Suggestions for desert dwellers

Since your soil is pretty sandy and too well draining, how about digging your hole, put in a kiddie pool, GENTLY put in some logs, and build your hugelgarden?

At least some of the water can be retained long enough for the logs to absorb it.

You could also use pond liner as well for a bigger setup.

Anything to hold the precious little amount of rain water you get.

Peat moss also can help hold water but it is considered a nonrenewable resource according to some.

You could also build a hoop house over your garden to help maintain humidity and less water loss.

Just try to have partial shade over the plants mid day to evening so you get the first 8-12 hours of morning sunlight.

Or build your garden below ground.

Just some suggestions that could make things easier.

Remember, plants don't like low or high humidity. Usually. Or too low or too high air temperature either.

Too low of a humidity and plants transpire too much. Too high and they won't take up enough nutrients.

https://en.wikipedia.org/wiki/Transpiration

Oh and adding in some Mykos would help too.

If I were trying to put a water-impermeable barrier under my hugelbeet, I think I would look for something a tad more natural, like bentonite clay, or agar, or maybe a mixture of each. I would seal the bottom of the trench like I would a pond, and it would keep the water around longer, and from a biological perspective, keep the water more relevant to the area surrounding the hugelbeet as the agar/bentonite clay layer gradually gets eaten by soil life.

That, and you don't have to worry about digging out a degraded old kiddie pool, or an old pool or pond liner, and you don't have to worry about what all that biological activity at the soil/air/water interface is doing to the material in question. Is it leaching into the beds where you grow your food? Are you slowly eating plastic with your carefully cultivated garden veggies?

Depending on where, a hoop house with shade cloth on it to dapple the direct sunlight from mid-morning to late-afternoon might be all that's needed, or that can be added to a vented glazed hoop house for added moisture retention in the air.

Peat moss is indeed a nonrenewable resource, as its formation timeline is closer to that of coal than of growing things. Coconut coir, though, is often useful in the same way, albeit without changing the soil's pH, and there are a variety of biomasses, banana fibre, for example, that are available in stupid quantities that nobody is using that could, with or without processing, fill in quite nicely.

I recently read on another thread how someone in a desert clime was growing prickly pear cactus as a living mulch. Growing your own mulch is probably the best idea in such a climate.

-CK