Actually, designing a swale or even a rain garden is a good idea for many gardens, even quite small ones. And it really doesn't make too much difference with the dryness - or not - of the climate. Unless already existing as a low lying, water accumulating area (i.e. a bog or swamp, perhaps only seasonally), a swale will simply catch and retain rainfall and allow it to be gradually absorbed into the soil. Among the reasons for this is to avoid seasonal flooding of the landscape, to reabsorb pollutants that are present in storm run-off before they can reach streams and groundwater or simply to retain and re-use infrequent rainfall. In your case, the latter seems most appropriate although when it does rain in SoCal, that rain can come fast and furious, so the other two reasons may be equally as valid as well, at least on a now-and-again basis.

A swale is just a low lying area......when rainfall does occur, the water will flow to the lowest point, the swale, so it does need to be constructed to be lower than current grade. If any slope to the yard at all, the swale should be located at the low point. If not, you need to grade your new "lasagna" planting to slope away from the structure and toward to the designated swale site. And you will need to excavate the swale to some degree, depending on the thickness you can manage with your lasagna method.

I doubt constructing swales was devised with a lasagna method of bed construction in mind. It does involve some work to do it correctly. Grading and directing water runoff to the correct area is critical. And you want something that slows water penetration yet provides good drainage at the bottom of the swale, not just soil - coarse sand and river rocks or coarse drainage gravel are common.

And yes, this is much more of a landscape design issue than just a soil question ;-) Do some research online to see how to construct an effective swale. Even check out rain garden sites - they are a wealth of information for this sort of activity if for a slightly different purpose.

One other thing you may want to do is investigate dry stream beds as a landscape feature. Not only do these make an attractive focal point to a drought tolerant garden, they will help to direct any rainwater runoff (even irrigation run-off) to the appropriate swale location where it can be dissipated evenly through the soil profile. Where moisture accumulates surrounding the swale and along its sides is where you can locate more thirsty plants.

If you don't already have a landscape designer, you might want to consider hiring one. They could be a huge help in the layout, design and construction of this kind of feature, as well as offering suitable planting suggestions.

Dig down in the swale area and spread the dirt over the rest of the area, or make a few mounds along the swale to get a creek bed look.

Thanks. I had wanted to put in a dry riverbed feature using river rock before we got the notion to do a swale, so we were planning to merge the ideas. There's a decent slope to the yard away from the house (maybe 8 inches down per 15 feet), so I don't think getting overflow drainage away from the main structure will be a problem.

I can see where having the swale at the lowest point of the yard would allow us to have maximum run-off from the whole yard into the swale. However, we may put the low part of the swale more at the midpoint of the yard. I think the overflow drainage will be fine, no threat to any structures, and the water we "lose" in terms of rainfall that will run off the yard instead of into the swale will be minimal. We hardly get any rain as it stands here in So. Cal., so I expect we'll need to use drip irrigation of periodic hand-watering to supplement rain for the swale plants (but my wife's more the expert on that).

Well, it sounds like there's no short-cut to digging up the lawn to create the swale shape before doing the lasagna layers. I still wonder if it'll be a waste to do lasagna layers below a bed of river rock, and if it'd be more efficient to amend the swale soil some other way before adding swale plants & river rock, and just doing the lasagna layers on the other parts of the lawn...

Ken

Some people recommend NOT amending the soil for California native plants. So you might not need the lasagna layering for plants in/near the swale if they are natives.

if your fear of digging the hole is that it is too much labor, remember that there are thousands of people who would LOVE to dig that hole and more for you for the same price as a few plants. half day of labor should cost less than $100. craigslist, nextdoor, day labor non profits.

its a great use of resources, keeps the money in the community, etc etc.

Hi Ken - did you ever do the swale? I am a west hills gardener too and would love to hear about your results :)