"Swales" are one of the most common water-harvesting devices used by Permaculturists.
Like most Permaculture techniques, the use of swales is extremely site-specific, so I'd like to give a few tips and thoughts about using them in my biome, the Great Lakes region. If you aren't already familar with swales, here's a great "basic" introdcution from PRI: http://permaculturenews.org/2012/05/16/swales-the-permaculture-element-that-really-holds-water/
(Water-harvesting micro-swale/ditch system at Lillie House)
The impact of swales can be nearly miraculous, especially in dry, wet tropical or Mediterranean climates where rain comes infrequently in large events and run-off is a big problem. But, as respected Permaculture teacher Patrick Whitefield points out, they might not always be appropriate:
In the Great Lakes region, we get most of our precipitation in the Fall and Spring, though we're lucky to get ample water through most of our growing seasons. Our relatively short growing season means there's a large advantage to having well-drained garden beds that warm up quicky and can be planted early in the season. Some areas in our biome may also have mucky clay soils that get easily water-logged. So a few indicators you might want to include swales in your design would be:
1. You have to deal with excess water from hardscape features such as a roof.
2. You're on a slope and you know you have a problem with erosion or losing water to run-off.
3. You want to establish a tree-based system like a food forest or orchard.
4. You have a lot of land to deal with and you need to collect water from a large area using "extensive" methods that are easy to implement.
Some counter-indications might be:
1. You already have trees where you want swales.
2. You have mucky soils and already expereince problems with draining. (Though in some special cases swales could actually improve drainage.)
3. You have a very steep slope or no slope.
4. You only have a small area to work with (as we'll see when we do the math below, a swale will be a better tool if you have a collection area of least 20 feet or more.)
If swales don't seem to make sense, there are other means of water collection and infiltration, such as ponds, "passive swales" (wood debris gabions layed on controur,) infiltration basins, keyline plowing, wicking beds, etc. For example, at Lillie House we use a network of a wide variety of tools to intensively manage water on our urban site.
Once you've decided to use swales, put some thought and planning into their design. Poorly designed swales do more harm than good. Make sure you get the size right. I have seen swales "blow out" because they were too small for the water they're meant to handle. These "swales" do the opposite of the Permaculture water mantra: slow it down, spread it out, infilltrate it--they actually concentrate water and direct it, possibly right towards your basement or favorite crop!
On the other hand, I've seen swales that were too large, completely destroying the landscape hydrology and infiltrating water so deeply that no plant will ever have a chance at getting to it. Over-sized swales can also be a trip hazard and a danger to humans and wildlife passing through the garden. I know of a whole set of swales right next to sidewalks where kids live and play, that look like 3-foot deep death pits. These were installed by a "professional landscaper," positioned where they'll only ever get about a gallon of water in a major rain, but are super-sized to hold hundreds of gallons at once! I've even seen some "swales" dug below the water table, becoming permanent ponds.
Worst of all, I've seen a few projects (in person, and AT LEAST a dozen more online) where folks dug deep swales right through established forest they intended to keep, needlessly wasting energy and harming the forest floor and roots of their trees to no benefit at all, as established forests in our region are reliably as good at harvesting and infiltrating water and stopping erosion as a swale!
To size your swales, there are several tools that can help you out. University extentions often have recommendations specific to their regions on sizing rain gardens, which essentially perform the same function of catching water and infiltrating it.
For most people, the rain-garden alliance is a good place to start: http://raingardenalliance.org/right/slope
Their tools will let you play around with your soil type, slope and catchment area. For example, assuming a clay loam soil, and a spacing of 20' between swales, it suggested a volume of 7.2 cubic feet per linear foot, or about 1,5' deep and 4' wide. On sandy soil, it recommended much smaller swales, just 3 cubic feet of volume.
(The basic math is pretty simple to figure out if you want to do it yourself. You need to know the size of your catchment area (in square feet) multipled by .6 will tell you how many gallons of water you'll collect for each inch of rain you receive. Next you'll need to test the infilltration rate of your soil. These two pieces of information will tell you how much water your swale needs to hold. Finally, 1.3 cubic feet will hold 10 gallons of water.)
This set of rotted wood swales is fed by our downspout and is planted with water-loving plants including Marshmallow and Valerian. In heavy rains, water cascades down the side of this system and into another set of swales.