I developed a Permaculture design for one of our local schools and as part of that they had a sloping area of ground where they wanted to put fruit trees, so to maximise the water retention for the trees the designed a series of interlocking swales called fish scale swales to plant them in. Of course for that sentence to make sense it pre-supposes that you understand what a swale is, but just in case you don’t, a swale is a long excavation forming a ditch and mound along the contour of the land which allows rainwater travelling downhill to be absorbed into your land and stay within the property rather than being lost as runoff.
Swales are quite often used on extensive properties and can run for metres or hundreds of metres depending on the need but they don’t usually find much application in urban or suburban settings, however the smaller fish scale variety can work very well into a small to medium back or front yard. Rather than form a number of lines extending along the contour in series down the slope they are made by digging a number of smaller (2-3 metres) interlocking curved swales along the contour. So that no water is lost each swale is designed to accept overflow water from any uphill swales and to overflow into down slope swales if they can’t hold the water flow.
Making the Swales
It helps with planning if you make a scale sketch of the area and draw in the swales to see how they fit, but if you are not that organised you can just go with the flow and see how it turns out. I also found it useful to have a spray can of water based making paint to mark out all of the swales first, because this makes it easier to get the location and orientation of each swale right in comparison to the others, before you start to dig. My original plan was for the swales to be in straight lines but in practice they were better being at an angle, so long as they were across the slope of the land, the slope of the land turned out to be more complex than I thought.
Once you have marked out your swales, dig down 20cm to 30cm, if you can avoid digging into the subsoil that is best, but if your topsoil layer is thin you might not be able to avoid digging into the subsoil a bit. The digging is best accomplished by pushing in a spade around the line of the swale and about 30cm uphill from where you expect the downhill face of the swale to be, start to dig and place the spoil downhill so that the mound of the swale is formed in front of (or on the downhill side of) the ditch. If you intend to plant into the swale then put most of the spoil towards the centre of the swale, thinning out towards each end, forming a crescent.
When the ditch is dug and a nice swale formed, to prevent the ditch filling up with crap or becoming a trip hazard, it should be filled almost to the top with coarse river sand, this will provide bulk in the ditch but still allow water to infiltrate into the ground. The sand can be washed out or crust over so to make sure that the surface remains open to water put down a 25 to 50mm layer of medium gravel (say 20mm) on top. The gravel will also form a good surface to walk on and even if it is eventually covered by grass it will be able to maintain an open structure so that the rainwater will be absorbed into the swale rather than running off.
Planting the fruit trees
The fruit trees are planted into the front, raised part of the swale rather than in the ditch so that even though the roots will find their way down into the swale and the rich water supply, the main part of the tree will be above the soil level and the tree won’t suffer from “wet feet” in times of high rainfall.
The centre of the swale should be thickest and that is where you need to plant the fruit tree. Dig down until you have a hole big enough to easily accept the root ball of your tree, place the tree in back fill then firm the soil down around the roots with your foot, mulch well, making sure the mulch does not contact the trunk of the tree and cause collar rot.
That’s about it! The swales will add interest to your property as well as maximising use of rainfall almost guaranteeing a great fruit crop. 
