Great input on this
thread already!
Here's the two cents of a desert rat.
Swales, or their flat-land cousins, "infiltration pits" are a huge boon in arid zones where soils tend to be sealed as in the case of tight clay or desert "glaze" or sometimes have too much gravel/sand that easily sheds water.
If your purpose is to rehydrate the soil, raise the water table, and grow trees to support a food forest; swales are awesome. Geoff Lawton stated in his
online PDC that they have noticed in their projects around the world, in all types of landscapes, it takes about 7 years to fully rehydrate the land.
And where I, personally, on my 1/6 acre lot in downtown Phoenix will never see a spring popping up on my property (unless one of my drip emitters blows a head), what COULD ultimately happen if we began to harvest water in nature strips, front and back yards, etc, is that we could slow and sink the water and being to rebuild the ground water so that the main river that runs through Phoenix - which RARELY has any surface water in it (Salt River) would start to show ponding in low spots that might last for several months. And probably a Mesquite bosc would start to grow there, and a small riparian habitat would grow. In fact, this is one of my goals for downtown Phoenix - to do these "green infrastructure" projects and eventually see a hint of water in the Salt River. I work with a group called "Watershed Management Group" and you can see a
project we did in downtown Phoenix here:
http://permaculturenews.org/2013/05/18/one-desert-city-turns-stormwater-into-an-abundant-oasis-using-green-infrastructure-practices/
Interestingly, Village Homes in Davis, CA (a fantastic
permaculture subdivision built in the 70s) uses swales between the homes to capture roof runoff and rehydrate the soil. They have been able to grow more food plants with less stress than they would have normally in their hot and dry (14" rain) climate.
"One remarkable element in the development of Village Homes involved the drainage concept of having natural swales and collection ponds to collect rain water and encourage it to seep down into the
local water table instead of being pumped through storm drains to an evaporation
pond. It was remarkable in part because during the early earth moving and shaping of the system, there was a severe drought and the system was untested. Finally, after two years, in November of 1977 it started to rain heavily. One day Mike was supposed to be conducting the "final walkthrough" for a newly completed house when he started pacing, looking outside, pacing, looking out. Finally he couldn't resist. He apologized for halting the inspection, saying, "I'm sorry, I just have to go out and see if the drainage system is working." It was and still is. In fact, the
city's streets flooded that day and excess water backed into the Village Homes system, which absorbed the city's water without any problem."
http://www.villagehomesdavis.org/public/about/history