10 liters, over 1 week's time, would be 16.8 hours per liter, or about 0.05 liter per hour, which is an incredibly tiny amount. I have never seen a drip control valve that could dispense that accurately at low pressure.

You might be able to use a fiberglass wick at the bottom ... wick dries out and lets in a bit of air, some water gets released, wick gets wet and air stops for a while.

Or insert the wicking through a hole in the side of the jug, to distribute the water slowly via capillary action.

Although, as long as they are getting watered once a week with 10l or multiples of 10L, it shouldn't matter if the jug empties before the week is over, because the water is down there for the roots, and where it can't evaporate. Only very young trees, those in their first year, would need more frequent watering.

I assume you are already using thick mulches to retain water? And using native, deep-rooted trees for optimal survival.

dwl,

I think LG has it right in my opinion. I use Earthtainers for growing tomatoes. Earthtainers waters from below with soil in a basket that touches the water reservoir below the soil. I went to wicking water from below with rope to decrease the amount of moisture moving into the soil by capillary action. It has worked very well for me. Experiment with burying the container between trees and have ropes buried sideways that will move water into the soil laterally from the jug. Experiment with the diameter of the rope needed. I used 5/8" diameter nylon but I think I will use 1/2" next time. JMHO Aloha