I have used the 1/2" cylindrical pipe with emitters either every 12" or 18" in two gardens and an orchard. The modern ones are so incredibly easy to set up, my fruit rows are close to 100 feet, I only had to make a few cuts, the whole system was set up in less than two hours, just by myself, 600 ft of pipes. In a garden there are many more cuts (the beds are only 12 ft long), with two helpers it only took 3 hours.

The pipes are under mulch, I expect them to last a long time. I do not see the need for spirals, each emitter drops close to 1 gph, 1 gallon/day will be OK for a newly planted tree, but a fairly developed tree has roots that reach several emitters, and therefore several gallons. Again, I suggest a straight line, it will be fine. I found last year that, with thick mulch, 1 gallon per day in clay is much too much for new trees, and it was a dry year. 2 gallons once a week was still more than enough, producing always wet or moist soil.

If there is any slope it is a good idea to have a dead one foot line at the point of lowest elevation. Currently, end caps are made with 8-shaped plastics, and it is a second to open and close one. This will assure that you can purge the line before freezing weather sets in. Do not buy an expensive valve, unless you have sectors with different watering needs.

Thanks glib.

I had done my original calcs for how much water a tree needs, by using the 1" of rain per week rule of thumb for the area under the drip line. For one of my mature SD apples, with a 25' diam circle, that if roughly 300 gal of water a week. which matches nicely with what I have read for water requirements for an apple tree in the SW. Small trees come to around 20g per week using this method. FWIW, in practice these numbers are close to what I have been putting on the trees during the drought this last summer and seem to be in the ballpark (no stunted growth and the soil was dry at the next watering).

I only mention this, because these numbers are quite a bit higher than those you mention. Of course, you may not be growing in the SW drought, and my soil is more sand than clay.

Doing a straight run of emitters for each tree would be a bit easier, but I am concerned about how the roots would react if/when irrigation is the only water source, and one side of the tree stays very dry, like that last year or two. With a sandy soil It seems that the "other" side may loose most of its roots.

I am curious, what type of connectors did you use, compression or drip-loc. And did you have any leaking problems?

I used compression in 2002, and spin-loc in 2012. I do not know how much compression fittings have changed in a decade, but IMHO spin-loc is 10 times better. No leaks, and each connection is a breeze. With compression, each connection was a wrestling match. although a leak developed in the garden at a spot where a T-spin-loc connector was being stepped on often (I placed some bricks around it). I can see your reasoning, that a tree with twice the canopy diameter needs 4 times the water, whereas its roots only reach twice the emitters, and I do have clay. Maybe a loop around each tree is what you need, or a short branch with extra emitters.