I setup a system using 1/2" PVC pipe and sprinklers at each tree. You can adjust get different types that you can adjust to get the water only where you want it. It was easier than watering each tree individually. You can trench it in very easy and since you don't have to worry about freezing it doesn't have to be deep.

You can also use drip emitters/hose. A 1/2" header pipe down the line, then at each tree, come off the header with some length of drip hose. The exact length will depend upon how much your water supply can handle, the spacing and flow rate of the emitters and how big an area you want wetted around each tree. A small circle or two of the drip line should suffice. By playing with the lengths and emitter flows and emitter spacing, you can design for a certain amount of watering time.

Drip would have the advantage of lower evap losses over sprinklers, and it won't wet the trees which may worsen some diseases.