Vegetables Gardening - Growing Organic Vegetables

We have used above ground grow boxes in our Mittleider garden for years, but here in southern Arizona, it gets beastly hot in the summer. It's 107 degrees here today, so I have to water the grow boxes twice each day as they dry out otherwise and the plants start to wilt. Has anyone planted in-ground, but instead of using your own soil, dug down through your regular soil in rows about a foot deep (we have worthless soil with rock-hard caliche) and then put your grow box soil into that trough for planting? My grow box soil is a mixture of sand and sawdust with some compost I have made over the years and purchased garden soil all mixed together. It doesnt dry out quite as fast as just sand and sawdust in our dry heat. 

Anyway, I have never attempted the above as I usually always have the grow boxes filled with veggies year'round, and the thought of digging 18" wide trenches a foot deep and 30 feet long doesn't sound fun in our hard soil, but I watched a video once of what I swear was Mittleider gardening in Israel, and since they get so little rain there like we get here, they had their beds dug down into their regular soil. In this heat it seems to make much more sense. The beds wouldn't dry out quite so fast and any time it rained, they would have water run to them if they are an inch or so below the surrounding soil.  We would have to lower the water lines that we have going to each bed which is another hesitation I have, but I'm sure it can be done. If I could save on my water bill as well as keep my soil beds damp longer, I think it could be a win in the long run. Right now in the heat, if I'm not home for the second watering each day, the plants wilt. We haven't installed an automatic watering timer yet. 

Heidi