soil is in deep shade for hours, making it available to plants for longer. By the middle of summer I’d seen enough, and installed another two, each with a retention pond.

During major rain events, overland flow runs into the swale and goes into the pond at its end. Once the pond is full the water back-floods the swale. Each swale and pond has a broad spillway, and once the swale is full, it slowly over-spills here, where it won’t cause any erosion, and is picked up by the next swale down. This summer I’ll fit a pipe monk to each of the swales, giving us the ability to decide whether we want to prioritise filling the pond or the swale ditch. This is our ‘switching off’ function I mentioned earlier. In drought conditions this could well make the difference between harvesting fruit, nuts and timber or watching trees die.

This spring I intend to expand the swale systems. I’ll put two new swales in, between the existing ones, so that every other swale has a retention pond. There is a large barn, not owned by us, uphill of the top swale. I’ve installed a small ditch to catch the runoff from this roof. This ditch is four inches higher than the top swale, and a pipe drains the water by gravity into the top swale. This gives us a huge amount of water to play with, so I’ll double the size of the pond on the top swale and run some pipe to give us pressure fed natural irrigation water across 90% of the croft. The three original swale berms are planted to hybrid willow, giving us easily processed stick wood for fuel, cuttings, woodchip and eventually material for producing biochar. The new swales, once installed, will be planted with fruit and nut producing trees and shrubs. We’ll graze between the swales, which will increase the organic matter in the soil and deepen the topsoil while animal manures are washed into the swale ditches, fertilising the trees. The trees and shrubs will push roots deep into the heavy clay subsoil, increasing water infiltration as well as also increasing the organic matter in the soil. Over time, the water holding capacity of the site will increase to the point where it’s productive even in drought years. I’ve been told repeatedly that drought just isn’t an issue here. I was being told it even last summer, while farmers were deeply worried about lack of grass growth. As I write this it’s early March 2019. Rainfall over winter has been far lower that usual, and it’s been so warm in recent weeks that the trees are already budding out. The land is far drier than is typical for this time of year. If we have another drought this year, things will get serious very quickly. Here in Caithness we have comparable rainfall to Kent, and the croft lies just below a ridge. without overland flow from uphill, we’re reliant on rainfall completely. The earthworks we have installed give me a massive amount of peace of mind.

So, why don’t people like swales? A number of reasons, most if not all of them perfectly valid.

  1.  They permanently change the land, and its hydrology. They do, sort of. But so does planting a tree. A tree sends roots down sometimes hundreds of feet, and permanently changes the way water is infiltrated as a result. A swale is just a ditch cut into the topsoil. I can fill that back in should I choose, and the field would be pretty much flat again. A swale is no more permanent than a drainage ditch.
  2. They can have unforeseen effects on land hydrology downslope. True. But here, downslope is Loch Watten. There are springs all over the place, and all the fields have mains water supplied to drinking troughs. It’s just not a problem.
  3. Keyline design does exactly the same, with less permanent change. Pretty much, yes. Keyline is a system of land design that equalises moisture across the landscape by the use of a specialised subsoiling plow that shatters the subsoil without disturbing the topsoil much at all. At the same time, seed is planted into the furrow. The plants send their roots down into the shattered subsoil, and over time the topsoil layer deepens. It is a remarkable technique that’s well proven. It does take a tractor and a keyline plow though. We could possibly hire a neighbour with a tractor, but importing the plow would cost a great deal for the 5 acres we have. The topsoil deepening effect can be achieved by rotational grazing, albeit more slowly, which we plan on doing anyway. It would take years for the soil to be deep enough to plant fruit trees without them drowning in winter. One of the major benefits of swales, for us, is the microclimate effect. Keyline does nothing to change microclimate. If we had another 20 acres it’s certainly something I’d use, as I’ve said it’s a proven technique, but it’s just not practical at the scale of our croft.
  4. They can cause landslides. Yes they can. In land that’s prone to slipping, swales can be disastrous. We’re on a gentle slope over solid clay here though, it’s hard to imagine land less prone to slipping.

So, what should you do? My advice is to listen to all the conflicting advice available. Weigh the pros and cons, and how they apply to your context. And that’s the key thing here. It doesn’t matter what any expert says, no matter how experienced the individual, when that person doesn’t know your land, and what you want to achieve. Blanket statements saying a particular technique is always appropriate are dangerous, but so are blanket statements always condemning a particular technique. These things come in and out of fashion, but fashion should have zero influence on your decisions when you’re going to be living with the consequences for a very long time, and so is whoever works your land after you.

 

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