My suspicion is that your concern is correct, and a low water table will make it more challenging to grow trees without irrigation. Especially with well-draining soil.
I live in Utah, which has a similar climate, but a bit wetter and colder. In my
city, we get an average of 18 inches of rain a year, most of it in the winter as snow. My soil is pure sand, so water disappears quickly.
I live across the street from a river, so the water table in my neighborhood is high. I can share a few observations I've made.
It's indisputable that there are a lot more wild trees near the river. I only see wild chokeberries and mulberries near the river, for instance.
Far away from the river, wild spaces in the valley don't have trees -- just bushes and grasses. The further you get from the river, the more you'll see that the only wild plants are winter annuals. (Rye seems to like growing wild here, for instance.)
My recommendation is that you add some natural sponges to your soil, so it doesn't drain as quickly. That will be almost like creating a high water table for your plants to draw from.
I recommend either
hugelkultur,
biochar, or preferably both. Got a bunch of sticks, autumn leaves,
wood chips, paper,
cardboard, or anything else like that? Dig a great big trench, fill it up to about a foot below ground level, and preferably add kitchen scraps and
urine at the same time. (The kitchen scraps and urine will provide
enough nitrogen to keep the
carbon sources from tying up any nitrogen from the soil. It seems to work that way for me, anyway.) Put dirt back on top. Plant seedlings into that dirt. Irrigate them only until you're sure their roots have reached that layer. Then taper them off and see how they do.
I'm given to understand that biochar is even better than wood, because it's actually a
permanent soil amendment. It won't go away over time. Thousands of years later, it'll still be there, acting as a sponge to keep water available up high for plant roots.
If you're willing to make a lot of biochar, and charge it by pouring urine into the pit with it and then covering it up with your soil, that would probably be a lot of work, but you'd only have to do it one time ever -- and your trees would benefit from it for their whole lifetimes, as would their descendants.
I think that would probably be your best option for growing trees without irrigation in the future.
If you're interested in trying it, I highly recommend this playlist:
https://www.youtube.com/watch?v=9eBMxk_gu_M&list=PL60FnyEY-eJAfBgRAHKXjsNiUwqD1GTFK
Skillcult has some really awesome stuff. He does a lot of research and experiments a lot.
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