Spacing: with 60inch of rain plants only need 10ft of rootspace to get all their water, with 30inch they probably have a 2x the rootspace (20ft) and with 15ft of rain they might need 4x (40ft) of rootspace. So don't plant your trees as close.
TapRoot: plant seeds and then graft a named cultivar, that way there will be a 40ft tap
root vs transplanted trees with
roots that only go down 3-4ft. Never reaching that deeper moisture.
Mulch: This will help cut down evaporation. And also help water to inflitrate
Swales: this will cut down sheeting on 'flat land" and give your water a change to soak in. You can plan right by te collecting
swale.
Mushroom: They actually take oxygen in and release WATER +
CO2 as waste. Yes it releases WATER. Water, glorious water.
Mushroom: The mushroom "roots" are better at getting minerals and water from the soil, so the trees actually like to outsource the job, for a bit of sugar which the mushroom love.
You might have to do some irrigation, just flood the swales once a week and/or drip irrigation