Keyline Swales – a Geoff Lawton/Darren Doherty Hybrid
A swale on Zaytuna Farm – © Craig Mackintosh
(Remaining images below © Cam Wilson.)
Geoff Lawton and Darren Doherty are the two highest profile people in Australian Permaculture when it comes to broadacre water harvesting earthworks. They’ve both had success in some very tough environments, and yet it’s interesting that their styles are quite different, particularly when it comes to infiltration strategies.
This article is a short comparison of their approaches, along with an idea I had recently for amalgamating the benefits of each.
To help illustrate, I’ve put a set of boundaries on a section of a topographic map (figure 1.1).
Figure 1.1 – Base Map
I realise that both Geoff and Darren would be salivating as they looked up the hill at the potential dam sites above, but I’ve deliberately left them out of the equation to keep things simple and limit the comparison to their infiltration strategies.
Similarly, although I haven’t marked it in, each of them would put in a small dam/wetland/silt-trap in each of the valleys to dissipate the flow coming on site and prevent their mounds blowing out.
Geoff Lawton’s approach
Geoff’s style for infiltrating water into the landscape is to use swales (often connected to dams but that’s another story). His aim is to catch water as high as he can in the landscape and use the dead level swale to spread the water across the length of the land. This water is held in the swale, giving it time to infiltrate into the soil, and it then plumes downhill, recharging the ground water for the benefit of trees planted below (figure 2.1).
Figure 2.1 – Soil water movement after swale infiltration
See this animation for more details
He often builds his swales with a bulldozer, resulting in a large capacity (eg a bulldozer blade wide and deep as in figure 2.2 – the back and front walls are battered on the subsequent passes).
Figure 2.2 – Front view of a bulldozer building a swale
This is well suited to the sub-tropics where 50-100mm events are common and also in arid areas where the few rain events that occur can be a deluge. A large volume of water is held in the swale, giving it time to infiltrate into the landscape, for the benefit of the trees planted below.
A design constant we can work with is that water flows at 90 degrees to contour, both above and below the soil surface. Each large red dot in figure 2.3 represents an even amount of water that has infiltrated along the length of the swale. The red lines show the path that the water takes as it moves down through the soil profile.
Figure 2.3 Swale infiltration (red) path
Natural water flow in the landscape
A natural pattern in the landscape is that valleys are moist whereas ridges are dry. You can see this in the vegetation in any undulating National Park you go walking in, with lush, moisture loving plants in the valleys, and dry sclerophyll forest on the ridges.
In figure 3.1, each large blue dot represents an even amount of rainwater that has infiltrated into the land above our boundary. The dotted lines show the path that the water takes (90 degrees to contour) as it moves down through the soil profile. This image clearly illustrating why it is that the ridges are much drier than the valleys.
Figure 3.1 – Movement of soil moisture
Darren’s argument against swales in some instances
In figure 4.1 below, I’ve overlayed the swale infiltration path (figure 2.2) over the top of the rainfall infiltration (figure 3.1). As you’ll notice, the swale tends to direct far more water towards the valleys and hasn’t really fixed the issue of our dry ridgelines.
Figure 4.1 Swale infiltration (red) in relation to moisture entering site (light blue)
Recognising this issue, Darren prefers to set out tree lines using a keyline pattern. In this aerial shot of George Howson’s agroforestry property, ‘Dalpura’ (figure 4.2), the tree mounds aren’t on contour but rather they gently slope away from the valleys (the naturally moist areas) towards the ridges (the naturally dry areas), therefore aiming to even out the moisture levels across the landscape.
Figure 4.2 Dalpura tree lines from above
He creates his tree lines using a ripper and mounder, common in forestry plantings, which have a small gutter on the upper and lower sides which help to direct the water. This is a cheaper and more fuel efficient option than a bulldozer or excavator, and works well in climates where rainfall events are generally consistent but small, such as in many temperate landscapes.
The green dots and arrows in figure 4.3 indicate the infiltration of the keyline mound during a small event. Water has been directed away from the valleys and encouraged to infiltrate on the ridge instead. You’ll notice that when combined with the water naturally moving down through the landscape from above, the moisture distribution is far more even than in the swale in figure 4.1
Figure 4.3 – Keyline mound infiltration (green) in a small rain event
Despite the obvious benefits, one downside I see to this approach is that the gutters on the sides of the tree mounds have a relatively small water holding capacity. If the landscape has dried out significantly, for instance during a long drought, it’s highly possible that the soils will become hydrophobic, and therefore there will be little water infiltrating as it travels along the gutters. During a large rain event, which occasionally come during the summer when moisture is most needed, due to the small capacity of the gutters, only a small amount of water will be held and given time to infiltrate. The rest will spill over the mound and down the ridge (figure 4.4). This would particularly be the case where there is a large catchment above as in the example used.
Figure 4.4 – Keyline mound overflow during a large rain event
(Note: At this point, I should mention that despite Darren’s mounds being smaller than Geoff’s swales, he places one for every line of trees, meaning that water infiltrates right at the base of each tree. Also, in the widescale forestry example of figure 4.2, the pasture in between the rows has been ripped using a keyline plow, which further increases the infiltration capacity. Similarly, when water does spill, it is in the best place possible – right up on the ridge where the water will fan out and have further opportunity to infiltrate)
The comparison in brief
Geoff’s swales – hold plenty of water in a large event but distribute the water less evenly in the landscape below
Darren’s keyline mounds – distributes soil water more evenly across the land, but holds and infiltrates less during a large event.
The keyline swale
With the benefits of each in mind, I came up with a hybrid, which you could call a keyline-swale.
It’s built just like a swale, set out on contour, except that the base of the swale isn’t level, rather it slopes from the valley out towards the ridges.
To build the keyline-swale, pegs are set out on contour. Starting at the ridge, a mark is made on each peg, rising at 1 in 500 towards the valleys. This is the guide for the blade depth (figure 5.1).
Figure 5.1 – Side section view of a bulldozer building a keyline swale
During a small rainfall event (figures 5.2 & 5.3), water is directed along the trench from the valleys to the ridges, where it infiltrates in a very similar pattern to Darren’s keyline mound.
Figure 5.2 Side section of a keyline swale during a small rain event
Figure 5.3 – Keyline swale (dark blue) infiltrating during a small rain event
During a large event, the water would fill up along the length like Geoff’s large swale, however the water depth wouldn’t be constant. One possible benefit of having a greater depth of water out on the ridges is that there will be more pressure here, causing water to infiltrate at a faster rate than it will in the valleys (figures 5.4 5.5). As the water level drops, it will of course infiltrate the remaining water on the ridge.
Figure 5.4 – Keyline swale full
Figure 5.5 – Keyline swale (dark blue) infiltrating during a large rain event
If this was a temperate climate where large rainfall events are rare, on this landscape I would go for a keyline swale at the very top of the property, and then use Darren’s keyline mounds parallel to this down the slope. This means you’ll get the benefits of water being infiltrated at the base of each of the tree rows (by the mounds), hydration of the ridgelines, while also capturing any large flows that enter the property, infiltrating them right at the top of the slope.
Cam Wilson runs Earth Integral, offering design, implementation and education in landscape rehydration and rehabilitation in the Southern Tablelands, NSW, Australia