As it is central to life on Earth, so water is of crucial importance to the permaculture plot, providing the plants, livestock and wildlife with an essential element for their survival and growth. Water is also a finite resource. It is precious and should be treated as such. Therefore, permaculture design seeks to preserve and use water in the most sustainable and efficient ways. This includes harvesting rainwater, mulching and composting soil to help prevent excess evaporation from the surface, planting native species that are adapted to the rainfall condition in the area, and reusing water from the home in the garden.
The permaculture gardener is also looking to slow the rate at which water moves off the site. The best place for water to be stored is in the ground, where it is available for plants and microorganisms to use, so it makes sense to try and keep water on the plot as long as possible to allow it to soak into the soil. There are various ways of doing this, such as planting slopes with trees and cover crops to increase percolation rates, and minimizing areas of paving or similar hard surfaces that increase water runoff. Another technique available to the permaculture gardener in this endeavor is the construction of swales.
A swale is essentially a small ditch located on a slope that captures water as it runs down the slope and stores it, allowing the moisture to penetrate into the soil, infiltrating the soil profile around the swale and recharging groundwater supplies. This also has the added benefit of reducing erosion of the topsoil on the slope, and potentially providing a water body that can be utilized for irrigation elsewhere on the site.
Swales are built along the contour lines of a slope. This uses the natural topography of the slope to the advantage of capturing the water and also makes it easier to create a level swale. There are several ways to mark out the contour line along which your swale will go. You can use a laser level, a water level or a simple A-frame. This device consists of three pieces of wood – two of equal length forming the legs and a third braced between them – with a plumb line hanging from the apex. ‘Walk’ the A frame across the slope, adjusting at each step until the line is dead straight, and mark the ground. These marks will provide the line of the uphill edge of your swale.
If you have a steep slope that experiences a lot of runoff, you may need more than one swale. As a general rule, the steeper the slope the closer together and deeper your swales need to be.
Consider what will happen if your swale overflows. This could cause significant problems on land further down the slope. It is a good idea to include a spillway channel in your swale that will divert overflow to a ‘safe’ area. This could be to another swale further down the slope (indeed, some permaculture plots have a series of swales down a slope with each feeding into the next one downhill from it – in this instance it is a good idea to stagger the swales across the slope rather than having them all in a straight line down the hill), to another water body that has the capacity to hold the excess, or to an area of ground that requires a lot of water and will absorb the runoff.
Dig the swale along the contour line you have marked. The width and depth will depend upon the unique conditions of your site and the analysis you have done of it, but you will be likely be digging a minimum of 1.5 feet down. (It can be a good idea to dig your swale to your estimated required depth then wait until a major rain event and observe the action of the swale. If it overflowed quickly, you can go back and make it deeper before the next rains come.) Digging by hand will take a lot of time, so you may want to consider sourcing a mechanical digger, particularly if you are getting close to the season when rain runoff is most likely to affect your site. Use a water level or your A-frame to make the base of the swale level, but do not tramp it down; leave cracks within it to aid moisture infiltration. And as you dig the swale, pile the excavated soil on the downhill side of the swale. This will create what is called a berm, which helps stabilize the swale and provides planting opportunities.
Planting on the berm is a very effective way to help stabilize the soil around the swale. Use a variety of different plants, with different rooting strategies in order to stabilize all levels of the soil. Typically, a tree (usually fruit) will form the centerpiece on the top of the mound. Choosing a fast-growing variety will aid the stabilization effect. Utilize deep-rooted perennials alongside, as well as herbs and leguminous ground cover crops to stabilize the topsoil. Many of them will also absorb water from the swale, helping to prevent overflow. By planting in this way, you are effectively instituting a guild. The variety of root depths means that plants are not competing for water and nutrients in the soil, legumes add nitrogen to the soil to help all the plants in the guild grow, the tree provides shade as well as support, and, of course, you will be able to harvest crops from the guild. An example of a guild on a swale berm would be a pear tree surrounded by asparagus, strawberries and rhubarb (which all have roots at different levels in the soil profile), as well as dandelion, comfrey, chickweed and thyme as shallow-rooting cover crops.
Mulch the base of your swale. You could use pruned branches, straw or any other mulching material. This will help slow evaporation of moisture from the swale when it is full. Mulch the berm as well to prevent erosion, supply your plants with nutrients and keep weed species in check.